HP 8903A DC Voltmeter or programmable gain amplifier problem - Page 1
Forum section: electronics / repair

author: aarons
author_rank: Regular Contributor
timestamp: June 02, 2025, 10:34:43 pm
content: Hello,I recently got my hands on a HP 8903A Audio Analyzer. While running through the performance tests (the ones I have the equipment for, anyway) I discovered some problems with the DC voltage measurements. Note: I did go through the volt meter tuning procedure.From a fresh reset, while applying 3VDC to the input, if I activate the DC level measurement I get a flash of ERROR 14 (input value out of range) and then it displays 1.7V. If I apply 30VDC to the input, again Error 14, but it shows 22.07V. If I apply 90V, it reads 89.9 (although the DVM I have in parallel with the input shows 88.Now, if I gradually reduce the voltage from 90, the 8903 will track along with the DMM (although maintaining the ~1V higher reading) until I get to 60.7V, at which point the 8903 will jump to 28.95V. Increasing the voltage back to 90 does not cause the 8903 to go back to showing the nearly correct voltage (it will show 33.11.) However if I toggle to display AC level and then back to DC, it will go back to 89.9V.Table 8-9 in the service manual (attached) shows the voltage ranges and the various attenuation applied to those ranges, along with the special commands to force programmable gain amplifier to that level. So with 90V in, it attenuates 40dB, and displays the correct voltage. At 60V, according to the table the attenuation should be 24dB, but when I activate that setting it shows 28V. At 30V that is where the 22V display comes from. At 10V input, I get 10.16V shown if I use the 24dB attenuation, but 6.06V if I select the (supposedly correct) 12dB. And similarly, the 0dB attenuation only seems to give accurate values if I'm below 1V.So it seems like the 8903 is choosing the wrong attenuation for a given input, and/or the attenuation isn't what it's supposed to be. I'm still working my way through the service manual, but if anyone has one of these and wouldn't mind checking the DC voltmeter readings as I did above, I'd appreciate the verification that I'm on the right track. Of course, any other advice would be appreciated as well.

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 03, 2025, 11:16:55 am
content: Yes, I would say your 8903A is screwed up.I took a 0-30V DC power supply and fed it to the input of my 8903A set to measure DC, automatic operation. The 8903A perfectly tracked my power supply with a response time of about 1/2 second as I randomly turned the voltage knob on the power supply. I didn't try it with other power supplies > 30V, but I expect it would perform similarly.Work your way through the service manual. You'll eventually find the problem.

author: aarons
author_rank: Regular Contributor
timestamp: June 06, 2025, 06:00:45 pm
content: So going through the tests in service sheet 1 I found first that the amplitudes of AC input was out of spec when looking at the left side of C19. At -12dB that should be 2-2.2V, but I measured 2.3, and at -24dB it should be 500-560mV, but I measured 720. All the various relays and fets seemed to be on/off as appropriate.The big problem is that the input to the differential-to-single-ended amplifier is way off. When the input to the 8903 is hooked to the 5V rail (~5.2V), and the input attenuation is set to 0dB, you should see 5.2V at the high side input, U6 pin 3. I was seeing 2.1V. There are only so many things on that node that could be pulling it down, since all the bias voltages called out on the schematic were correct. I pulled Q2 and Q3 off the board to test them. According to the DCA75 Q2 is:P-Channel JFET with symmetrical source and drain.V_g(off) = -4.2VV_g(on) = -0.02Vgfs = 2.5 mA/VI_DSS = 5.04mAR_DS(on) = 355 ohmsQ3 is:A voltage regulator with V_out = -1.682VBut then I changed the order of the wires on the DCA75 and it tested as:P-Channel JFET (not symmetric)V_g(off) = -3.86VV_g(on) = -0.19Vgfs = 2.4 mA/VI_DSS = 5.46mAR_DS(on) = 358 ohmsSupposedly it shouldn't matter which wires go where with the DCA75, so this was curious. In all but 1 configuration it was recognized as a voltage regulator. Also, Q2 and Q3 are the same part, so I'm guessing Q3 is bad. The part is described as "Transistor J-FET P·CHAN D-MODE Si" and the HP (and MFG) part number is 1855-0082. The 2N5461 looks like the equivalent, so I'll try swapping that in.

author: aarons
author_rank: Regular Contributor
timestamp: June 07, 2025, 09:55:19 pm
content: I replaced the Q2 transistor with a 2N5461 and that has fixed the DC voltmeter. It is pretty much deadon with my bench DMM. However, now if I connect the input to the output (with a good quality cable) I am getting a sig/noise of only 88.23dB, and values as high as .06% distortion. Could the new transistor be making the internals noisy? I foresee a game of whackamole.

