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 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.

[vintagekiki]

Gary tremolo unit is Q10/Q11.
Put signal from generator or guitar to jack P5. Reverb delay line is disconnected. If you have low audio output power amp Q6..Q9 or theirs components is defect. Direct (guitar) signal from preamp Q1-Q2 power amp receive from Q3 (ReQ3=R16), Reverb mix control R24 and via C18 go to power amp.
In maintenance any amps check units step by step from speaker to inputs
Are you connect guitar to amp via P5 connector? Do you have high or low audio output?

[Garydean]

Correction, it has trem and reverb. I don't have the pan. This unit came in with no pan.

Base on Q3 is -500mV at the moment.
emitter is -1v
collector is 650mV


I'm going to try what kiki suggest next.

Thanks guys

[Garydean]

OK. I plugged the the guitar signal into P5 and was able to control the volume via the reverb pot. The volume was higher than going direct to the input jacks but still low power.

Gary

[52 Bill]

Gary:
Earlier you noted that the voltage on the base of Q4 was 1.77 volts. Q3 is direct coupled to Q4 so there should be the same voltage at Q3 collector and Q4 base. Is there continuity there on the board?

Still, check the reverb return signal and let us know what happens.

[Garydean]

Yes Bill there is continuity between Q3 collector and Q4 base on the board. I did the reverb test..... I plugged the the guitar signal into P5 and was able to control the volume via the reverb pot. The volume was higher than going direct to the input jacks but still low power.


thanks,
Gary

[Garydean]

I've been looking for equivalent transistors for this amp. I guess I could spend $8 a pop thru the Thomas site. Would you guys know what alternate works?

86-5044-2 Q1,2,3,5
86-5050-2 Q6,10,11
86-5073-2 Q4
86-5075-2 Q7
86-5089-2 Q8,9

thanks.

[vintagekiki]

All preamp transistors in your amp is low power NPN silicon type transistors
For replacement maybe use BC414 (used in VoxBass100 model 1204) or BC107-109 or BC182-184 or BC550 (some of this used in Vox Foundation series). For idea look their schematics.
Q7 is general purpose NPN silicon transistor as 2N3053 or 2N3054 (look type case!).
Visual check PCB on cool solder.

[Garydean]

I've got a few 2N2923 NPN transistors in stock. Will they work? They have the same pinning and are general purpose small signal transistors.

thank you,
gary

[Garydean]

Another question. How can I test the output transistors to be sure they are ok?

Thanks,
Gary

[52 Bill]

Another question. How can I test the output transistors to be sure they are ok?

You can pull them out of their sockets and test with your meter, but I'd fix the pre-amp problem before moving on to other things.

As Vintage Kiki stated Q3 is a basic NPN transistor, and almost anything will work in that position. I use 2N3391A's, because I happen to have a lot of them, and they are based with the same pinout.

Q4 however, is the output transistor for the reverb drive circuit and will need to be a higher power transistor, maybe 500mW. If you want to reuse the same heatsink, use a TO-5 cased unit. If not, a TIP series transistor will probably work as a replacement.

[Garydean]

I'm still stumped. I replaced

Q3, Q4, Q5, Q6, and Q7.

R15 and R17

C3, C5, C9, C11, C13, C16, C19, C20, C22, C29, C30, C31

The collector of Q3 and the Base of Q4 STILL have only 2.07V. The rail is reading 15V.

Q1 all voltages are close to spec
Q2 all voltages are close to spec
Q3 emitter 67mV, C 2.07V, B 667mV
Q4 emitter 1.5V, C 25.6V, B 2.07V

I'm loosing hair!!!

[guitician]

Hello, I'm new here. And this thread caught my eye. That .22 C18 may be the trouble if it's leaky. Lift one end and see it the voltages on Q3 fall in spec. How do the rails look while playing the amp? I'd monitor them under load to see if the power supply could be at fault. I've worked on amps that have been real pains in the rear, and it has always helped if I set it aside and come back to it another day and approach it from a fresh view. Good Luck

[Garydean]

Thanks for the tip. That was not it:(. The voltages to the board all measure in spec.

[guitician]

I'd look at Q3 on a scope, there my be some occillations that are above the audio range happening.

[52 Bill]

Man, you've been busy!

I'd look at Q3 on a scope, there my be some occillations that are above the audio range happening.
Interesting point. Did you replace the missing reverb tank? Try loading the secondary of the reverb drive transformer with a resistor.

If you clip a cap to the volume control and send the signal out to another amp, and your pre-amp is working you will hear a good strong signal.

Have you checked R16, R19 and the reverb pot?

[Garydean]

Update. Following the audio signal thru the preamp, I get a signal off the emitter of Q2 yet I have no signal at Q3's base, Just very low voltage. I am getting my generated signal thru the speaker loud and clear. The volume is adjustable with both the vol pot and the reverb pot . I'm now fearing the replacement transistors I put in yesterday are not correct.

[Shontsy]

I'd check for bad (lack of) ground on both input jacks and speaker output jack(s), or shorting to ground (sending signal to ground due to short in jack) of the same jacks.

Everything else seems to be OK, but everyone overlooked these more simple explanations. These simpler causes have flustered me a couple times lately!

Good Luck!

[52 Bill]

Update. Following the audio signal thru the preamp, I get a signal off the emitter of Q2 yet I have no signal at Q3's base, Just very low voltage. I am getting my generated signal thru the speaker loud and clear. The volume is adjustable with both the vol pot and the reverb pot . I'm now fearing the replacement transistors I put in yesterday are not correct.

