vacuum tube signal generator rf - Page 1
Forum section: electronics / repair

author: davy peleman
author_rank: Regular Contributor
timestamp: January 31, 2018, 04:55:30 pm
content: I purchased a signal generator that i wanted to repair. changed caps, checked resistors contacts cleaned. now it works but the upper half of the wave bounces up and down (amplitude changes) and the bottom halve is clipped. I cannot check the tubes and they're expensive to buy. could it be the output tube (6AR5)? because i already replaced the double triode tube (12BH7)and it stays the same. since i checked everything else i cannot think of another reason for the wave being clipped this way. also the audio wave has shifted its midpoint the audio wave is going way more negative and the negative halve is pointed ( more triangular) whereas the upper halve is nicely curved (sine). Also as mentioned the amplitude of the wave bounces up and down. I'm going to order the tubes anyway. But i'm not sure that will remedy the problem.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 31, 2018, 06:00:20 pm
content: Did you replace all the caps or only the electrolitics?Schematic ?

author: davy peleman
author_rank: Regular Contributor
timestamp: January 31, 2018, 06:10:27 pm
content: replaced most of them but checked the others and they are ceramics so i doubt they've failed since they had the right values. the scematic can be found online , i'm sorry but could not upload it. the signal generator is model TE- 20. pretty simple but i should have new tubes to check if they give the same results. since they've never been replaced as far as i can tell, they've probably failed. Correction: the wave goes negative but is clipped the upper half bounces in amplitude.

author: woodchips
author_rank: Frequent Contributor
timestamp: January 31, 2018, 07:09:47 pm
content: Bouncing could be due to it using a lamp bulb rather than a thermistor to stabilise amplitude, would take longer to warm or cool down. So long as the bouncing stops within a second or so then probably have to live with it.Measure the grid bias and other grid voltages, compare with the data value. Clipping the top half says the valve isn't turning off, too much bias from the cathode resistor?

author: davy peleman
author_rank: Regular Contributor
timestamp: January 31, 2018, 07:21:41 pm
content: the top half is bouncing constantly. not only when i change freq. and it's the bottom half that's clipped. i'll check the bias voltage on the tubes and measure voltages at cathode and anode and see they're into spec. but my question was: can it be a bad tube? can the bias be off because the tube has failed? should i by the tubes and do the test? that's what a wanted to know. because i think these tubes are really old ones and i have no tube tester.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 31, 2018, 07:40:24 pm
content: Quote from: davy peleman on January 31, 2018, 06:10:27 pmDid you check them for leakage - i.e. with working voltage across them? Sometimes the anode to grid coupling capacitors become so leaky that, in effect, you have a D.C. amplification path - i.e. unintended gain at D.C. and therefore instability.The circuit that I found does not have component labels! I would be paying attention to the cap from the anode of V1 to the next grid and similarly the 50\$\mu\mu f\$ that couples the tuned circuit to the grid of V1. If you do not have the means to check for leakage - try testing the voltage on the grids - it should be close to zero.

author: ferdieCX
author_rank: Regular Contributor
timestamp: January 31, 2018, 07:56:00 pm
content: I think, that the clipping probably is correct. There is no cathode resistor, it seems to be designed to have grid current, like most tubed RF generators. The only automatic level control seems to be, the DC voltage that the grid current develops over the 50 pF capacitor, although I would have expected a 100K grid resistor, not just 10 K . I have a Heathkit IG-42 and clipping is the normal behavior. The bouncing can mean a parasitic oscillation, does it happen also without modulation ?

author: davy peleman
author_rank: Regular Contributor
timestamp: January 31, 2018, 08:53:15 pm
content: So it's not the tubes? I'll check the mentioned suspects. thanks alot everyone.

author: davy peleman
author_rank: Regular Contributor
timestamp: February 01, 2018, 04:17:25 pm
content: yes it happens also without modulation. Well i'm gonna replace the 2 caps of 50 picofarads (the only ones i didn't change) and see what that gives me. but my question remains unanswered: "can it be the tubes???

author: Andy Watson
author_rank: Super Contributor
timestamp: February 01, 2018, 04:26:22 pm
content: I would expect the tubes to fail towards complete non-functionality - i.e. I think their gain would be reduced to the point that they would not oscillate. But this is more of a guess than knowledge.

author: davy peleman
author_rank: Regular Contributor
timestamp: February 01, 2018, 05:13:32 pm
content: ok thanks. tubes are so past that not many people use them. so it' difficult to find good info. i'm told tubes can fail in many ways. a good tube tester has many parameters to test. they can get microphonic, have anode to cathode leakage, etc. but in my case nothing happens on the scope when i gently strike them with the back of my screwdriver. that's the easiest way to check for microphonics (i'm told). first i thought they were, because of the bouncing of the wave. leakage can be checked with a meter while powered, still have to do that. i'll change the 50 pf caps and order some tubes anyway, since they are really old, and maybe still good, will not last much longer i suppose. thanks again i'll update when the work is done. have to order the tubes but the 2 tubes will cost me 45 euros. i'm ordering the best which is a balanced tube so the triodes will be within the same operating specs. i think it is needed because the occilator uses them like 2 transistors i think (but not sure).i use a variac, isolation transformer and series light bulb to keep me safer. bought 2 toroidal isolation transformers that came out of a medical apparatus for 30 euros. they give me 230v and 21v at 500va so i think that was a bargain. they still look like new and work fine.

