hybridfree/eevblog-posts huggingface dataset:
HP 8593E phase noise - Page 1
Forum section: electronics / repair

author: bistromath
author_rank: Newbie
timestamp: March 31, 2025, 08:45:54 pm
content: Hi all,I have an old 8593E spectrum analyzer that seems OK except for a huge amount of phase noise at bandwidths closer than about 100kHz. Here's what it looks like.I've been down a number of rabbit holes and have the Artek manuals. The PSUs have been recapped. I've attempted to confirm the power supplies are clean, I've tried driving the YIG coil from a battery supply to rule out FM noise from the power supplies, I've done all the calibrations and tried removing the counterlock board as well. The noise is on the first LO at 4.223GHz as well as every subsequent signal.So, before I unload the results of a week's worth of diagnostics here, have any of you wily old campaigners seen something like this before? Should I resign myself to it being a bad YIG and move on?Nick

author: Perrold
author_rank: Contributor
timestamp: April 01, 2025, 07:04:04 am
content: Gut feel is your YIG is fine. From the pic, it looks like the noise occupies discrete sidebands and there might be higher level noise hiding inside the filter curve at half the observed offset frequency. No idea what is causing it though and you've certainly done all the due diligence at this point. Doesn't take much noise riding on PS rails or getting in through the YIG coils to cause this though. Some unaccounted for ground loop still being present with the external supplies is a possibility. The YIG in these spectrum analyzers is also very susceptible to magnetic field pickup, at least at 60Hz. A small shaded pole motor running about a foot away caused me to do a completely unnecessary troubleshooting deep dive.

author: grendle373
author_rank: Contributor
timestamp: April 14, 2025, 05:50:36 pm
content: looks like a noisy yto or bad res bw board set. loop the cal out to the input and run the self cal. If it passes it's most likely the yto. Noisy yto's are common on the 60 and 90 series. They can be found used or rebuilt. The res bw board sets usually fail for amplitude but we have seen them get noisy a couple times. My bet would be they yto. Third converter could be a possibility as well.
HP 8593E - Hard to push buttons? - Page 1
Forum section: electronics / repair

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: May 20, 2019, 04:31:05 am
content: Hi All, I just came in to posetion of a HP8593E analyser. It powers up fine and I get a trace and all seems good so far, although I still need to find a cable to do the self-cal.Anyway one thing that I cannot figure out is the Frequency button does nothing? The span and Amplitude buttons works. I can hit SPAN + 8 + MHz and that sets fine.I also notice the numbers 1 and 0 on the bottom left of the numeric key pad do not work, and the others require a hard push to get to work.So my guess is the frequency and probably all other buttons need a good clean. But I just wanted to check with the FREQUENCY button if it's not somehow locked by some system setting.All other buttons and the wheel seem fine.ThanksDave

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: May 20, 2019, 04:46:21 am
content: Strike that.. They all work I just had to put all my weight on to them. So I guess some cleaning tips would now be appreciated.ThanksDave

author: GreyWoolfe
author_rank: Unknown
timestamp: May 20, 2019, 12:40:03 pm
content: You can simply try operating the button a bunch of times. Sometimes this works to free up the buttons. Judicial application of DeOxit into the switches if you can get to them should free them up. Take care to try not to get contact cleaner everywhere.

author: Neganur
author_rank: Unknown
timestamp: May 20, 2019, 12:46:00 pm
content: if it's the 'rubber keypad' type, don't push them too hard since it will only make it worse.From what I recall there is some conductive liquid that should be used to refresh the carbon domes.Perhaps someone else can remember what exactly it was.

author: MarkL
author_rank: Unknown
timestamp: May 20, 2019, 02:19:05 pm
content: Quote from: Neganur on May 20, 2019, 12:46:00 pmhttps://www.digikey.com/products/en?keywords=cw2605

author: David Chamberlain
author_rank: Frequent Contributor
timestamp: May 21, 2019, 05:57:59 am
content: Thanks for the product link, I will give it a try.Yes after pushing them a few times they are not so bad now, and they are rubber buttons.I found a test cable (turns out n-type connectors sort of fit over bnc's) and the unit passes self calibration and confidence test so I might have a good one on my hands.
HP 8594A Spectrum Analyzer  - Page 1
Forum section: electronics / repair

author: Ghastly
author_rank: Contributor
timestamp: March 17, 2024, 01:04:44 am
content: Hello. Recently I had bought my first spectrum analyzer to learn RF stuff. I had plugged in the cal out to RF in. Nothing was showing up so I hit the cal button and still nothing. I factory reset the configuration and still nothing. I plugged my scope to cal out and yet I saw the waveform. My error is SRQ110 and signal not found. Are those gold connectors that do not have any cable attached suppose to have them? I'm completely new to this stuff and I don't even know where the rf input cable connects to. Any help is appreciated. Please see photos for references. Thank you all.

author: 807
author_rank: Frequent Contributor
timestamp: March 18, 2024, 06:44:52 pm
content: Can you measure the frequency of the cal out on the scope? The resolution of your photo isn't quite high enough for my old eyes to see. The label on the cal out socket should tell you what it should be (probably 300MHz @ -20dBm). Your scope won't have the bandwidth for an accurate level reading, but should still give an accurate frequency reading. (You might need to use the cursors if it doesn't give you a frequency readout).

