Gulbransen(Seeburg) Select-a-Rhythm white noise

[pontiacpete]

I have one of these on the bench with too much 'white noise'.
I was wondering if anyone had any experience with one of these.
Here's a sound clip of the problem: https://www.youtube.com/watch?v=uzOCN8r0Vh4

The service manual: http://ericarcher.net/wp-content/upl...ice-manual.pdf

Filter caps are certainly old, but would changing them out solve this problem.
ON page 64 there's talk of the noise generator. Once I figure out where it's located, is there anything to check for?
Thanks

[Enzo]

Make sure you are chasing the right problem. As I recall this system ( and recall gets fuzzier every day), and without studying the manual you linked (thanks), it is a rhythm machine - an old version of a drum machine. It was VERY basic. To make a snare drum, they set up a white noise generator, and then gate it for the drum hit. Now it is possible there is a noisy section just making hiss, but it is also quite possible you have a gate stuck open, so the endless snare hit is going on. Same with a cymbal hit, gate some white noise. The master noise generator can be routed through various gates and filters to affect the tone.

page 72 of the file, figure 84:

Upper right is the noise generator. Just a transistor used as a diode, the collector is left not connected. It is a nose diode now. The sound exits the benny to the left, and all those lettered signals from the left are controls which will gate the noise.


Listening past the noise, are all the drum sounds working or is there one or more missing? In other words is the stuck on noise supposed to be off and gated on for one of the sounds?

I could be wrong, of course, but i think it more likely you have lost a gate than have a real noisy stage. But...


FIlter caps dry out and cause hum. You get hum?

[Mick Bailey]

What does the term "Benny" mean? Don't think I've hear this before.

[Enzo]

I haven't heard it yet. Where did you see/hear it? The you tube link came up as private for me, so I couldn;t view it, in there?

[nashvillebill]

Make sure you are chasing the right problem...

Upper right is the noise generator. Just a transistor used as a diode, the collector is left not connected. It is a nose diode now. The sound exits the benny to the left, and all those lettered signals from the left are controls which will gate the noise.

....

I was wondering about it myself! I suspect a typo?

[pontiacpete]

Listening past the noise, are all the drum sounds working or is there one or more missing? In other words is the stuck on noise supposed to be off and gated on for one of the sounds?

I could be wrong, of course, but i think it more likely you have lost a gate than have a real noisy stage. But...


FIlter caps dry out and cause hum. You get hum?

It sounds like one of the cymbal sounds are either holding too long or just on throughout.
I'm still trying to figure out where things are, like the noise generator.

No hum so I'm holding off on changing filters

Sorry about the youtube link, it should be unlock and public now.

So what do you mean by "lost a gate"?

[52 Bill]

So what do you mean by "lost a gate"?

He means that one of the internal switches that controls the sound of let's say the cymbals is not turning off the sound as it should, so you are hearing the generated noise all of the time. It has lost its' gating or switching action.

Could Benny be a typo for Genny (generator).

[Enzo]

Oh, yes, sorry, I meant "genny" for generator. Yes, a typo.

The noise generator is where I described earlier in figure 84, upper right. Really, it is nothing more than a transistor with the collector left loose, and that amplified by a transistor.

Just imagine you have a constant white noise source. Now turn it off and on with a switch. it would sound kinda like a snare drum beating. hat is what they did here. The various transistors turn the sound off and on - thus gating the sound. If one4 of those gate transistors gets stuck in the on condition, the noise will remain steady instead of pulsing like a drum. That is what I mean by "lost a gate" I mean there is a gate circuit that has failed in the on state.

I listened to your clip, I hear a hiss in the mix, which could be like a ride cymbal stuck on, or indeed just a noisy stage.

[pontiacpete]

Well I'm making progress finding the position of components in the schematic on the board. Found the transistor that has only 2 legs connected.
I can trace my way now to the other transistors lower and to the left.
Right now I'm checking resistances of the components in that area.
The transistors are all 2n2926 except for the one that the collector is not connected, can't make out what is it says on it.

Would one of those 2926 be open or have a very high resistance?

Could I shunt one of those collectors to ground with a capacitor to see if it will shut off without damaging anything?

