Farfisa TR-70 OS high pitch problem

[Abraxas]

Hi,

I have a Farfisa TR-70 OS. It has a organ channel and a guitar channel, the organ channel works fine, but the guitar channel when you turn the volume knob to 1 it produces a high pitch sound and it's like that from 1 to 4, from 5 to 10 it stops but volume is the same there is no difference between 5 and 10. I already changed three volume potenciometers changed a dozen of capacitors and it is all the same, could the fault be in the integrated circuit ?

Thanks

[Jazz P Bass]

That IC is a strange duck with it's comp pins.

Generally, if the output pin is making the noise, then the ic is bad.
Although with those odd pins, it may be a comp capacitor.

Which caps did you change?
I see electrolytics going to the volume pots.
They may be dried out.

[J M Fahey]

1) I guess "Guitar channel" is channel 1 on schematic.
2) get a short piece of wire with tinned ends and do the following tests, one at a time:
a) short Ra4, the one which goes to Ic1n Pin9
b) short Ra9 (Ic1 pins 8 to 13)
any change?
3) pull shorts, then add an electrolytic cap, 10 or 22uF , 16V or more, in parallel with Ca4 , nominal 6.8uF (which yes, must be quite dry by now).
Any change?
Leaving it there:
4) tack solder a .1uF ceramic cap (25/50V) in parallel with Ca10 (Ic pins 10 to 11)

In all cases, tack solder new parts to the underside of the board, to the original pads, avoid pulling and resoldering parts (until we are certain which one is bad) , or you'll destroy the irreplaceable PCB .

5) IF the IC is bad (which I doubt very much) you can still get a new one from:
Semiconductor: TBA231 (TBA 231) - DUAL LOW NOISE OPERATIONAL AMPL...
for just U$10 + freight.
If you buy that one, carefully scratch pins on both sides , inside and outside until you get shiny metal, before soldering.
20/30 y.o. parts tend to have the tin plating oxidized and do not solder well, creating even more problems than the original ones.

Personally I'd replace it with a modern TL072 but it requires a little "creative mounting/modern sculpture" approach, because they are not pin compatible.

That said, not sure it's bad until proven otherwise.

[Abraxas]

Thank you for your advices I will try that and let you know the result !

[Abraxas]

I bought the IC that you told me. And the the high pitch stoped and the amp is much louder, but another problem occured. When you hit the strings hard it is great, but when the sound starts to fade it acualy stops immediately there is no slow fade out. And if you want to play your guitar quite there is not enough signal for the amp.