author: Michael-VK2BEA
author_rank: Regular Contributor
timestamp: March 23, 2024, 06:53:10 pm
content: A common problem is the switched attenuator.First off, I would try selecting manual attenuation setting and then change the attenuation to exercise the relays. (just switch from 0 to 70dB up and down a few times).If that doesn't give you a clue (you might see signal on 20dB that you don't see on 10dB if the 10dB attenuator or switch is bad).Michael
HP 8594E spectrum analyzer interesting problem  - Page 1
Forum section: electronics / repair

author: GigaHurts
author_rank: Contributor
timestamp: March 02, 2025, 06:19:08 pm
content: Just curious about everyone's thoughts on this problem that keeps showing up on one unit:After CAL there is a certain power level (around -55 dBm) where the power will jump randomly by 10dB to -65dBm. This happens with noise floor (if it is increased to that level by AMPLITUDE control, as well as with the CAL signal, where it shows as two distinct 10dB kinks in the sidebands of the signal, see pictures. Span, rbw, vbw dont have an impact on this, as long as signal is in that band. Sometimes this causes confidence check to fail as well with RBW and attn. errors.RF attn.? IF level control? Is there a name for this type of step jump in amplitude in the signal sideband?

author: GigaHurts
author_rank: Contributor
timestamp: March 02, 2025, 06:20:51 pm
content: Here is the pic

author: GigaHurts
author_rank: Contributor
timestamp: March 02, 2025, 06:21:51 pm
content: And the noisefloor

author: rg58
author_rank: Regular Contributor
timestamp: March 02, 2025, 08:02:14 pm
content: Something's definitely not right with this unit. Did you go through the complete alignment procedure? Mind you, it took me roughly 2 days!

author: GigaHurts
author_rank: Contributor
timestamp: March 02, 2025, 10:41:54 pm
content: I am planning to do the whole alignment. But i do not expect it will fix the issue, more likely it is some component in the IF section that is on its way out.Will update here when alignment is done (it will take me more than 2 days)

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 02, 2025, 11:13:36 pm
content: I think this is one of the HP analyzers that had the "O" rings in the relays in the attenuator problem. I think you have the manual (relay) attenuator which contains the tiny "O" rings..I had a somewhat similar problem with my VNA and the one "O" ring was just not there. replacing the rubber "O" rings cures the problem..It does take a bit of time to do this repair.

author: MarkL
author_rank: Unknown
timestamp: March 02, 2025, 11:25:24 pm
content: Do you get similar spectra if you turn off corrections (CAL --> CORRECT --> OFF)?If the strangeness clears, an error probably occurred during self cal. It could be an issue with the attenuator, as Wallace points out, or something else that might be causing a discontinuity in amplitude response.If there is a problem during self-cal, the unit will use the bogus corrections determined at the time and make the sweep look strange. There's not a lot of checking for nonsensical results during self-cal.

author: GigaHurts
author_rank: Contributor
timestamp: March 03, 2025, 12:25:17 pm
content: Yes, it goes away when the faulty cal factors are removed. I have to check the o-rings, thanks for the tip!
HP 8594L CRT issue - Page 1
Forum section: electronics / repair

author: WB8PFZ
author_rank: Regular Contributor
timestamp: March 09, 2020, 09:20:44 pm
content: I while ago I purchased a HP 8594L spectrum analyzer with a problem. The screen would show the image but the display was shifted up about 2 inches . The good news is it was an easy fix once I located a bad cap. Now the issue is the displayed image is not plum or square. It is twisted 5-10 degrees from plumb. There are a few adjustments on the monitor position, brightness, hold and a few others. Does someone know how to twist the image. I thought there was an installation issue but I reinstalled and the same problem. Thanks Mike

author: helius
author_rank: Super Contributor
timestamp: March 09, 2020, 09:47:55 pm
content: Most CRTs use magnetic deflection with a yoke around the neck of the tube. This needs to be adjusted by turning it while the picture is visible to present the correct rotation (the yoke is then held in place with dots of lacquer paint).If the instrument has an electrostatic deflection tube like a scope, trace rotation is electronic and there will be a pot somewhere to adjust it.

author: WB8PFZ
author_rank: Regular Contributor
timestamp: March 09, 2020, 10:38:52 pm
content: Well the yoke is glued and that does not look like much of an option. Hoping someone knows where the adjustment (if there is one) is. There is a coil with a tuning slug with no marking on the board. All other pots are labeled on the circuit board. Mike

author: WB8PFZ
author_rank: Regular Contributor
timestamp: March 09, 2020, 11:11:21 pm
content: The display is made by Omni Vision model LP0615E3Y if that helps. Mike

author: helius
author_rank: Super Contributor
timestamp: March 10, 2020, 06:37:33 am
content: A yoke on the tube means magnetic deflection. Maybe there is a shorted turn or a break in one of the yoke coils? That can cause vertical/horizontal collapse, but I don't think I've seen it cause rotation. When you say "twisted 10 degrees from plumb", that could mean rotation, or it could mean a geometry problem like trapezoidal or pincushion misalignment. The geometry is controlled by dynamic circuits that change the deflection timing.The other part of the magnetic deflection system is the purity magnets for color tubes. Those are placed around the neck, sometimes a pair of ring magnets are used, other times they are small disc magnets stuck on plastic pegs. They don't rotate the picture, but if misplaced they cause color smearing.
HP 8595E Attenuator problem - Page 1
Forum section: electronics / repair