[Enzo]

The unit works, so you know the noise generator works. Figure 81 and surrounding text explains the generator. You can shunt most anything to ground with a capacitor. Just using a wire can cause trouble.

Look at fig.84. at bottom, four drum hit gates. Those are all emitter followers, so grounding the collectors with a cap does nothing. They are impulse generators, and probably not involved in your hiss. But to try it, use your grounding cap on the emitter instead.

At top is the noise genny, Benny the Genny, and three "noise keyer" circuits. The three transistors on the right are the actual gates. The four transistors left of them are the controllers - triggers. Note just right of each trigger is a cap and resistor. They have different values to set the different gate times.

Note the triggers and gates are DC connected, the noise is coupled in through caps. If it were me, and I wondered if a gate was stuck, I might look at the three gates and just check DC on the three legs of each to see if one was different. Then the four triggers, same deal, any with a DC difference?

[pontiacpete]

The gate transistors have equal dc on all legs the collectors are at 14v. The bases are all about 1.9v and the emitters are 1.3v.
I tried the .1uf cap to ground on the base of the 3rd one down(sure wished they numbered these components), and alot of hiss is removed. When grounding the emitters of these gates nothing happens, they are all connected to the 8uf electrolytic at the bottom.

When I ground the bases of the first and 3rd gate at the same time more hiss is removed, but the lower one is the loudest.

The voltages seem to be equal on the triggers .62vdc on base, 0 on emitters and .1v on collectors.

[Enzo]

Cool

See the control inputs down the left, and all those diodes? Now look at the triggers. Each base is pulled up towards B+ by the 470k. Ther is a 10k series with the base. This tells me the base sits high. Thinking in logic terms, where high (or 1) is near B+ (+14 did you say?) and low (or 0) is near ground. And it is active low, meaning to activate the thing the vase has to go low - ground it. The voltage at the vase will stay at the junction drop of the part, but it has the positive current which ir down;t when held low.

Note also the named signals, snare, cymbal, etc. The column of lettered inputs can activate more than one trigger, the diodes prevent unwanted cross-activation. So for example D input activates brush and snare, while E activates snare and something else farther down, but not the brush. A diode "or" circuit.

The trigger base sits pulled high, which turns on the transistor so the collector is now close to ground - it is low. Any time you ground that base - go ahead - it turns the transistor off and the collector then rises to B+ - it goes high. And that positive signal turns on the base of a gate or two.


The noise is injected into the base circuit of each, so shunting that signal to ground through a cap tells us the gates is open to some extent. If the gate were shut, then you could mess with the noise all you wanted and it wouldn;t be heard past the gate. Like playing guitar with the volume on zero.

Use your grounder to ground the collector of each gate, that will tell you which one(s) is/are PASSING the noise. And in case: look at each gate, there is a diode in its base leg. The left end of each is the anode, try grounding each of those diode anodes and see if any turns of its gate transistor. That is also the point each gate gets its noise through a 0.022uf cap.

[pontiacpete]

Cool
Use your grounder to ground the collector of each gate, that will tell you which one(s) is/are PASSING the noise. And in case: look at each gate, there is a diode in its base leg. The left end of each is the anode, try grounding each of those diode anodes and see if any turns of its gate transistor. That is also the point each gate gets its noise through a 0.022uf cap.

The 1st and 3rd gate seem to react to the grounding of the collectors, changing the tone of the hiss. The third one(the 'brush') seems to have more of a change in tone, deeper.

Grounding either of the anodes that are connected to the each the gates' bases, also, the one through the 100k, the hiss is eliminated.

[Enzo]

So you have the base diodes of the gates, and grounding the anode end kills the noise? Interesting, we may be chasing the untamed...

We already know your triggers have only 0.1 on the collector, not enough to enable the gates. But we don;t know what is on those anodes you just grounded. See if there is any DC there. Like on the brush gate, right at the point where the three trigger diodes join in. I am strating to wonder if any voltage is coming in the sandblock route, terminal A.

[pontiacpete]

There's 1.3 to 1.7vdc on that anode, brush gate base diode. .... the others have negative voltage -1.4v