Vox AC30S1 Very Faint Output

[misterc57]

This amp had two issues.

1. the input jack was broken and very loose, but still functioning
2. pilot light and reverb not working

Before doing any repair the amp was working but had the pilot light and reverb issue. I found that there was no power going to the little effects reverb board which also send V to the pilot light. But after probing those connections a subsequent turn on did have the reverb and pilot light working. Great.

I could not find an exact replacement jack so I rigged a cliff jack by soldering extension legs to a new one. After installing the jack, I have very faint output and the pilot light and reverb issue is back, no power at the effect board.

For now I want to focus on the faint output.

I have a strong signal at the first half of V1 (pins 2 and 1) and a strong signal at V1 pin 7, but a faint signal at V1 pin 6. There is nothing between pin 7 and 6. So I would expect a gain factor of 100. Instead it seems like a loss factor of 100. I swapped tubes, same result.
At V2 there is barely an audible signal at pin 2 or the rest of the pins.

All V seem good

V1 pin 1 = 95V
V1 pin 6 = 165V

V2 pin 1 = 148V
V2 pin 6 = 145V


service manual below


ac30s1.pdf

[g1]

What is V1 pin 8 voltage? And signal will come out at pin 8, not pin 6, it is a cathode follower.

[Helmholtz]

I have a strong signal at the first half of V1 (pins 2 and 1) and a strong signal at V1 pin 7, but a faint signal at V1 pin 6.

Second half of V1 is a (DC coupled) cathode follower, so no signal at the plate (pin 6).
Output is the cathode pin 8. What do you see here?

There is nothing between pin 7 and 6.

Not sure what you mean.

So I would expect a gain factor of 100.

How did you come up with that number?
Gain from pin 2 to pin 1/7 should be 62.
Gain at pin 8 a little bit lower.

[R.G.]

Since the input jack was dingled up, maybe there is some persistent problem in the wiring to the input tube. Is the amp wired in a way that would let you remove the wire to the input jack and sub in a signal generator onto the wire to the first tube?

It's a bit of divide and conquer. After the test, you would know whether the preamp stage was messed up (if the low signal persists) or if the input jack, etc. is messed up (if the signal is correct from the input tube on).

[misterc57]

My bad. Pin 8 is 98 V and there is a strong signal there.

[misterc57]

Narrowed the problem to the fx loop board. Signal gets to the board but does not come back, even with a jumper cable.

The internal cable connecting both boards is okay.

[g1]

Do you have good signal at the send jack? Do you hear signal if you plug the guitar into the Return?

[misterc57]

Do you have good signal at the send jack? Do you hear signal if you plug the guitar into the Return?

No on both.

Finding all negative V at P8 on main board which feeds the FX board which is not right. Trying to find what components set P8 V on the schematic.

[Helmholtz]

What do you mean with P8?
A connector or a pin?

[g1]

P8 is the connector on the Power board. It carries +/-15 and 5V supplies to FX loop board. It comes off D6 and D7 zeners on pg.6 of pdf.

[misterc57]

P8 V at the pins are off.

My bad again. I did not realize that P12 was disconnected from the PT. After reconnecting same issues, but different crazy V as follows traced back to the following

F4 has 14.7 V, good
D9 cathode and D10 anode 14.6 V, good
D10 cathode 31.5 V, bad
D9 anode -2.4 V, bad

D9 and D10 are 1N4007

[Helmholtz]

D9 cathode and D10 anode as well as F4 should all measure something like 14V AC to ground.
D9 anode should measure something like -18VDC to ground.

[misterc57]

D9 cathode and D10 anode as well as F4 should all measure something like 14V AC to ground.
D9 anode should measure something like -18VDC to ground.

Right...

D9 cathode and D10 anode as well as F4 are 13.2 VAC
D10 cathode 31.5 VDC
D9 anode -2.4 VDC
D6 cathode and D7 anode are 14.6 VDC
D6 anode is -0.38 VDC
D7 cathode is 30 VDC

Are D6 and D7 causing the imbalance?

[Helmholtz]

D6 cathode as well as D7 anode are connected to ground, so must read 0VDC.

[misterc57]

I have lost connection to ground and I cannot find it.

On the board I have continuity from P12 pin 2 ground to the diodes and U3 but not from these components to the input jack or the board screw hole that also grounds the board where see about a 1M resistance. Is this the only path to ground from this board? See markups I made on the board.

I have a lifted pad on the input jack so I connected with a wire per schematic, even removed I still have the 1M resistance.

These are out of circuit readings, in circuit I was getting different resistance and moving high resistance readings but never zero to ground.