HP 608C Repair. - Page 1
Forum section: electronics / repair

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 12:34:14 am
content: I have a 608C on my bench today and the problems include;A) No measurable output.B) Squirrelly meters.C) Black beauty paper caps.To start i am going to replace the paper caps with new ones, though i would appreciate advice on the first two problems. Meanwhile, here are some pictures to ogle at.In order,A picture of the transformer in this thing, note the quarter for reference.The rear.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 12:37:58 am
content: More pictures.Top viewRight side view.Just a note, this thing weighs 100 pounds. The manual says 60 pounds but thats for the non rack-mount version, don't ask me where the extra 40 pounds is but i get the impression they beefed up the chassis.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 01:16:12 am
content: Right side paper caps replaced with capacitors up to modern snuff. At least the capacitors are, not sure about my soldering but neither the Heathkits nor the Hickoks have complained about it.

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 01:52:57 am
content: Quote from: neo on February 26, 2018, 12:34:14 amI expect that the transformer might be an oil filled one, was an ex military unit? I had an old oscilloscope that had oil filled transformers and capacitors as well. These photos show the unit, these are not my photos but some I found on the web.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 02:00:29 am
content: Quote from: Specmaster on February 26, 2018, 01:52:57 amHonestly, i don't know about either. It wouldn't surprise me if it was oil filled transformer and it definitely wouldn't surprise me if it was military. The only things i have ever seen that were completely devoid of stickers has been military, my bet is either the navy, the air force, or a very specialized lab.It was made in 1973.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 02:14:25 am
content: Looks like it is pretty good performance for a 1954 RF oscillator design. They were still making it in 1973? Drift less then 0.005% over 10 minutes. They really worked hard on the stability - low tempco Invar steel, split stator variable capacitor so there were no sliding electrical contacts. 1% accuracy and 0.1% resettability. The 608D model includes a crystal so you get 0.01% accurate reference points every 5MHz. I gather you used headphones to zero the beat frequency.http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1954-08.pdfOnly cost $850 new in 1954. Probably half a family car.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 02:17:30 am
content: Quote from: amspire on February 26, 2018, 02:14:25 am$850 in 1954 WAS a family car. Accounting for inflation that is $18k

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 02:31:03 am
content: Output accurate to 1db across all frequencies from -127dBm to +7 dBm. Below 0dBm, the accuracy is typically double or more. 10Mhz to 480MHz. That is a good generator!

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 03:54:03 am
content: I can't find the last 4 paper caps, they hid them well.Those 4 may only be in the 608D, thats the only manual i can get.C# uF ESR VALC52 15 3 10C52 12.44 3.4 10 second halfC42 106 0.56 80C55 30 1.8 10C40 112 0.49 80C75 1300 0.21 1000C45 53.9 0.59 45C46 15.62 2.5 10C7 26.36 2.3 10C7 23.04 3 10They did something odd with C75, i had to pull it to test it. If you say it's good i'll put it back in taking extra care to get it exactly as it was, without a manual i stand a snowballs chance of actually understanding it.The rear with new caps and the left side.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 03:56:55 am
content: Left side without shield and transformer.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 04:22:48 am
content: Quote from: neo on February 26, 2018, 03:54:03 amAh! You have had the modifcation for regulated DC filament operation:http://bama.edebris.com/download/hp/608c/HP_608C_Service_Note.pdfC75 is a 1000uF 25VW electro for the filament supply. Could be worth replacing, but for the job, your existing still looks adequate. It probably only powers the filaments of one or two valves so it is not anything special. The reason for the mod was to reduce ripple on the RF output. This mod means your valve VHF oscillator is infected with a transistor!

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 04:30:16 am
content: Quote from: amspire on February 26, 2018, 04:22:48 amThanks!As i said it was made in 1973 (as far as i know), a transistor in the ointment was inevitable.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 04:41:54 am
content: Quote from: neo on February 26, 2018, 04:30:16 amI can believe they were selling this after 19 years of manufacturing. It is a very good VHF oscillator today - except for the size and weight. There was still a lot of valve stuff available in 1973.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 05:16:17 am
content: I've still not found a culprit.Replaced all paper caps, checked all eloctrolytics, put everything back as it was and i turned the thing on.Still no output, percent modulation meter is still screwy. Everything glows, v15 which is a 5651 particularly brightly.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 05:28:17 am
content: Looking at the front panel for the first time in months i stand corrected there are stickers and it wasn't made in 1973, it was cataloged in 1973.It was owned by the United States Navy, circa 1973. So now it just jumped to the top of my priority list to get this thing working.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 05:37:22 am
content: Luckily, it is fairly easy to debug with basically one valve per function. If I were debugging this with an oscilloscope, I would probably be using one of the cheap 100x high voltage probes that are available for about $20. You could make a simple capacitor isolated 1N4148 RF detector probe and just go searching for RF at the oscillator, buffer and power amplifier.Also the resistors can drift high - probably worth checking. Many of them look like carbon composition resistors which are often not particularly stable.The RF band switch is critical - if the contacts need attention, that will kill the oscillator.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 05:40:57 am
content: Quote from: amspire on February 26, 2018, 05:37:22 amThe only thing of those i know how to do is check the resistors, though that will take some time. I'll get started on that.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 05:46:31 am
content: Quote from: neo on February 26, 2018, 05:40:57 amWhen you are checking them, remember most are +/- 20% tolerance. You are probably used to 5% or better. So if a 1K resistor is somewhere between 800 ohms and 1.2K, it is still within tolerance. If there are only 3 colour bands on the resistor with no tolerance band, they are 20%.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 05:57:08 am
content: Quote from: amspire on February 26, 2018, 05:46:31 amThank you for reminding me and making sure i was aware, though i already knew that.I ran the band switch through a few times with contact cleaner and it didn't make a difference, still no output.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 07:36:27 am
content: It might be of note that when i got this thing it worked, squirrelly meters not withstanding. Then it just stopped, i suppose it could of been on the edge and just said "enough"...

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 07:44:18 am
content: Quote from: neo on February 26, 2018, 07:36:27 amHave you had the tubes tested, it could be as simple as replacing a duff tube or two.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 08:03:24 am
content: Quote from: Specmaster on February 26, 2018, 07:44:18 amUnfortunately I've no tube tester and the drug store variety has long since gone the way of the dodo.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 08:14:48 am
content: Even without a tub tester, I think it will be a very interesting exercise in fault finding, because you can just work through it stage by stage. Are the supply voltages OK? Is the 225V series regulator OK. Is the 400Hz modulation oscillator OK? 1KHz? The Pulse generator? The transistor DC filament regulator OK? The Oscillator? The Buffer? The Output Amplifier? The Level meter?You will get there.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 08:20:49 am
content: Quote from: amspire on February 26, 2018, 08:14:48 amAll of which would be easier with the correct manual, all i can find is the 608D.It still will be a very interesting experiment and exercise.Edit to note: I'm not relying on the meters to tell me if i have an output, i put the output on my scope and got nothing.

