Vox Berkeley ll Low output

[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