Randall RG-120 No Tremolo (& More) Help

[misterc57]

Sorry. I was reading them backwards...

TIS58 Q22
S 2.4V
G 1V signal 88mV
D 10.6V signal 1.2V

TIS98 Q23
C 23.7V signal 6mV
B 10.6V signal 1.2V
E 11.3V signal 824mV (does not look right on scope, see pic)

First pic is Q23 B, next pic is Q23 E

[Helmholtz]

If Q23 base voltage is lower than its emitter voltage, it can't work right.

[misterc57]

B node 24.3V

I tried various values with R70 (47K). No matter the voltage change to the base, the emitter is always about .6V higher than the base. Should I be focusing on R71 instead?

[Helmholtz]

If resistors are good (always measure disconnected from circuit) there's no reason for a redesign .
Remove Q23 and if it tests bad, replace it.

[misterc57]

Q23 removed and tests good.

R71 lifted and reading 9.8K instead of 8.2K per schematic. Is that a big enough difference to replace it?

[Helmholtz]

Q23 removed and tests good.

That's strange.
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?

Replace it.

And be prepared to also replace Q24. But first check DC voltages.

[misterc57]

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

[misterc57]

Helmholtz, reading your last post I missed the part about replacing Q23. Can I use any of the NPN below?

TIS98 datasheet attached.

Thank you

tis98.pdf

[Helmholtz]

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?

Schematic suggest a gain of 10/0.425 = 23.5.

[Helmholtz]

TIS98 datasheet attached.

Use BC546 (or BC547). Observe pinout.

[misterc57]

Schematic suggest a gain of 10/0.425 = 23.5.

Sorry. I have much to learn in the transistor space. So the gain correlates to the hFE? TIS98 has an hFE of 100, can I use any on that list with a similar hFE as long as it meets the V ratings?

Thank you

[Helmholtz]

Sorry. I have much to learn in the transistor space. So the gain correlates to the hFE?

Signal gain is voltage gain.
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.

[misterc57]

Use BC546 (or BC547). Observe pinout.

Woops did not see this. I installed a BC337. I can go back and use your suggestion.

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).

[Helmholtz]

BC337 should be ok here.

But first of all I hope you're not measuring DC voltages with signal? Signal shifts DC levels!

A different Q23 must not change Q24 voltages. Otherwise C42 is leaky.

[misterc57]

But first of all I hope you're not measuring DC voltages with signal? Signal shifts DC levels!

Yes I made that mistake. No signal and V readings are back to where they were.

Still no reverb and still not seeing the 10V signal at the input of the pan.