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 08:39:19 am
content: Quote from: neo on February 26, 2018, 08:03:24 amHmm, I know what you mean, but I've discovered that a local company to me that specialises in sourcing hard to find parts actually has a tester for their own use and will also test for customers. Try looking on line for companies that sell tubes in your area, they will also more than likely be able to test them for you.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 08:42:02 am
content: Why do i need a 100x probe exactly? My scope has a maximum input voltage of 500V ac coupled, and even if there is 500V present then with a 10x probe it becomes 50V, well within spec.Am i missing something?*NOTE* scope rating at 1 kilohertz or less, can't find a rating beyond that and the manual don't say.Quote from: Specmaster on February 26, 2018, 08:39:19 amI don't even know how to begin looking for that... yes the google-fu is weak in this one. Just try searching for tubes by location perhaps?

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 09:02:14 am
content: Quote from: neo on February 26, 2018, 08:20:49 amI think the only differences is that the 608D has the crystal oscillator and headphone amplifier that is not in the C version, and the output stage is different. Perhaps the 608C does not have the buffer stage - you will work it out.QuoteJust be careful with the scope. You can destroy the scope input very easily especially if you are using x1/x10 switch-able probes.Probes like this make your scope pretty safe:https://www.aliexpress.com/item/100X-P4100-Oscilloscope-Probe-High-Voltage-2KV-Oscilloscope-Scope-Passive-Clip-Probe-Kit-100MHz-With-Marker/32850502604.htmlThe maximum voltage that ever reached the scope inputs is 2.25V with this oscillator - that is very safe.To test the oscillator, you should be able to put the scope on the band switch contact going from the oscillator stage to the grid of the next stage. If there is a signal there, then you have a problem in the amplifier stages. No signal and the oscillator has a problem.The oscillator valve gets its filament voltage from that DC mod circuit, and that could fail, particularly if they used a germanium PNP power transistor. The voltage on the transistor emitter should be about 9.3 to 9.7 volts DC. If it is 12-16V, then the transistor is shorted and the filament in the oscillator valve may have been damaged. Hopefully it is OK - that will be a really hard valve to replace:http://www.r-type.org/exhib/aaa1026.htmThe valve came out at the same time as the oscillator, so the valve manufacturers may have designed it to HP's spec. Definitely not a standard valve.Be really careful probing anywhere near the valve or the DC filament circuit. One slip could destroy the valve.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 09:06:29 am
content: Quote from: neo on February 26, 2018, 08:42:02 amIf you never make a mistake, and if the x10 switch never gets accidentally knocked to x1, you probably will not destroy your scope. Put some tape around the switch so it cannot ever go to x1 and test it.If you use x1/x10 switchable probes near high voltage without disabling the switch, you will one day make that 1 second mistake.

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2018, 09:18:30 am
content: Quote from: amspire on February 26, 2018, 09:06:29 amYep, do yourself a favor and limit usage of 10x probes to less than 200V.With 100x probes; voltage rating, frequency derating, scope and personal safety are much enhanced.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 09:28:18 am
content: Quote from: amspire on February 26, 2018, 09:06:29 amHow fortunate then that my good probe isn't switchable and is 10x only. Thanks for all your help up to this point, i'm not a stranger to high voltage but neither am i an expert so your help and input is valued. Especially since my knowledge of vacuum tubes is iffy.Getting a 100x probe would mean putting this thing back on the shelf for a couple of months, for financial reasons I'd rather not explain. So unless you say that using a 10x probe, as a last resort, to fix this thing is suicide (scopeicide?) then i will use the 10x, as a last resort.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 09:32:41 am
content: Quote from: amspire on February 26, 2018, 09:02:14 amI thought the output was limited to a volt?

author: tautech
author_rank: Super Contributor
timestamp: February 26, 2018, 09:34:06 am
content: Neo, I have in the past used 10x probes on mains but I'm reluctant to do so now.If you take a small step back andthinkbefore you connect you'll be OK if you know what you're doing.Get the DMM out for a quick sanity check rather than risk your scope or yourself.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 09:38:07 am
content: Quote from: tautech on February 26, 2018, 09:34:06 amYes, exactly the words i was looking for. Save the oscilloscope for a last resort option, and only if the point in question has already been examined and found to be within bounds.Use anything but the scope on the inside of the 608C unless i have to. Just a little bit of common sense goes a long ways."What does this do?" Famous last words.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 09:59:56 am
content: Make up a little probe like this:For the input capacitor, use something like 30pF to 100pf instead of 100nF. Try and find a 500V capacitor if you can. A 50V capacitor is not enough.or this:http://www.electroschematics.com/659/rf-probe-circuit/Use common signal diodes such an 1N4148 or 1N914. Nothing special as long as it is a fast low power diode.The construction can be quite crude - parts soldered together and wrapped in tape. Just make sure the input capacitor that takes all the DC voltage cannot short out on anything.In both cases, the negative rail has to connect to the chassis. A clip on the probe end would be handy. Keep the probe end short if possible.Connect it to your multimeter on the volts range.If RF is present, you will get a voltage out. If it is not, you will get zero volts out.For detecting Audio signals in the 608C, it should be OK to use the AC voltage range of your multimeter. It probably has a frequency range too that you can use to check the 400Hz/1000Hz.

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 10:06:26 am
content: Quote from: neo on February 26, 2018, 09:38:07 amThat sounds like good common sense to me

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 26, 2018, 05:51:21 pm
content: Quote from: amspire on February 26, 2018, 05:37:22 amCheck the contacts of all rotation switches to see if there is black silver tarnish on them. Switches of that vintage tend to be silver plated. They go high-resistance / open-circuit if that's the case. Inspect the wiper and the contacts.With the unit unplugged and discharged an ohmmeter between the wiper and the contact will usually work well. Check at the wires or terminals. If a connection is suffering from tarnish turn the knob a little, and the resistance will jump all over. I like analog meters for this type of work.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 08:38:27 pm
content: Quote from: Tomorokoshi on February 26, 2018, 05:51:21 pmHow do i hold two probes in place and turn the knob? None look tarnished, testing their connections now.

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 08:46:22 pm
content: Quote from: neo on February 26, 2018, 08:38:27 pmGet a pair test leads with croc clips?

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 08:49:09 pm
content: Quote from: Specmaster on February 26, 2018, 08:46:22 pmGood idea, but the way the contacts work it isn't exactly feasible. Nothing to clip on to.

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 09:01:21 pm
content: Solder some temporary tags on it?

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 10:00:46 pm
content: Quote from: Specmaster on February 26, 2018, 09:01:21 pmIt is a thought, though for now i think i have checked the contacts well enough. No visible tarnish, no noticeable erratic nature to them. Besides all else they were in a sealed case for the years.So moving on to basic voltage checks before i start pulling resistors to test.

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 26, 2018, 10:28:57 pm
content: Don't remove resistors unless you have to, you'll make a mess. Check them first, if they are higher then rated (by tolerance band) then they are obviously bad. Only remove them to check if they read much lower than marked.You can use a scope if your B+ doesn't exceed 300 volts (the max for most 10x probes).

