Vox Berkeley ll Low output

[Garydean]

I'm working on a Vox Berkeley ll solid state head from 1966. The symptom is low audio output. I've replaced all the larger electrolytics on and around the preamp board as they were showing their age as well as measured boarder line. The amp came to me as a chassis, no box.

I have a few questions please.

1. The lamp is missing. What is the replacement?

2. I'm plugging into the aux speaker via 1/4". The molex speaker plug is there but I'm not tapping into it. Does this matter?

3. I measured 16vdc on one of the output transistors, outer case to chassis ground. The other one gave me no reading on the case. Why is this?

Does anyone have a schematic?

Thank you

[vintagekiki]

Vox Berkley II v1081

Vox Berkley II v1081 schematics
Missing lamp is TS#1892
Connect speaker as auxiliary speaker via jack J7
Check all volltages on output Q8 / Q9 and driver transistor Q7;
UcQ7=27V; UcQ8=16V; UcQ9=0V (collector is grounded)
UeQ7=10V; UbQ8=33V; UbQ9=16V
If you have not sound check C22/500uF

[Garydean]

thanks for the info. I do have sound output is low.

[vintagekiki]

If sound is clean, if voltages on all transistors correct, but output is low check capacitors C22; C19; C18; C11; C5; C1.

[Garydean]

The sound is clean, but it's distorted when driven hard and turned up. The voltages on the transistors look good.

Q7 c 25.9
e 9.34
b 9.97

Q8 c 16
e 31
b 31

Q9 c 0
e 16
b 16

thanks again for your help

[Enzo]

I'd change out those small signal path electrolytics - C3, 5, 9, 11, 19.

I'd sub the speaker output cap C22.

I'd verify the trem was not involved by lifting C31, the term output cap. Also verify the DC readings around Q6 are reasonably close.

In fact, check DC readings through the amp.

[Garydean]

I checked voltages of Q1, Q2, Q3.....and Q3 has a problem. I'm only getting 600mV at the collector. The 17v supply to that transistor goes thru R15 27k on the supply side I get 16v and the collector side only 740mV. where should I be looking next?

[52 Bill]

I checked voltages of Q1, Q2, Q3.....and Q3 has a problem. I'm only getting 600mV at the collector. The 17v supply to that transistor goes thru R15 27k on the supply side I get 16v and the collector side only 740mV. where should I be looking next?

I'm sure you've already figured this out, but the obvious thing to check is R15, but also check Q3 and R16 as well.

[Garydean]

R15 and R16 are good. Q3 is not shorted. I replaced all the small electrolytics and tested the obvious resistors. I'm going to test it as soon as the board gets back in. Do you guys connect jumpers to the 20ish leads on the board instead of soldering each lead in before testing? Not that it takes much time, but the leads get a bit buggered up after a few meltdowns.

Gary

[52 Bill]

Do you guys connect jumpers to the 20ish leads on the board instead of soldering each lead in before testing? Not that it takes much time, but the leads get a bit buggered up after a few meltdowns.

That's why these things are such a pain to work on. All solid core wires on top of it.

Did you check Q4?

[Garydean]

Thanks for sticking with me Bill. Q4 I'm reading 26V on the collector 1.16 on the emitter and 1.77 on the base. I'm testing it in circuit and the collector and emitter are conducting both ways using the diode mode on my DMM. I really should remove it and test it out of circuit, but I have a feeling it's Q4 that has an issue.

R16,17 & 19 are fine

C11 and 13 I replaced.

[vintagekiki]

Do you have reverb sound? Connect your guitar to amp via P5 (reverb male connector). Reverb R24 go to max. Do you have sound? If you have sound power amp (Q6-Q9) and reverb preamp Q5 is OK. Check Reverb (Balance reverb/clean) control R24. If is OK check with ohmmeter Q3 and Q4. If b/e Q4 shorted UcQ3 getting low voltage.

[Garydean]

I removed Q4, 3, 5, 6, 8 and all tested fine.

[Garydean]

this unit does not have tremolo, just reverb.

[52 Bill]

this unit does not have tremolo, just reverb.

I've never seen that before?

Did you try what Vintage Kiki suggested about the reverb signal? Is the power amp ok?

Your voltages around Q3 and Q4 are still off. The entire pre-amp signal feeds into the power amp from the emitter of Q3. What is the voltage on the base of Q3? Even though they test ok, I'd try replacing Q3 and Q4.