author: ferdieCX
author_rank: Regular Contributor
timestamp: February 01, 2018, 06:49:01 pm
content: The 50 pF capacitors are probably ceramic ones, they usually don't fail. The left half (in the drawing) of the 12BH7 is a Colpitts oscillator, the other half is a cathode follower that gets also grid modulated.It could be, that the power supply has too much ripple and you see that as a bouncing in the amplitude. If your scope allows it, look with the scope at the 10 uF/150 V capacitor and measure the ripple. Please BE CAREFUL. Measure first the DC level with the DMM, and then check if your scope + test probe allows for measuring this voltage.If your DMM is True RMS, you can also use it to measure the ripple of the power supply.

author: bd139
author_rank: Super Contributor
timestamp: February 01, 2018, 07:15:49 pm
content: Check the supply voltage with an AC coupled scope. If it’s one of the ones with selenium rectifier in it, this can cause instability as the impedance of the supply changes constantly. I had the same thing on my Heathkit generator. Then the rectifier blew up. Smelled terrible.

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 06:02:22 pm
content: i never used a scope to check mains, so no experience. and really it scares me a bit. i have a velleman hps50 portable scope (so it's isolated from mains, no earth connection). the maximum voltage x10 is 100v. so i'll have to use some resistance on my probe i guess, but i have no experience with this. maybe i shoud replace the rectifier along. i have good replacements in my parts bin. i would really like the generator to work factory. keep in mind that the 400hz tone is also not perfectly sine, so that's an indication too.next week i'll have the new tubes. but if something else is wrong i could damage them and thats what i fear since they cost 45euros.gonna replace the ceramics anyway since i have them laying around and i cannot check for leakage. and what about the clipping on the negative halve of the wave?

author: ferdieCX
author_rank: Regular Contributor
timestamp: February 02, 2018, 06:35:55 pm
content: DON'T use your scope hps50 to check the ripple of the power supplyIt only accepts a peak voltage DC+AC of 100 V, and the power supply in the RF generator has for sure more than 100 V DC.To just put a resistor is always tricky. I have seen 39 years ago, how a coworker destroyed the vertical attenuator of a Tektronix 465. It only accepts 250 V and he tried to measure 300 V AC using a x10 probe in a switching power supply.The 400 Hz are generated using the 6AR5 as a Colpitts oscillator, this requires the use of a large inductance and has no automatic level control, so don't expect a nice waveform. Mi IG-42 uses the same method and the 400 HZ don't look nice either. When I want a good looking modulation, I use an external audio generator.You can measure using a DMM the DC voltage at the output of the power supply and then switch it to AC and measure the AC component. Even if your DMM is not true RMS, you will have some idea of the ripple. If it is comparable to the amplitude of the bouncing that you are seeing, probably the problem is the power supply. If this is not the case, the 12BH7 "could" not be completely OK and make the oscillator work unstable.Tubes are quite resilient. To damage the new ones just by using them some minutes for testing, would require more power that the one that the power supply of this generator can usually deliver.The clipping is normal in this kind of tubed generator, with grid current. The amplitude of an oscillator depends of the loop gain at the oscillating frequency being " 1 " (Barkhausen criteria). Without AGC, this involves some clippingTo get a perfect waveform, you need a much more elaborated generator, like the HP-606B

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 06:59:36 pm
content: thanks for saving my scopeok the 400hz is not to shabby so now i know it's ok. i'll check the power supply with your method. i think i'm getting to the source of the problem with everybody's help. with all this info i'm pretty assured i'll get it in tune again.thanks to you all!!! since it's so old i"ll better replace all the other parts that could have gone bad, since i'm planning on using this device for a long time.I really love tubes. all my guitar amps are tube driven and i really like them. they're more mystical then the transistor amps. but i'm aware that the so claimed better sound of tube amps is a myth. there are alot of transistor amps that sound superb. But what i really learned here is that older tube driven devices weren't all that precise, or am i wrong??? please correct this if i"m wrong, otherwise i"ll keep searching untill the generator spits out perfect waves.the reason i started electronics is because i couldn't find a good tech to repair my amps, so i did it myselfit is also much cheaper to do it myself hehehe. why is it so hard to find a good amp repairman??? first project was to mod my mesa mini rectifier which had some bugs. mesa has a good help desk for sure; with their help i was able as a newbie to repair it.now my friends ask me to repair their amps toomaybe i could make a living of itdon't get me wrong my friends do not have to pay anything except for the expensive parts

author: ferdieCX
author_rank: Regular Contributor
timestamp: February 02, 2018, 07:25:23 pm
content: The components more likely to give problems are the two 10 uF/150V capacitors in the power supply and also the rectifier, if it is of the Selenium type.Tube circuits are mostly from an era, where the good resistors had 5 % tolerance. Anyway, they can perform awesome good. I have a tek 310A scope that I seldom use, and it is incredible stable.You will find this two excellent books in the archive.orghttps://ia801601.us.archive.org/16/items/ElectronicDevicesCircuits/MillmanHalkias-ElectronicDevicesCircuits.pdfhttps://ia601400.us.archive.org/20/items/RadioEngineering/Terman-RadioEngineering.pdf

