Prophet-600 No Sound

[genx]

Thank you so much for your replies, Rasputin and Mooger. They have been insighful and very helpful. I hope you don't get tired of commenting on this thread until all the issues get resolved.

I have been at work these past few days and will get back to the synth by Friday. I'll do all that you have suggested and post the results then.

Best Regards.

Genx

[genx]

The analog switches are controlled by a single IC, U404. Excepth the triangle, I have to find where the command signals are coming from.

The CV for PW are dmuxed by U418 and "sampled and held" by U412. Check for DC signals at all those ICs.

Just had enough time today to do a quick reading on the IC pins:

U418 (4051)
Pin 1: 1.17vdc
Pin 2: 1.14
Pin 3: 1.06
Pin 4: 1.17
Pin 5: 0.61
Pin 6: 4.83
Pin 7: -4.93
Pin 8: -0.0
Pin 9: 4.23
Pin10: 4.28
Pin11: 4.28
Pin12: 1.17
Pin13: 1.17
Pin14: 0.0
Pin15: 1.14
Pin16: 4.91

U412 (UAF772)
Pin 1: 1.30
Pin 2: 1.20
Pin 3: 0.81
Pin 4: -14.62
Pin 5: 1.14
Pin 6: 1.16
Pin 7: 1.28
Pin 8: 14.99

U404 (MC14504B)
Pin 1: 4.91
Pin 2: -0.0
Pin 3: 0.0
Pin 4: 14.99
Pin 5: 4.90
Pin 6: -0.0
Pin 7: -0.0
Pin 8: -0.0
Pin 9: 0.0
Pin10: -0.0
Pin11: -0.0
Pin12: -0.0
Pin13: 2.07
Pin14: 4.90
Pin15: 14.99
Pin16: 14.99

[genx]

Make sure the PW knob is turned clockwise, select the square wave and measure again at both sides of the capacitor C456. The 3340 are indeed heavily attenuated. But never mind, if the square wave reached the 3376 and if there´s nothing wrong with the filters/vcas, then there would be audio signals present at the synth´s output.

I was able to trace the audio up to capacitor C456. The audio was present on one side of the capacitor but absent on the other side. The audio was weak so I used a multimeter to double check the signal. I got a reading of 5.6VAC on one side and 0.0VAC on the other side.

The capacitor was a 1uf 50c electrolytic so I thought the capacitor was dead and was not allowing the audio to pass. I happen to have a capacitor of the same type and value on hand so I replaced it. When I checked the signal, I got the same result: 5.6VAC on one side of the capacitor and 0.0VAC on the other side.

I did the same check on all the other voices and got the same result. The signal is present in VCO's 1 and 2, up to one side of the capacitor, then it disappears and does not reach the input of CEM3372.

Any suggestions will be most welcome at this point. Thanks.

[Mooger5]

Set the the DMM to a lower range then, 5VAC. Even better, build the audio probe. The easiest version is just an audio cable, some aligator clips and a small capacitor. Nothing fancy.
Of course, turn all waveforms on in the frontpanel.
Even if no audio is reaching the 3372, the VCF should be audible in self resonating mode. Turn reso fully clockwise and set cutoff in mid position. Presssing any key and the synth should output a sine wave. Unless there´s something wrong with the VCA.

Based on your previous readings I tracked a probable suspect. C309 next to the battey, is a tantalum cap. If it shorts the Vnm to ground, it prevents u308 from sending a reset signal to u311, which in turn prevents u404 from sending the command signals to the analog switches that turn the waveforms on and off.
The cap in question looks lik a blue drop. See if there´s continuity across it, with unit turned off. Then, check the battery. 20 years on and it must be fully discharged by now.