Prophet 5 Rev 3.2 S/H Issue

[Volter]

Hello,

I been working on my P5 trying to repair a problem with the Osc A of voice 3 being out of tune and unstable (even after tuning), this problem only occurs after the P5 has been left on for about 10-15mins to warm up. The other voices are working fine and Osc B of voice 3 is also working correctly. I've narrowed down the issue to a problem with the sample and hold circuit for OSC 3A discharging too quickly.

I'm trying to identify which component is to blame before taking the board out, interestingly the op-amp U403 (which I assume to be a TL082) does not match the others which are TL072.

My plan is to order a replacement polystyrene capacitor, as I think the most likely cause is the capacitors age, if so would this part be a suitable replacement: Rapid order code: 12-0546?

Would anyone have any advice or experience fixing this kind of issue? I've attached images of the board and some oscilloscope readouts, Thank you! - Volter

S/H channel working correctly:



S/H channel with issue:



Clearer picture of op-amp U403:


Schematic of S/H circuit, red box shows the oscilloscope connection for the OSC 3A S/H (with issue):

[synthRodriguez]

I'd first suspect the cap, but polystyrene's are pretty long-lived. The 4051 and TL082 should be considered as well. I'm a bit surprised though that you don't drain the "good" one when testing, it doesn't take much to make it fall off, even with a good, high-impedance scope.

One thing you could do help to lead you down the right path would be to take a 10 nF or 22 nF film cap and alligator clip it across the suspect polystyrene leads and see if it either fixes it or helps a lot. If it's still really crappy, I'd consider swapping out the CD4051 and/or the TL082. I've had to change out lots of these in the Rhodes Chroma. More of the 4051's though.

The TL072 vs TL082 difference is of no consequence.

Edit: Looking again, I've never seen that brand of TL082. Hmm. And the 4051's are different dates as well like maybe some work was done on it in the past. You can't go wrong updating to modern day components.

[Volter]

Fixed the problem!

In the end the (U403) 082 op-amp IC was to blame.

After removing the board I unsoldered one leg of the S/N capacitor and tested its capacitance, in normal range.



After finding the capacitor was fine, I soldered the leg back and moved onto replacing the 082 op-amp with a socket and a new TL072 IC.


Finally after replacing the IC and re-installing the board, I left the Prophet 5 running for about 20mins, and then probed the same point where the ripple was present. Now, the ripple is gone, and the 3A oscillator keeps in tune with the others.


My guess is that with age the 082's non-inverting pin started to leak to ground, hence causing the S/H capacitor to discharge instead of holding the voltage.

Hope this post helps anyone with a similar issue in the future. Enjoyed learning more about this great synth, may be tempted to preform the 3.3 rev modification soon.

Thank you synthRodriguez for your advice!

-Volter

[synthchaser]

Good job on the repair. FYI, your method of testing the capacitor didn't actually tell you the capacitor was OK. It measured capacitance, but not leakage current, which is the critical property of the cap in this application. Film caps, including the polystyrene are more likely to fail by developing leakage than by reduced capacitance.

[madtheory]

Great thread! Top notch knowledge and experience from every poster here, kudos.