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 10:36:57 pm
content: Quote from: Cyberdragon on February 26, 2018, 10:28:57 pmThank you, though the way the resistors are soldered in there is no way to lift an end and check the value. To do that, check the value, they pretty much have to be removed.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 10:54:50 pm
content: I checked theheater V, 6.32 VAC+225 V, 226.7 VDC-165 V,-165.6 VDCBy all logic this thing SHOULD work.... so why doesn't it?Also oddly the transistor is 2n1554, i thought it was meant to be a PNP?Short of taking this thing to bits i am running out of things to check, should i check the resistors now?

author: Specmaster
author_rank: Super Contributor
timestamp: February 26, 2018, 11:17:57 pm
content: Quote from: neo on February 26, 2018, 10:36:57 pmPersonally, I'd be inclined to snip the longest leg of the resistor through, measure the values and if they are within spec, make a solder bridge across the cut, otherwise replace them.

author: amspire
author_rank: Super Contributor
timestamp: February 26, 2018, 11:29:52 pm
content: Quote from: neo on February 26, 2018, 10:54:50 pmIt is - a 90W Germanium PNP transistor.That definitely needs checking. If you look at the circuit, there is a 7 ohm power resistor on the emitter of that transistor. If you can read the voltage to both sides of that resistor it will check that the DC regulator is working and if current is flowing to the oscillator valve. If there is about 9.4 volts on one end and something less on the other end, then it is working. If it is 9.4V both ends, then we have a problem.If you use a multimeter to check the junctions of the 2N1554, you will get a lower voltage drop then usual - perhaps 0.2V for a power transistor. Much lower then silicon.

author: neo
author_rank: Super Contributor
timestamp: February 26, 2018, 11:56:42 pm
content: Quote from: amspire on February 26, 2018, 11:29:52 pmI cant find that resistor, it looks as if it just goes straight from the emitter through.EDIT: to fix a brain fart.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 12:11:51 am
content: Quote from: neo on February 26, 2018, 11:56:42 pmDo you mean the emitter?

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 12:14:08 am
content: Quote from: amspire on February 27, 2018, 12:11:51 amI did, thank you for catching that. Besides no resistor there is also no voltage on the emitter of that transistor.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 12:22:50 am
content: 1.5 VDC on the base5.8 VDC on the collector.075 VDC on the emitter.15 VAC on the emitter.06 VAC on the collector1.2 VAC on the base

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 27, 2018, 12:29:55 am
content: Obvious question time...But are all the tubes glowing? If they are, then there are no problems with any heaters.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 12:33:02 am
content: Quote from: Cyberdragon on February 27, 2018, 12:29:55 amI've already said it, but yes, all that do glow are glowing. The oscillator however uses a regulated supply and the oscillator tubes don't (visibly?) glow.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 12:43:41 am
content: Quote from: neo on February 27, 2018, 12:22:50 amWell there is your problem. The oscillator valve has no filament power.Do a resistance check from the emitter to chassis (with no power). If you get a resistance under 100 ohms, then the filament is OK.Check the emitter-base and collector-base diode junctions to check the transistor is OK.It is obviously not shorted which is great. C75 should connect to either 4 separate diodes or a bridge rectifier. The bridge rectifier should have 12.6V AC coming in and probably over -15V DC out. You are measuring +5.8V. (is it plus?). It is all odd, but there are not many parts to look at. If the other valves are working, the 12.6V is probably fine that would suggest the bridge rectifier may be a problem.If you go back to that pdf describing the mods for the schematic.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 12:45:14 am
content: Quote from: Cyberdragon on February 27, 2018, 12:29:55 amThe oscillator valve is a weird thin rf coaxial valve. You probably cannot see its filament, but it clearly has no filament voltage right now.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 12:49:02 am
content: 5.5 ohms. going on to the check everything else.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 12:52:58 am
content: Quote from: neo on February 27, 2018, 12:49:02 amLooks like the oscillator valve will live! Everything else is fixable. I would probably disconnect the wire from the DC regulator to the valve for now until the regulator is working again. Replace is with, say, a 1K resistor from the emitter to ground. Do not reconnect the valve until you see something like -9.4 volts on the emitter.

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 12:53:54 am
content: Quote from: neo on February 27, 2018, 12:33:02 amEven with a regulated supply, the heater will glow, if you can't see it normally then look at in a darkened room, if there is no glow, there is no heating, that is a smoking gun right there, check the heater voltage to see if there is a supply to the heater, if so, then replace the tube.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 12:57:34 am
content: QuoteNo it is negative, my mistake. and it is now 7.5.Quote from: Specmaster on February 27, 2018, 12:53:54 amAs said it's a special tube, on a regulated supply separate from the rest of the tubes. As also was said there is no voltage there; so even if it wasn't a weird type of tube it wouldn't glow anyways, since there is no voltage

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 12:58:51 am
content: Quote from: Specmaster on February 27, 2018, 12:53:54 amThis is the oscillator valve:Depending on how it is mounted, the glow may be invisible. The valve was released the same year as the oscillator, so it was probably custom designed for this oscillator.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 01:17:39 am
content: Let's take a trip to oscillator-ville!

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 01:33:21 am
content: 8050A on 2kpositive to base;negative to emitter; 1.58negative to collector, OLnegative to base;positive to collector, .1042positive to emitter, .1042positive to emitter, negative to collector .0509reversed, .6200my gut says to replace it, but with what?

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 01:42:06 am
content: Quote from: neo on February 27, 2018, 01:33:21 amThese two look perfect. The rest is odd, but I assume it is still connected to the other parts.QuoteYou can just replace it with a silicon PNP. It will only drop the valve filament by about 0.3 to 0.4 volts and that will not matter.Get a TO3 PNP transistor such as the 2N2955, MJ15003, 2N3772, 2N3773, 2N5884If you find a TO3 silicon PNP transistor, it will almost certainly be OK. As long as it is not something like a 1.5kV TV transistor with a low current rating and gain. Just don't get a transistor from Aliexpress or a Chinese ebay store - there is a good chance of a fake with little power handling and reliability.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 01:48:01 am
content: Quote from: amspire on February 27, 2018, 01:42:06 amI have a large quantity of vintage parts, how about a 2n665 from 1964?It tests out with a hfe of 11, i'll keep looking unless this is fine.