author: gopman
author_rank: Newbie
timestamp: November 22, 2020, 03:08:05 am
content: Working on a HP 8595E with an attenuator problem. Only the 40dB attenuation seems wrong. All of the other attenuation settings appear correct. I don't understand how that could happen since (as I understand it), the 40dB attenuator is enabled with 40, 50, 60, and 70 dB attenuation.I checked the voltages at the ribbon cable with the attenuator in place, and they match the table in the service manual.Here is a video showing the attenuation steps from 10-70 and back again.https://drive.google.com/file/d/1sG4pwMffKFmfUx6XFGPa_2zrX-nayi_I/view?usp=sharingThanks for any help!Rich

author: MarkL
author_rank: Unknown
timestamp: November 22, 2020, 04:40:06 am
content: Try turning off corrections and look at the levels again. If amplitude calibration was run with the attenuator broken, you can't trust anything you're seeing.

author: gopman
author_rank: Newbie
timestamp: November 22, 2020, 01:45:19 pm
content: Thank you! Looks great now. Hopefully that's a mistake you only make once...

author: MarkL
author_rank: Unknown
timestamp: November 22, 2020, 05:00:04 pm
content: You're welcome! It happened to my 8595E too and I was ready to start digging into the ADC circuitry before figuring it out. The sharp discontinuity around the skirt of the signal at some of the attenuator settings in your video was a dead giveaway.If you looked at the "AUX VIDEO OUTPUT" signal on the back, it also would have behaved sanely since correction factors are not applied to that output. Corrections are done by the processor after the signal is digitized. Examining that output is what made me realize it was not an RF issue (after fixing the attenuator, of course).

author: gopman
author_rank: Newbie
timestamp: November 22, 2020, 06:12:03 pm
content: That's good to know. There is a lot to learn about these SA's.
HP 8595E Spec Ann - unusual lockup when FREQ UNCAL error arises - Page 1
Forum section: electronics / repair

author: Tracker
author_rank: Regular Contributor
timestamp: August 11, 2022, 02:15:47 pm
content: Hi guysI am looking for some direction with this fault please.The 8595E spectrum analyzer (normally 9khz to 6.5 GHz) appears to work well in low band up as far as 2.7 GHz, but beyond that frequency the FREQ UNCAL error pops up.according to the manual 'FREQ UNCAL' indicates a YTO issue...But the unusual bit is that the whole machine locks up and will not respond to any button press except 'PRESET' which resorts default settings.I tried to run the YTO cal routine which takes about 5 minutes, but when complete it appears to mess up the low band also by adding +300 MHz offset to Low band and it still goes FREQ UNCAL on Band 1I also ran the Service Diagnostic checks and can see a small but noticeable step in the signal traces.Photos to follow.Any suggestions? I am hopeful the YTO is ok and there might be a Logic issue here because of the lock-up.Thanks

author: m k
author_rank: Super Contributor
timestamp: August 20, 2022, 08:55:29 am
content: I don't know anything about the device and seen the clip only once, but anyway.Based on your description I guess it can be just a wrong cal source.If device is seeing its own errors it really can't do much more than revert to default.Is it ANA_TEST where internal cal input is finally available?TP2 is shaded but there's also SWEEP+TUNE.If cal input is there and the problem is those DACs you should see something unusual there also.Is TP3 of mux U15A something where you can stuff your own cal check signal?

author: Tracker
author_rank: Regular Contributor
timestamp: September 05, 2022, 07:42:09 pm
content: ok so after some research and some more testing I can confirm its a bad Yig.I removed the YIG and ran it externally with three PSU's, two for voltage and one for a current limited source to control the main coil (didn't bother with the FM coil) .This proved to me that the YIG has two problems, it wont cover the required operational frequency span (it has gaps) and then it shuts down after 5 minutes of operation from cold.So a new YIG is needed.This seems to be the most common fail on these SA's, 2nd hand Yig's can be got from the far east but you takes your chances there!!!A company is CA can provide recondition units with a guarantee, provided you donate the faulty Yig.But it's a tough call to know if the Analyzer is worth the 1000 Euros (approx) it would cost for a Yig.
HP 8596E repair, Freq Cal pass, Amp cal fail on low signal  - Page 1
Forum section: electronics / repair

author: lfsrouger
author_rank: Contributor
timestamp: May 05, 2021, 04:05:28 am
content: Check the cal signal, looks normal w/o correction, distort w/correction. Could be a wrong calibration in past? Any advise?

author: MarkL
author_rank: Unknown
timestamp: May 06, 2021, 04:47:06 pm
content: The first thing to try is defaulting the cal data and re-running the self-cal.You will need to enter a passcode to load the defaults. The procedure is described in the User Manual and also in the Assembly-Level Repair Manual. Search for "default cal data".And while you're at it, if you haven't backed up your correction constants yet (this is not the same as the cal data), I recommend performing the backup procedure in the Assembly-Level Repair Manual (chpt. 3).

