New problem discovered on my HP 8903A - Page 1
Forum section: electronics / testgear

author: aarons
author_rank: Regular Contributor
timestamp: August 20, 2025, 11:46:44 pm
content: I was testing a receiver the other day, and decided to check the phono input using my 8903A (rather than by just hooking up a turn table, as I normally do). I connected the 8903A output to the phono input, and the 8903A input to the speaker output of the receiver. I set the signal amplitude to 2mV, 1kHz. I also had the speaker outputs connected to my oscilloscope and an 8ohm dummy load.As I turned up the volume I noticed the signal was 'bouncy' and at a certain point it becomes completely unstable. Video here:https://monosnap.com/file/t40DZjCYgrBLLtL5CWgioXBA74gcO1Unsure where things were going wrong, I brought over my "modern" NAD amp which is very clean signal-wise, and used that to drive the dummy load, but this time the input was coming from my quant aslyum QA403. The 8903A input was also still on the speaker outputs. In the video below, the top trace is the speaker output from the NAD amp. The middle trace is the output of the 8903A, set to something in the 10s of mV, and the bottom trace is the input into the NAD amp. You can see the amplitude of the 8903As output start to oscillate in proportion to the signal it sees on it's input. This can only really been seen at very small 8903A output amplitudes. And apparently, when output and input frequencies are precisely matched, the constructive interference causes the output to go unstable.Video:https://monosnap.com/file/rah98ho0uFNQUhRY3diQW5q1zusvkISo, I'm curious, does anyone else with an 8903A or B see the same behavior? And does anyone have some ideas of how the input signal could be getting coupled to the output signal? I'm going to check the supply rails to see if the high input voltages appear there, which is the obvious one, but if that's not it I'm not sure what the next step is.

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 21, 2025, 12:40:40 pm
content: I tried something similar to your experiment. Took my 8903A output and put it into an old Insignia (Best Buy house brand) receiver. It didn't have a phono input, so I used the CD input. Hooked the receiver output to a speaker and connected the 8903A input and a scope at the speaker. Set the 8903A to 1 KHz, 5 mV.Cranked up the volume control on the receiver and watched the scope. Nothing really bad happened, although eventually I saw some low frequency modulation on the 1 kHz signal. It looked like about 60 Hz, so my guess is that there are massive ground loops in the experiment that I set up.A second experiment I tried was to hook up the 1 KHz, 5 mV 8903A output to a scope. Then I took another signal generator with precise frequency control down to the millihertz range, set that signal generator to about 100 mV and feed it into the 8993A's input. I wanted to see if I slowly tuned the signal generator's frequency, whether or not I could cause the 8903A's 5 mV output to fluctuate. I didn't see any fluctuation, however, I couldn't control the signal generator's phase, so constructive signal addition might not have been met. Thus, this experiment was sort of inconclusive.Reading through the specs of the 8903A, it doesn't appear that it is optimized for low signal amplitudes (for example, it needs 50 mV to do a distortion measurement). So I wouldn't be surprised if there were some misbehaviors at low levels. On the other hand, knowing HP's engineering quality, I would bet that there is something wrong with either your experiment or your 8903A. What could be wrong, I haven't a clue. Maybe someone smarter than me can chime in.

author: aarons
author_rank: Regular Contributor
timestamp: August 21, 2025, 07:43:21 pm
content: So the 1kHz is definitely finding its way onto the +-15V rails. You can see it clearly on the spectrum analyzer, and also on the scope. In the scope video below, the yellow is the +15V rail as measured at the test points, the blue is the output, green is the FLT HI testpoint on the A6 board, and the red is the input to the 8903 (attenuated by 16dB). The scope is triggering on the OSC 3 testpoint on the A5 board which remains stable even when everything else goes crazy.Video:https://monosnap.com/file/E1xKfOhrOrNRxJWo2jVTLLftdex2NW

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 22, 2025, 12:43:45 pm
content: For what it's worth, I hooked up both a scope and spectrum analyzer to the +/-15V rails and set the 8903A output to 1 kHz, with varying amplitudes of 50 mv to 1 V. Saw absolutely nothing on the scope/analyzer.Maybe a bad bypass cap on one of the A5 op-amps? I haven't a clue.

author: aarons
author_rank: Regular Contributor
timestamp: August 27, 2025, 02:17:59 am
content: Ok, so the problem with the rails was operator error. My cables/adapters were tying the speaker- to ground. I haven't reproduced the runaway instability I was seeing initially, but now I know how to avoid that.However, while I was digging into this I discovered something else which doesn't seem right. I wanted to check the balanced output of the 8903A and hooked both high and low into my scope and there are two things that don't seem right. 1) The high and low outputs are about 118deg out of phase, and I'd expect 180. and 2) the peak to peak amplitude of the low output is always, well, low. With amplitude set to 1V, the high output will have an amplitude of 2.8Vp_p, but the low output is 60mV.So I removed F1 and F2 from the A6 board and then measured from TP2 and TP3 (FLT LO and FLT HI) and the lo output is better at 610mV, it's still not what I would expect for a balanced output, and the phase is still wrong. The output amplifiers are not exactly symmetric on the schematic, and the service manual on this circuit essentially says "this is too complicated to get into, here is an article on it" and points you to page 12 of thishttps://hparchive.com/Journals/HPJ-1980-08.pdfMy board has a bodge on it, but I believe it was done at the factory, as I've seen pictures of this board with the same modification. I've drawn that modification on the schematic here:I've gone through service sheet 11 and everything checks on there (apart from logic of Q12 and Q11 being swapped in table 8-59, which I believe is an error in the manual). Can anyone verify what their high and low outputs look like with a 1V 1khz output set (and the switch set to float)? This is what I'm seeing.

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 27, 2025, 11:51:52 am
content: I got pretty much what you got, although my low-side signal is a bit stronger.

author: aarons
author_rank: Regular Contributor
timestamp: August 27, 2025, 01:17:37 pm
content: Interesting. It's good to know mine isn't necessarily broken. I'm curious about this circuit now. Your phase shift is closer to what I would expect as well. I think I'll try simulating this to see what it comes up with.