Onboard Guitar Preamp Oscillating

[mhuss]

I'm fixing an old Jackson/Charvel 80s guitar with an on-board active preamp for a friend. All three pots were frozen. I replaced these and wired everything back together. The signal is pretty weak, and if the volume control is near maximum or minimum, it oscillates (9v low frequency semi-square wave). When the volume is in the middle, it does not oscillate. I tried adding 10k in series with the volume wiper, but it made little or no difference. Any ideas/suggestions?

[big_teee]

Don't know much about preamps.
I noticed you said when vol. max or min it oscilates?
When the vol is turned off it should ground the sig according to the schematic?
Wouldn't the ground turn off the signal?
Can you check for the ground?
A250k Tone, A50k Volume, and a B50k Balance.
Is it possible you have 500k pots instead of 50s?
I've done things like that.
T

[pdf64]

Failing power supply decoupling caps are often the cause of low frequency oscillation, ie the 10uF at the 4.5V reference.
As there's no decoupling on the 9V line, a good battery is needed.
Pete

[R.G.]

pdf is right. Replace the 10uF with a new 22uF, and put a 47uF to 100uF across the power supply.

You're seeing motorboating.

[pdf64]

What's that 2nd stage / balance control doing?
My guess is humbucker emulation.
Pete

[mhuss]

> When the vol is turned off it should ground the sig
It does, but the second stage (which I believe is the oscillator) is after the volume control.

> 2nd Stage?
It's a mid range boost (both stages are that way, actually).

Thanks all, I'll pop a couple caps into it and see if that helps. ;-)

[Mick Bailey]

Bear in mind that these preamps worked fine when the guitar was new, so whilst it's worthwhile improving the decoupling my approach would be to fix the problem first. The 1uf coupling cap will often go very high ESR and reduce the volume - eventually to almost zero. So replacing both electrolytics would be a good idea anyhow. Measure the voltage at the IC pins 4 & 8 and check that VGND is at half the supply voltage. The cheap battery clip will sometimes go high enough resistance to cause problems. If you add a 100uf cap across the supply before checking this it can mask the underlying fault

Another thing to check is that the circuit grounds are in place. I've just repaired a motorboating Ibanez guitar preamp that someone had rewired and all that was needed was a missing ground connection.

Lead dress can cause problems, but mainly produce a higher-frequency sine (ish) waveform. Low frequency oscillation generally involves either electrolytics or the battery, which behaves like a rather poor electrolytic (hence once the problem is fixed to bypass it with a 'proper' electrolytic).

May also be a good idea to check the resistors. Just in-circuit. Near-enough-is-good-enough. Did you double-check the wiring to the balance pot, and it's value?

[mhuss]

I had some more time to work on this tonight (being somewhat snowbound).

I see V+ and 0v on the op-amp corners, and VGND is exactly half V+. Other spot checks of circuit voltages look reasonable.

All external grounds meet at one central point. That point and the battery minus both connect to the zero volt PCB trace.

1) I replaced the VGND bypass cap with a new 50uF, no improvement.
2) I tried adding a bypass to V+, no improvement.
3) I tried using a regulated 9v supply instead of a battery, no improvement.
4) I disconnected the volume wiper, no improvement. Oddly, in this condition it oscillated when open, and stopped oscillating when it was grounded.

The second half can play the LFO by itself (see test 4), although the two op-amps still share the same V+, VGND and ground connections. There's clearly enough RC phase shifts around this oddball circuit to potentially cause problems, but as Mick noted, it must have worked when new. I would also expect it to be a normal high frequency sine-ish oscillation.

Manipulating the balance control from end to end changes the shape and frequency of the oscillation subtly, even though the only load is the 10M scope probe.

Any other suggestions/ideas?

[oc disorder]

I have one eyed deer... I am not familiar with that particular preamp but was wondering if the pots are sliders/faders that
will allow for debris to fall in ?

My suggestion is to replace all three well maybe you could rig up some external pots of any shape but the required value to see if that's the cause.

I had one ages ago different brand but on close inspection could see "green" oxidation on the tiny rivets that connect the track to the terminal.

I guess some spit or drink gradually accumulated causing the problem.

Other than the above I guess Deoxit in the pots may help or even a good scrubbing down of the pcb with isopropyl alcohol and an old tooth brush
may help.

Just an idea.. for when you are running out of them !

[MarkusBass]

I had the same problem in similar preamp when the balance pot had incorrect value. It was 50k instead of 250k. Please notice that the pot is in the positive feedback loop so incorrect value may cause oscillations. Are you sure that the value 50k on the schematic is correct? Try using 250k. If it works, you may still need to simulate the preamp to see if the filter works as expected. But this is easy.

Mark

[mhuss]

I had to replace all three pots (frozen), that's what started this project. Thanks for the suggestion, though.

[mhuss]

I actually am using a 100k in place of the 50k (since 50k long-thread linear pots are pretty scarce), and I paralleled it with a 100k resistor early on to see if the diff was a problem, but that didn't make any difference. I'll try a 250k.

--mark

[MarkusBass]

In my opinion the pot has incorrect value. I had the same problem with the Eric Clapton Mid Boost preamp. The pot forms a positive feedback loop and too low value of the pot causes oscillations. As a first step you may just remove the parallel resistor (it takes about 5 seconds to do it), and in the second step you can add a 100k resistor in series with the pot, not with the wiper(this will take 10 seconds). As a third step you may try 250k pot (this may take about 1 minute to test). And report results back.
However, if you think that the 50k value is correct, you may take a look a the 4n7 capacitor in the negative feedback loop of the filter.

Mark

[52 Bill]

Are these pots pc mounted or connected with wires?

If they are pc mounted, could there be a broken trace on the board? If they are wired, could you have mixed up any of the wires?

If you disconnect the balance control and jumper the output from the volume wiper to the output jack does it pass signal and not oscillate?