HP 8903A Audio Analyzer Mods and Upgrades

Status
Not open for further replies.
Jump to Latest
#41
OK, first hurdle, capacitors c9/20 are 0.18uf 200vdc +/- 1% and c10/21 are 0.0223uf +/- 1% they look like poly caps, looking at the schematic are these tolerances strictly necessary?

Why replace them I hear you say? Well, the limited info on people repairing these mostly have had problems with these type of capacitors, and if I'm going through it these are getting replaced as well.
 
#42
reddish75 said:
Did any of these mod's get done?

I'm going to try and restore my 8903a and could use some help/tips on what to replace or modify?

Going to start on the oscillator board A5, going to replace the capacitors on there, are there any opamp upgrades that could be done to this board whilst its out?

Thanks

Chris
Click to expand...


I haven't had much time to do anything lately, but after creating some mods to solve the ramp-gate glitch issue, I became a bit disillusioned with it. The A4 board is very badly designed. There are many high dv/dt signals right next to sensitive analog stages.

Probing around with the oscilloscope, I then realized that the task ahead was going to be an uphill battle due to the sheer amount of noise being generated in different parts of the unit.

For example, there's a very noisy digital board in the last slot, radiating a tonne of crap directly into the analogue power supply. HP should have really fitted another aluminum shield there, but for reasons unknown didn't bother.

My next experiment was to substitute a +/-15V supply temporarily using a lab power supply to see if that has any noticeable effect on performance. Maybe you could try this?

If that works, then perhaps creating and fitting an aluminum shield might be helpful.

But I suspect, there's no low hanging fruit. I believe you'd need to make a large number of modifications to improve the unit's performance overall.
 
Last edited by a moderator:
#44
Kramtweeter,
maybe just add some extra grounding and some shielding around sensitive chips or polluters but if the noise is in the traces more than RFI/EMI then you're stuck I think.

Myself if I had the time, I would try to make the notch filter deeper and narrower. But I agree that if the monitor output is polluted then it means you have to take that with a grain of salt in some cases.

cheers
Paba
 
#45
paba said:
Kramtweeter,
Myself if I had the time, I would try to make the notch filter deeper and narrower. But I agree that if the monitor output is polluted then it means you have to take that with a grain of salt in some cases.

cheers
Paba
Click to expand...

That's why I posted post #37, an FFT of the monitor output. You can see from that, the notch filter could do with some improvement. You can also see mains harmonics there.
 
#46
Was any progress done?.. I have recommendation on power supply mod🙂 with Tps7a4701 and.. For negative rail. Here on diy i find Ales who sels those boards... We could next to literarlly just plug that board in.. Im half way designing same - ish board for myselfe when i find his design.. That leave us only 5 V supply... Mybe similar chip for more current.
 
Status
Not open for further replies.
Top Bottom