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 27, 2018, 01:51:05 am
content: Quote from: amspire on February 27, 2018, 01:42:06 amDo not assume this circuit can tolerate a voltage shift! If it's this specialized it might go haywire!NO! You most certainly can't just throw in any transistor that fits! There's no feedback, it's expecting an exact gain!You can get these transistors as surplus (or the equivalent NTE121), for about $35 USD. If you use a silicon transistor, make sure the gain is the same.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 01:53:48 am
content: The first 665 can be assumed to be a dud, i got another one with a gain of 87.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 01:58:58 am
content: Interestingly the equivalent of the 665 is the NTE121EDIT: OK i have two like new 2n665 transistors, the equivalent of which is the NTE121. One test with a hfe of 61 the other 84. Both on a Chinese transistor tester, both like new.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 02:13:23 am
content: Quote from: Cyberdragon on February 27, 2018, 01:51:05 amIt is just a simple emitter follower regulator using a zener reference on the base - it is not expecting an exact gain.The transistor only powers the filament which is inside a cathode - I didn't think changing the filament from -8.7v to -8.4v would change things that much. I could be wrong.If that transistor shorts, it will probably destroy 2 or 3 irreplaceable valves. I just trust modern silicon transistors more then PNP Germanium transistors made in the 1960's.What I can now see is that this circuit drives all the filaments in the Oscillator assembly - not just the oscillator valve. That explains the low cold resistance of 5 ohms. The current when hot could be about 1 A or less.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 02:19:55 am
content: Quote from: amspire on February 27, 2018, 02:13:23 amSo will a 2n665 work? I got two and they seem to be a near direct replacement.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 02:31:57 am
content: You would be replacing a 30A transistor with a 5A pre-1963 era transistor. I do not like it.I would disconnect the base and emitter and test the 2N1554 transistor again.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 02:35:54 am
content: Quote from: amspire on February 27, 2018, 02:31:57 amThey were already disconnected. I took the transistor out of circuit and tested it on my tester, the aforementioned Chinese one, and it said "broken".Guess i'm going to the garage and looking for a transistor.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 02:42:00 am
content: Does the Chinese tester understand Germanium power transistors? They can have much higher leakage then silicon. Also the gain figures will be at milliamp currents - you need to have 30+ gain at 1.5A. The 2N665 has a 30+ gain at 500mA, but it may drop above that.Were the Fluke 8050A measurements done out of circuit?

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 02:46:54 am
content: Quote from: amspire on February 27, 2018, 02:42:00 amWith the base and emitter disconnected, yes.

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 27, 2018, 02:50:20 am
content: This won't be anywhere near 30A, it shouldn't be a problem. Don't be paranoid about the transistor, bipolars like to fail open (FETs like to short). Even if it does short, the diode will clamp it.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 03:24:43 am
content: Quote from: Cyberdragon on February 27, 2018, 02:50:20 amTransistors short all the time - it is the usual failure mode. Often when you see open transistors, they shorted and the blew a wire internally.The diode is no protection - if the transistor shorts, the diode will hopefully blow too. It would be trying to drag a transformer output down.I would still go for the silicon power transistor. It will be much more reliable then 50 year old Germanium transistors and it cannot harm the oscillator. If the circuit is extremely touchy, it is possible that the slightly lower temperatures in the valves affects performance, but it will get the valves working again.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 03:36:40 am
content: Quote from: amspire on February 27, 2018, 03:24:43 amOK lets try a bit of a compromise, i put in the 2N665 now and get the thing working and then at the soonest possible convenience put a new one in? or perhaps a fuse?

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 03:51:35 am
content: A fuse and a thyristor latch circuit to clamp down on the voltage is not a bad idea. Protecting those filaments is key to keeping that oscillator alive. A circuit consisting of a thyristor from emitter to ground, a resistor from gate to ground and a zener to the output would do the job. A capacitor from gate to ground can be used - you do not need instant response.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 04:20:39 am
content: Quote from: amspire on February 27, 2018, 03:51:35 amCould i trouble you for a schematic of that?

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 04:51:52 am
content: http://axotron.se/index_en.php?page=26I just picked of a ST TN1610H-6T from Digikey (85c price)The values of parts is not critical. If you go for the thyristor-zener-resistor-capacitor circuit, the resistor can be 1K. The capacitor could be 10nF. If the thyristor switches on at 1.3V, and you want to stop greater then 10V, then you want a 8.7V zener. Get both a 8.6V zener and a 9.1V. Try the 8.6 first. If there are spurious latches, then use the 9.1V.It might be better taking the anode of the SCR to across the 1000uF capacitor. Shorting the emitter might just cause the transistor to current limit instead of blowing the fuse.The TL431 based circuits can be set to trigger within 1% of a voltage you set while the one above is within about 10%.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 05:58:10 am
content: Quote from: amspire on February 27, 2018, 04:51:52 amI replaced the transistor and left the emitter unconnected, the V out into a 700 ohm load hovers around -9.5 to -9.7 VDC. Referenced to the chassis.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 07:10:22 am
content: I'm still at a loss understanding that fuse, maybe i'm a bit dim. Seems to me it would be just as effective if i added a half amp to the load and then used that as the value for a slow blow fuse?

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 07:22:37 am
content: and on the note of confusion, why does the schematic show a 7 ohm resistor in series with the filament and it's not there in my unit? Besides that, wouldn't that just form a voltage divider and limit the filament current?

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 07:26:52 am
content: Quote from: neo on February 27, 2018, 07:22:37 amYes to both your questions and as regards to the 7 ohm being there on the schematic, that might be because you don't actually have the correct schematic do you? If so than that might well be correct.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 07:45:27 am
content: Quote from: Specmaster on February 27, 2018, 07:26:52 amIt is a 608C service note i got it from.

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 07:50:45 am
content: Quote from: neo on February 27, 2018, 07:45:27 amWell in that case it will limit the current to the filament as well as the voltage but on your model I'm afraid I'm batting in the dark.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 08:35:44 am
content: Something weird is happening.small is 400R, across voltmeterlarge is 5R, resistor not filaments and only applied to the output.Emitter, small load; 5.14Emitter, large load; .070Collector, small load; 5.51Collector, large load; no changeBase, small load; 5.20Base, large load; -.05

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 08:52:38 am
content: Quote from: Specmaster on February 27, 2018, 07:50:45 amThe service note actually explains that it should be chosen to get the filament voltage down to 6.5V for the 608C.I think someone installed the mod, and didn't bother to put a resistor in.It wouldn't have been provided with the mod hardware as it is a select on test.6.5V is a very normal sounding filament voltage so I would go with that. 9V sounds high. You always want to start with the lowest voltage as it will prolong the life of the oscillator, so I would add the resistor.My guess is about 3.9, 4.3 or 4.7 ohms 2W. I find 7 ohms hard to believe, unless the mod only does one valve in the oscillator section. We do only have the 608D schematic.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 08:57:37 am
content: Well it kind of looks like it is only for one, each filament connection is brought out individually and this only goes to one. Hows that account for the voltage oddities?

