Peavey KB-1 issue

[misterc57]

I installed a new 4560 op amp. I am still having the same issue on channel one. With volume set at zero there is no noise but as soon as I turn it up there is that noise. The sound reminds me of the old tv days when you would tune a channel that was not there and you would see gray static and hear that wind/surf noise.

Any other thoughts on what to try? I sprayed all the pots which made no difference also.

Thanks!

[Jazz P Bass]

About the only thing left is electrolytic coupling capacitors.

[52 Bill]

Have you checked for any dc voltage on the volume control?

[DrGonz78]

Still sounds like a possible input jack problem to me. Did you re-solder the legs on the input jack? Did you try using a signal tracer though the ch1 circuit? Yes check coupling caps and especially good to check for DC on the volume control.

[misterc57]

.43 VDC at all three legs of both volume controls.
15 VDC at all three legs of tone controls.

[Jazz P Bass]

For all of the non EE's out there , may I suggest that you post the corresonding voltage readings (when one is taken) from the working channel.
Just a thought.

[misterc57]

These were identical readings with the working channel, sorry I did not make that clear.

Everywhere I have taken readings on both channels have been identical.

--------

Channel One (bad)

VR1 .43 VDC
VR2 15 VDC
VR3 15 VDC
Between R13 and C21 13.6 VDC

--------

Channel Two (good)

VR4 .43 VDC
VR5 15 VDC
VR6 15 VDC
Between R14 and C21 13.6 VDC

--------

R28 18.4 VDC

--------

I am suspecting the Volume Pot, VR1. I have tried to bypass it with no success by placing a jumper across R2 and R10, but I get no amplification of the input signal.

I have previously said that the noise in suspect is affected by the volume pot. I also want to note that it is effected by the tone pots as well.

I have tested all resistors around VR1 and they are all within 10% of spec. Also since my end signal of each channel matches at 13.6 VDC I would think that all the resistors in the circuit would be good.

All caps appear good.

This is not a critical fix but I sure want to figure this out and enjoy the learning process.

Thank you for your help. Mark

[misterc57]

The input jack looks clean and good at all the solder points. I get 0 ohms resistance from the tip (plug inserted) to R19. I get 46 ohms resistance from the ground of the jack to the chassis. These readings are identical on both channels.

I ran a 440 hertz tone through both channels.

Definitely have different voltages between Channel One and Two, I can also hear that the noise "starts" at pin 3 of the Channel One Op Amp.

Looks like voltages are too low on Channel One.



Voltages Channel One Op Amp (pin # and VDC)

1, 6.5 VDC
2, 6.5 VDC
3, 6.5 VDC
4, .43 VDC
5, 15 VDC
6, 15 VDC
7, 14 VDC
8, 32 VDC

Voltages Channel Two Op Amp (pin # and VDC)

1, 15 VDC
2, 15 VDC
3, 15 VDC
4, .43 VDC
5, 15 VDC
6, 15 VDC
7, 15 VDC
8, 32 VDC


UPDATE:

I found the voltage drop on channel One at R5 (220k). 15 VDC in and 6.5 VDC out. On corresponding Channel Two at R4 (220k) 15 VDC in and 14.74 VDC out.

The resistors both look good and test at close to 220k ohms. Both corresponding caps C19 and C13 test the same at about 22U.

I am perplexed as to what is causing the voltage drop. Hopefully this drop in VDC is the cause of the noise problem.

[misterc57]

Please see my other response in this thread outlining a voltage drop.

[Jazz P Bass]

Is there any voltage on the minus side of C19?
There should be none.

[Enzo]

Hiss or white noise is not going to be caused by a resistor off value a little.

This doesn't could like a volume control problem to me, but it is simple enough to take the two volume controls and trade places. Now if channel 2 has the issues instead of channel 1, then you know it is the pot. While the suspect pot is off the board, measure through its element for something remotely cloe to value, then measure from the wiper to either end and rotate the control to verify it makes contact.

You see diodes D9 D10 right there next to the IC and R5? Try unsoldering one end of each and lifting it off the board. Did that kill the noise?

Your voltage drop to 6v is more likely to be a symptom than a cause, but is probably a clue.


I am thinking one of your clamping diodes has a funny junction, is leaky, and turned into an RF detector. Since the voltage is low (and this circuit runs single supply) I suspect D10.

[misterc57]

.43 VDC on the minus sign of C19 and corresponding channel C13 as well.

[misterc57]

You see diodes D9 D10 right there next to the IC and R5? Try unsoldering one end of each and lifting it off the board. Did that kill the noise?

I am thinking one of your clamping diodes has a funny junction, is leaky, and turned into an RF detector. Since the voltage is low (and this circuit runs single supply) I suspect D10.

Will try that, probably will not be able until tomorrow.

Thanks!

[Jazz P Bass]

Start lifting those diodes.

[misterc57]

D9 and D10 lifted on one side and the noise is gone! OP amps on each channel now have same VDC readings.

I plan to replace both D9 and D10 unless I receive other advice. I do not have the 1N4148 but do have some 1N4007, are they compatible?

Thank you!