Vibrolux reverb blowing fuse

[bruno]

Hi, need a little bit of help.


I have a vibrolux reverb currently blowing the main fuse.


I've disconnected all the secondary lead to make sure it wasn't the transformer and all good!


Then with all the tubes removed, rewired all the secondaries and disconnected the lead from the rectifier pin 8 to the first filter cap... all good!


Still with no tubes, I reconnect the wire from the first filter cap to pin 8 of the rectifier socket (standby switch down, so no connection to OT or choke) and the fuse blows...


I've tried other caps in the first position and same result, any ideas where I can look next?


Since the standby switch is off, I'm assuming it's not the OT, so the issue should be somewhere where those two first parallel 16uf+16uf caps are wired.

[mresistor]

Have you tried replaceing the rectifier and then connecting pin 8 ?

[bruno]

There is no rectifier in the socket...


Also when I test connectivity with a multimeter from the 5v tap ends to ground the multimeter beeps. I was under the impression these should test as open.


Is my 5v secondary shorted?

[mresistor]

what year is this Vibrolux R? Schematic? 

[bruno]

Circuit is AB763 is that relevant to the question?

[sluckey]

Then with all the tubes removed, rewired all the secondaries and disconnected the lead from the rectifier pin 8 to the first filter cap... all good!

Still with no tubes, I reconnect the wire from the first filter cap to pin 8 of the rectifier socket (standby switch down, so no connection to OT or choke) and the fuse blows...

That only leaves a couple possibilities. Shorted filter cap(s), shorted STBY switch, or shorted wire. Should take about 2 minutes to figure out which, a little longer to fix.

[sluckey]

Circuit is AB763 is that relevant to the question?

Not relevant but I don't think Fender ever made an AB763 Vibrolux Reverb.

[bruno]

Tested the standby switch, seems to be good. Not sure how to test what else could be shorted.


Could it be the 5v tap? however the fuse didn't blow when I isolated the secondaries.

[bruno]

Sorry I meant ab568.


Is it normal to test continuity from the 5v taps to ground?

[sluckey]

There should not be any resistance from the 5V filaments to ground. But you've already eliminated that when you disconnected the lead from rectifier pin 8 to the filter caps.
 

That only leaves a couple possibilities. Shorted filter cap(s), shorted STBY switch, or shorted wire. Should take about 2 minutes to figure out which, a little longer to fix.

Use your ohm meter with one probe connected to chassis and the other probe connected to filter caps, or STBY switch lugs, or wire. DON'T CHECK FOR CONTINUITY!!! Check for actual resistance. One of those 3 items will likely show a very low resistance to chassis. You'll need to separate them to determine which one is the culprit.

[bruno]

Neither the switch, nor the wire/cap show a small resistance to the chassis when isolated.


The 5v taps show a resistance of about 30 ohms to ground. Does that means the 5v tap is shorted?


However the fuse doesn't blow if I plug it in with all the secondaries lifted from the circuit, which is strange.

[mresistor]

was just wanting to know if it was a re-issue or not.   Not sure what problem is but last thing I'd think would be the PT windings.  However they can be shorted internally.  I think I'd hook up pin 8 and disconnect the lead to the standby switch to verify it's not the standby switch. That would leave a reservoir filter cap breaking down or a problem with the 5v winding ..however Sluckey has mentioned it's not the 5vac recto filament winding  so that would leave just a filter cap.  Have the large filter caps ever been replaced on this amplifier?

[sluckey]

The 5v taps show a resistance of about 30 ohms to ground. Does that means the 5v tap is shorted?

OK, guess we really didn't eliminate the 5V winding. Disconnect both 5V wires from the tube socket. Now use your ohm meter again. One probe on ground, the other probe on one yellow wire, then the other yellow wire. Still got about 30Ω? If so, the PT is bad. If not, check pins 4 and 6 of the tube socket.

[bruno]

Yes I have 30ohms to ground on each 5v wires, however, the fuse doesn't blow if I leave the secondaries floating, or connect everything up to the first filter cap.


Shouldn't the fuse be blowing in this case, given that PT seems to be bad?

[Dave]

What's the voltage on the 5volt leads unloaded?


Dave

[bruno]

I'm getting silly values...


5v tap is measuring 312v and the high voltage tap has one side reading something very low like 2v and the other sode measuring something incredibly high which my meter is has no range for.

[mresistor]

Bruno  if you wouldn't mind  would you please take some pictures of the two reservoir filter caps before the standby switch and post them.   

[bruno]

Hi, sure I can, here it goes...

[tubeswell]

Hi, need a little bit of help.

I have a vibrolux reverb currently blowing the main fuse.

I've disconnected all the secondary lead to make sure it wasn't the transformer and all good!

Then with all the tubes removed, rewired all the secondaries and disconnected the lead from the rectifier pin 8 to the first filter cap... all good!

Still with no tubes, I reconnect the wire from the first filter cap to pin 8 of the rectifier socket (standby switch down, so no connection to OT or choke) and the fuse blows...

I've tried other caps in the first position and same result, any ideas where I can look next?

Since the standby switch is off, I'm assuming it's not the OT, so the issue should be somewhere where those two first parallel 16uf+16uf caps are wired.

There’s a short to ground around the Stdby switch or a shorted filter cap of a short to the chassis hidden behind the filter cap board or something. Use your R-meter to ohm-out the power supply rail points to ground. You’re looking for a zero/low resistance that shouldn’t be there.

[AlNewman]

Don't forget to check your bias circuit.

[sluckey]

The 5v taps show a resistance of about 30 ohms to ground. Does that means the 5v tap is shorted?

LOOK NO FARTHER! The PT is faulty. Replace it.

[bruno]

Tks guys

[mresistor]

I would still be concerned as to why the 5v filament winding is shorted? Could be internal insulation breakdown due to heat and age.  But...


Did the amp see a rectifier failure?  I'd want to make sure the rectifier tube was good prior to reinstalling it after replaceing the PT.


Hawk Electronics has the Hammond replacement for that PT.

[sluckey]

There is another cheaper solution. Disconnect the 5V filament wires from the rectifier socket and tape the ends. Now switch to silicon diode rectifiers. Use four diodes. You could just solder two series connected diodes directly on the socket between pins 4 and 8. Repeat with two more series connected diodes soldered between pins 6 and 8. Done.

Even easier, just buy a SS plugin rectifier and simply plug it into the socket.

[mresistor]

Well heck yeah that works great too..