Gulbransen(Seeburg) Select-a-Rhythm white noise

[Enzo]

ALL right.

From that point I only see four diodes - three to triggers and one back to terminal A. And their cathodes are all connected to the brush gate diode anode, right? So go to the anode end of each of those and find out where that voltage is coming from. I am wagering on terminal A.


If in fact it is the terminal A branch, try shorting across the 150k resistor (same as grounding the anode of its diode.)

[pontiacpete]

Taking measurments from all four anode shows no dc.
But on the .022 coming down from the noise generator, which has 6.2vdc on the collector there.
The last .022 at the node where all those diodes meet is the 1.2-7vdc. It might be that this cap is leaking. What do you think?
The other .022s with the 6.2v on the same side are not showing voltage on the other side.

It's a ceramic disc not very easy to pop out.
I'll clip something in and see if that cleans it up.

No clipping a cap in parallel did nothing. hmm

[pontiacpete]

Although taking voltage readings on these diodes with the DVM reduces the hiss when touched with the probe giving a better sound to the brush or cymbal.

[g1]

Clipping a good cap in parallel will not help a leaky cap. You must disconnect the suspect.
Clipping caps in parallel is only for caps suspected of being "open".

[Enzo]

If a cap leaks DC, clipping another in parallel with it will do nothing to stop the DC leak.

All three of those caps are joined on the 6v side to the noise amp collector. The third one down is in fact the remaining link to that node with the unwanted DC. Only thing to do is unhook one end and lift it. It stops th DC or it doesn't.

[pontiacpete]

ok this is strange. First I tried just putting in a new cap(mylar), since the board had to come out and feeling confident that this will solve the problem. Instead I find that the dc is only reduced a small amout, bouncing between .9v and 1.1v as oppose to the 1.2 - 1.7v. And still too much hiss. I dig up an old ceramic .02 cap and the same results.
So I go with Enzo's suggestion of lifting one leg and no dc and hiss is a lot better, but not all together gone(don't know how much hiss I should allow for here).
With one leg lifted up I'll try a few other caps.

[pontiacpete]

Could it be that the diode on the base of the brush gate is leaking the dc?


With the .022 cap lifted on one end there is still dc but it pulses with the rhythms from .6 to almost 2v on the anode.

[Enzo]

I suppose it could be, I have no direct knowledge of this circuit, I am just applying troubleshooting technique to this unknown circuit. I know the basic idea of the thing, but have no experience with it. So I am open to guesses as well.


I would expect voltages to appear there when it was playing, I don;t really expect any when it is paused. Does your hiss only appear while in play or is it constant even when paused?

[pontiacpete]

It hisses when paused.

[Enzo]

Then I'd work on it that way, as then the pulses from the rhythms won;t confuse us.

[pontiacpete]

Ok I'm back at it.
I decided to pull the lower gate and try a replacement, a 2N5089.
This cleaned up the hissing sound completely but there's no cymbal/ brushes. The tamborine(I pretty sure it's a tamborine) works on some rhythms, which seems odd that it works at all.
The collector and base are both showing 12vdc and 1.5v on the emitter, this can't be good. THe other 2 gates are getting 12v on their collectors, 2v less than before the sub.
So does this show that this gate was the source of the hiss?
Since the 2N5089 is not working, is there a modern equivalent to 2N2926?

[Enzo]

Pretty much any NPN small transistor ought to work.

You replaced a gate and the noise stopped, that says to me the gate was passing the noise. Now whether it had help is an additional question.


Remember how this works, we bring a letter input on the left to a low state and it triggers whatever. So ground post B, Brush, and it ought to turn off trigger 4 and thus turn on gate 3 allowing brush noise through. Try it.

At the bottom is post R, the cymbal. Ground that and it grounds the base of trigger 2, and thus allows gate 1 and 3 to open. Cymbal noise.

If your 5089 can turn off and on, then it works.

Are you sure the new transistors are in right? The old round style transistors are EBC around the edge, but modern 2Nxxxx are EBC straight across. Verify that the base of the new transistor is in the hole connecting to the 470k. And the collector to the inductor pin.


Split it up. test each trigger by itself. Watch the voltage on its collector, and then ground and unground the base, or at least the 10k base resistor. Does the collector toggle high and low opposite the base? Note trigger #1 has a cap in the base line so it should only pulse. So you'd need yo ground the actual base, not the 10k resistor for this test.

If your trigger collectors go from low to high, then follow the collector voltage to the right. Do the voltages toggle at the bases of the gates?

[pontiacpete]

Had the pins in the right holes but the wrong transistor, a 2n4125, duh (
So the hiss is back with the the 2n5089 in place. Sorry for the confusion.
I'm trying the test you suggest, grounding with out the cap, R turns off cymbal and lots of hiss.
The voltages drop a 1/10 of a volt or so on the base of the lower gate. Also, similar result when grounding A.
I go down the line grounding each post takes out a sound. When I ground R the brush or cymbal cuts out but reducing hiss but not eliminating it. That is, background hiss remains.

[Enzo]

Grounding a letter input will prevent the system from playing that sound, yes, but grounding it is what the system does to turn it on. In other words - at least as i read the schematic - the letters all remain high until pulled low to activate.

I thought we were doing this while paused so we don;t have confusing sounds? At pause, are all those letter inputs sitting at some good positive voltage? Like +12 or something? grounding each one ought to produce the sound it controls.

By exercising each trigger and each gate separately we should discover what is hanging up.

[pontiacpete]

When you say 'pause' I think you mean when all the bottons for the rhythms are in the 'out' position, in this position there is hiss. If I hit the foot pedal it turns off the hiss, but the unit is still on, there's voltages around board. Which is the right 'pause' here I'm confused?
Although doing the test, both positions get the same results when grounding the 4 trigger bases, the bases of the gates go up a tenth of a volt except for the top gate, 2 of the triggers are only moving that base less than a tenth of a volt.