Vox AC4 blowing main fuse. done stuff, need more help

[Kenrod]

attached again, i hope

[g1]

Yes that worked.
Now what is connected still with the fuse blowing? OT? Caps on either side of R16? R16?
Disconnect all those and see if it blows fuse.

[R.G.]

If you've replaced the rectifier diodes, this won't be hard. Remove all tubes. Temporarily pull up the transformer-side lead of R14, R17, R10, and R11. I'd prefer that you open the CT to the PT too if you can. Did you say that there was an undocumented 125ma cap between PT CT and ground? If so, remove it.

This removes all secondary loads from the transformer, saving only for solder shorts on the secondary winding. I'd prefer you remove all the secondary leads, but I don't know if you can on this PCB. Now turn on the AC power. There is no secondary load on the PT. IF the mains fuse (not shown on the schem, BTW) blows, you have either primary-side wiring issues, or an internally messed up PT.

If all is well, reconnect R10 and R11. Turn on AC mains power. Fuse blow? If yes, problem with R10 and R11 or the heater wiring. If not, reinsert the undocumented fuse, and one tube at a time and see if any tube causes a fuse blow. Heaters can sometimes short. IF no tube causes a fuse blow, remove all the tubes.

Re solder one of R14 or R17. UNsolder one lead of R16. Power on. If the fuse blows, something about R14(or17 if you did that one), it's diode, or the `100uF/350V first rectifier, or the wiring from there to the EL84 plate. There's nothing else connected.

There is no load on the cap, so it should go to full DC voltage. If it does and no fuse blows, turn off AC and reconnect the other resistor/diode, and test again for full DC voltage on the cap. Notice that you MUST make the 100uF/350V cap leak down between power-ups. It's handy to solder a 220K to 470K resistor across this cap PERMANENTLY to ensure that the high voltage leaks down if the tubes are out and power is applied. This makes it leak down to safe in a couple of minutes after the AC power is turned off. The life you save could be your own.

DC ok, no fuse blown with both rectifiers/resistors in? Hook R16 back up and try again.

The process is to open up any path for current to "overload" the PT, restoring one level at a time until fuse blowing starts again. When that happens, you're just past the bad part's level.

[Kenrod]

Thanks for the input guys. I have a busy weekend ahead, but hope to put some time on this and report back soon.

[Kenrod]

Yes that worked.
Now what is connected still with the fuse blowing? OT? Caps on either side of R16? R16?
Disconnect all those and see if it blows fuse.

I pulled the 100uF/350V cap, the PT side of R16, the OT primary leads, and powered up, nothing blew, but that isolated the secondary of the PT (and recification) from the rest of the circuit.

I suppose that meas PT is ok?
Then I started into some of the trouble shooting per the next post (RG)

[Kenrod]

If you've replaced the rectifier diodes, this won't be hard. Remove all tubes. Temporarily pull up the transformer-side lead of R14, R17, R10, and R11. I'd prefer that you open the CT to the PT too if you can. Did you say that there was an undocumented 125ma cap between PT CT and ground? If so, remove it.

This removes all secondary loads from the transformer, saving only for solder shorts on the secondary winding. I'd prefer you remove all the secondary leads, but I don't know if you can on this PCB. Now turn on the AC power. There is no secondary load on the PT. IF the mains fuse (not shown on the schem, BTW) blows, you have either primary-side wiring issues, or an internally messed up PT.

If all is well, reconnect R10 and R11. Turn on AC mains power. Fuse blow? If yes, problem with R10 and R11 or the heater wiring. If not, reinsert the undocumented fuse, and one tube at a time and see if any tube causes a fuse blow. Heaters can sometimes short. IF no tube causes a fuse blow, remove all the tubes.

Re solder one of R14 or R17. UNsolder one lead of R16. Power on. If the fuse blows, something about R14(or17 if you did that one), it's diode, or the `100uF/350V first rectifier, or the wiring from there to the EL84 plate. There's nothing else connected.
.

First, all the PT secondary and OT primary leads just unplug from the PCB so this was much easier to isolate, something you couldn't know just from looking at the schematic.

i think I accomplished this in my update this morning to the post from g1. I pulled the OT primary leads, the PT side of R16 and pulled the 100uf/350v cap completely.

This isolated everything past the recifier portion of the power supply, from the rest of the circuit. no fuse blow.

. Did you say that there was an undocumented 125ma cap between PT CT and ground? If so, remove it.

.

Here are the fuse locations. only on secondary side of PT. 1 between PT secondary and R10 (heater), 1 between secondary PT and R17 (B+?)

.


Re solder one of R14 or R17. UNsolder one lead of R16. Power on. If the fuse blows, something about R14(or17 if you did that one), it's diode, or the `100uF/350V first rectifier, or the wiring from there to the EL84 plate. There's nothing else connected.

Found something. when tracing wires/connections, i found I have continuity from pin7 of El84 socket to ground. That can't be right, that's the output pin of the tube correct?

tubes out, OT still unplugged, R16 still lifted, 100uf/350V still out.

direct short to ground. Did i find the problem?

[52 Bill]

Found something. when tracing wires/connections, i found I have continuity from pin7 of El84 socket to ground. That can't be right, that's the output pin of the tube correct?

tubes out, OT still unplugged, R16 still lifted, 100uf/350V still out.

direct short to ground. Did i find the problem?

Yes, it sounds like it.

[Kenrod]

I think I mis-spoke. I retested and found that I only have the short when the OT is still plugged in. I unplugged the OT and tested from yellow lead (normally connected to pin 7) and it is shorted to ground. When I look at the possible paths, cant' be through the 100uf/350V because it's still off the board, R16 has a leg pulled out at same cap.
I suppose the OT is shorted across the windings to ground?

[The Dude]

Sounds right- primary to secondary short. To be sure, test primary to secondary with everything unhooked. OT's are expensive. It's best to be positive.

[g1]

Something sure went sideways at post #3 then.

Edit: nevermind, I see C8 turned out to be shorted as well. Unfortunate combination of faults.

[Kenrod]

So I bought this OT from Musical Power Supplies.

OT10SE USA STD
5K/7K primary
4/8/16 ohm output
3.125" mounting center

see also this thread

http://music-electronics-forum.com/t39112/

Originally this amp is intended to run at 16 ohms. This new OT has multiple output taps.

Can/should I use the 8 ohm tap to add flexibility in using different cabs (8 ohm)? Is it still safe to use with 16 ohm if wiring up the 8 ohm secondary?

OT is installed and waiting to hook up that last lead.

[Kenrod]

update: yes I'm still tinkering on this and really want to wrap it up.

Here's the latest: blows with power tube installed, but not without.

I changed the power tube out for a new one.(although original tested good). still blows. New output transformer.

on another note, why don't i see a way to edit my first post to add some findings and save folks some time in sifting through the entire thread if not desired? I'm just not seeing it.

thanks, Kenny

[Chuck H]

Please check resistance from pin 8 on the power tube to ground.

[Kenrod]

Please check resistance from pin 8 on the power tube to ground.

Open

[Chuck H]

My bad. The schematic says pin 8, but it's an el84 (palm to forehead emogi). Sorry. Please do the same test from pin 3 to ground.