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Randall RG-120 No Tremolo (& More) Help
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Originally posted by misterc57 View PostQ23 removed and tests good.
Q23 should not be conducting at all with emitter voltage higher than base voltage.
Maybe the problem doesn't show with low test voltage.
I'd replace it.
R71 lifted and reading 9.8K instead of 8.2K per schematic. Is that a big enough difference to replace it?
And be prepared to also replace Q24. But first check DC voltages.
- Own Opinions Only -
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I clipped in a pot set at about 8.2K to substitute R71 and that did bring the base voltage about .6V above the emitter.
However signal levels did not change and still no reverb effect.
There is gain taking place at Q24, input around .9V and output about 3.4V. If I am reading the schematic correct I should be seeing a much higher gain there?
Q24 2n1132
C -19.5V
B -1V
E -0.38V
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Originally posted by Helmholtz View Post
Schematic suggest a gain of 10/0.425 = 23.5.
Thank you
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Originally posted by misterc57 View PostSorry. I have much to learn in the transistor space. So the gain correlates to the hFE?
Hfe is current gain, typically having little effect on voltage gain.
Voltage gain is mainly determined by circuit values, but the circuit may require a minimum Hfe.
Regarding replacements look for sufficient minimum Hfe and voltage ratings as well as sufficient collector current for the circuit.
Also max. power dissipation may matter.
Last edited by Helmholtz; 08-15-2022, 08:03 PM.- Own Opinions Only -
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Originally posted by Helmholtz View Post
Use BC546 (or BC547). Observe pinout.
This has certainly become an exercise in perseverance!
After installing the BCBC337 I am seeing no signal changes. One supply changed, V at the base of Q24 is now +1.4V (no longer -1.0V).
Below images are signal at base of Q24 (left pic) and Q24 output (right pic).
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Originally posted by Helmholtz View PostBut first of all I hope you're not measuring DC voltages with signal? Signal shifts DC levels!
Still no reverb and still not seeing the 10V signal at the input of the pan.
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