author: lfsrouger
author_rank: Contributor
timestamp: May 06, 2021, 07:22:17 pm
content: Thank you! Will try as suggested.
HP 8601a substitution for unobtanium IC - Page 1
Forum section: electronics / repair

author: neil
author_rank: Contributor
timestamp: August 21, 2019, 03:22:53 pm
content: Moving along with the HP8601a sweeper (seized attenuator shaft now resolved), it appears that the IC used as the loop amp in the A3 module is deceased. I tried reseating it several times, but no joy. I gather that these are unobtainium, so am wondering about a modern substitute. This sweeper would be for non-critical hobby use.The amp (labelled "E1" on the schematic) sits in a PLL loop, between a mixer and a divider chain (which starts with a 2.2uF blocking capacitor). It has a +20v power line and signals in the 0.1 to 110 Mhz range. The input is shown as -20 dBm and output at least 2 dBm.I wonder if I can substitute a modern chip, perhaps a video amp of some sort on a small daughter board, with appropriate voltage-shifting circuitry as needed? Any suggestions for favorite chips?

author: fzabkar
author_rank: Super Contributor
timestamp: August 21, 2019, 10:14:54 pm
content: I couldn't find the part (1820-0151) in the cross references:https://www.sphere.bc.ca/download/hp_xref-free.pdfHowever, here is something along the lines that you propose as an alternative solution:https://f6aoj.ao-journal.com/cariboost_files/hp_20hybrid.pdf

author: bd139
author_rank: Super Contributor
timestamp: August 21, 2019, 10:59:39 pm
content: 1820-0149 is referenced as well. See:https://susumu-oiso.blog.so-net.ne.jp/2017-04-05for a substitute. Looks like they used a standard MMIC as a replacement. You will need to use google translate.

author: retrolefty
author_rank: Super Contributor
timestamp: August 21, 2019, 11:20:09 pm
content: I too own this sweeper and when obtained used had like half output power but otherwise worked fine. Traced with a scope that A3 was not putting out full gain. I added a Mini-circuits standard MMIC before the input to the hybrid. I just wired it up dead bug style and it seems to work just fine. It's not a lab quality piece, but extremely useful for sweeping filters. The step attenuator is very nice mechanically and electrically.

author: neil
author_rank: Contributor
timestamp: August 22, 2019, 03:08:11 am
content: Thanks for all comments.It certainly looks like a simple mmic amp would be the first thing to try. My first thought was something from Minicircuits, but I don't know if they do small orders. However, Mouser lists a BGA2874 with 31 dB gain over a DC-750 Mhz range, and at the cost of 41 cents in single-unit quantities, it's hard to turn down.

author: Tony_G
author_rank: Super Contributor
timestamp: August 22, 2019, 06:33:07 am
content: Minicircuits will do small orders. Sometimes they even have dev boards for the part if you want to use one.TonyG

author: djsb
author_rank: Super Contributor
timestamp: April 09, 2021, 11:44:54 am
content: Did you get this sorted? I might want to buy one of these units. Thanks.David.
HP 8640B  power supply problem - Page 1
Forum section: electronics / repair

author: sweatybetty
author_rank: Contributor
timestamp: August 05, 2021, 04:52:05 pm
content: Hello all!Picked up a 8640b last week. Not getting any output nor am I getting +44V. I think I have that part figured out.While looking through the FSM, I found a term I have never seen before and am hoping someone can enlighten me to the meaning or point me in the right direction. The term is "crowbar"It is shown on the schematic and in the troubleshooting section as well.Appreciate the help!

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2021, 05:09:40 pm
content: A crowbar is a fast-acting circuit that shorts the output should the voltage exceed a threshold, to protect the load or regulator. It’s analogous to throwing a crowbar across the third rail on the subway.

author: sweatybetty
author_rank: Contributor
timestamp: August 09, 2021, 02:45:37 pm
content: Thank you Tim.I found more info after changing search words around
HP 8640B sig gen 512...1024MHZ range not working - Page 1
Forum section: electronics / repair

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 04, 2023, 01:16:14 pm
content: Have a 8640B in which the range 512..1024MHz is not connecting, output frequency stays the same as in range 256...512MHz.Everything looks fine inside, before I read the service manuals is there anyone who has had this problem before?

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 04, 2023, 02:04:19 pm
content: Oh I know what happened, the metal strips in the rotary switches are falling down...

author: factory
author_rank: Super Contributor
timestamp: August 05, 2023, 10:41:40 am
content: Not had this problem, but neither of mine (8460A & 8460B) have the internal doubler option fitted, the control has labelling for the 512 to 1024MHz but requires an external doubler.The switch contacts falling out is a very common problem with both the 8640 and SA plug-ins for the 180/181/182 series SA.David

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 07, 2023, 03:07:05 am
content: I disassembled everything and secured all contacts with UV glue, now works fine.Everyone with this equipment should do this as preventive maintenance, it’s bound to happen sooner or later

author: Zenith
author_rank: Frequent Contributor
timestamp: August 07, 2023, 09:58:04 am
content: The other notorious problem with the HP8640A/B is the nylon gears, which crack with age and can't be easily fixed with glue. There are articles on moulding new ones and at one time someone sold brass replacements. The ones on my 8640A are cracked. I just take it very easy when I use it as I can't face up to replacing them.The analogue scale on the 8640A is rather delicate and suffers from age as well. Fortunately I found a replacement for mine.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 07, 2023, 10:27:29 am
content: Quote from: Zenith on August 07, 2023, 09:58:04 amThat can be fixed more or less with very fine steel wire mesh. Cut it so it has the shape of a half moon and then apply it after heating it to red to the flat side of the gear so that it melts enough to get inside the plastic. The trick is finding this fine wire mesh, I do repair lots of cracked plastic this way

