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FDP Forum / Fender Amps: Vintage (before 1985) / '68 Bassman blowing the circuit breaker when switched On

Pirate
Contributing Member
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It's a Time Machine

You never have to wait for breakfast!
Jul 16th, 2016 03:18 PM   Edit   Profile   Print Topic   Search Topic

Been away from this for a while again and can't remember where to start trouble shooting.

When I switch the amp on it blows the circuit breaker in the wall socket. I played it last Sunday night and it was fine, but I think it tripped the breaker when I turned it off and I just didn't notice it.

Put the volt meter to the switches and they seem to be fine.

Need help with where I should start looking for the trouble. It still has the Death Cap so I'll be removing that too. It was serviced in '82, filter caps look good. Need to change the coupling caps as they are starting to show some small bulges - I'll be looking through my goody bag for those tonight.

So, any help getting back on the repair track are appreciated.
Thanks!

Pirate
Contributing Member
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It's a Time Machine

You never have to wait for breakfast!
Jul 16th, 2016 07:11 PM   Edit   Profile   Print Topic   Search Topic

So, I replaced the coupling caps, then, with the amp unplugged and the power switch in the ON position I checked the end of the plug for continuity between the prongs. At first, I had continuity between prong A and Ground, but not from prong B to Ground. Then I had no continuity between A to Ground or B to Ground. Now, after plugging it back in and popping the circuit breaker again, I have continuity between A and Ground, just like in the beginning.

I undid the Death Cap so that was out of the loop and I even bypassed the switch so it's just there for looks now.

I'm hoping it didn't give me one last play that Sunday night and now a transformer is gone. I really, really, really want to keep the original transformers, but I can't find what's leaking to ground.

roadhog96

USA / CT.

Jul 17th, 2016 08:30 PM   Edit   Profile   Print Topic   Search Topic

Just curious about something. When you say the circuit breaker in the wall socket is tripping are you referring to the GFCI tripping in the AC 125V wall receptical, if it is have you tried the amp in a different receptical other than the one that keeps tripping. Only asking because I've seen a few of these GFCI trip because they are faulty not because they are working correctly. Sometimes they won't reset and sometimes they keep tripping out and it's not because of what's plugged into them, it's because they are malfunctioning. Just a thought..

(This message was last edited by roadhog96 at 12:09 AM, Jul 18th, 2016)

6G6

Texas

Fender power to the people!
Jul 18th, 2016 07:17 AM   Edit   Profile   Print Topic   Search Topic

It sounds like the A to ground continuity is the problem.
Not sure why it is intermittent.
Worn cord?
Worn switch?
Try attaching your DMM leads to the plug with aligator clips and moving the cord and wiggling the swtich toggle.

roadhog96

USA / CT.

Jul 18th, 2016 10:32 AM   Edit   Profile   Print Topic   Search Topic

I re-read your post and it sounds simular to an issue I had with a power cord. Intermitting AC and I wasn't getting 120v at the end of the cord before the switch. It turned out it was either corroded or broken connection with one of the wires to the plug.

(This message was last edited by roadhog96 at 05:19 PM, Jul 18th, 2016)

guitarcapo

U.S.A.

Jul 18th, 2016 03:46 PM   Edit   Profile   Print Topic   Search Topic

I'd definitely check for a short from the either primary in the transformer to ground and secondaries as well. short in the rectifier tube or socket? Also any short in the filter caps to ground like somewhere the b+ rail is getting grounded?

Hammond101
Contributing Member
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So. Cal. USA

Jul 19th, 2016 12:04 PM   Edit   Profile   Print Topic   Search Topic

I think your short is in the A/C power cord or mains connections in the amp. There could be a mis-wire if the cord has been replaced. Since the amps fuse is not blowing I think all is well with the power transformer.

Peegoo
Contributing Member
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Eat. Sleep. Guitar.

Repeat
Jul 19th, 2016 02:33 PM   Edit   Profile   Print Topic   Search Topic

Unplug the amp from the wall. Remove the rectifier tube. Plug the amp into wall power again and flip the power switch. If the breaker doesn't pop, it might be a shorted rectifier. They sometimes go dead short.

FDP Forum / Fender Amps: Vintage (before 1985) / '68 Bassman blowing the circuit breaker when switched On




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