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roggli

Blowing power (T-2A) fuse in my 100 RD head.

Hi!
The fuse (Power T-2A) blowes immediately when I turn on "POWER", the other switch is in "STAND BY". I do not know if it has any significance, but it is written in some places that there may be a fault in the power / output stage tubes, but then the fuse should not go directly, but the fuse blows even if I have no tube connected. The large MALLORY capacitors seems to be perfectly ok. The resistance across the transformer from the voltage is about 3.5 Ohm.
All solders look very good. Can any of you have a good idea about this and help me? Thank you and good night from Roger / Sweden

mgriffin155

Check the Low Voltage Power Supply

I checked my 120V RD65 and the Input Xfmr primary measured 2.8 ohms. With yours being an export model (220V), 3.5 ohms sounds reasonable. Standby mode disconnects AC input from the HV Power Supply so probably no fault there. Suggest that the low voltage power supply be visually and ohm checked for burnt and/or shorted caps and diodes, including the zener regulator diodes. Also ohm check the secondary Xfmr windings if possible. Wish I could be more help. Are there any smokes or funky smells when the fuse blows? Good luck. -mgriffin

roggli

MM 100 RD

Thanks.
No, unfortunately, you don´t see anything and it doesn´t smell anything, it doesn´t have time to get hot when the fuse blows directly, black inside the glass on the fuse, of course. Nothing seems to be damaged and everything looks very good. Yes, you should probably be much more knowledgeable than I am, that's for sure. Cheers

bill_moore
bill_moore's picture

Fly back diodes?

Check the fly back diodes on the power tubes, when I serviced my RD100, I powered it up first without a limiter, and immediately blew a fuse. After checking, I found one diode shorted! I couldn't find the original, and settled for one rated for less HV.
Working OK!

roggli

MM 100 RD

Thanks, what was the diod you use ?
Cheers from Roger

bill_moore
bill_moore's picture

I think they were rated for 3KV.

I believe they were used by Fender, and maybe others. Peavey used some rated at 2KV.

roggli

MM 100 RD

Thanks ! // Roger

mgriffin155

Fuse Question

Is it a Slo Blo type fuse in the amp? Also, has a slightly bigger 3A Slo Blo fuse been tried? -mgriffin

roggli

No, I don´t dare .... the

No, I don´t dare .... the 2AT (slow blow) fuse blows immediately, then I can probably destroy, more, right?

mm210
mm210's picture

First thing I would do is

First thing I would do is build a current limiter out of a light bulb, socket, a plug and a duplex outlet. that will keep you from blowing fuses while you look for the draw/short. Attached drawing/pic
It will glow bright as hell when ready to blow a fuse and when you get it fixed, it will briefly flare up and then settle down to a dull glow. I use different bulbs myself for two tubes and four tube models.

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roggli

MM 100 RD

Thank you very much, I'll take care of that. Cheers! // R

mm210
mm210's picture

I don't turn on ANYTHING that

I don't turn on ANYTHING that is being worked on without first powering up with mine.

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