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 09:26:57 am
content: Quote from: neo on February 27, 2018, 08:35:44 amDo you mean negative voltages? There shouldn't be any positive voltages.The mod is powering just the oscillator - that does make sense. Check the other filament supplies in the oscillator section are 6.3V AC.Did you put the 1000uF C75 back properly? The negative rail of the capacitor has to go to the collector. -5.51 on the collector is definitely wrong. I think it is something more. I think there is either something funny about the bridge rectifier, or there is not 12.6VAC going into the bridge rectifier. Until you have proper fullwave power going to C75, the transistor cannot work.The base bias to the base of the transistor is from -165V through a 18K resistor (R127). If that is missing, you will not get the -9V across the zener. If you short the base to ground with a meter current range, you should get -165/18K = -9.2mA. The power transistor will need a gain of at least 40 at about 300mA.If it is all too much, scrap this bodgy mod and get a LM337 TO3 adjustable negative voltage regulator. Make it adjustable with a potentiometer. See the data sheets. C75 + the bridge rectifier remain. You have to sort out the 12.6VAC/Bridge rectifier/C75 issues before anything else. That should be simple - just three things to check.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 09:29:30 am
content: I'll check again, i referenced them to the chassis.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 09:34:13 am
content: Quote from: neo on February 27, 2018, 09:29:30 amThe chassis is fine. All the voltages should be negative compared to the chassis.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 09:36:28 am
content: Quote from: amspire on February 27, 2018, 09:34:13 amI had the leads reversed, my mistake. Yes they are all negative, the one marked as negative is positive.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 09:50:14 am
content: FL3, 226.4 DC+ 0VACFL2 165.44 DC- 0VACMy mistake assuming FL meant filament, so yeah this mode powers the whole oscillator section apparently.Completely unloaded I.E Emitter not connected,Emitter, -45Collector, -54Base, -45700R load on the Emitter,Base, -9.65Collector, -18.71Emitter, -9.5400R load on Emitter,Emitter, -5.2Collector, -14.6Base, -5.23

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 10:05:37 am
content: Between the negative of the bridge rectifier and the collector is a 51 ohm resistor, not on schematic.The negative end of the bridge with400R, -14.25, .027VACNC , -10.8 , .4VAC700R, -18.64, .26VAC

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 10:16:01 am
content: Where is the 9V zener?51 ohms to the collector? That is crazy. I think you have to draw this circuit from the transformer to the valve filament.Before this mod, the 608D used a multivibrator valve circuit to generate the filament current (V17). Is that circuit still present?Have you looked for R127 to the base?

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 10:19:06 am
content: Quote from: amspire on February 27, 2018, 10:16:01 amI drew it out when i saw that 51 ohm resistor, all is exactly as it should be with the exception of that one resistor. I didn't check the transformer connections, didn't suspect them, but i can if you like.

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 10:30:37 am
content: Is the schematic of the mod as provided byamspirecorrect to as per your unit?I'm trying to follow your measurements and descriptions but they don't seem to be relevant to that schematic in my opinion, so are we all talking about the same thing or do you actually have a slightly different circuitry there? Perhaps if you do have something different, it might be a good idea to draw it out and post here so we can actually see what you can see.Another thing is could you also clarify just what the voltages are the red and green, red is connected to the left of R125 (7 ohms) and green (FL3) is not shown connected to anything on this drawing?I also take it that the junction of CR18 and CR17 is connected to the filament of the tube and the other connection to the tube from the junction of CR19 and CR16 which is DC, so are all the other voltages here AC?

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 10:44:08 am
content: I've said what was on the FL's, red is filament green is FL3 which is 226V.As I've also said, though just to be entirely clear. The schematic is almost dead on but some of my circuitry is physically different.One end of the filament is the chassis, which the junction of CR17 & 16 are tied, all voltages unless otherwise marked are to be assumed to be DC.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 10:49:55 am
content: OK this is odd, across CR20 i get -6.8V, i move the ground clip to the chassis it somehow becomes -45V

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 10:53:33 am
content: Someone was messing around in here long before me. This is all kinds of screwed up.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 11:09:47 am
content: A connection to ground was ignored, i fixed that and now i getNo load;Emitter, -7Collector, -17Base, -6.84700RBase, -6.84Collector, -16.5Emitter,-6.74400REmitter, -6.71Collector, -16.20Base, -6.855RBase, -3.75Collector, -12.72Emitter, -3.35I don't know about you but i would call that progress.With that i bid you adieu, it is 5 am in these parts and i need to be up by my definition of early.

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 11:11:32 am
content: Just a note if the filament is 5.5 ohms, which it was when i measured it, then -3.75V can deliver 750mA to it.

author: amspire
author_rank: Super Contributor
timestamp: February 27, 2018, 11:18:49 am
content: Quote from: neo on February 27, 2018, 11:11:32 amThat is the cold resistance. The resistance increases a lot as it gets hot.

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 11:29:57 am
content: Quote from: neo on February 27, 2018, 10:53:33 amIt would really help everyone trying to help you if you could sketch out the circuitry feeding the tube heaters on the affected tubes. What we are currently doing by the look of it is trying to describe a paperclip clip and telling a person who cannot see it, how to draw it. If you have not ever tried this, try it and you will not believe the drawing that the drawer will give you back. To you its simple because you can physically see it, but we can't, so we are trying to draw a picture in our minds based on what your telling us.I gather from picking through the thread that you have something similar to the schematicamspireprovided but what we don't know is how that differs from what you physically have for that part of the circuit?I attach that schematic with some notes on it that would be useful to know what the volts are at those points, BUT only if you have this circuit, if not then you really will need to draw it for us all to understand what you have there?

author: neo
author_rank: Super Contributor
timestamp: February 27, 2018, 05:36:55 pm
content: Quote from: Specmaster on February 27, 2018, 11:29:57 amWhat i meant was that there is a resistor missing, a resistor added, a connection to ground missing. At the time i thought more was missing until i understood what they did, which is in compliance with the schematic.Voltages on the labeled points,No load;Emitter, -7Collector, -17Base, -6.84700RBase, -6.84Collector, -16.5Emitter,-6.74400REmitter, -6.71Collector, -16.20Base, -6.855RBase, -3.75Collector, -12.72Emitter, -3.35FL3, 226.4 DC+ 0VACFL2 165.44 DC- 0VACFL1, RED, Emitter.All voltages referenced to the chassis.

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 06:24:52 pm
content: Ok, thanks for this so we can ignore FL3 completely for this section as it plays no part of this equation and is shown unconnected, thus we ignore it altogether.So essentially you have the same circuit as the one supplied byamspirebut you resistor is located in the collector leg, not in the emitter leg and that this resistor is a different value, and also the capacitor in yours is only 1 micro farad as opposed to 1,000 micro farad correct?So the tube heater is connected to the emitter and the other side of the heater is grounded yes?What I'm struggling with here is what do the 700R, 400R and 5R relate to? Is the a selection of resistors that you have connected between emitter and ground to simulate the heater load?

