Repairing my HP8903a - bad distortion in source - diyAudio
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Old 12th September 2013, 04:08 PM   #1
dmnieto is offline dmnieto  United States
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Default Repairing my HP8903a - bad distortion in source

Hi All,

I just bought a "working" hp 8903a from ebay for a low price ($260), but it is showing horrible distortion in the range 20Hz->3Khz (4% at 20Hz, 0.18% at 1KHz, 0.05 at 3KHz, 0.02% 10KHz following) which render it unusable for my purposes.

Before I give up and ship it back to the seller ( he claimed it was in perfect condition) I want to be sure I cannot repair it. I have the Service manuals for the baby and they describe what to do when the distortion is slightly off, but nothing about this level.

I am starting to feel that it is the source the issue, or maybe that the notch is really badly calibrated, but following the service manuals it appears to be ok. ( I followed BD1 and BD2)

What I do see is that the signal in A5OSC1 is clean at 1KHz, also at A6 AMP1..., but I see a significant second order harmonic (-54dB) when I probe at A6 AMP3.

Any hints of where should I check? For information, this is a prefix 2420 so it has the 3 relay A6 board.

What do you think is the problem? Source, notch or amplifiers?

Thanks,

David.

Just for info, i plan to document in this thread what I am doing and my efforts, it could be interesting.
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Old 13th September 2013, 01:51 AM   #2
dmnieto is offline dmnieto  United States
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Not much luck tracking the source of the distortion, it would be great if I had a frequency reference I knew was good, so I can see if the problem is
a) The notch
b) The power supply being very noisy.
c) Something in the source.

At least I managed to verify that the system is failing the test in:
Service Sheet 9.8 and 9.9

but I don't know if that would create that huge distortion
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Old 13th September 2013, 06:32 PM   #3
dmnieto is offline dmnieto  United States
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The good news is that I know now that the problem is in the source.

I used a Keithley 2015 to compare, and the measurements are accurate.
But when I connect it in loopback, it shows the horrible distortion in the low end of the spectrum
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Old 14th September 2013, 06:29 AM   #4
dmnieto is offline dmnieto  United States
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Nailing it down, it appears that the source of the distortion is that there is at least one transistor leaking on the oscillator board (A5).

I need to find a way to probe those transistors...
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Old 16th September 2013, 03:34 PM   #5
grali63 is offline grali63  Italy
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Hello, my hp8903a also has the same problem. After losing a week, I think that the problem is U6 (MC1595L) to A5. As soon as I get the chip'll let you know. I hope it's that .... you also see service sheet 9 point 12 and Hint. good luck
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Old 17th September 2013, 03:16 AM   #6
dmnieto is offline dmnieto  United States
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I am not sure... I am pointing more towards the discharge circuit around A5U4, I see multiple components that can go haywire.
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Old 17th September 2013, 06:13 AM   #7
grali63 is offline grali63  Italy
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it is possible, try to put ALC test point to gnd, with 20hz and 3volt amplitude, measure the distortion, let me know ... the first solution that is talking on the forum. There are many hp8903a in the world.
See you soon
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Old 17th September 2013, 11:37 PM   #8
dmnieto is offline dmnieto  United States
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with ALC grounded still a lot of distortion,

I changed VR1, VR2 and C12, C13 with no change.

I measure around 30 KOhm when probing CR3 and CR4... do you have the same?

I am hoping that it is the rectifier circuit still
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Old 18th September 2013, 12:42 AM   #9
tomchr is offline tomchr  United States
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Check the supplies for excessive ripple. The supply caps tend to dry out over time.

As I recall, the oscillator in the 8903A is a state-variable oscillator. It's not the easiest circuit to debug.

$260 for an 8903 that isn't mechanically damaged and actually powers up is not a bad deal. I'd still pester the seller about the "good working condition" though. But since you've decided to tear into it, I'm guessing you're not returning it...

~Tom
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Last edited by tomchr; 18th September 2013 at 12:45 AM.
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Old 18th September 2013, 05:25 AM   #10
grali63 is offline grali63  Italy
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Hi, I read 30 ohms too, but I can tell you that I have detached and are ok, if you can do this additional test, the offset voltages correspond a5tp6, a5tp4, a5tp8 a service sheet 8 point2? ... Let me know, ciao
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