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

roggli

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

Full Name
Mike Griffin
Music Man Equipment
MM 112-RD-65

Member for

10 years 9 months

mgriffin155 Tue, 01/04/2022 - 12:03

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

Full Name
Roger Glimsjö
Music Man Equipment
100 RD Amplifier head

Member for

3 years 1 month

roggli Wed, 01/05/2022 - 07:05

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

Full Name
Bill Moore

Member for

10 years 9 months
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bill_moore Fri, 01/07/2022 - 13:37

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!

Full Name
Roger Glimsjö
Music Man Equipment
100 RD Amplifier head

Member for

3 years 1 month

roggli Sat, 01/08/2022 - 04:01

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

Full Name
Bill Moore

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10 years 9 months
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bill_moore Sat, 01/08/2022 - 11:33

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

Full Name
Mike Griffin
Music Man Equipment
MM 112-RD-65

Member for

10 years 9 months

mgriffin155 Fri, 01/14/2022 - 11:54

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

Full Name
Roger Glimsjö
Music Man Equipment
100 RD Amplifier head

Member for

3 years 1 month

roggli Sun, 01/16/2022 - 05:11

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

Music Man Equipment
MM 2-10-65

Member for

10 years 9 months
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mm210 Tue, 01/18/2022 - 15:38

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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Full Name
Roger Glimsjö
Music Man Equipment
100 RD Amplifier head

Member for

3 years 1 month

roggli Wed, 01/19/2022 - 06:43

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

Music Man Equipment
MM 2-10-65

Member for

10 years 9 months
Profile picture for user mm210

mm210 Thu, 01/20/2022 - 04:28

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

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