Tuning shift over fifths and low volume

[MMM]

Pin 12 - 16 reads 10k

Unpowered, I hope? Looks like we are getting closer; this is way, way too low.

[njosnari]

yes, unpowered.

[MMM]

Okay, final test for now: With R78 installed and R79 removed, please measure the resistance from each of the R79 solder pads to pin 16 of IC9. If my guess is right one should give 500k and the other one the 10k you reported.

[njosnari]

You are correct. 10k and 500k.

[njosnari]

Sorry, I mean to say is I get those readings from R79 and pin 12, nothing from 16.

[MMM]

Sorry, I mean to say is I get those readings from R79 and pin 12, nothing from 16.

Please check again. Once side of R79 (and hence one of the solder pads) connects directly to pin 12. So if you measure between pin 12 and those two solder pads, one of them should give a "continuity" reading (read: no resistance). But what's more important is the resistance between the 5.33 volt power rail (which connects to pin 16 which is why I was using this) and the various points of the resistor network. You could also try to measure between pin 16 and both side of the (installed) R78 (with R79 removed).

How do the other resistors measure with R79 removed and R78 installed?

[njosnari]

I had pin 12 and 16 confused, the readings were as you said. R78 reads 200k and 10.

[MMM]

Good.

If you look at the schematic you'll see that all the wrong resistor values went through a path between pin 12 and the bottom of R78 (which is the 5.33 volt rail). With either R78 or R79 removed the values were correct.

This means the problem is not WITHIN the resistor network but happens before (and has something to do with pin 12 and the 5.33 volt supply rail).

Which points more or less to IC9. Either this one's the culprit or there is some strange connection over a 10k resistor between pin 12 and the 5.33 volt rail - which I consider unlikely given the layout of the PCB.

Unfortunately more or less the only way to find out is to remove IC9...

[njosnari]

Unfortunately more or less the only way to find out is to remove IC9...

How can I test IC9?

[MMM]

Unfortunately more or less the only way to find out is to remove IC9...

How can I test IC9?

If it's removed you can again measure between pin 12 and 16. If there is still 10k the IC is faulty. On the other hand if you measure 10k on the PCB (between the pads for pin 12 and pin 16) then the problem wasn't with the IC.

There's not much you can do with the IC installed - mainly because we are trying to find out if the problem is with the IC (which is my guess) or the PCB. And for this to find out we naturally have somehow to "separate" them both...