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 Post subject: New old VRM trouble....
PostPosted: Wed Apr 21, 2010 2:51 am 
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Hi guys,

since I´m still sitting at home due to the volcanis ash cloud here in europe, I had some time to fix some things on my Tweed Deluxe....

I re-wired the input jacks, replaced the coupling caps as suggested by TuTone, AND hooked up the VRM again with all new wiring and mosfet (courtesy of Stephen, Thanks !)

I don´t have a clue if it´s quieter now, or if the caps solve my woofy-ness problem since the VRM is giving me the exact same problem I had the first time.

Power on, Standby on ---- NO PROBLEM

Turn Stanby OFF ---- Frying / Sizzling sound and Rectifier tube starts to light up like Luke´s lightsaber !

I was quick enough this time to turn it off again before the rectifier popped, and the mosfet didn´t burn up like the first time, but I really don´t know what to do any more.

I triple checked all connections against the documentation and pictures off of the forum, and was confident this time it would work......

I will get some pictures up asap, and would love some input from you guys. (or maybe it´s time to just ask Stephen to sell me a completed unit.....but then I would be giving in, wouldn´t I..!?)


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PostPosted: Wed Apr 21, 2010 5:47 am 
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OK,

I had a bright moment, and started some measurements. 0 resistance over the outer lugs of the vrm pot. So I de-soldered the pot, thinking it was faulty. Measured it again (desoldered) and it had about 900 Kohm resistance. Not quite 1Meg, but ok.

The measured from the point were pot is soldered to the drain of the mosfet: ) resistance. So it was shorting out....

Unscrewed the vrm board a little bit solved it. But as soon as I screwed it back down; same problem. I took it out completely, but could not find any point were it could be shorting out. All leads were well betweeon the body of the mosfet and the eyelet board.

I messed with it some more, without succes, and then decided to isolate the underside of the board were the eyelets are with some electrical tape. Now it's ok.

VRM all the way down to all the way up give me a spread of 49 to 395 V. that seems ok, doesn't it ? This is with a new TAD 5Y3GT, since I was afraid to blow my NOS Hytron rectifier. I gues that one can go in now !

BTW, is there any (physical) danger in case the mosfet shorts out to the chassis again, when playing the amp ?


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PostPosted: Wed Apr 21, 2010 6:58 am 
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Boom,

You should have installed the mica washer between the MOSFET & chassis. Check your package for that or a grey 'sil-pad' that does the same without the silicone grease .
I would not run that with electrical tape.
emasil me if you need something.

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PostPosted: Wed Apr 21, 2010 8:00 am 
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Hi Stephen,

the mica-washer is installed between the mosfet and the chassis with a little bit of heatsink-compound.

I just taped off the bit where the bare leads from the mosfet go into the eyelets. You think that will be a problem ?

What I found weird is that it would short out when screwed tighter onto the chassis. I mean, that only presses the body of mosfet tighter to the chassis, it wouldn´t bring the leads closer to it....

thanks


Last edited by Chester on Mon Apr 04, 2011 2:51 am, edited 1 time in total.

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PostPosted: Wed Apr 21, 2010 11:39 am 
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Boom wrote:
the mica-washer is installed between the mosfet and the chassis with a little bit of heatsink-compound.


Good to hear!

Boom wrote:
I just taped off the bit where the bare leads from the mosfet go into the eyelets. You think that will be a problem ?


That should be OK

Boom wrote:
What I found weird is that it would short out when screwed tighter onto the chassis. I mean, that only presses the body of mosfet tighter to the chassis, it wouldn't bring the leads closer to it


Strange yes, but something is making contact with the chassis. The MICA washer might be cracked? A long lead somewhere? I would want to sort what it is before you close it up tho. You don't want it to happen when you cant save it or a rectifier quickly.

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PostPosted: Thu Apr 22, 2010 7:48 am 
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Well, actually there was a little crack in the mica washer. I didn't think anything of it..... :oops:

Perhaps I can get hold of one of those silpads you talked about locally. Thing is, I don't have a clue what they look like and what to ask for in the shop. Is there someone with a picture of it, or maybe a manufacturer's name ?

It is running really well now. I'm loving the VRM functionality. It way better than the BBQ Mini Mass I had hooked up before.

Also all my gripes from before are gone (noise, hiss, too much bass with HB's, etc.) I rewired the the input jacks and replaced the coupling caps for both channels. The Tweed Deluxe now does everything better than I hoped for !

So now I guess I'm ready to start thinking about the next kit...... :D (don't worry, it WILL be a Trinity) I would like to have something that does clean really well, but is still low power. Time to do some research and listen to all the clips again ! The Tramp would be too close to the Deluxe I guess, wouldn't it ?

Yoeri


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PostPosted: Fri Apr 23, 2010 2:51 pm 
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Boom wrote:
The Tramp would be too close to the Deluxe I guess, wouldn't it ?


Well, I dont think so, but you can get those Tweed sounds. We did compare the two and you can discern the heritage. But, it does clean very nicely.

Of course, if you really want clean, there is always the TRIWATT. Not low power or quiet tho! :D

The Sil Pad is grey, was made by Bergquist , Digi-Key Part Number BER204-ND, They were for TO-220 . Dimensions are 1.00" (25.40) L X .750" (19.05) W & I got them from joeyvelour :D . http://dkc3.digikey.com/PDF/CA2010/P2485.pdf

http://media.digikey.com/photos/Bergqui ... %20104.jpg

A mica insulator is fine.

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PostPosted: Mon Apr 26, 2010 11:53 am 
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Thanks for the info about the sil pads Stephen, I´ll get on it.

So the Tramp may be in the picture after all... The second sample bgroup posted earlier today has a very nice clean tone indeed. (not sure what effects are used though, if any)

If I put the Tramp against one of the 18Watt versions you have; which would have the better / nicer clean tone, and STAY cleaner longer. I like the cleanish tone I can get with the Deluxe (I´m getting better and better at using the two volume controls...) but it is of course a limited clean range. I would like to have something with a little more range as well.

I was feening for the Triwatt befor, but I think that would be a waste. I think I´ll never be able to use it properly. I have neighbours, and I don´t gig. I´m far to uncertain about my playing to have anyone else besides my daughter and wife listen to it.


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