Need help debugging big box micro synthesizers

[Kevin Mitchell]

Next step - lift R98 (47k) Q4 side, then connect to wiper of a 100k linear pot. Connect outside lugs of the pot to 0v and -9v. The VCA should now work. If it doesn't , something is definitely amiss around CA3094 A14

I gave it a shot again with no luck. There's a very small trace of the signal at the output but it's distorted. The pot isn't really doing anything.

If the signal sounds fine around C29 maybe there's something going on at the input of A14 that I'm not catching?

-KM

[Slowpoke101]

This may seem silly but have you actually replaced R98?
It may measure as being 47K but it may be failing when current is applied.

[Kevin Mitchell]

This may seem silly but have you actually replaced R98?
It may measure as being 47K but it may be failing when current is applied.

I feel that's very unlikely the issue but certainly worth trying. I'll swap it this afternoon and report back.

-KM

[anotherjim]

It is possible for a resistor to read well at uA but fail at mA, so it is worth checking.
The control input to an OTA is into a BJT base, isn't it? So if the resistor is passing control current, there should be a junction voltage drop above the OTA -Vs?

[Kevin Mitchell]

It is possible for a resistor to read well at uA but fail at mA, so it is worth checking.
The control input to an OTA is into a BJT base, isn't it? So if the resistor is passing control current, there should be a junction voltage drop above the OTA -Vs?

Sounds like what's shown in the scope shot on the previous page? That's on the BJT side. The other side is stuck low when A14 is installed. Otherwise it looks okay. So maybe it is the 47k resistor?

I've dealt with damaged resistors before but never one that looked and measured good but wasn't. So we'll see what happens!

-KM

[anotherjim]

You can also get PCB's with strange faults.
It may be getting to the stage where it might be worth building the basic OTA VCA offboard, or at least in a small protoboard. As it appears the required control swing and audio input is there, just copy the circuit from the input cap to an output jack so the existing OTA and its pcb area is bypassed.

[Kevin Mitchell]

You can also get PCB's with strange faults.
It may be getting to the stage where it might be worth building the basic OTA VCA offboard, or at least in a small protoboard. As it appears the required control swing and audio input is there, just copy the circuit from the input cap to an output jack so the existing OTA and its pcb area is bypassed.

I'll do that out of curiosity but I will not except that as a fix. I have the intention of keeping it as original as possible.

The resistor swap did nothing by the way. I still have a 100k pot tied there just to try to get the VCA to open up.

The listing said it was functional but obviously that was a load of shit. I'm starting to wish I had returned the unit instead of just shrugging it off and thinking it's something I could handle. It's becoming a nightmare. And it's SOOOO CLOSE TO WORKING.

-KM

[Scruffie]

So it's not the OTA, it's not the Attack Decay circuit, it's not the 47k... even though you say it tests fine I would consider replacing C29 as ya know, 40 year old tantalum...

[Kevin Mitchell]

Replaced C29, no cigar.

I'll replace the 47k after C29 and see if that does anything. Maybe that's the one that's failing and the signal should be more pronounced than what I've observed at the VCA input. The VCA is producing noise when I turn the pot it's just not amplifying the very small signal at the input. I'm wondering if it's trying to amplify anything at all - this whole time I assumed it wasn't. So I'll either bypass the filter or input an external signal into the VCA and see if it shows up on the other side.

I swear if it's that resistor I'm going to spend my weekend headbutting a wall.

-KM