author: Zenith
author_rank: Frequent Contributor
timestamp: August 07, 2023, 10:57:14 am
content: That's well worth remembering and is a lot better than moulding new gears or getting hold of brass gears. It would be nice to see at least photos of the technique being used, especially with these particular gears.I find it extremely irritating that they built these fine sig gens, which cost the price of a small house, and cheaped out with nylon gears, rather than use brass for a few bucks more. There and again, I doubt it occurred that they'd still be in use 40 plus years on.
hp 8642a issues - Page 1
Forum section: electronics / repair

author: sharonfs
author_rank: Newbie
timestamp: February 09, 2022, 03:39:04 pm
content: I'm hoping someone can point me in the right direction. My HP 8642A Opt. 001 decided to post mutable errors yesterday. A6A1, A9, A11 and A12 all failed with Out of Lock. I checked the 10 Mhz reference and it's spot on. I'm thinking this may be a Power Supply issue. Any suggestions would be great.Thanks,Sharon

author: sharonfs
author_rank: Newbie
timestamp: February 09, 2022, 06:05:57 pm
content: Sorry for the stupid request, opened up the unit and seen that 50 Volt supply was down. Replaced the .25 amp fuse and all is good.Thanks,Sharon
HP 8642A Signal Generator Output Clipping - Page 1
Forum section: electronics / repair

author: sfoster
author_rank: Newbie
timestamp: April 08, 2024, 09:51:48 pm
content: I just picked up an HP 8642A for doing AM/FM tuner alignments. Yes, it's a bit of overkill but the price was right. I'm also using a Sencore SG-165 for external modulation and an HP 334A to measure distortion. All was working well, though I have an issue with the 334A balance control...another thread perhaps. While working, the output of the generator was shorted due to a homemade banana plug adapter for the 334A input. The correct one has been ordered. Now I have clipping at the top of the waveform below 5kHz. This decreases with frequency but the sinusoidal is not symmetrical. The modulation and 10 mHz output look good. Also when I perform the Instrument Level Self Test, after a few minutes the display is "No Messages". This means all modules have passed.Perhaps there was no circuit protection for my sloppiness. Attached is the output on my scope of 1kHz. I have seen this before on an audio receiver when an output transistor's emitter resistor goes open.I have all the manuals and am starting to go through them, but this unit looks quite daunting compared to what I'm used to. Any thoughts for troubleshooting are appreciated.Thanks -Steve
HP 8644A Signal Generator - Problem with step attenuator on 5 DB scale. - Page 1
Forum section: electronics / repair

author: aa7ae
author_rank: Contributor
timestamp: January 28, 2022, 05:36:24 am
content: I received an HP 8644A from E*BAY today and everything seemed to be in order. I ran a complete diagnostic routine with code SPCL 320 and it came back with +0 as in all is good. However, when I started looking at the output of the signal generator on my spectrum analyzer, I notice that when I move the knob for DBM the signal drops completely out on the 5's of the DBM scale. It works fine on even 10's, but in between on the 5's it's gone. I can hear a click, so I think the relay is fine, but the pad must be faulty for this range. Does anyone know what the part number of this is and how hard it might be to repair the bad 5 DB pad?Thanks

author: wn1fju
author_rank: Frequent Contributor
timestamp: January 28, 2022, 01:10:17 pm
content: Google "HP step attenuator repair." There are some articles, YouTube videos and eevBlog posts that might help you. I don't know the part numbers of the individual pads, and even if Keysight has any for sale, so you might be forced to buy a similar old HP attenuator (on eBay) and salvage the parts. But I would start with a gentle cleaning and inspection of the plunger's O-rings (if the attenuator is the kind that has them). I doubt there is a way for an average user to "fix" an individual pad.Fortunately, my unit has the electronic attenuator, not the mechanical one, so I am spared this ordeal. Good luck.

author: aa7ae
author_rank: Contributor
timestamp: February 13, 2022, 07:59:02 pm
content: My attempts to repair this signal generator has taken a bit of a turn for the worseI was able to revamp all the o-rings in the step attenuators with new ones and clean up the contacts. However, when I put everything back together again, there was no RF output.In my attempts to troubleshoot why there was no RF output, taking the boards in and out, etc. now the I/O board seems to be having problems as I'm getting a fault 15,022,110In addition, the hardline coax that goes to the input of the step attenuator suffered damage to the SMC connector, now I have to find a way to replace it.
HP 8647A Signal Generator - randomly some keypad buttons not working - Page 1
Forum section: electronics / repair

author: odium
author_rank: Contributor
timestamp: April 22, 2025, 09:48:50 am
content: Hi all,My HP 8647A Signal Generator has a weird keypad issue ...randomly the bottom half numeric keypad mostly the "0 , the "." and "+/-" keys don't work.after several power cycles it works again ! then after several minutes it fails again !a cleaning of the flex ribbon connector didn't change anything.any ideas what is causing this ?Thanks in advance !

author: odium
author_rank: Contributor
timestamp: April 23, 2025, 07:16:44 am
content: zero comments on this issue ?

author: odium
author_rank: Contributor
timestamp: April 24, 2025, 12:23:37 am
content: up ! need advices

author: benj38
author_rank: Frequent Contributor
timestamp: April 24, 2025, 08:02:09 pm
content: As far as I know this is not a common problem, which may explain the lack or responses.The service manual is available online, so plain old troubleshooting is the way to go. Should not be too hard given the very few components that are involved.

