Randall RG-120 No Tremolo (& More) Help

[misterc57]

Q19 E with 10 mV at tank output

[Helmholtz]

Q18
E 1.4 V
B .09 mV
C 15.6 V

Q19
E 20.7 V
B 15.5 V
C 16 v

Sorry, I made a mistake.

These voltages are not good at all.

Did you mix up transistor legs?

Please measure again and also re-measure point B power supply voltage.

[misterc57]

Q18
D 1.4 V
G .11 mV
S 16.1 V

Q19
E 21.3 V
B 16.1 V
C 16.6 v

B node 24.8 V

[misterc57]

I think R52 (47K) has very high resistance with V applied. In the M ohms. Overtime the R drops after shutting off the amp.

[Helmholtz]

Q18
D 1.4 V
G .11 mV
S 16.1 V

Q19
E 21.3 V
B 16.1 V
C 16.6 v

B node 24.8 V

Especially Q19 voltages don't make sense.
Collector voltage must be higher than emitter voltage.
And as the collector is directly connected to point B (it's an emitter follower), voltages must be the same.

[Helmholtz]

I think R52 (47K) has very high resistance with V applied. In the M ohms. Overtime the R drops after shutting off the amp.

What voltage applied?
Resistors in circuit always measure too low. And measuring in a powered circuit is complete nonsense.

Always disconnect at least one lead from circuit while measuring R.

[misterc57]

What voltage applied?

The circuit V.

B node is 24.8 V. Reading about 21 V (not sure why the big drop here) going into R52 and 16.1 V coming out.

What are your thoughts on the updated readings?

Thank you

[Helmholtz]

The circuit V.

AS said, never use an Ohmmeter in a powered circuit.
Any voltage other than the meter's own voltage will upset the meter, give nonsense readings and may even damage the meter.

B node is 24.8 V. Reading about 21 V (not sure why the big drop here) going into R52 and 16.1 V coming out.

Doesn't make sense. Wrong schematic?

Two per schematic connected points must have the same voltage - or the connection is bad.

What are your thoughts on the updated readings?

Q19 E and C reversed?
Check datasheet for pinout.

[misterc57]

A few discrepancies on schematic. I was following the pinout on the schematic (lower right corner) which has the flat side, left to right as EBC, the datasheet shows CBE. So I was reading the numbers backwards.

Reading on Q19 - TIS98

C 21.3 V
B 16.1 V
E 16.6 v

There is a 150 ohm difference from node B to R52. So it turns out that this recovery circuit is connected to node A (not B as shown in the schematic).

Node A is at 21.3 V (schematic shows 25 V with a 24V Zener)
Node B is at 24.8 V (schematic shows 28 V).

Q19 E shows 14 V on the schematic, I am reading 16.6 V

I replaced R52.

Still no reverb.

[Helmholtz]

What did R52 measure?

So it turns out that this recovery circuit is connected to node A (not B as shown in the schematic).

Sure this is original wiring?
Still wondering why point A voltage is that low.
Is C49 good and correct polarity?
Is the 24V zener good?

Do you see amplified signal at Q19 base?

[g1]

Is it an actual TIS98 in there? It's odd that you are reading .5V between emitter and base, but in the wrong direction.

[misterc57]

My camera has better eyes. Q19 is a TIS58. What does this mean regarding the circuit and issue?

R52 measured correctly but a leg broke off so I installed a new one.

In the pic Q18 is o the left, Q19 is on the right.

I do not see any mods to the circuit etchings on the board so I assume this is all original wiring.

Thank you

[Helmholtz]

Ok, so with 10mV signal scope the gates of Q18 and Q19 (1X probe, AC coupling).

Emitter follower or source follower shoudn't make difference.

[misterc57]

scope pics. Not sure I got the AC coupling to stick.

pic 1 is signal entering board, pic 2 is Q18 G, pic 3 is Q19 G

As before in this test I can hear the signal at the speaker. Level rises with Reverb and Master Vol knobs.

[Helmholtz]

So your signal doesn't even reach the actual recovery input (Q18 gate).

Is the input signal shown when connected to the amp?
Where exactly do you feed the signal, before or after the tank cable? Was the tank connected?

Carefully check C32, R8, R49 and look for bad connections and shorts to ground.