Champ blowing fuse

[mort]

Just received a fixer upper '66 Champ AA764 and it's blowing fuses. With the tubes removed and a 2A slow blo in there I can get a read on the feed to the rectifier socket of 744vac. May be a stupid question but is this the nature of how a PT can go bad? Layout is showing 320vac


http://www.flatearthguitars.com/file...764_layout.gif

[52 Bill]

So with the tubes removed, it's not blowing the fuse?

The voltages will be higher than the schematic, but it will also be reduced by the load of the circuit.

Replace the tubes one at a time, rectifier first and see when the fuse blows.

[mort]

ok, when just the recto tube goes in the voltage drops to 585v, still hums, and after 10 seconds or so it sounds like something is sizzling(audible through the speaker)

[mort]

So with the tubes removed, it's not blowing the fuse?

It blew a 1A slo-blow after a few seconds(last one). I have 1A fast-blow that go instantly even with tubes removed, and I also have 2A slo-blow that don't blow within 20 seconds or so and then I turn it off when it starts to sound sizzly(see last post)

[Jazz P Bass]

O/K. So the power supply supply holds up o/k without any tubes installed & it blows the fuse when the rectifier is installed.
Correct?
If yes, with power off, all tubes removed & the power supply drained, check all of the major nodes of the B+ rail.
Use your volt meter & check them to ground.
Then check the cathodes of the tubes.
Compare everything to the schematic.
What you are searching for is something that does not seem correct.
If all looks well, install a new rectifier tube.

[52 Bill]

If you get a sizzling sound through the speaker with only the rectifier tube in place, I'd check the output transformer for shorts.

Are you reading the AC voltage from the transformer across the tube socket (red wire to red wire) or with one lead grounded?

[mort]

If you get a sizzling sound through the speaker with only the rectifier tube in place, I'd check the output transformer for shorts.

Are you reading the AC voltage from the transformer across the tube socket (red wire to red wire) or with one lead grounded?

red to red

[mort]

wait.... I'm supposed to check from lead to ground, right? If I do that I get about 370vac on each lead with no tube installed.


I worked on mechanical circuits for years and still sorta do. It's force of habit to check from lead to lead...


btw, while I'm ignorant to amp circuitry and troubleshooting, I at least know how not to hurt myself. That much I am sure of

[52 Bill]

Most schematics list voltages referenced to ground, so the AC readings that you are getting sound about normal.

[riz]

But do you know how not to hurt an amp? Never put a higher rated fuse in there than what the amp calls for. It's SUPPOSED to blow when something's wrong. This sounds like a job for the "light bulb limiter". First thing that crosses my mind is a bad filter cap.

[Steve Conner]

Yes, Electronics 101. If the fuse blows, don't just put a bigger one in. You're just upping the ante until some more expensive part burns out.

An important detail that was missing: how did it behave when powered up with the rectifier removed, but the right sized fuse, not the oversized one? If the fuse stayed intact, it's a bad filter cap or rectifier. If the fuse still blew it's more likely to be a bad PT.

If you keep messing around with oversized fuses, you will soon have a bad PT if you didn't already.

[mort]

Yes, Electronics 101. If the fuse blows, don't just put a bigger one in. You're just upping the ante until some more expensive part burns out.

An important detail that was missing: how did it behave when powered up with the rectifier removed, but the right sized fuse, not the oversized one? If the fuse stayed intact, it's a bad filter cap or rectifier. If the fuse still blew it's more likely to be a bad PT.

If you keep messing around with oversized fuses, you will soon have a bad PT if you didn't already.

With the recifier removed the fuse does not blow.

I knew I was running a slight risk with the 2A fuse but it was all I had and I only left it on for a few seconds... shortly after that I made a trip to the store and got proper ones.

btw, I completly removed the OT from the amp and I'm getting 256 ohms and 155ohms between the primary leads and the xformer housing. Is this normal or could this be my culprit? And I made sure that my body wasn't causing the reading by touching the test lead ends...

[Steve Conner]

No, that's bad, the OT primary leads shouldn't have any connection with the core. It would short your B+ to ground, which would explain the symptoms you're seeing.

[mort]

Ok, cool. I'm getting somewhere then.

One more question for now, if I lift one side of the filter cap and read it I'm getting 26.35uF, rated @ 25uF. If I'm getting a good read does that always mean that the cap is good? It looks a little darker than the other same value caps in the amp, but it reads ok. Could it still be hypothetically bad anyway?

[mort]

Ok, it took what seemed like forever for my parts to get here but alas, they have arrived.

I replaced the OT and a resistor that I found to be wildly out of spec(2.7k on the layout).

http://www.flatearthguitars.com/file...764_layout.gif

All of my hum has gone and the amp is working, but seems underpowered and sounds a little off.

Voltages in and out of rectifier, along the B+, and past the 100k resistor where the voltage is supposed to drop to 200v are all a bit high, but seem normal in conjunction with each other. I've tried swapping out tubes and it sounds the same.



Please excuse me, I don't know all the terminology to effectively communicate circuit locations but I'll give it a try.

Into recto = 356vac
out of recto = 440vdc
pin3 on 6V6 = 424vdc
pin4 on 6v6 = 432vdc
leaving can cap(A) = 401vdc
pin1 at 12ax7 = 260vdc

http://www.kbapps.com/audio/schemati...champaa764.gif

Where else should I look for trouble and is there a way to get these voltages to calm down some?

Thanks for any help.