author: srb1954
author_rank: Super Contributor
timestamp: June 07, 2025, 10:29:27 pm
content: Quote from: aarons on June 07, 2025, 09:55:19 pmSee if the signal/noise and distortion readings improve if you switch in the 400Hz HPF and/or the 30kHz LPF. In both cases put a scope on the monitor O/P on the rear panel to view the residual signal after the fundamental signal has been nulled out as this will indicate what remaining signal is degrading the readings.A significant improvement with the HPF switch could indicate that the oscillator O/P is has excessive hum and ripple and that some electrolytics need replacing. An improvement with the LPF switch could indicate a noisy front end or, most likely, pickup of environmental noise e.g. LED lighting, cell phones, WiFi etc.

author: aarons
author_rank: Regular Contributor
timestamp: June 10, 2025, 05:59:56 am
content: So, at 1V, 1kHz, fed back into the unit, both input and output grounded:No filters:Distortion bounces around between .13-.2%Sig/noise: 88.24dBSINAD: About 56 dB400Hz High Pass:Distortion: UnchangedSig/Noise: 88.36SINAD: Unchanged80kHz Low Pass:Distortion: Maybe slightly lower bouncing around between .10-.18%Sig/Noise: Improves to 96.28dBSINAD: Unchanged30kHz Low Pass:Distortion: Same as 80kHz low passSig/Noise: Improves to 98.63 dBSINAD: UnchangedAnd I've attached what I see on the monitor output. It looks like noise to me.

author: srb1954
author_rank: Super Contributor
timestamp: June 10, 2025, 08:19:00 am
content: Quote from: aarons on June 10, 2025, 05:59:56 amDefinitely not all noise as you can see some repetitive patterns in the monitor waveform. The periodicity is confirmed by the regularly spaced peaks in the FFT trace.The fact that the peaks on the FFT only roll off slowly indicates that the source of the distortion is a sharp discontinuity in the signal producing a high harmonic content. You can see some relatively sharp transitions at some points in the monitor waveform. Display the source waveform on the scope as well as the monitor output and see if the sharp discontinuities in the monitor O/P correspond to any noticeable glitches e.g. clipping, crossover distortion, in the source waveform.Measure the frequency of some of the peaks on the FFT and see how that relates to the source frequency. Try adjusting the source frequency to see if the peaks on the FFT move with the source frequency, which indicates harmonically related components in the source waveform or distortion components created in the front end of the analyser. If the peaks stay put then there is some external signal causing the poor readings.Try increasing the output level from the oscillator to its maximum of 6V and repeat the measurements. This will help lift the repetitive pattern out of any random noise to help identify the source.Lastly, increase the scope input to 1Mas the monitor output is 600output impedance and is excessively loaded down by the 50you have your scope set to.

author: aarons
author_rank: Regular Contributor
timestamp: June 10, 2025, 06:24:53 pm
content: Oh, so I forgot to mention that the peaks in the FFT are harmonics of the 1kHz test signal. I'm not sure if it's expected that there be harmonics of the test signal or not, but that's where they're coming from and I confirmed that by changing the frequency and watching them all move.Interestingly, when I turned things on just now, still using a 1V amplitude, the distortion reading is coming in at .026%, so an order of magnitude better than last night. SINAD is 70dB and Sig/Noise is about the same at 88.37dB. But as I've been sitting here, I see now the distortion is creeping up and is now approaching .06%, so perhaps there is some thermal component to this as well.Here are 2 more scope captures, both at 6V, 1kHz. The highest peaks of the monitor output do occur in phase with input signal, but if there is a pattern to the timing of them beyond that I don't see it.And in the time I've been getting these new scope captures, the distortion reading has gotten back to where it was last night.Finally, I have tested with some external function generators as well. The one built into my scope yields similar numbers, and also has the harmonic peaks on the monitor. However using my rigol AWG (with the same output settings as the 8903 -- the scope FG is limited to 5Vpp) I get back down to .065-.07% distortion, and down to around .05% with the 30kHz low-pass. Apparently the rigol has a lot of noise in the signal. I believe the peaks are still there, but since the noise floor is up, they stand out less.

author: srb1954
author_rank: Super Contributor
timestamp: June 11, 2025, 08:36:39 am
content: In order to determine whether it is a faulty signal source or faulty distortion measurement can you post some more scope trace captures just showing a single cycle of the 8903A oscillator O/P together with the monitor O/P showing the resultant distortion signal on another channel? If there are any sharp discontinuities in the source signal you might be able to see them on the scope trace.To be able to see the maximum detail in the signals expand the traces to fill as much of the screen height as possible and use the maximum averaging settings to enhance the vertical resolution and eliminate the stair-steps in the waveforms.If there is nothing obvious to be seen in these traces then you may need to get another known low-distortion signal source to isolate the fault to the generator or detector. Most likely this will require a high-quality analog oscillator as an AWG is not likely to be sufficiently low-distortion for this purpose unless it is super high quality unit with 14-bit resolution or better. A computer sound card with a tone generator program may also be suitable as a clean signal source.

