Polytone tone control issue

[gbono]

I don't usually post the same issue to multiple forums (MEF, etc) but ............

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).

[Roly]

 

With the EQ set flat, is the output of IC1A pin 1 the unity gain inversion of the signal at IC1B pin 13?

[DrGonz78]

So does "no tone controls working" mean that both channels are exhibiting the same symptoms? IF the other channel has a working tone stack then you could compare the two.  Do the tone pots have any effect on the signal at all, even the slightest?

[gbono]

So does "no tone controls working" mean that both channels are exhibiting the same symptoms? IF the other channel has a working tone stack then you could compare the two.  Do the tone pots have any effect on the signal at all, even the slightest?

Single channel preamp - pots have no effect

With the EQ set flat, is the output of IC1A pin 1 the unity gain inversion of the signal at IC1B pin 13?

With Vin=170mV f=1000Hz
V1 = 70mV
V13= 83mV

didn't look at phase

[Roly]

didn't look at phase

The phase is what is critical; need a CRO and using external triggering off the signal source.  The signal on the output of the Baxandall tonestack should be phase-inverted on the input, it is basically an inverting stage.  If the op-amp in the tonestack isn't working it is possible you are seeing direct leak-through from the previous stage.

I must say that this has got me seriously puzzled because I'm trying to see a single failure that could disable the entire tonestack and still pass (flat?) signal.


I got curious enough to introduce some faults into my LTSpice model of a Baxandall control, and shorting the op-amp inputs together causes the control ranges to be seriously reduced, the treble almost entirely and the bass mostly.

So my hit pick is the op-amp in that tonestack.  Maybe.

[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. 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???

[Roly]

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???

Yo!   :dbtu:

That would make more sense than not there at all, and if it were way down it might be hard to hear any effect.

So yeah, it looks like you found the old "sick op-amp" trick.