Vox AC15C1 Burning 5 Watt Resistors

[Pavels07]

Hello all,

I have a Vox AC15C1 that is stumping me.

It came in not powering on (no light or sound). Customer had said that he forgot to turn it off one night. Then after turning it off, we had a nasty line of storms come through that caused power outages. After everything was restored, he amp wouldn't function.

I checked the fuses, power cable, switch, inrush current limiter...everything seemed ok.
I then ran some tests on the transformer (using the process outlined on the GeoFx site) and the thing was burned wide open. Easy enough. Searched replacement Power transformers and see that the Hammond 290CZ is a direct drop in and that the stock ones are notorious for failing.

Get that installed, everything fires up and it survives test playing. I come back to it the next day, except now it has a distortion that sounds as if a fuzz pedal is being played through it (going directly in). I open up the back and see that my 5 watt 180R resistors (R87, R88) ink has been burned off and one of them has cracked in half. R79 (5w 120R) has also shown signs of heat. I install new 5 watt resistors and fire up the amp. It sounds good, except after about 5 minutes, I get the distortion sound again and notice a burning smell. R87 is burning up again. I go down the line and check/replace the diodes, filter and bypass caps on that line, the screen resistors and the tubes themselves. I get everything together and fire it back up. 2 minutes before the distortion and burning smell this time. I noticed the board around the power resistors getting charred, so I then dug all that out thinking conductivity in the circuit board.

Fired up again and the problem still persists.

Anyone have any thoughts besides throwing this in the lost causes bin?

[Platefire]

If it were mine, I would pull out all the tubes and take the load off and try it without any tubes.If didn't start to smoke, I would put a tube back in one at a time and see which one started the trouble back again. Then wiring related to that tube or the tube its self, would be where to look for the problem. If it stayed on without smoking without the tubes, then I would also check the voltages with schematic. Would be some higher without the tube load but
might spot a problem.

If it started to smoke with all the tubes out, it's something shorted in the wiring or components.
That's all I can think of??

Story: One time I had a SE EL84 amp with a 12AX7 Preamp
and got mixed up and put the 12ax7 in the EL84 socket
and the EL84 in the 12AX7 socket. It did smoke!:>)
I doubt you've done that but in the heat of the moment
in amp adventures we can do some crazy things

[PRR]

You can also disconnect the OT CT (J11, 1/4" Spade) which gets the power plates and OT offline. If it still smokes, you got bad diodes or caps.

EDIT: sorry, bad idea. Don't do this.

[Latole]

You have to redo the tests without any tubes.

You seem to have tested many things don't nee to, but not the main one; the power tubes.
There is a good chance that this is the problem.

If without any tubes, these resistors still heat up too much, you have to disconnect the output transformer and see.

Screen shot show R87 , R88

[mresistor]

I just had a similar issue with an AC15CC1 that was left on for almost a day and then was blowing the mains fuse. The filaments and 25v supply and circuit was working. The problem was in the HV area. With the following disconnected;


standby switch
C32
C33
TT7
FS2
W13
FS1
R71
TT5
W21
All tubes removed
speaker disconnected
replacement power transformer


and even then the LBL lit up like aircraft landing light when TT5 was reconnected.


BTW mods  I have a copy of the schematic and it needs to be added to the schematics list but is too large to post on the forum 
I snipped it to show it here

[tubeswell]

Resistors burn from too much current. What is sinking all the current? The fact that both resistors are burning indicates a common denominator in the power rail. filter cap voltages may indicate where the current is disappearing.

[kagliostro]

Ciao Mresistor

You can send the schematic (in .pdf format) to Doug and he will add it to the database

if you can't obtain a .pdf version, please host the full resolution .jpg at IMGUR, then post the image link here

I'll transform it in a .pdf file and send it, for you, to Doug

EDIT:


I've just find one copy on my database and I'm sending it to Doug

Franco

[Pavels07]

UPDATE:

I've cracked back into this one after a fresh eyes reset.

Ran a test without power tubes, one resistor (R88) got warm, but not to the point of smoking. Put in a brand new set of EL84s and R88 got so hot it started to smoke. So issue is still persisting, but my test equipment isn't showing any signs of drawing too much current (~1amp @ 120v).
Looking over the schematic, I have replaced every.single.component. on that line including the tube sockets and 8 ribbon push on connector (I probably didn't need to, but sometimes the nuculear option is the best option). And I'm still running into that one resistor overheating and smoking.
Out of curiosity I tried swapping the Orange leads where the power transformer connects to the board (J18/J19) and now R87 is the one that is overheating and smoking while R88 is (seemingly) ok.
Did I happen to get a bum replacement transformer from Hammond? Is this just a crummy design flaw? Would an over spec resistor or possibly chassis mouning a resistor to dissipate heat be a solution?

Clearly I'm grasping at straws.

EDIT:

I also have a Fender HR Deville on my bench. The current draw on my test equipment for that amp (2 x 6L6) is only .5 amps, So the 1amp draw for the AC15C1 running 2 x EL84s seems high in compairison. Do what you will with that information.

[sluckey]

Out of curiosity I tried swapping the Orange leads where the power transformer connects to the board (J18/J19) and now R87 is the one that is overheating and smoking while R88 is (seemingly) ok.
Did I happen to get a bum replacement transformer from Hammond?

That seems to point to the PT but I'm not easily convinced. Measure the AC voltage between one orange wire and the white wire (center tap). Now repeat for the other orange wire and white wire. Do this with the amp in standby mode. Then repeat the whole test procedure with the amp in operate mode. Be quick so as to not make much smoke.

You should have four voltage readings and they should all be about the same. What do you actually have?

[Pavels07]

Out of curiosity I tried swapping the Orange leads where the power transformer connects to the board (J18/J19) and now R87 is the one that is overheating and smoking while R88 is (seemingly) ok.
Did I happen to get a bum replacement transformer from Hammond?

That seems to point to the PT but I'm not easily convinced. Measure the AC voltage between one orange wire and the white wire (center tap). Now repeat for the other orange wire and white wire. Do this with the amp in standby mode. Then repeat the whole test procedure with the amp in operate mode. Be quick so as to not make much smoke.

You should have four voltage readings and they should all be about the same. What do you actually have?

Standby measurements:
White --> J19 Orange = 306 volts
White --> J18 Orange = 134 volts

Live Measurements:
White --> J19 Orange = 296 volts
White --> J18 Orange = 6 volts (!!!!!!)

I think we just confirmed Sluckey's theory.

[AmberB]

Well, that certainly doesn't look right!
(Captain Obvious)