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 07:30:22 pm
content: the caps are replaced by new electrolitics (10uf, 350v) the rectifier i will do tomorrowi'll check the archivecan i use common rectifier diodes???

author: ferdieCX
author_rank: Regular Contributor
timestamp: February 02, 2018, 07:34:26 pm
content: If it is a silicon rectifier, it is ok. The problem are the old Selenium oneshttps://en.wikipedia.org/wiki/Selenium_rectifierI replaced the one in my IG-42 with a bridge rectifier, to have full wave and less ripple.The Selenium type have more internal resistance than the silicon ones. When you make such replacement, you have to add a series resistor to keep the output voltage of the power supply in the same value. In your case, you can increase the 2 K / 1W resistor

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 07:39:28 pm
content: do you mean i could replace the resistors with more precise ones to give better output???

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 07:44:43 pm
content: ok good you remind me that fully rectified has more voltage otherwise as newbie i would not think off that. i think i'm going to do that mod. full rectification, off course less ripple. nice advice, definately going to do that mod. because, yes it's one of these old selenium ones that's inside.

author: ferdieCX
author_rank: Regular Contributor
timestamp: February 02, 2018, 08:05:48 pm
content: No, you don't need to replace the resistors, only if they are out of tolerance.What I wanted to say was, that when most tube devices where designed, a good VOM had 2 % precision and a VTVM usually 3 %. The worst DMMs have today 1% or 0,5 % precision. Everything was designed allowing for some dispersion in the characteristics of the components and the devices where adjusted before leaving the factory. If a device had to be really precise, there existed expensive 1 % resistors and other precision components, like industrial and military grade tubes.I will be away some days, I hope that you find the problem in the meantime.

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 08:12:45 pm
content: ok got it. will not replace any resistors if i don't have to (like mr carlson always says).have a nice time

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 08:14:33 pm
content: Quote from: ferdieCX on February 02, 2018, 06:35:55 pmGood point. Just to note, when I did this I had an HP 1740A which is specified to 500v p-p at under 500Hz.

author: ferdieCX
author_rank: Regular Contributor
timestamp: February 02, 2018, 08:21:09 pm
content: Quote from: bd139 on February 02, 2018, 08:14:33 pmI used a Tek 453 for repair those power supply, specified for 600 V. I am still wondering, why my coworker used the 465

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 08:24:46 pm
content: Probably because it was shiny. I have a 465 and it's nice and shiny

author: davy peleman
author_rank: Regular Contributor
timestamp: February 02, 2018, 08:50:40 pm
content: hehehe i like old dusty stuff. seems more interesting to me, and it's a real pleasure if you can get this stuff to work. like reanimating the dead and get to talk to them. yes i'm a real nuttcase they say; at this moment "real steel" is on the tele how appropriate

author: bd139
author_rank: Super Contributor
timestamp: February 02, 2018, 08:57:15 pm
content: Couldn’t agree more.

author: bob91343
author_rank: Super Contributor
timestamp: February 04, 2018, 11:30:54 pm
content: Many cheap designs clipped normally. The instability, however, can be traced to some intermittent connection.I assume there is a variable tuning capacitor. Those units rely for their ground connection on a clip that is slid over the rear of the shaft. Looking down on the capacitor, you will see, from the front, the knob, the shaft, the bearing, the plates, and then the clip at the rear just before the frame. Try some contact cleaner on that area. (NEVER use WD-40!) Sometimes just exercising it by spinning the shaft many times will help.Also check the tube pins as they are inserted in the sockets. A bad tube is unlikely.Bob

author: davy peleman
author_rank: Regular Contributor
timestamp: February 05, 2018, 05:16:50 pm
content: already contact cleaned and looked at the inserts of the tubes. they make good contact. even gently squeezed them together just to be sure. will have a good look at the tuning capacitor and check other connections.

author: davy peleman
author_rank: Regular Contributor
timestamp: February 05, 2018, 05:53:11 pm
content: Bob i think you are right. doing alot of moving on the tuning capacitor i clearly see on the scope that it's inconsistent. on the end of the bandwith the signal doesn't bounce anymore. in the beginning of the bandwith the signal sometimes vanishes or looses alot of amplitude. really everytime i power this thing it behaves differently. so it was hard for me to describe the symptoms. today the amplitude doesn't bounce all that much. but the clipping is worse. i'm going to clean it a couple of times. tommorow i'll power it again and see what kind of signal it spits out.so many things that can go wrong, not easy to diagnose these old things.