Vox AC30 Vintage Repair.

[Joe bags]

Hi guys

I'm having a bit of trouble repairing a Vox Ac30 amp. The initial problem was bad sound from the amp.
I had a check inside and found some bulged power supply capacitors. So I replaced them.

I made some notes on the orientation of the caps I replaced, some were the 3 pin ones but subsequently lost the notes.
I have some schematics so think I have it wired up ok but not 100% on it.

Another thing I found was a badly burnt 10 watt resistor located beside the bridge rectifier circuit consisting of 4 x 1N4007 diodes.
Looking at a schematic I believe the value of this is 22 ohms.
I've replaced the resistor and the diodes just in case!

Testing the amp again slowing bringing up the voltage with a variac and the 22 ohm resistor heats up real hot.
So don't want to leave the amp on too long.

So the questions I have are
Why is this resistor heating up so much?
Is my wiring correct?

Also when connecting the load there are 3 connections, green 15 ohms, blue 4 ohms and black common.
What way do I connect up a speaker to this? Don't have the cab at the moment.
Can I just connect blue or green with the black as common?

If anyone is familiar with the insides of this amp and cold have a quick look over the photos to see if the wiring is ok
I would be much obliged.

Thanks for reading!

I'll post some photos in a bit.


Joe

[Joe bags]

Here's some photos.

Any comments welcome!


Vox 1 by Joebags1, on Flickr


Vox 2 by Joebags1, on Flickr


Vox 3 by Joebags1, on Flickr


Vox 4 by Joebags1, on Flickr


Vox 5 by Joebags1, on Flickr

[pdf64]

The cap in 'vox 2' and 'vox 3' has got the positive end to ground, which is unlikely to be correct. If it is wrong then it will be effectively a short across the HT, so things will get hot.
The charred area of tag board may be conductive and needs to be removed, cut out or the board replaced.

Could you sketch out how the diodes of the bridge are connected, as the wire links aren't clearly visible?

'Can I just connect blue or green with the black as common?'
Yes

Have you left the old can cap in place, by the el84s? Or is a new can cap?

Pete.

[Joe bags]

Nice one

I'll change that cap with the positive end to ground tmw.
The cathodes of all the diodes are pointing towards the el84's.
I'll have a closer look tmw and try and sketch it out.
Cap beside the el 84's is new.

[pdf64]

'I'll change that cap with the positive end to ground tmw'

Best to replace it really, it's likely to have been damaged from significant overheating. It may work but would be 'walking wounded', can't be relied on.
However, I'm not sure it's needed anyway, looks like it may be in parallel with a section of the can cap; if that's 32uF + 32uF, then the 'backwards' cap needn't be replaced, just removed.

I can only see 3 100ohm 2 watt resistors on the tag board? Should be 4, they're the screen grid supply resistors. Without a screen grid supply, one of the EL84s won't work, so there won't be sufficient current to adequately bias the other 3, and they would run super hot.
Pete.

[Joe bags]

"I can only see 3 100ohm 2 watt resistors on the tag board? "

The fourth one is under the big white 22ohm resistor.
I swapped around that cap and turned it on, That resistor didn't seem to heat up this time.
Then noticed one el84 wasn't lighting up went to take it out and the glass cover came off.
Need to replace this before further testing

[Joe bags]

So

I got a two new el34's stuck em in. Took out that cap with the +ve end to ground.
Put a signal on the input and it sounds pretty nasty! In a bad way!

I took some measurements.
All 4 x 100 ohm resistors to pin 9 on output tubes measure ok (in circuit)
All 4 x 1.5k ohm resistors to pin 2 on output tubes measure ok (in circuit)

402 Vdc on pin 9 of all output tubes
409 Vdc on pin 2 of all output tubes
35 Vdc on pin 3 of all output tubes

Had a sine wave on input getting triangular wave on pin 7 of output tubes. ( not sure what the crack is with this??)

I went through all the pre-amp valves they all seem good. I swapped them around with good ones.
Took some measurements here too.

Not sure how to number these at the moment.
But with the output transformer on the left and moving right from there.
V1 being closest to output transformer and V6 being the furthest away.

V1
Pin 1 = 180v
Pin 3 = 1.5v
Pin 6 = 261v
Pin 8 = 20v

V2

Pin 1 = 150-170v
Pin 3 = 1.7v
Pin 6 = 253v
Pin 8 = 27v

V3
Pin 1 = 165v
Pin 3 = 2.5v
Pin 6 = 165v
Pin 8 = 2.5v

V4
Pin 1 = 75v
Pin 3 = 3.7v
Pin 6= 73v
Pin 8 = 3.7v

V5
Pin 1 = 270v
Pin 3 = 72v
Pin 6 = 310v
Pin 8 = 72v

V6
Pin 1 = 218v
Pin 3 = 1.6v
Pin 6 = 350v
Pin 8 = 218v

Anything here jump out as being off?
I guess the voltages on pins 1+6 and 3+8 should match.
Seems to be high irregularities on V1 V2 V5 and V6.
I think V5 is the phase inverter tube.
Reckon I should try and trace voltages from power supply and see where they are falling down??


Appreciate any thoughts.

Joe

[pdf64]

35V on the EL84 cathodes - something is wacky.
What is the grid Vdc (pin 2)?
Do the grids have a path to ground (220k resistors ok)?
Measure the cathode resistor value.
What are you using as a load for the amp on these tests?
Pete.

[Joe bags]

Will make some more measurements tmw.
For the load I have an 8 ohm speaker connected to black and green.

Joe

[Joe bags]

What should the voltage on the EL34 cathodes be?
Changed a cap it's down to 28v now.
Grid is 0.2v
Not sure about grid's path to ground.

I referenced the Pre amp tube voltages with Stephen Grosvenor's service guide.
They seem to be ok all are slightly higher but seem to be in line with each other.

Ran a sine wave on input i'm gettting the wave on the input of the output tubes.
Getting a triangular wave on the output.

Should I suspect the output transfomer?

[soundguruman]

When you get that kind of waveform, something is typically out of phase, wires reversed on the OT?

[pdf64]

Cathode voltage should be about 12V static, max of say 15 when cranked.
Try changing the 50 ohm cathode resistor, whether it checks good or not; the value may change when it's hot.
A cathode voltage of 28V indicates something is way wrong, weird waveforms to be expected.
Pete

[Joe bags]

Yea changed the 50 ohm cathode resistor, still the same.

[pdf64]

Well, something is causing the EL84 to conduct fully.
Confirm the grids have got a ground reference via the 220k resistors.

[Joe bags]

Think you were right about the ground connection. Checking the wiring again I had the 50 ohm cathode resistor soldered to the wrong point on the board. Grid voltage is now 12v. Amp sounds a whole lot better now.
Although one tube is now red plating I think this is down to a bad tube.
I'll have to do some further testing on this issue.

Thanks a million for the help and comments don't think I could have gotten this far without them.
Really appreciate it