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September 22, 2017, 07:42:16 pm

Author Topic: Fuse blown during use  (Read 618 times)

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Offline slm

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Fuse blown during use
« on: May 08, 2017, 11:17:08 pm »
Hi All

Kaiju has been singing since january and last night fuse blew during use . Pulled tubes and performed some resistance checks and found 6.2 ohms from terminal 37 to 38 . also zero ohms across diode on 38l . That doesn't seem right ? is that diode and capacitor gone down - is it likely cap shorted and took out diode ?

Do I just replace both ? is it likely to have damaged anything else ?

Thanks in advance

Simon

Offline Caucasian Blackplate

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Re: Fuse blown during use
« Reply #1 on: May 12, 2017, 11:04:42 am »
Did you try replacing the fuse and turning the amp back on?

If a diode is "gone", especially if it is shorted, you will know because it will explode and there won't be anymore diode in your amplifier.   Capacitors are much the same!
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline slm

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Re: Fuse blown during use
« Reply #2 on: May 12, 2017, 01:02:41 pm »
Yes tried again and fuse blew instantaneously . That was the only screwy measurement I could find . Visually they look normal . Not sure how else to investigate cause ?

Offline Doc B.

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Re: Fuse blown during use
« Reply #3 on: May 12, 2017, 01:34:58 pm »
Disconnect the rectifer diode from 38, then measure its resistance from one end to the other, without being connected to the cap. That will tell you if the rectifier failed or the cap.
Dan "Doc B." Schmalle
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Offline slm

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Re: Fuse blown during use
« Reply #4 on: July 12, 2017, 02:56:59 pm »
Hi All
Couldnt follow up earlier as I haved moved house and geared was all packed


I sat down today to trouble shoot.I disconnected the diode at 38 and it still reads as a short . quick trip to electronic supplies and replaced diode- still blows fuses .

I disconnected all secondaries from pt and switched on - all good

Reconnected filament heaters - still all good .

I pulled out the terminal block 37-41 and rebuild it. Caps test fine resistance wise - I can't test capacitance and they look fine so I reused them . Resistors test 270.1 k so used those . replace diodes again and connect diodes to pt , terminal 41 to pt and connect ground to 38. Did not connect the resistor at 40 - wanted to isolate rest of amp  - double check my wiring and switch on - fuse blows . Am I right? this has to be one of caps even though does not measure as a short and looks fine ? Is there any way this is something else ?

Thoughts very welcome at this stage because I want my music back .......

Offline Caucasian Blackplate

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Re: Fuse blown during use
« Reply #5 on: July 12, 2017, 03:44:46 pm »
I would suggest posting photos of the area around that terminal strip.

I have never had a new capacitor that wasn't good right out of the box.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline fullheadofnothing

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Re: Fuse blown during use
« Reply #6 on: July 12, 2017, 05:10:46 pm »
What value fuse are you using?
Joshua Harris

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Offline slm

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Re: Fuse blown during use
« Reply #7 on: July 18, 2017, 08:18:03 pm »
Ok So next attempt  :(

The fuses are 1.5 amp slow blow

I have completely rebuilt the voltage doubler - new caps new diodes , new resistors new tag strip.

Still blowing fuses

Have taken photos during the ( 4th ) rebuild as I add the diodes and the caps . I also rewired the heater supplies just in case .

Without the voltage doubler attached the fuse is fine the secondaries on the transformer read a little high but otherwise as expected . With the doubler in place the fuse blows immediately . it is not attached to the rest of the circuit except the ground wire . I cant see a mistake .

Offline Caucasian Blackplate

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Re: Fuse blown during use
« Reply #8 on: July 19, 2017, 05:25:15 am »
Can you double check the AC voltage on the high voltage terminals on the power transformer without the diode attached? (See bottom of page 33)

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man