author: aarons
author_rank: Regular Contributor
timestamp: June 12, 2025, 10:37:54 pm
content: I am going to get back to this tomorrow, but can you recommend a good analog oscillator for this purpose? Ideally something that can be had for not too much on ebay? The AWG I have is a rigol DG 922 Pro, which has 16 bit resolution, so maybe it's fine, but it's a mid-tier AWG at best, so I wouldn't be surprised if it had some issues. I can try with the function generator on my phone, too, just for kicks.

author: srb1954
author_rank: Super Contributor
timestamp: June 13, 2025, 03:53:31 am
content: I use a Tektronix SG505 when I need a low-distortion audio source but these are rarer than hen's teeth and very expensive to buy these days.I haven't had any experience with other types of low-distortion oscillators so I can't recommend a particular model. There are some low-cost Chinese modules offered eBay but I don't know if these live up to their claimed specification. I suggest you search through this forum and other audio forums to get recommendations.The Rigol AWG wouldn't appear to be suitable for testing the ultimate performance of the 8903A as, although it is a 16-bit AWG, the signal distortion specifications aren't that great.

author: aarons
author_rank: Regular Contributor
timestamp: June 13, 2025, 09:15:29 pm
content: So I hooked up my phone to the analyzer and it also does substantially better with the distortion number than the 8903A source (0.02% vs 0.1%). What is confusing me is that I thought the monitor output would remove the fundamental signal from the input, but with the phone signal as input, the monitor output shows a noisy sine wave. Is that expected? I also noticed that while the frequency counter of the 8903A seems to perfectly match the frequency the phone is sending, the 8903A output is about 1.7Hz off.Regardless of whether the input side of things is working to spec, it seems clear to me that the generator of this unit has some issues.

author: aarons
author_rank: Regular Contributor
timestamp: June 13, 2025, 11:12:28 pm
content: I just received an NTE326 jfet I had ordered previously as a better substitution for A2Q2. I swapped it in, and immediately the distortion level was .02% and the waveform on the monitor looked like a normal sine wave. Then I left the analyzer on as I tended to some other things, and when I came back I found the waveform looking pretty janky again, and distortion had gone up to .04%. It still doesn't look as odd as the tests from earlier today, so I'm going to leave it and see if get worse. If so, I might need to source the original JFET, because while this one is closer to the original part, it's not identical, and has a bit more resistance when it's on (450 ohm vs. 355). Meanwhile I'm going to continue to look at the oscillator since even the .02% is higher than I am expecting.

author: srb1954
author_rank: Super Contributor
timestamp: June 13, 2025, 11:37:15 pm
content: Quote from: aarons on June 13, 2025, 09:15:29 pmNormally there is a little bit of the fundamental frequency visible after the notch filter but your trace shows more fundamental than I would normally expect. There could still be some minor issues with the operation of the notch filter. Once you get the generator section fully working again you could perhaps have a closer look at the notch performance. There are a set of procedures in the service manual that allow you check the performance of the notch filter and ancillary circuitry including special commands to take manual control of the notch filter tuning.QuoteThe source oscillator is tuned in discrete steps by the microcontroller rather than being a fully synthesised as a signal generator might be. Consequently, there is generally a small error in the indicated frequency versus the actual frequency but this has negligible effect on the instrument's measurements of distortion etc.QuoteThe single cycle trace of the generator O/P and resulting distortion components on the monitor O/P clearly shows the sudden glitch creating all the numerous harmonics. There ought to be a corresponding glitch on the generator waveform but this is too small to be seen on the limited vertical resolution of the digital scope trace.Do you have an analog scope to view these signals? Small perturbations might be more easily seen on an analog scope with a good sharp trace. Failing that you could perhaps step down the volts/div on your digital scope and use the position control to reposition the expanded waveform to get a closer look at the waveform detail.

author: aarons
author_rank: Regular Contributor
timestamp: June 14, 2025, 04:14:08 am
content: I do not have an analog scope at the moment, and I really should take a break from buying old test equipment since I just bought a 3562A signal analyzer yesterday on top of this audio analyzer, the 3455A dmm, a 5005A signature analyzer I thought i might need for fixing these things, and 5372A that I have no need for but the guy was basically giving it away and I thought I'd play with it a bit before selling it. <Takes breath>And now I'm looking for some low distortion oscillator (i haven't found anything with the numbers of the SG505 as far as old test gear goes. There are the 1kHz DIY boards that boast -140 dB distortion or better, and there are audio interfaces with -108dB). But all that said, a local guy is selling a tek 2467B that I covet. But I'm a little concerned about the test waveform in the picture.