author: Aheld
author_rank: Regular Contributor
timestamp: April 27, 2025, 03:10:33 pm
content: I'm not sure if this device has rubber buttons?At my R&S spectrum analyzer, I fixed it with a soft lead pencil toi renew the contact.This works since 3 years without issues.Br.Axel

author: coppercone2
author_rank: Super Contributor
timestamp: April 27, 2025, 03:20:56 pm
content: that expenisve $30 kit on digikey is awesome but its expensive for rubber buttons
HP 8648A - Error 616 - AM Modulation Path Failure.  - Page 1
Forum section: electronics / repair

author: BradF
author_rank: Newbie
timestamp: September 17, 2023, 09:12:49 pm
content: Hi all. I decided to look at the generator and this post again. I don't believe I got a reply. I thought it would help if I repost this request. ... My HP 8648A during startup shows error 616 - "AM Modulation path failure". Per the service manual due to supposedly bad A3. The motherboard! All voltages from power supply are good per the schematic. I reseated everything. Cleaned inside. Not much in the service manual for A3 repair. Was working fine for years. Went to use it after being in storage and this error came. RF out is ok but will not modulate with internal 400 or 1KHz or external ac or dc. Has anyone solved this? Does anyone have an A3 for sale? Thanks.
HP 8648B AM suddenly very noisy - Page 1
Forum section: electronics / repair

author: route20
author_rank: Newbie
timestamp: April 29, 2023, 02:50:14 pm
content: Picked up a an 8648B about a month ago and it's been excellent, until today when out of nowhere the AM output became a mess:It occurs with internal or external modulation (mine does not have the programmable internal modulation source and is limited to 400Hz and 1kHz). The above captures are using internal 1kHz, below is 400Hz:All captures so far have been at 100% AM depth, it decreases as depth is lowered. Here is 1kHz at 80%:No errors on power on self test or returned via GPIB SYST:ERR?. Happens across entire frequency range.Hoping someone has seen something like this before on one of these units as the service manual is pretty useless (all adjustments done through ATE software)

author: gnif
author_rank: Unknown
timestamp: April 29, 2023, 03:37:48 pm
content: @route20, aka@dos... care to explain why you created a puppet account to post this?See rule #8https://www.eevblog.com/forum/chat/forum-rules-please-read/

author: route20
author_rank: Newbie
timestamp: April 29, 2023, 04:18:11 pm
content: Quote from: gnif on April 29, 2023, 03:37:48 pmJust go by something else now (did not see a function to change username) and didn't realize it was a problem as on other forums it's only an issue if it's for some malicious reason like ban evasion or spam. You're right, I should have checked the rules first. Can my old account be deleted so I only have this one? There's nothing worthwhile there.

author: gnif
author_rank: Unknown
timestamp: April 29, 2023, 04:19:46 pm
content: You can request your username to be changed, however creating a new account to operate under is not allowed.Edit: Account banned, but user is welcome to continue here under their old account.

author: dos
author_rank: Regular Contributor
timestamp: May 01, 2023, 10:46:59 pm
content: So I found one of these at work (probably hasn't been used in decades) and hooked it up to an E444 series SA and observed exactly the same behavior, starting to think this may be just how they are and I didn't really notice it initially, or I had a lower modulation depth set or something. Either that or they both have the same fault. All I know is the newer RF generators at work don't do this at 100% and neither does my 33120A function gen, but I have heard that noise was a problem with these 8648s. I think this is case closed unless another owner of an 8648 chimes in with a clean capture on their SA or scope's FFT.Apologies again for the duplicate account, forum staff
HP 8648B ERR 616 AM Modulation path failure - Page 1
Forum section: electronics / repair

author: petercut62
author_rank: Contributor
timestamp: February 15, 2018, 02:24:36 pm
content: Hello,I've got my RF Signal generator HP8646B in trouble.It display at power on : 616 AM Modulation path failure.According to the troubleshooting section of the service manual : Either the main board A3 or the detector has failed.As it's still doing AM fine, i think something around AM detector has failed or partialy failed. May be the AM diagnostic side is faulty...I've teardown the unit. Fortunately for me the power supply is in bad shape, full of dirt and some resistors are almost fried. So i will repair the power supply first.For AM side, as there is no schematic of the A3 mother board on the web, i don't know where i must have a look first :- Cabling- Follow the AM signal path trying to find a kind of detectorAny advices are welcome.Thanks

author: FloridaJo
author_rank: Newbie
timestamp: July 03, 2018, 06:59:50 pm
content: In my 8647 manual it says:"The AM path detector indicates a failure at the output of the A3 board.Either the signal output has failed, or the detector has failed.Replace the A3 board."

author: mmagin
author_rank: Frequent Contributor
timestamp: July 03, 2018, 07:03:05 pm
content: Sadly they don't provide board-level information on that series. I'd first check supply voltages (including for excess ripple) and re-seat (cleaning if necessary) all the relevant connectors.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: July 05, 2018, 03:27:56 pm
content: full of dirt... yes, they are! Dirt is a main culprit in these, particularly if they are ON 24/7.No Filter fan draws air in from the front panel holes around the soft keys, and dirt / contamination destroys the key membrane printed circuit switches too. I have repaired 2 of these now. Happened to find the membrane switch assy on ebay. It is possible to repair, but far easier to replace!