author: ArthurDent
author_rank: Super Contributor
timestamp: February 27, 2018, 06:29:40 pm
content: It has been many years since I owned any of the HP608/HP606 series signal generators but at the time they were really great pieces of equipment. Where the HP608 was produced for almost two decades, HP made many changes over that long production run. This is why it is very important to have the schematic to the unit you actually have.Looking at the schematic you will see the filaments for the osc, buffer, and amp tube are in parallel. The two 5675 pencil triodes (6.3V at .135A each) draw .27A combined and the 6BC4 draws .27A for a total of ~.5A. If you have a transistor/zener circuit putting out 9 or so volts into a 7 ohm resistor, the resistor at .5A drops 3.5V and places 5.5V on the filaments of those three tubes. That is why the mod note says to parallel resistors with the 7 ohm to bring the voltage up to about 6.5V. If your version of the mod doesn’t have the 7 ohm resistor to drop 3.5V then you adjusting the circuit to output 9V directly to the filaments will probably be fatal to the tubes, if not instantly, then soon. If your mod doesn’t have a 7 ohm resistor then the zener should be about 6.5V, not 9V. If the gain of the pass transistor has dropped over time that could cause the output to die as well.When tube filaments are cold and the power is applied, there is a high inrush current that can cause the tube to fail at turn on. The 7 ohm resistor acts like a cheap current regulator and allows the filaments to heat up more slowly and this will increase their life. Initially the filaments are low resistance (they draw more current cold) and most of the 9V will be dropped across the 7 ohm resistor and as the filaments warm up, the drop across the 7 ohm resistor you’ve trimmed drops until there is about 6.3-6.5 volts on the three tubes.A good simple way to check the tubes out is to use a metered D.C. bench supply and connect 6.5V to the filter (FL) that supplies the filaments and see if the current is about .5A. If it is about .5A then the tubes are probably good. If the current is lower by .135A then one of the 5675 triodes had died. Unplugging the 6BC4 should drop the current to .27A if the two 5675s are good. I would only remove the pencil triodes as a last resort.Also the transformers are not oil filled but tar filled. Using the simple diode probe suggested probably won’t work because of loading and detuning. A FET input/bipolar follower probe is a far better choice. I’ve attached a photo of a probe like this I designed years ago that I built into a retractable pen case. The input capacitor voltage rating should be high (1KV) for safety.

author: Specmaster
author_rank: Super Contributor
timestamp: February 27, 2018, 06:48:56 pm
content: Quote from: ArthurDent on February 27, 2018, 06:29:40 pmThis seems to make more sense to me at least. Its always best to get the right schematic if possible but I believeneohas tried and failed to locate one, a free one at least, not sure about paid options.He does not have the 7 ohm resistor, instead he has a 51 ohm in the collector, I presume that the location is not that critical as it is still in essence in series with the filter but the value of 51 ohms, could this be something to do the gain of his particular pass transistor? also the capacitor is only 1 micro farad not the 1,000 could this be playing a part by not storing sufficient energy to reduce the ripple?

author: ArthurDent
author_rank: Super Contributor
timestamp: February 27, 2018, 08:44:59 pm
content: The function of the 7 ohm resistor is far different than the function of the 51 ohm resistor (which doesn’t seem like a reasonable value). The transistor/zener forms a regulator circuit and could be thought of like a 3-terminal regulator like a 7909 if it uses a 9V zener or as a 7906 if it has a 6V zener. A resistor is often placed in series with the input of a transistor regulator circuit or 3-terminal regulator to reduce dissipation in the regulator by dissipating part of the heat loss in the series resistor. With the resistor before the regulator and the load connected directly to the regulator output, the voltage across the load remains constant. If the 7 ohm resistor is on the output side of the regulator in series with the load, it is used to control inrush current and the voltage at the load (filaments) will vary, first being quite low, and then rising to the set value once the filaments are up to temp.If you had 51 ohms in series with the input to the regulator and you needed an output of 6.5V at .5A for the three tubes, the 51 ohm resistor will not work because the value is much too large. For example, if you wanted a filament voltage of 6.5V at .5A and you just used a resistor to drop the 13V from the bridge down to that value you’d have to drop 6.5V through a resistor and that would require a 13 ohm resistor. I could believe the value being 5.1 ohms, not 51.The load test neo did with various resistor loads was reasonable except for the last step where he used 5R. The regulator is designed for ~.5A and the 5 ohm resistor was more than double the load the regulator should see. I’d be interested to see what a 10-15 ohm load would look like.

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 01:44:47 am
content: Quote from: Specmaster on February 27, 2018, 06:24:52 pmThose are loads I've selected to test the circuit before reconnecting the heater.The zener is about 6.5V.1mF is 1000uF.

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 02:24:49 am
content: 5R is for a worst case scenario.Measured the resistor in question, i read the markings at 51 but it is in fact 1 ohm, the marking is 5 1. The 5 is presumably the wattage, i read it wrong and failed to test it, my mistake apologies for the confusion.As requested,25REmitter, -6.58Collector, -14.56Base, -6.8412.5RBase, -6.84Collector, -14.56Emitter, -6.58I searched for a manual, all i got was one for a 608D. Couldn't find one for the 608C, paid or otherwise.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 28, 2018, 02:37:02 am
content: http://www.ko4bb.com/manuals/73.236.124.51/HP_608C_Operating_and_Service_Manual_Service_Manual.pdfIs this the manual you need?

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 02:48:42 am
content: Quote from: Addicted2AnalogTek on February 28, 2018, 02:37:02 amI probably spent a week looking for that, thanks!

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: February 28, 2018, 02:51:38 am
content: If there's nothing else that I'm good at, I'm at least good at finding things on the internet....

author: ArthurDent
author_rank: Super Contributor
timestamp: February 28, 2018, 04:13:50 am
content: Neo, I see that with both the 25R and 12.5R loads the output of the regulator is constant-a very good sign. It looks like you're going to save it!

author: Cyberdragon
author_rank: Super Contributor
timestamp: February 28, 2018, 05:26:00 am
content: I still think there's something weird going on in this circuit. The base is powered through the -165V rail, whereas in a normal series regulator, it's just powered by the input. It's either a safety shutoff for PSU failure (why would the heaters need this?), or it's using it as a stable reference for the zener, which can be affected by load, and temperature (it's in with tubes after all!). I don't know how the load would change when it's just tube heaters (besides on start-up), but it's definately a sign that it's somehow sensative!You seem to have it regulating, so if it's at the correct voltage, then leave it alone. (No transistor upgrades or other shenanigans!)

author: amspire
author_rank: Super Contributor
timestamp: February 28, 2018, 06:10:27 am
content: Quote from: Cyberdragon on February 28, 2018, 05:26:00 amI think there are two reasons for using the -165V for the base/zener bias:The whole reason of this mod was to eliminate mains ripple noise from the oscillator valve. The -165 is regulated so it will not inject noise into the base. A resistor from the collector will inject main some ripple.Secondly, they might have done that so that the transistor acts like a constant current source on overload (like on startup). The base current is more or less a constant -165/18K= 9ma due to the high source voltage. Even if the transistor is at the top end of the gain range at 100, the short circuit current is only 900mA or about 14W. Given that it is a 90W transistor, shorts on the emitter will not blow anything up. No need for fuses or current limits components.

