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#1 2014-04-11 15:44:09

rogerslodger
New member
Registered: 2014-04-11
Posts: 3

ZZap bang goes R54 and melted standby

Hello All

I'm new round here so go easy on me please...

I'm in the UK and Ive got me a scruffy old VT22 I'm trying to fix up. Ive recapped the psu and thought Ide power her up on a variac today. So started slowly and was measuring voltages.. All seemed well ... power light on standby light on .... No valves in. ( correct me at any point if I'm doing this wrong ) I get to 200v after 5 mins and then FFZZK Bang jump back variac down.. Ive now got a melted standby light and a blown away R54 .

I know enough about electronics not to kill myself I hope but I'm no master. Not sure where to look apart from D5 ( which isnt shorted )

Any pointers much appreciated.

Dave

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#2 2014-04-11 16:22:09

rogerslodger
New member
Registered: 2014-04-11
Posts: 3

Re: ZZap bang goes R54 and melted standby

just a note, I had replaced R54 ( what did look rather small to be a 1/2 watt but I was assured when I purchased it was ) as it was missing when I received it , with a 1/2 watt resistor. Doing some more research it seems that perhaps a 180/200K 1 watt resistor could be best to replace it with.

I was thinking if it is just the circuit for the standby light then I can power up and check the amp without it ?... Is there however a possibility that it was caused by something else and I will damage the amp by powering it up again and stressing something else ?

Dave

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#3 2014-04-12 06:53:38

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: ZZap bang goes R54 and melted standby

Those resistors are a common failure for the v series,  I always up the resistor to 1 watt myself.  Haven't seen one of those go yet.   If the shop gave you a 1/4 watt resistor that thing would go up in smoke for sure.

When you are ramping the voltage up,  are you monitoring power supply point A?  Might be a good idea, just so you can follow what is happening inside the amp a little.

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#4 2014-04-12 14:34:43

rogerslodger
New member
Registered: 2014-04-11
Posts: 3

Re: ZZap bang goes R54 and melted standby

Thanks for the advice Hangman ! I will replace with a 1 watt and find another silco lamp ( shame the postage for those things is so much on ebay... cant see why as they are tiny and weigh nothing... )

I powered the beast up with preamp tubes in and it seemed happy . I get heaters that I can see on all but cant quite see the 12au7 ( might chuck a mullard in instead of the national one see if I can see em ) And then the reverb tubes .. all lit up ok . Point A measures 545 at around 220v input . Bias seems a little low at -52 ...

I guess the next thing is to measure some output tubes and try them in place. I seem to remember that having speakers connected to the output was best when doing powering up properly or am I mistaken ? ( or was it an input ?.....

Dave

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