author: Zinger Liu
author_rank: Newbie
timestamp: June 21, 2021, 12:47:45 pm
content: I got the same issue. I measured the A3/J11-1 and A3/J11-2 for the frequency. it was 1 MHz instead of 200 kHz.I'm not sure is that correct?
hp 8648B firmware - Page 1
Forum section: electronics / repair

author: hackwell
author_rank: Contributor
timestamp: September 20, 2021, 07:11:17 pm
content: Hi everyoneI'm in search of the firmware for my HP 8648B which throws a ROM checksum error.Does a life saver here have it ? Sure enough it's nowhere to be found on the internet or I goofed it ?There are 4 27C010 OTP ROMs on the firmware board.

author: hackwell
author_rank: Contributor
timestamp: October 07, 2021, 06:32:42 pm
content: This firmware seems to be impossible to come by...Weird enough the generator works fine so far. I read the ROMs and disassembled it with Ghidra which is a pain in the neck for 68K code. Unfortunately I can't find anything wrong so far.I'm still working on this and can provide the contents of my ROMs in case you're interested. The firmware is corrupted but it could be the only one around for now.
HP 8648B repair - Page 1
Forum section: electronics / repair

author: TheDefpom
author_rank: Frequent Contributor
timestamp: July 02, 2019, 03:51:07 am
content: Hi All,I am in the process of repairing a HP 8648B RF Signal Generator, it is giving error 608 in boot up and after much confusion with differences in the service manual and the actual unit I have traced the fault to the A5 "Sig Gen Synth" board, specifically to the VCO which is not oscillating.I am recording video about the repair for my YT channel, but I cannot see an obvious reason as to why the VCO isn't working (at least not yet, I am still digging into it).I still have some voltage checks to do within the VCO block itself to be sure the rails are actually getting there, but I have confirmed the VCO control voltage is (although not present) not the cause, as there is a jumper (JP2) which will inject 9V into the circuit, and that still doesn't start it working.If you have any experience with this kind of oscillator circuit (or repairing the 08648-60115 or 08920-60373 board) please take a look, I could use some pointers, right now I am suspecting the actual oscillator transistor Q13, I found and have ordered some NOS ones.There are no circuit diagrams for the HP 8648B that I can find, but in a live stream one of my subscribers pointed me at the HP 8920 which uses the same (or at least very similar) boards, so that has been a big help.Note attached is the block diagram from the HP 8648 service manual, and the circuit diagram of the VCO section from the HP 8920 which is a similar (but earlier) version of the board, component values are different, as is some of the architecture, but it is VERY similar.

author: TheSteve
author_rank: Unknown
timestamp: July 02, 2019, 04:21:04 am
content: Have a look at U20(if you have an equivalent in your circuit) for sure, when those amps fail all kinds of interesting things happen.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: July 02, 2019, 04:24:29 am
content: I have tried probing from C63 before U20, there was no oscillation there, but of course that doesn't mean that U20 isn't dragging it down.

author: richardb
author_rank: Newbie
timestamp: August 27, 2019, 03:01:14 am
content: Hi TheDefPom,I am repairing an 8468. Can you please link to where I can download a copy of the 8920 manual you mentioned - the one I found on the internet is very blurry.Thank youRichardb

author: TheDefpom
author_rank: Frequent Contributor
timestamp: August 28, 2019, 08:19:26 am
content: I will try to find it.I did put it along with some datasheets etc on my Patreon page for my supporters at the time.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: August 28, 2019, 08:21:01 am
content: Ok found it:https://www.keysight.com/upload/cmc_upload/All/8920_Component_Level_Information_Packet.pdf
HP 8648C Sig Gen Ebay purchase repair story - Page 1
Forum section: electronics / repair

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: October 13, 2019, 06:41:26 pm
content: I purchased a 8648C off Ebay with the description saying it had no display so unable to test. Considering the prices these units go for, I think I got a great deal at $250. The picture showed the unit powered on with no display. It is the LCD model and the backlight was on. Well, once I received the unit, it was exactly as described. No display with backlight.First repair: Must get a display.I was hoping for an easy fix by adjusting the display contrast pot on the rear of the unit. No luck because there is NO pot. WHAT? You could also tell that one was never installed from the factory. I am assuming at some point, the front panel was switched as the main board was for the VFD display and not a LCD display. After some research, the main board seems to be the same except for the contrast adjust pot. So I installed a 5K pot and presto, I now have a display.Second repair: Error code.The display is showing a 608 synth error. I just happened to watch Defpom's YouTube channel on the repair of his B model with the same error. Since I had the same symptoms, I decided to first check the inductor that opened on his unit. Yes, the same inductor was open on the Synth board. Thanks Defpom for saving me a lot of troubleshooting time. So I replaced the inductor and now no errors.Third repair: Keypad issues.Now I have a display and the unit powers up normally with no errors. The RF output also is working fine. Now about a third of the front panel buttons do not work. They are just unresponsive. I remove the front panel and disassemble the keypad. Well, the problem is the flex circuit membrane has corrosion. Something is eating away at the traces. To get the front panel working now, I used a circuit pen to jumper the corroded open spots on the traces. And to my amazement it worked. The buttons work now. I will probably order a new one from China even though they want like $60. I'm not sure how long the repair will hold since the membrane will continue to corrode.Anyway, just thought I would post this repair story.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: October 20, 2019, 07:27:57 pm
content: UPDATE:Well, a 4th problem has been found.Previously, I just did a quick RF test of the frequency & amplitude capabilities. After a more extensive test, I found that below -75.9dbm the signal would just drop out completely. This unit is capable of going down to -130dbm.I disassembled the attenuator and discovered the tiny o-rings on the plungers were in terrible condition. 1 of the o-rings was missing and numerous others were cracked & hard. Now, I'm glad I looked inside because more problems would have occurred as the o-rings got worse.Anyway, I replaced all the o-rings and now it works properly all the way down to my measuring capability of -125dbm.
HP 8648C erratic attenuator - Page 1
Forum section: electronics / repair

