My VS100 has a heartbeat??

[allante666]

So, was going to call this cured after fixing the channel switching problem due to TR1 but then I noticed what sounded like scratchy pots. decided that it wasn't scratchy pots though and was probably due to there being DC somewhere there shouldn't be.
I checked around and found that the worse ones were the gain pots foro both OD channels VR5 and VR7 an dfound there was -3V on the wipers of these. Am I correct in thinking thats not right? Anyway, while digging around I found that if I turned the volume up I would get what literally sounds like a pulse! a thump every 2 seconds. Bear in mind the amp works fine other than the scratchy sounding pots when you adjust them.

I found that on the positive of C51 electrolytic there is a dc voltage that oscillates slowly between -12v and +12v every 2 seconds.
Anyone have any ideas?

i think ive attached the diagram!

[oc disorder]

QUICK ! Grab a defibrillator !

Idea 1. (DC on pots)

4300 series cmos. Replace 4316 via fitting good quality IC socket.

[allante666]

I can do that, I don't have one in but I can get one. Can I ask the logic behind it? what lines are you thinking along? Or are you thinking this is just for the DC on the pots and not the strangely rising and falling 'heartbeat'?

[Jazz P Bass]

I found that on the positive of C51 electrolytic there is a dc voltage that oscillates slowly between -12v and +12v every 2 seconds.
Anyone have any ideas?

i think ive attached the diagram!

Kindly point out where C51 is.

[oc disorder]

C51 3/4down D across ...output IC7a

"what lines are you thinking along?"

cmos susceptible to breakdown .. cheap... connect to negative end C51 via pots

low freq feedback .. that's about as far as the line went !

[allante666]

Kindly point out where C51 is.

[Enzo]

You have DC on your pots. They are isolated from everything by caps except for that chip. The 4316 is then the only potential source of the unwanted DC. Pull that IC from the board and I'll wager your 3v disappears.

[allante666]

ok, looks like that chip is next in line for the bin! I'll get one ordered next week and ill pull it in the meantime to test the theory. Do you have any idea why it would cause the slow regular swing from -12 to +12 and back again? just curious as to the theory behind that one. Ive had caps repeatedly build up slowly and then collapse though a faulty diode before but never a positive to negative swing.

[Enzo]

Because it is bad?

[allante666]

Well, not quite as scientific as I was hoping for but it will do!

[allante666]

UPDATE
Ok, so, I pulled IC5 the favorite suspect and yes, as Enzo wagered correctly the 3 volts on the pots disappeared. The strangely pulsating 12 volts also disappeared. Replaced with a new IC and all great! well, to start with! Checked it all out, played through it a little and no issues. Put it back together and tried it again and much dismay!
OD channel is fine, great, reverb on OD channel also great.
Clean channel on the other hand, whilst great to begin with, gradually started motorboating. I dont mean it floated away I mean it made a noise like a motorboat at mid throttle. The volume made no difference, the only difference was the FX mix pot would change the frequency of said sound.
Much much louder with the reverb tank plugged in. My first thought was IC1 a quad op amp in the reverb circuit? I happened to have one so replaced it but no different.

Ive seen enough of this thing for one day, time to chill! However, just looking over the diagram and does anyone else suspect IC2?

Or maybe IC4a?

[Enzo]

I don't suspect, I isolate. If the mix pot affects freq, then it is within the feedback loop. WHich pot is that, and where on the print?

[allante666]

I don't suspect, I isolate. If the mix pot affects freq, then it is within the feedback loop. WHich pot is that, and where on the print?

Its on the daughter board, VR1. Ive added the diagram here.

[allante666]

Still at a loss with this. I have however found that with no lead plugged in and switched to the clean channel, its quiet. Yet, if I bring my finger within a eighth of an inch of pins 1-4 of IC2 I can get it to make the noise. If i just touch the pins briefly it will make the noise and carry on for about a second after I move it away. unless I touch the chassis with my other hand which stops it immediately. Looking earth related but not sure the best practice to track it down?

[allante666]

Can anyone offer any advice on this? I appreciate what Enzo says about it being in the feedback loop but Im struggling to see exactly what the loop consists of. Would it more than likely be a small cap within the loop gone bad?