Prophet T8, repaired pop noise - but overall sound too hot?

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WhinyLittleRunt
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Prophet T8, repaired pop noise - but overall sound too hot?

Post by WhinyLittleRunt » Sat Sep 30, 2017 6:15 am

Just an update to my Prophet T8 - I was having issues with two things, one being that the overall sound seemed overdriven, and two, there was a static "pop" coming from somewhere that would appear in every couple notes, or so it seemed.

I was able to track down the pop to voice 4 on the left-side board, by closing the decay and sustain all the way and raising the cutoff (the square output made this more apparent than the saw) so I could hear a nice pop with some tone to it. Every few voices I'd hit a note where it was nothing but a "pop" and no tone at all. The only way I could follow which voice was firing was by scoping out the oscillator output on a particular note and then playing the keyboard with successive notes until I saw the wave change on the scope. Then I was able to follow the change around the rest of the voices until I got to the bad one. I had initially swapped out all the chips from one voice to another, etc, changed out a few op-amps (for the other problem), nothing was fixing it and it still followed voice 4.

Then I found the broken capacitor... :roll: :lol:

This one, rinky-dink, un-matching 100n cap I moved out of the way and saw the leg was busted. Took the board out, put in a new 100n cap, put it back together.... pop is gone. Yay!

Now - I don't know if this "overdriven" sound I am hearing with the filter closed most of the way is normal or not on the T8, and I'll never have a physical comparison. I'm really leaning towards it being an overall master output problem, however I did change out both of the summing op-amps in the output stage. There is a CEM3360 VCA sitting at the end of the line also...
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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by madtheory » Sat Sep 30, 2017 8:24 am

Um, check output caps?

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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by WhinyLittleRunt » Sat Sep 30, 2017 1:22 pm

I replaced everything on the output board...

A complete re-cap is in order, I had already replaced the caps in the funky voice but I noticed the caps they used originally all have different markings and look like they face all different directions. I’m pretty sure if I do a connectivity test between a ground point and what I think is the negative of the cap I’ll be ok but, would be nice if there were markings on the board or at least a board layout in the service manual...
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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by schmidtc » Thu Oct 05, 2017 12:51 am

Wine country told me a few years back that they had a collection of docs that were destined for a full up service manual, but they wanted like $80 for it. The CEM3360 datasheet should be helpful in determining if that's why it's running a little hot/distorted in its output levels. My studio is a little scattered from a recent move, but I've got a T8, with some spare parts and documents up in Boston. To my ears it has less distortion and a cleaner sound than other synths of this era---definitely not what you've described.

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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by WhinyLittleRunt » Mon Oct 09, 2017 2:06 am

I still haven't gotten around to recording a good sample of the T8 for comparison...

Last night I re-capped the voice cards and found no difference. I wasn't really expecting one, but it was worth it to me to change out 32 aging caps.

Tonight I decided to finish the recap with the power supply. That's where I ran into a problem. Recapped the entire supply. Checked voltage, was getting somewhere between 39 and 49v at the +15 test point. Checked my work, everything was soldered clean and looked right, polarity correct etc... adjustable trimpot for the +15 rail did not work. Through some discussion between a couple of friends it was decided that a voltage regulator may have popped. Didn't have any adjustable regulators on hand, so I checked a few more things. Plugged back in the supply, grounded this time at the negative leg of a different cap... voltage correct. Cool. Plugged it all back in, fired it up, got the nastiest feedback loop ever, volume control not working. Uh oh... turned it right off.

I'm hoping I didn't damage anything but... when I ground at the power supply, I didn't think grounding at the chassis was going to work. I grounded at the - leg of a cap. If I ground at a different cap... I get double the voltage reading (30v) ??? If I ground at the chassis, same thing. I'm praying I didn't just cook something else...
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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by AK79 » Tue Oct 10, 2017 9:13 am

don´t you have the schematic ? Make sure you choose the correct reference points. Chassis will not work as a ground reference here.

Apart from the regulated voltages the PSU also makes the unregualted DC of both rectifiers available at the connectors.
that would explain your 49V.

br
Arne

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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by WhinyLittleRunt » Sat Oct 14, 2017 9:28 pm

AK79 wrote:don´t you have the schematic ? Make sure you choose the correct reference points. Chassis will not work as a ground reference here.

Apart from the regulated voltages the PSU also makes the unregualted DC of both rectifiers available at the connectors.
that would explain your 49V.

br
Arne
I do have the schematic and full manual, but I was unclear on where to catch the ground point. I was testing with it grounded to the negative lug of the 6800uf cap, which was then giving me 15v/5v, trim pots were working and adjustable. In fact, I even replaced all the regulators just to be on the safe side, and both trimmers. Got everything set correct, checked the voltages at the pins going to the rest of the synth, looked good... then I fired it up and got nothing but a hum and then, fading in, a feedback loop. I was poking around at the output board and with the power off but plugged in, I got shocked on my finger at the CEM3360 VCA.

The power supply does have a feed that goes directly to the output board. The rest of the synth front panel seems to function but no output sound other than what I described.

This may sound really stupid, but if an op amp is fried in the output stage (or the VCA), if I removed it would I hear anything at all to help diagnose if my problem is just at the output?
I like vintage synths....

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Re: Prophet T8, repaired pop noise - but overall sound too h

Post by WhinyLittleRunt » Sun Oct 15, 2017 1:39 am

At the risk doing more damage but realizing it doesn't work anyway, I decided to pull the 2 op-amps (LF356N and 353N) on the output board and the CEM3360 VCA. Powered on, got nothing. No hum, no feedback. Ok, good. I put the VCA back in, powered on. Works! Get sound, everything works it's just fairly low volume. I did both op-amps one at a time and everything is now functioning the way it was before. Thank God! :shock: :shock:

I think the output distortion got worse after this little mishap, which is making me believe the VCA is damaged in some way. I'm going to start troubleshooting that as best as I can now because having a spare part available isn't easy...
I like vintage synths....

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