author: Oscillosmoke
author_rank: Newbie
timestamp: August 07, 2022, 10:14:00 pm
content: First... I'm somewhat of an electronics newbie with access to and some experience setting up and using a variety of test equipment.So, the problem is an attenuator that is not acting as it should in an 8648C sig gen. I know these are notorious for attenuator issues but I'd like to give a repair a shot for a change. The output is not linear at all! As I moves the amplitude in the upward or downward direction the output jumps all over the place. So I opened it up and connected a known good sig gen to the attenuator input. The output to a know spectrum analyzer and controlled the attenuator (33322-60011) to an HP 11713A attenuator driver - also good. This is a 4 section attenuator that is configured as 10 / 20 / 40 / 60 dB. What I'm getting is 40 / 20 / 40 / 60 dB... and it is very accurately that! So it doesn't seem like a dirty pad or pad/contact issue as I was expecting. But before getting to far in the weeds taking thing further apart, thought I'd ask for advice / thoughts. Thanks in advance!

author: m k
author_rank: Super Contributor
timestamp: August 17, 2022, 07:34:14 pm
content: Manual hasA11 Attenuator (8648B/C/D)The attenuator is a 4-section attenuator (10, 20, 30, and 60 dB sections) that provides 130 dB attenuation in 10 dB steps.Calibration EEROM on the motherboard contains calibration data specific to this assembly.It needs to be updated whenever the attenuator is replaced.Some simple selector problems?

author: edpalmer42
author_rank: Super Contributor
timestamp: August 17, 2022, 07:49:37 pm
content: 10, 20,30, 60 dB doesn't get you to 130 dB in 10 dB steps. You can only get to 120 dB.10,20,40,60, dB can get you to 130 dB in 10 dB steps.Oscillosmoke, it sounds like either your 10 dB pad is in trouble or the driver is confused. Yes, it's suspicious that the 10 dB pad is measuring exactly 40 dB, but coincidences do happen. Is the loss through your 10 dB pad precisely the same as the loss through the 40 dB section? To the nearest 0.1 dB? That would suggest that the driver is activating the 40 dB pad when it thinks it's activating the 10 dB pad. You should be able to power the sections manually to help isolate the problem.Ed
HP 8640B replacement gears? - Page 1
Forum section: electronics / repair

author: wd5jfr
author_rank: Regular Contributor
timestamp: November 02, 2021, 12:31:52 am
content: I need to fix my 8640b and I see that there's a seller in India selling brass gears without screws for $55.00, Has anyone tried them and how many hours does it take?ThanksHan

author: jonpaul
author_rank: Super Contributor
timestamp: November 02, 2021, 06:06:44 am
content: Bonjour, I have many of the HP 8640B excellent RF generators, with the original gears.Mechanical gear work requires complete removal of front panel and all the concentric gear shafts and knobs.Great risk to loose a tiny part or fail to properly reassemble.The complete manual and forums threads have more details on the procedure, perhaps 50-100 steps and many pages.NOT FOR THE FAINT OF HEART!I would Post to the groups.io with the Indian ebay gear link and photos of your gear situation.https://groups.io/g/HP-Agilent-Keysight-equipment/topicsBon chanceJonPS your title has a type 8649B

author: wd5jfr
author_rank: Regular Contributor
timestamp: November 02, 2021, 01:28:56 pm
content: Hey BobThanks for the tips. Next month I'll be 81 so age has effects but I think I can do it. I can sell it for the parts if I don't succeed. Two gears are split at the set screw.Hank

author: Stray Electron
author_rank: Super Contributor
timestamp: November 02, 2021, 02:13:53 pm
content: Hank,Keep us posted. I'm interested in what those Indian gears look like and how your repair goes. There's a 8640 in surplus store nearby that's been calling my name. But I'm SURE that some of the gears are split. They're not one of HP's better designs.
HP 8656B Successful Repair? - Page 1
Forum section: electronics / repair

author: OldVolts
author_rank: Contributor
timestamp: April 11, 2021, 02:31:49 am
content: I'm about to take a look at a friend's HP 8656B signal generator. It's an option 1 with a 2838 serial.He reports that he can't get full output anywhere but he believes the attenuator is working.Aside from the attenuator assemblies, has anyone had any experiences and success / failure in this arena?Thanks,Bill
HP 8657A replacement battery - Page 1
Forum section: electronics / repair

author: Exeter
author_rank: Newbie
timestamp: March 15, 2023, 04:17:50 am
content: Can anyone suggest a replacement for the on-board 2.8 V (0.35Ahr) Lithium battery in this frequency generator?

author: Swake
author_rank: Frequent Contributor
timestamp: March 15, 2023, 09:48:37 pm
content: What does it look like? What size is it?