I'm confused here, "no signal at Q3's base...generated signal thru speaker loud and clear". So the amp is working now, but the Q3's voltages are off?

[Garydean]

Further development. I jumped the generated signal from the input jack to the center lug of the volume pot the bass of Q3 and the volume increased dramatically. A much larger wave showed up at bass of Q3. Volume still adjustable via reverb pot and vol pot.

[Garydean]

I jumped the signal from the emitter of Q2 to the base of Q3 and the generated signal is loud.

[guitician]

I jumped the signal from the emitter of Q2 to the base of Q3 and the generated signal is loud.

What is L1 made of? Could it be shorted?

[Garydean]

in circuit it does have continuity. Shall I remove it?
thanks for your help.

Gary

[Enzo]

Stop and think. An inductor is a coil of wire. It will therefore have continuity.

Turns could be shorted together, but then it might also show signs of melting.

[Garydean]

true true. Looks fine no signs of melting.

[guitician]

I meant can it be shorted to ground? The case, or outside winds, to a metal surface or shield. Also, wipers of R12, or R14 may be open or dirty.

[Garydean]

After putting this amp aside for a week I decided it time to get back into it. I just replaced 14 or the non-polar caps with lemon drops (sorry I don't know what else to call them). I was hoping one had shorted....but no, still no correct voltages after Q2.

I figured it's now time to trace the signal again thru the preamp. At Q3 base I do get a very faint signal which is increased by the volume control. I jumped ahead to the speaker jack and the signal was loud and not distorted. So I'm getting a clear signal thru the circuit but due to low voltages beyond Q2, the signal remains low until it hits the output transistors. The biggest ball buster is ,in order to swap a component via flipping the board, I have to remove 20 wires. The power supply at the board is very close to spec. The only caps I haven't replaced are the ceramics, none of which are shorted. The inductor is not shorted to ground. I've lifted and checked most every resistor in question.

I replaced the following transistors, here are the subs I used...I know, I know NTE is crap.

Q3 NTE 123AP .......stock was 86-5044-2
Q4 NTE 128............stock was 86-5073-2
Q5 NTE 123AP.........stock was 86-5044-2
Q6 NTE 123AP.........stock was 86-5050-2
Q7 NTE 128.............stock was 86-5075-2

I performed all the suggested tests, yet I'm stumped.

Gary

[52 Bill]

I figured it's now time to trace the signal again thru the preamp. At Q3 base I do get a very faint signal which is increased by the volume control. I jumped ahead to the speaker jack and the signal was loud and not distorted. So I'm getting a clear signal thru the circuit but due to low voltages beyond Q2, the signal remains low until it hits the output transistors.

So what are you saying here, the signal dies somewhere in the controls or in the Q3/Q4 stage?

If you take the signal generator and inject the signal into the power amp at the wiper of the reverb control, does the power amp drive to full output? What happens if you inject the signal at the wiper of the volume control?

In diagnosing a dead or weak signal, I always start at the back and work forward til I find where the signal stops.

[Garydean]

A signal at the wiper of the volume control produces a very loud signal. So, it's somewhere before that. I should work back now and see where the signal stops?

signal at wiper of the reverb control....very loud

[52 Bill]

A signal at the wiper of the volume control produces a very loud signal. So, it's somewhere before that. I should work back now and see where the signal stops?

signal at wiper of the reverb control....very loud

Forward from the volume control. Inject the signal at the negative end of C5.

[Garydean]

very faint at neg end of C5 Bill

and faint sig at the emitter of Q2

[52 Bill]

very faint at neg end of C5 Bill

and faint sig at the emitter of Q2

Check all of the tone pots and related wiring, etc.

Inject the signal at the wiper of the treble control and then at the top of the treble control at C8. Inject the signal at the top of the bass control. Be sure to sweep the controls through their full ranges while doing these tests.

Did you test Q1 and Q2?

[Garydean]

thanks Bill,

The signal is very soft on the signal side of J1's 68k resistor. The output side of that resistor I get a louder signal. On J2 it's the same scenario but the signal is stronger but not as strong as the wiper of the volume pot. I get a good signal around the controls and is fully adjustable.

I am getting an OK signal when I inject at the emitter of Q2. Before really cleaning the pots I did not . When I inject at the emitter of Q1 it faint and scratchy.

[52 Bill]

The signal is very soft on the signal side of J1's 68k resistor. The output side of that resistor I get a louder signal. On J2 it's the same scenario but the signal is stronger but not as strong as the wiper of the volume pot. I get a good signal around the controls and is fully adjustable.
The three input jacks are wired like a Fender, so that when you use a single jack (J1) all three input isolation resistors are paralleled across each other through the N.C. switches on the jacks. Make sure that you clean all of the contacts on the three input jacks.

I am getting an OK signal when I inject at the emitter of Q2. Before really cleaning the pots I did not. When I inject at the emitter of Q1 it faint and scratchy.
Test Q2? Inject a signal at base of Q1.

[Garydean]

the signal is coming through but not as strong as the base of Q3. As I work toward the jacks I loose strength.

[Garydean]

Here's something that's interesting. I replaced Q1 and Q2 with 2n4401 transistors. Not sure why but.... Q1 Used to have 15v on the collector, now has 4v. Q2 dropped in voltage as well. I flipped the board to see if something had gone wrong and everything is visually dandy. I'm supposed to be getting 17v to the board which feeds Q1. I have 4v. I checked the specs of 2n4401 and it seems like it should work.

This is getting old.

Gary

[Enzo]

Are the legs in the same order as the old parts? ANy chance the transistor needs to turn the other way? Vollow the trances to other components and verify teh base, collector and emitter are connected to the right spots.