1981 TS9 gating effect problem

[pete_g]

I know it would be easier to post an audio or video of the problem, it is basically gating out at the end of the sustained note/chord. Its a 1981 TS-9 so its pretty old. What would be the best place (component) to start and hopefully trace the faulty part? It sound like a normal ts-9 should, but it starts to gate as it sustains away.
Thanks

[R O Tiree]

In a unit that's 37 years old, I'd be tempted to start with any electrolytic caps.  Replace them all.  There are probably 2 Tantalum ones in there as well.  Those are said to be self-healing, and indeed they are, but they also have their own failure modes over (a long) time, so I'd replace those on spec as well.

[pete_g]

Is there any one electrolytic in a specific spot that would be responsible for the gating sound vs the others, or can any one of them be the cause?

[thermionix]

Can you post voltages from the IC and transistors?  You can ignore the bypass FETs for the time being.

[R O Tiree]

Electrolytic caps (the ones that look like little soda cans) are "lifed items".  There's a fairly complex formula for figuring out how long they'll last in any given situation.  For example, say a cap is rated at 1000hrs@85C.  That means that, if you run it at 85 degrees C, it is guaranteed to last for 1000hrs.  If it happens to last longer than that, fine, but it is living on borrowed time.  Of course, if you run it no hotter than, say, 20 degrees C, then it will last a lot longer (and it's not a linear relationship).  Similarly, if you have a 10V-rated cap in a 9V pedal, it will last as long as the manufacturer guarantees it and probably not a lot longer.  If you put a 35V-rated cap in a 9V application, again, it will last an awful lot longer.  There are a lot more terms in that complex equation, not just those 2, but they serve to illustrate part of the problem with these caps - treat them gently, give them lots of headroom and keep them cool and they'll last for ages.  Run them at or close to their ratings, and they won't last anywhere near as long.  But, even just simply sitting un-powered, doing nuttin'-a-tall, they will still degrade over time.  And 37 years is a really long time as far as these caps are concerned.

There are a couple in that circuit at the power-supply stage - one from +9V to GND (100uF) and one from Vref to GND (47uF on the schem I'm looking at).  There's a 10uF cap at the output and there are 2 x 1uF caps, one after the input buffer and one on the output from the active tone filter.  Mind you, they might have put more in there - there's a 0.1uF connected between the Vol pot wiper and the base of the output transistor and they used electrolytics of that value quite often, back in the day, because they were smaller than an "ordinary" cap of the same value.  These days, you can get metal-film caps at 1uF, maybe a bit more if you hunt around and are prepared to accept a BIG form-factor, but they should all be replaced anyway, IMHO.  Even if you found "the one" that was causing the problem, what's to say that the others won't also fail in the near future?  Not a bet I'd take.

Lastly, there are the 2 tantalum caps (little, light-brown, blob-shaped ones) both at 0.22uF, and both attached to the Tone pot - one to the wiper and one connected to one of the outer legs.  As I said before, they should be self-healing, it sez 'ere, but they can still fail over time.

Of course, all your caps could be absolutely peachy, right now, and replacing them all would not, therefore, solve the problem.  That said, even if they are fine right now, they could fail tomorrow, next week/month/year/whatever, and you're back to square one.  Also, if it isn't the caps causing the gating, then at least you've eliminated 7 or 8 variables from the problem - often, knowing what it isn't is just as valuable as knowing what it is.  And you're only out of pocket for a couple of $$ and 10 minutes of your time, all told.

So, get the voltages that thermionix asked for, then replace the caps and take the voltages again.  Test the pedal to see if the problem has gone away, or maybe improved a bit?  If you could then post all the results (voltages before and after replacing the caps and improvement or not) and we'll see where we go from there.