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 09:52:03 am
content: At 7 volts the filament draws 286mA

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 10:07:27 am
content: With the filament connected i get a response from the output volts meter, nothing detectable on the output however.

author: ArthurDent
author_rank: Super Contributor
timestamp: February 28, 2018, 03:04:07 pm
content: When I previously discussed the current drawn by the filaments I assumed there was a 6BC4 buffer between the oscillator and the output amp but after checking the schematic for the C model, this is not the case. There are just the two pencil triodes so the current drawn at 6.5V would be .27A not ~.5A as I first thought. The 6BC4 buffer was added in the D model to eliminate the load pulling the oscillator off frequency. Ampsire is correct that the relatively low current supplied to the 6.5V zener by the -165V line would allow the filament current to be limited if there was a short, thus protecting the solid state devices that were just starting to be used in this mod that came out after the HP 608C was made.Now as to the missing output, a fairly common problem with a number of signal generators is that some people use them to check transceivers and accidentally hit the transmit burning out the signal generator's output attenuator resistors. I would start looking in that area for damaged/open resistors.

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 07:23:05 pm
content: Quote from: ArthurDent on February 28, 2018, 03:04:07 pmThe problem being is that there WAS a measurable output when i got it.Then it just stopped working.

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 07:52:33 pm
content: Just so that we are all on the same page, at least temporarily,Obtained free at hamfestIt worked, so was left alone on shelf and occasionally turned onOne day when wanted for an experiment it wouldn't output anythingTook cover offSaw paper capsPut it in repair que and ordered new caps to replace paper capsSat in repair que until properly boredTaken off repair que, replaced capsNo changeLots of head-scratchingReplaced original transistor because it had gone faulty, used 2n665AHA! A change! the output meter now responds...... but wait there is no outputBack to head-scratchingAlso just a note i think the filaments might have taken double the current briefly.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 28, 2018, 08:14:50 pm
content: Quote from: neo on February 28, 2018, 07:52:33 pmSince the output meter is an RF indicator there is RF present which makes me question how you concluded that there is no output...?

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 08:31:48 pm
content: Quote from: PA0PBZ on February 28, 2018, 08:14:50 pmUsing a 10X probe i put my scope on the output connector and got nothing.

author: Specmaster
author_rank: Super Contributor
timestamp: February 28, 2018, 09:41:00 pm
content: Quote from: neo on February 28, 2018, 08:31:48 pmIt sounds like like the output stage is where the problem is then to my mind. Can you detect any sign of a signal prior to the output stage at all?

author: neo
author_rank: Super Contributor
timestamp: February 28, 2018, 11:52:56 pm
content: Quote from: Specmaster on February 28, 2018, 09:41:00 pmI'm actually having trouble actually physically finding where the signal comes from, yeah i'm an idiot but tracing the cable lead me to mechanics not circuitry.

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 12:37:40 am
content: OK having figured out what i'm looking at, i'm left wondering which would be a good spot to probe? The attenuator is deeply buried circuitry.

author: amspire
author_rank: Super Contributor
timestamp: March 01, 2018, 12:40:26 am
content: Quote from: neo on February 28, 2018, 11:52:56 pmIf you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 12:51:44 am
content: Quote from: amspire on March 01, 2018, 12:40:26 amInfinite in all ranges.On the attached picture, the connection under the red circle was where i probed. Set to 12.5MHz and got 12.45MHz, it changes frequency as i adjust.I think i have just narrowed the problem down to a bit of circuitry i'll have to take the thing to bits to get to.

author: amspire
author_rank: Super Contributor
timestamp: March 01, 2018, 01:23:47 am
content: Quote from: neo on March 01, 2018, 12:51:44 am.Looking again at the circuit and the attenuator is capacitively coupled so you will not see resistance.They are easy to damage, which is why the service note mentions how to replace them.To check it, you would need to feed in probably 10MHz or more from a 50 ohm source and see if the source voltage halves as it should. If you do this,Watch The Level!. Accidentally connecting RF transmitter outputs to these generators is exactly how these attenuators get blown.It is not the easiest thing to check as the RF gets via a pickup coil - there are no input connections. To do the ranges, a switch seems to physically switch different output transformer coils.

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 01:30:42 am
content: Quote from: amspire on March 01, 2018, 01:23:47 amYeah... i don't know how to do that.That switch would the range selector.

author: amspire
author_rank: Super Contributor
timestamp: March 01, 2018, 01:43:40 am
content: Quote from: neo on March 01, 2018, 01:30:42 amOK. We can just improvise. Do you have a digital oscilloscope? Attach a 470 ohm resistor to the output and attach the oscilloscope across the output. Set the scope to a single trigger on 500mV and touch 1v to the resistor. You should get an instant jump to just under 0.1V and then there should be a curve up to 1V that takes about 10uS. If you see that, then the attenuator probably works.Might have to take a few goes to capture it.If the voltage shoots straight up to 1V in nanoseconds, then things do not look good for the attenuator.

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 02:12:04 am
content: Quote from: amspire on March 01, 2018, 01:43:40 amUnfortunately i don't have a digital scope, i do have a (very nice) analog one, and if i understood your instruction clearly then the attenuator would appear to be blown. I tried a 10k just to be sure, still went high fast.EDIT, the two labels i forgot to edit are Scope Probe and RF out

author: ArthurDent
author_rank: Super Contributor
timestamp: March 01, 2018, 03:54:37 am
content: The coils for the different frequency bands are on a turret and these coils are switched in front of an output port that is just a hole in the cast osc/amp assembly. On the outside of this hole is a cylinder that the attenuator slides in and out of. There is also a fixed coil on the inner end of this cylinder that runs across the opening and that coil may even be a straight piece of wire and is the pick-up coil for the meter circuit. The attenuator has a one-turn coil that is moved in and out on a smaller concentric metal tube.These two coils are inductively coupled to the switched turret coil. The fixed coil’s output is rectified to drive the meter and the movable coil is moved in and out to determine the attenuation by varying the inductive coupling between the turret coil (primary) and movable coil (secondary). The movable coil is capacitively coupled to the output as previously pointed out (on the C, not the D model). The output impedance (or A.C. resistance) is 50 ohms but you can’t see that on a DMM which measures D.C. resistance. Amspire has presented a great way to check it but you’d need a second signal source of greater than 10Mhz to do that. If you don’t have access to another source there is another way to troubleshoot the problem.Seeing that the meter circuit and the attenuator get their signal from the same coil inside the osc/amp assembly, if one of them works properly the problem is outside the assembly. If you can adjust the output meter level to the set line and you still have no output, the attenuator has to be defective. Depending on the amount of use even the coax may have been flexed too many times but I’d be looking at the resistors and coil inside the attenuator.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 01, 2018, 04:21:39 am
content: Oh, one other way to check out part of the attenuator is to use a component tester or DMM with a capacitance range. It appears the capacitor in series with the resistor/coil is .01 microfarad (10nF) if I read the drawing in a previous post correctly. If you can't read that capacitor you have an open circuit. I tried reading a .01 through a 100 ohm resistor with both instruments and it does work.

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 04:41:03 am
content: At this point i am fairly sure the attenuator is blown, not sure what caused it because it worked when i got it though it does appear to be blown now.So under my assumption that it is blown, do i take it apart? Sub in new parts? Give up and accept defeat? What should i do?

