Kustom 250 Tremolo Issues

[gbono]

I'm working on a Kustom 250 amplifier and having everything working except the tremolo vibrato circuit. I get a signal up to and through I3A so I know the inverter and follower (I1B/A) are okay. The transconductance amp (I2) appears to be working but I haven't been able to figure out how to check the modulation bias (pin 5).

I checked Q2 and it appears to be okay though and shorting the input (pin 9 on J1) doesn't seem to get things going. I think I'm missing something simple here but what is it?


Kustom PC5068.pdf

[R.G.]

I'm working on a Kustom 250 amplifier and having everything working except the tremolo vibrato circuit. I get a signal up to and through I3A so I know the inverter and follower (I1B/A) are okay. The transconductance amp (I2) appears to be working but I haven't been able to figure out how to check the modulation bias (pin 5).

I checked Q2 and it appears to be okay though and shorting the input (pin 9 on J1) doesn't seem to get things going. I think I'm missing something simple here but what is it?

You can't check the modulation bias at pin 5. The ICs are obviously CA3080s from the pinout, and pin 5 will sit about 0.5V higher than the minus power supply. Well, I guess you can check that the voltage between pin 4 and "-12" is about half a volt. If it's zero or large, the chip is kaput. Pin 5 is a current input, and the only way to check bias is to check the voltage across R31 and R36 leading to the pin 5s. The output of the chip is also a current, so the output voltage is whatever the output load resistor pulls it to.

Check to see that there is a modulating voltage on the non-chip sides of R31 and R36, and that the pin 5s are at about 0.5V.

A quirk of the 3080 is that if more than a milliampere EVER goes into pin 5, the chip dies. It causes the outputs to cross-conduct catastrophically.

[gbono]

I don't see any issues with the transconductance amp (I2) - voltage is balanced between +-12 rails and there is 11.70 V at pin 5. It looks like the 18Vpp is not getting up to the bias input of I2 since there is no voltage at pin 7 of the I6B. IC6a/b appear to be another osc (vibrato) and the two dual gate MOSFETs have 18Vpp across their drain and source which supply the bias pin of I2. Need a way to test this theory.

[52 Bill]

There is only one LFO that works for both vibrato and tremolo. Does the vibrato work but the trem doesn't?

Does the reverb return work? It is mixed with the straight signal at I2 and then the mixed signal is trem'ed.

[gbono]

Neither tremolo or vibrato work but reverb is okay. The problem has to be LFO circuit. I have +-12 at I6a/b but no output at pin 7 of I6b.There is no signal (18Vpp) at R34 either. I'm going to check all the 1% resistors, etc. Again it seems like I'm missing the obvious ...

[Enzo]

1%??

None of this will work without the LFO, and that means you need to get your 18vp-p at I6 -7. These op amps are 5558. Are they really any different from 4558? Shouldn't just about any dual op amp work as an oscillator? Not saying they are bad yet, but if I6 has +/-12v reaching its power pins, something ought to be happening there. If I had any doubt at all, I'd sure stuff a 4558 in there. Their part is round. Sometimes the round ones fit into two parallel rows of four for a DIP8. But if the leg pattern is round on the board, get an 8 leg wire wrap socket, or tack wires onto a DIP socket to fit.


I suspect all that 1% stuff is more to make the wave symmetrical than it is to make it function. We need to make it work, then later we can worry if the waveform has lumps.

Those two op amps feed each other. If you have no 18v at pin 7, you won't have any at pin 1 either.

The footswitch just turns it on and off to the signal path. The oscillator should still run regardless.

[gbono]

I6 is LM1458 not 5558 as shown on schemo - I don't see as well as I used to and missed this when I did a visual on the board. The LM1458 looks like it was original since date code is correct and solder joints also look like wave solder. I'll put in a socket and try a few alternate OP Amps.

[Enzo]

OK a 1458 is just an earlier version of 4558, more or less. SO it lends credence to my theory that any op amp ought to work. It ain't rocket surgery. I don't think it much cares what op amp you use, as long as it is a functioning one

[gbono]

Yes it was the 1458 - put in a socket and tried a few OP Amps and the tremolo/vibrato work. I don't believe I've come across and amp with both types of modulation that can be mixed together. The reverb also just started acting intermittently and it was a broken wire on the input. Even the power switch lights up now THX for any and all help

[Westrek]

I'm working on this same board. I can't get a dry signal from in to out with all the controls down. I get signal at pin 7 of IC 1B but no further. I can inject signal to the reverb return and see it at the output once the rev controls are turned up. Also the tremolo functions, so the LFO is working. Installed sockets and tried new IC's #1,4,5. Quite stumped on this one.
Any thoughts appreciated. Thanks.

[52 Bill]

If you have signal at pin 7 of 1B then check the resistor and the two caps that couple the signal to pin 3 of 1A. Maybe broken solder joint?

If there is reverb return signal, that proves that that I2 and I3 are both working.

[Westrek]

Thanks. Those items test fine. Am I wrong in assuming that dry signal should pass in to out with IC 4 and 5 removed?

[52 Bill]

Thanks. Those items test fine. Am I wrong in assuming that dry signal should pass in to out with IC 4 and 5 removed?

Yes, I believe that you are correct, I4 and 5 are the vibrato phase shift stages and there should be enough signal going directly through IC 1B to hear it.

[Westrek]

Thanks! The board was virgin to start with. This may be one of those "can't see the forest for the woods" situations.

Mike

[Westrek]

As I thought, I created some of the problem. I removed the soldered in IC 1 and installed a socket in which pin 1 of the socket folded under and was then not soldered to the pc board. With that corrected, I could see dry signal in to out with IC 4 and 5 removed, but not with them installed, which I'm sure was the original problem. As another poster stated, those transconductance opamps blow very easily, as I went through 4 of them in the course of chasing my tail. Also, the NTE 996 replaces original 80848/CA3080A/LM3080T.

Thanks to those who helped!
Mike