Peavey VSS-20 amplifier with chorus noise problem

[jpcar]

I have a problem with the chorus on this amp. When the chorus is switched on it comes with a white noise type of hiss. The volume controls don't change the noise. Thankfully most of the 8 pin ICs are socketed, so I was able to easily change U3 (TL072). Same problem. I'm wondering if I should change the capacitors next: C52, C47, C48, C49. Might bad capacitors cause noise like this?
Or any other parts I should look out for?

Thanks,

[Enzo]

U3 and all those caps are the chorus clock circuit, they are not in the audio path.

U2, U7 Q6, Q7 are the chorus path.  MAybe Q5 too but I doubt it.  Turn chorus on, you get hiss.  Power off, remove U2, power on.  His still there?  You won't get a chorus signal with U2 gone but we are looking for the noise now.  Still noise?  Remove U7, same question?

Make SURE not to install ICs backwards.

[jpcar]

So,

Removing U2 the noise remains, but removing U7 and the noise is gone.

Now I need to find a MN3007 online.

Thanks!

[edvard]

Make sure you buy from a reputable seller; there are some counterfeits ever since the MN-series bucket-brigade delay chips went out of production, though on eBay you're more likely to encounter salvage chips, which are a good deal if they aren't dead.  If they're actually listed as salvage and are cheap enough, I'd order two or more to make sure you get at least one good one. 


YouTube link: (Part2) Detecting Fake Ebay ICs Chips - Sound Test




Smallbear carries genuine NOS stock at somewhat of a premium ($10.95 at the time of this posting), but at least you know you're getting the real deal. 
Xvive is making re-issues, but they may be a little hard to find, and are on the expensive side ($25 and up from what I could find).  They sound as good as the originals, from what I hear.

[jpcar]

Thanks for the advice!
I found a Xvive reissue on Ebay for $12 + shipping so I went with that. Should arrive in a few days.
I'll post how it turns out.

[jpcar]

I’ve replaced U7 (with a new Xvive reissue MN3007) and the hiss remains.
Hmm. What to do next.

Should I tackle Q6, Q7?

[Jazz P Bass]

Have you checked the 'digital +9vdc supply'?
It should be pretty free of any ripple voltage (Vac).

[jpcar]

I checked the Digital 9V supply.
There is noise there. It’s not really ripple but a very high frequency noise (scope says around 170 KHz). It’s around 150mV pp. I’ve marked on the schematic where this noise is present. It’s both sides on R74, one side of L1. Pin 4 of U9. Pin 1 of U7.
Then there’s also a spiky looking wave shape around 60KHz and around 600mV pp.
It’s present at U7 pin 7,8; Q6 B, E and the other side of C35. I think this is the signal path so it’s probably shouldn’t be there?
—————

With U8 removed:
Noise is gone and the Spiky wave is gone

With U7 removed:
Noise still there’s a different shape at 30KHz.

Should I look to replace U8?

[Jazz P Bass]

I find it odd that you have 'less noise' at the ground connection of U8 & U9.
There should be 'nothing' there.

[jpcar]

Ah yes, thanks for spotting that.
That’s a good point. It doesn’t make much sense to see anything there if it’s supposed to be ground. I tested some of the ground locations and there’s about 1 ohm between the ground side of speaker output terminals and that ground from U8, U9.

I tried retesting with the scope ground reference on the C46 neg side. Now there’s about half the amount of noise. It’s around 50-70mV pp on the scope. Maybe that’s not worth worrying about.