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FDP Forum / Amp Mods, Repairs, and Projects / 66 Bassman fuse blows immediately

Mars Hall

USA NW Indiana

Telefunkin'
Dec 1st, 2018 09:31 PM   Edit   Profile  

When I came across this amp, the 2 prong cord was cut, the main power caps were blown (discharge from vent holes). After installing all caps under the pan, installing tubes, replaced screen grid resistors and upgraded with a 3 prong cord (defeating the death cap), it still blows the fuse immediately when powering up. Also ruled out the PT as a possible culprit.

Peegoo
Contributing Member
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Results not typical.

Do not try this at home.
Dec 1st, 2018 10:42 PM   Edit   Profile  

Does the fuse still blow with no tubes installed?

Did you remove (or completely bypass) the ground switch?

Did you remove (or bypass) the AC receptacle?

Have you tested the power switch to rule out a short to ground?

Have you lifted the board out to check for shorts to the chassis?

Elmo66
Contributing Member
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Midwest USA

Dec 2nd, 2018 08:22 AM   Edit   Profile  

Solid state recification, right? Check all 6 diodes with amp unplugged. Next, triple check that the filter caps are installed with correct polarity.

Elmo66
Contributing Member
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Midwest USA

Dec 2nd, 2018 08:28 AM   Edit   Profile  

Also, if you want to stop blowing fuses to test the amp upon power-up, make yourself a light bulb current limter.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 2nd, 2018 07:26 PM   Edit   Profile  

Peegoo, I'll pull the tubes and try another fuse tomorrow. Do I remember correctly, if the fuse blows, without tubes, equals bad OT?

The ground switch and receptacle are still in the circuit.

I'll check switch and board tomorrow as well.

Elmo66, It is solid state rectifier. I did use a light bulb limiter when I tried it after replacing electrolytics. It did not blow fuse with limiter.

So just check diodes for continuity?





Elmo66
Contributing Member
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Midwest USA

Dec 3rd, 2018 06:19 AM   Edit   Profile  

Let's take it one step at a time to isolate the short.

As mentioned, pull the output tubes and power up with current limiter. If the light bulb glows bright and never dims, you still have a short. You have just saved your fuse.

Do this and report back.

Roly
Contributing Member
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Whitehorse Canada

I don't get out much
Dec 3rd, 2018 03:07 PM   Edit   Profile  

Disconnect all the secondarys and see if the fuse holds.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 7th, 2018 09:43 AM   Edit   Profile  

The fuse blew without tubes.

BbendFender
Contributing Member
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American Patriot

About as ordinary as you can get.
Dec 7th, 2018 01:48 PM   Edit   Profile  

Mars Hall. My light is on, I might be able to provide an answer for you.

Elmo66
Contributing Member
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Midwest USA

Dec 7th, 2018 03:04 PM   Edit   Profile  

Next step: Check the caps under the dog house for correct polarity. All should have the same polarity except the furthest right one should be reversed.

After this step we'll check the output transformer.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 7th, 2018 04:38 PM   Edit   Profile  

I goofed, reversed the other cap.

Elmo66
Contributing Member
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Midwest USA

Dec 7th, 2018 05:22 PM   Edit   Profile  

"I goofed, reversed the other cap."

I want to make sure you're looking at the chassis from the correct orientation.

Make sure you're looking at the caps with the tube sockets between you and the doghouse. The furthest right cap should be positive going down.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 8th, 2018 10:54 AM   Edit   Profile  

Yes of the 2 larger caps, I accidentally reversed the inner one.

Hammond101
Contributing Member
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So. Cal. USA

Dec 10th, 2018 11:47 AM   Edit   Profile  

Easy to do. I did it on a Showman once years ago. It's the kind of error you make only once :-)

FDP Forum / Amp Mods, Repairs, and Projects / 66 Bassman fuse blows immediately




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