Author Topic: Old School: LFO Yes, Trem No  (Read 726 times)

Droogie

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Old School: LFO Yes, Trem No
« on: April 21, 2018, 09:47:33 AM »
Test fired the Old School and LFO is happening, LED varies with speed Knob, but no volume change. All my voltages are good EXCEPT collector of T1 which reads 9.8 instead of 17.xx. The voltage going into R7 (4K7) is @22, coming out is 9.8. Is that correct or should I try a different value for R7?

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madbean

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Re: Old School: LFO Yes, Trem No
« Reply #1 on: April 21, 2018, 10:29:53 AM »
Actually, that's correct. I just pulled mine out and re-checked and I wrote down the wrong voltage for the collector. I re-measured that transistor and I have:
C - 9.51
B - 3.43
E - 2.82

Everything is so close together I probably touched the lead to something else when I took the initial voltages and just did not pay attention to the huge discrepancy between the base and collector.

So, your voltages being correct, perhaps there is something else. A couple of pics of the top of the build would help.

PS- I've updated the doc with the corrected voltages.

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Droogie

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Re: Old School: LFO Yes, Trem No
« Reply #2 on: April 21, 2018, 12:36:19 PM »
I re-checked all components since many will not be visible in photos, and all good. But nobody is perfect:




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madbean

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Re: Old School: LFO Yes, Trem No
« Reply #3 on: April 21, 2018, 01:10:44 PM »
I don't see anything obviously wrong, either. Except that 4k7 - I can't make out the color bands. Is it yellow-purple-black-brown-brown? The 4th band looks black in the pic. But, I think it's my eyes playing tricks (and since your voltage is right it probably is right).


Are you getting just plain audio out (no trem effect) or no audio out at all?
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Droogie

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Re: Old School: LFO Yes, Trem No
« Reply #4 on: April 21, 2018, 03:07:37 PM »
The 4k7 is correct. I get audio but no trem. The speed knob works; can't really tell about any other control. Does the depth knob change the intensity of the 5mm LED? If so, it's not working.
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madbean

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Re: Old School: LFO Yes, Trem No
« Reply #5 on: April 21, 2018, 05:52:25 PM »
The Depth knob should not change the brightness of the LED.

But, you've narrowed the problem area. It's most likely somewhere in the highlighted area below. What the problem actually is...hard to say. I'd look closely at the soldering in that area. Reflow everything, look for any small solder blobs, etc.

One thing to try - temporarily jumper pins 3 and 2 of the depth knob (just wrap some wire around them so they are connected). That will bypass the depth knob...might help identify the problem. And, make sure the Starve control is all the way down when testing.
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zgrav

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Re: Old School: LFO Yes, Trem No
« Reply #6 on: April 23, 2018, 08:28:30 AM »
have you checked to see if you are getting variable resistance from the opto-coupler?  the flashing LED suggests you are getting variable voltage going to it, but the part may be defective.

madbean

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Re: Old School: LFO Yes, Trem No
« Reply #7 on: April 23, 2018, 02:06:08 PM »
have you checked to see if you are getting variable resistance from the opto-coupler?  the flashing LED suggests you are getting variable voltage going to it, but the part may be defective.

That should not be causing the effect he's describing. The opto is only for the ramping control and the internal photocell only reacts to input...it won't flash on its own (it's separate from the LFO). Rather, the JFET (q1) is used as a variable resistor to modulate the signal amplitude for the trem effect.
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zgrav

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Re: Old School: LFO Yes, Trem No
« Reply #8 on: April 23, 2018, 02:42:33 PM »
OK -- I misunderstood the role played by the opto-coupler in the circuit.  In that case, seems like the question would be whether Q1 is functioning properly (input and output).

You might also check that the wires coming into the board have not strays touching something they should not -- I've had that problem before.