Polytone mystery

[gbono]

Polytone tiny mini brute with proper output levels from power amp and preamp but no tone controls working? Checked all connections and components and I have an output signal from pin 13 and 1 on U1A/B. head scratch icon here

The preamp circuit is the lower one in the attachement - very simple volume, bass and treble.

Last effort is to swap U4 and U1 (RC4739).

[Jazz P Bass]

Does the 'Dark/ Brite' switch have any affect?

To me, that tone circuit is anything but 'simple'.

I would try to monitor pin U1/ pin 7 & compare it to pin 1.

Not sure why you are thinking about changing U4.

[52 Bill]

Not sure why you are thinking about changing U4.

Jazz, I think he means to swap the positions of U1 and U4 to see if the problem follows the op amps. Like swapping tubes.

[gbono]

Jazz, I think he means to swap the positions of U1 and U4 to see if the problem follows the op amps. Like swapping tubes.

Bingo - I will also check to see that I do have +-15V rails but I assume the RC4739's (U1 & 4) would not pass signal without both supplies. Something simple but it's getting past me for now...........

[TimmyP1955]

If U1A fails, the signal will be high-passed but a significant portion of the mids and highs will get through.

[gbono]

Here is what I've done:

Swapped out U1 and U4 and I now have a working tone "stack" but very low preamp output level. I believe that the U1 opamp is suffering from some type of degradation. Solution : I "rewired" the PCB to accept a RC4558 and I appear to have a working preamp with tone controls.

I assume that the degraded U1 could not supply enough gain and my "active" filters were "there" just -20dB down in level???