Randall RG-120 No Tremolo (& More) Help

[misterc57]

Hey Folks

My RG-120 has no tremolo and possible no reverb. I am focusing on the tremolo first.

Schematic attached. If anyone has a more legible copy they could post that would be great.

It looks like the Tremolo will operate without the footswitch inserted (I tried both ways). I noticed that at the footswitch jack yellow wire, (no pedal inserted), the tremolo circuit is at about 380 ohms to ground, this seems to low to me and could be why it is not engaging? With a pedal inserted that circuit is about 3 ohms to ground.

Thoughts? Something in the circuit to close to ground? Suspect parts?

I am reading +40 VDC at the base of Q21.

Thank you, MarkO

[nosaj]

Turn the reverb up and give the amp a good ENZO slap on top what do you hear?

nosaj

[The Dude]

If you look at the schematic, from tip of the footswitch jack the path is through a 300 ohm resistor, then the lamp of the LDR, then ground. So, I'd expect something just over 300 ohms when the jack is not shorted/switched.

[g1]

The low resistance with pedal connected would be tremolo turned off. Toggle the footswitch and I think you will get the other reading, same as with no footswitch.
Suspect parts in trem circuits are always capacitors.

[misterc57]

So in this case it looks like C36, C37, C38 are the tremolo caps. 3.3 uF, 3.3 uF and 10 uF. Much higher values than what I usually see in tube amps.

[misterc57]

So in this case it looks like C36, C37, C38 are the tremolo caps. 3.3 uF, 3.3 uF and 10 uF. Much higher values than what I usually see in tube amps.

Reading 4.8 uF, 4.6 uF, 10 uF on those caps.

The 3.3 uF caps had ESR 3 ohms and VLOSS 7%. I swapped some new 2.2 uF caps in there but still not working.

LDR reading about 42K ohms across one side and 80 ohms across the other side. Two of the legs are connected to ground.

[Helmholtz]

Only one half of the roach is an LDR (light dependent resistor).
The other 2 leads belong to the (blinking) light source (lamp, LED).

Can you see the light blinking in the dark?

Is the Q21 collector voltage pulsing?

What is the base-to-emitter voltage of Q21?

[Enzo]

Higher value caps but lower value resistors. The whole point of that circuit is the three RC stages feeding back. You can have a 0.02uf and a 1 meg resistor or a 2uf and a 10k resistor and the time constant would be the same.

When you lower the capacitances like that, you will speed up the trem.

[misterc57]

Only one half of the roach is an LDR (light dependent resistor).
The other 2 leads belong to the (blinking) light source (lamp, LED).

Can you see the light blinking in the dark?

Is the Q21 collector voltage pulsing?

What is the base-to-emitter voltage of Q21?

I do not see a blinking light. the unit looks to be totally encased.

Q21 collector voltage is not pulsing

Q21 B to E 6.3 V
Q21 Base 33.5 V
Q21 Emitter 40 V
Q21 Collector -0.1 mV

[Helmholtz]

Q21 looks bad. Remove from circuit and test.

Did you verify supply voltages?
The schematic is confusing, D voltage should be -40V, C voltage: +40V, B voltage: +28V, A voltage +25V.

In your first post you reported Q21 base voltage of +40V? What changed?

Is C30 good and correct polarity?

Is the collector voltage of Q20 pulsing?

[misterc57]

Q21 looks bad. Remove from circuit and test.

Did you verify supply voltages?
The schematic is confusing, D voltage should be -40V, C voltage: +40V, B voltage: +28V, A voltage +25V.

In your first post you reported Q21 base voltage of +40V? What changed?

Is C30 good and correct polarity?

Is the collector voltage of Q20 pulsing?

I need to pull the board later to remove Q21

D -40.5
C +40.5
B +24.5
A +21

I may have been on the wrong leg when I earlier read +40

I think you meant C39? Reading 2.2 uF

All the legs of Q20 are pulsing

[Helmholtz]

D -40.5
C +40.5
B +24.5
A +21

B and A voltages look low, especially as A voltage should be regulated by the 24V zener. But that's for later.

I think you meant C39? Reading 2.2 uF

Probably C39, hard to read. That cap must not be leaky. Use Ohmmeter, resistance should be in the Megohms in both directions (out of circuit, of course).

All the legs of Q20 are pulsing

That's good and means that the LFO works.

[misterc57]

I removed Q21 and it tests bad. MPS 6522. Are there any available inexpensive replacements transistors? I have 2N3906, MPS 6533. I know it is PNP but what other characteristics do I need to look for?

Thank you

[Helmholtz]

You could use a BC560C:
https://www.onsemi.com/pdf/datasheet/bc560c-d.pdf

Check for different pinout.

I wouldn't risk to use a 25V transistor with a 40V supply like they did. Not surprised it failed.

[misterc57]

I need a sanity check. Getting ready to order some PNPs.

The MPS 6522 and the BC560C show negative V in the specs, but I have positive V from the C node at this part of the circuit. I am confused by this.

Also I have a good used MPS 6533 (spec below) with higher V ratings and would like to know if I can use it to replace the 6522?

Thank you