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 06:56:19 am
content: OK I may or may not be losing my mind. Capacitor now a bit ugly but still functional and i'm at a loss with fixing this thing.

author: Specmaster
author_rank: Super Contributor
timestamp: March 01, 2018, 09:19:20 am
content: Quote from: neo on March 01, 2018, 06:56:19 amLooking more and more likely you may have a dodgy tube there, I'd personally at this juncture be looking for a way to test them. At least then you could eliminate them from the equation if they turn out to be OK, one more suspect ruled out of contention. Caps, resistors, transistors and coils etc can be checked normally using the tools you already have, but tubes are not that easy.

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 09:50:29 am
content: Quote from: Specmaster on March 01, 2018, 09:19:20 amTwo problems,I've made a mess of the attenuator probe.No tubes in the way, i have oscillation but not an output. The output is there but not going through the attenuator probe.

author: Specmaster
author_rank: Super Contributor
timestamp: March 01, 2018, 09:53:01 am
content: Quote from: neo on March 01, 2018, 09:50:29 amYou proved that the output was leaving the tube then?

author: neo
author_rank: Super Contributor
timestamp: March 01, 2018, 10:04:55 am
content: Quote from: Specmaster on March 01, 2018, 09:53:01 amI probed the connection in the red circle of this picture and got the frequency.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 01, 2018, 02:27:07 pm
content: The suggestion of removing either of the two 5675 pencil triodes from the osc/amp assembly is a very bad idea and as I previously stated should be a last resort. First, there is probably a 0% chance you can find a highly specialized tube tester that could test them, they aren’t your usual plug-in tubes. Second, in circuits like this, manufacturers generally will say not to even test tubes because if they work in the circuit that is all you’re interested in. Plus most tubes will work well outside of what a tube tester will say is weak or bad.As I said before, the best way to check the tubes is to check the supplies feeding them plus, in this case, the R.F. output meter. The question I’ve asked before is whether the output to the meter can be adjusted to the set line and is that reading stable. The manual states the meter is reading the voltage at the input to the attenuator so if the meter indicates proper output, forget the tubes. If the meter indicates properly the problem is external to the osc/amp assembly and messing with it is a good way to really screw the HP 608 up beyond repair. The HP608 was calibrated after the tubes were properly installed and even moving some of the connections at these higher frequencies could really change calibration. Checking the filament voltage and current then checking the B+ will check the inputs to the tubes and the meter will verify the output.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 01, 2018, 08:55:05 pm
content: If the 5675 tubes need to be replaced this could require recalibration of the signal generator. The tubes are selected for specific characteristics from a number of those available. If you just buy one it might not be within the circuit specs.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 02, 2018, 12:16:37 am
content: Did you check L17 and R58?

author: neo
author_rank: Super Contributor
timestamp: March 02, 2018, 05:26:41 am
content: LET ME BE ENTIRELY CLEAR!I've found the problem! I know what the issue is but not what is causing the issue. I don't want to and i have not once thought of removing the the tubes. The issue is not in the tubes!THERE IS OSCILLATION!The problem is exactly this,it uses inductive coupling and an attenuator probe in a metal tube to pick up the frequency. For some reason it is never picked up.EDIT to add;Sorry if this was over the top.It might not help that to me this method as well as most VHF and up is perceived as a kind of black magic.

author: Specmaster
author_rank: Super Contributor
timestamp: March 02, 2018, 10:20:11 am
content: Do you think the message has sunk home yet?Can the attenuation probe be dismantled at all to gain access to the components inside or does it have to replaced as a sealed unit?

author: neo
author_rank: Super Contributor
timestamp: March 05, 2018, 08:33:54 pm
content: It, as far as i can tell, doesn't appear to be something that can be disassembled. Unless the variable capacitor in the thing is in the order of a few puff then i think it went short.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 06, 2018, 02:03:11 am
content: Quote from: neo on March 05, 2018, 08:33:54 pmHighly unlikely, it's a tiny mica cap, which won't short unless there's corrosion. Did you actually try pulling (hard) on it from the end where the components stick out? Is there any solder around the rim you can melt off?

author: neo
author_rank: Super Contributor
timestamp: March 06, 2018, 03:27:51 am
content: There is nothing to pull on.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 06, 2018, 03:40:03 am
content: Quote from: neo on March 06, 2018, 03:27:51 amThe end comes off somehow (look at the pic above). There should be a seam. It might be covered in solder though.

author: neo
author_rank: Super Contributor
timestamp: March 17, 2018, 10:55:44 pm
content: Sorry if i seemed to have abandoned this project, i just needed a break to sit and think. I got an idea and so i'm back at it, i fed 13 MHZ into the attenuator and it attenuated. Meaning that it just refuses to pick up the oscillation, i'm beginning to doubt my ability to resurrect this boat anchor

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 18, 2018, 02:28:39 am
content: Is L17 blown?

author: neo
author_rank: Super Contributor
timestamp: March 18, 2018, 03:16:54 am
content: Quote from: Cyberdragon on March 18, 2018, 02:28:39 amAs far as i can test it, no.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 18, 2018, 05:18:40 am
content: Where did you feed the attenuator input when you saw it work?Also did you check the wire continuity?

author: neo
author_rank: Super Contributor
timestamp: March 18, 2018, 05:22:23 am
content: I probed this pointhereand got the dialed in frequency.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 18, 2018, 10:20:02 pm
content: Doesn't matter.Did you check the wire that connects L17 to the attenuator for continuity?

author: neo
author_rank: Super Contributor
timestamp: March 18, 2018, 10:51:46 pm
content: Continuity across the inductor but i'm not getting a connection to the tines.EDIT; there is continuity to the tines.

author: neo
author_rank: Super Contributor
timestamp: March 18, 2018, 11:21:58 pm
content: The thing just sprung to life, it seems all i needed to do to fix it was rule out every possible motive except for old and cranky.

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 18, 2018, 11:46:30 pm
content: Exactly as I suspected, bad wire or solder joint. You poked it and restored continuity.Jiggle it a little bit, the bad connection is still there.

author: neo
author_rank: Super Contributor
timestamp: March 19, 2018, 12:14:39 am
content: I've jiggled the coaxial every which way and taken a good long look at the joint on the attenuator probe, all kosher. Then i tweaked the single wind inductor on the thing and it sprung to life, reattached it the sliding mechanism it and it still works.

author: Specmaster
author_rank: Super Contributor
timestamp: March 19, 2018, 01:08:55 am
content: Quote from: neo on March 19, 2018, 12:14:39 amExcellent news, so you now have a working unit.From mobile device so predictive text might have struck again

author: neo
author_rank: Super Contributor
timestamp: March 19, 2018, 02:41:46 am
content: Quote from: Specmaster on March 19, 2018, 01:08:55 amMostly anyway, the percent modulation meter is still screwy but i'm not sure i want to dig into THAT without a tube tester. Time will tell how well it is fixed, i suppose.

author: neo
author_rank: Super Contributor
timestamp: March 19, 2018, 06:13:50 am
content: Having knocked it around plenty getting it back in the case and onto it's shelf i think it'll keep working. With how much i moved it for it to still work eliminates a short, almost definitely. Fingers crossed.