author: aarons
author_rank: Regular Contributor
timestamp: June 23, 2025, 10:00:43 pm
content: I got the signal analyzer, and when looking at a 1khz output from the 8903A vs either my rigol AWG, or the source built into the analyzer, the 8903 seems to have some noise on it. Here is a brief video showing some of what I see:https://monosnap.com/file/W175eTCW8A5ZvLHuAO4qDMZr4WGVKRSo I guess I need to look at the generator section of the analyzer, right?

author: srb1954
author_rank: Super Contributor
timestamp: June 24, 2025, 12:36:35 am
content: Yes, but it would help if you had a few more scans with your 3562A showing both a wider frequency range to show any harmonic content and a zoomed-in scan to show any noise side-bands around the 1kHz generator signal.I still think the main problem is the harmonic content of the generator rather than some irregular random noise so you should concentrate on that first. Unfortunately, the captured 3562A scan width of only 2kHz doesn't show any of the harmonics of the 1kHz signal. Try a wider frequency scan of at least 5kHz to pick up a few harmonics.To pick out any smaller harmonic content from the signal it might be necessary to use averaging on the 3562A to minimise effects of noise and quantization error on the captured traces. It should be noted that the 3562A doesn't have a wide enough dynamic range to fully check the ultimate performance of the 8903A source as you would need a signal analyser with a dynamic range of at least 100dB rather than the 80dB dynamic range that the 3562A provides.The fact that the width of the centre peak of the 8903A trace appears significantly wider than that of the Rigol possibly indicates that there is also some low-frequency noise or side-bands riding on the 8903A 1kHz signal. Try zooming in on the fundamental signal to see if there are any side-bands or excess spreading of the signal peak. Discrete side-bands spaced at 60 or 120Hz away from the 1kHz signal would indicate power supply ripple modulating the amplitude of the oscillator or noise pickup from the mains supply. More diffuse spreading of the peak might indicate a problem with level control circuitry in the oscillator causing random variations of the oscillator amplitude.

author: aarons
author_rank: Regular Contributor
timestamp: June 24, 2025, 02:36:36 am
content: Yeah, I'm not seeing any harmonics, which is why I had such a narrow window. Here is the full range of the 8903, and then a narrow range of both the 8903 and internal source.

author: srb1954
author_rank: Super Contributor
timestamp: June 24, 2025, 07:39:30 am
content: I think I can see some harmonics in the first division of the scan but the scan is too wide to see any details. Try again with a scan reduced to a 10kHz span so we can more clearly see any harmonics and, if any exist, try to measure the amplitudes of each of the visible harmonics relative to the 1kHz fundamental. The uniform "grass" in the rest of the scan is probably artifacts from the quantisation distortion produced by the limited dynamic range of the 3562A.The narrow scan is zoomed in a bit too far to see if there are any power related side-bands. Try again with a scan covering at least 850-1150Hz to see if there are any 120Hz power supply ripple induced noise sidebands.The narrow scan does show that there is some extraneous phase noise around 1kHz. This could be due to low frequency noise on the power supply to the oscillator or instabilities within the oscillator levelling loop. You should go through the performance test and troubleshooting procedures listed in the manual for this PCB.

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 24, 2025, 02:16:55 pm
content: For what it's worth, here's a photo of my HP 8903A (1 kHz, 1V) feeding my HP 3562A for comparison to your plots. I did use 8-averages which might differ than yours. Otherwise, I used Hanning window and the same x- and y- scales. Sorry for the poor photo (light reflections), but despite having tons of vintage test equipment, I still find it easiest to simply point my camera at the screen rather than try to export data!

author: aarons
author_rank: Regular Contributor
timestamp: July 01, 2025, 10:08:18 pm
content: Update on this:I bought another oscillator board and another notch filter board to verify mine are problematic, and hopefully provide a useful comparison for figuring out why mine are problematic. The oscillator board was the same PN, 08903-60006, but the notch filter is not -- the original was 08903-60005, and the new one is 08903-60163. I couldn't find a reference to that new part number is any of the manuals I've been able to find for any version of the 8903. But I verified that the A and B versions have the same pinout for those A3 cards (at least where it counts) and tried it anyway.Then I did a THD+N vs. Frequency plot using the old boards, the new notch filter old oscillator, and both new boardsNow I guess I should go through the entire tuning procedure, since it appears the major problems are fixed!One of these days I'll try to figure out what's wrong with the old A3 and A5 cards, but that will have to wait until I finish a couple more pressing repairs.