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 Post subject: Tweed VRM pictures
PostPosted: Tue Aug 30, 2016 10:57 am 
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Hello,
Could someone either post some good close ups of a well done VRM instal or direct me to something that's already been posted? I've found a few things on the forum but there seem to be lots of different ways people do the project. I'm looking for what would be considered the optimal install.
Thanks in advance for any help you can give me.


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 Post subject: Re: Tweed VRM pictures
PostPosted: Wed Sep 14, 2016 11:09 am 
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There are some Tweed photos at this link. Scroll down. If you have a new board (black), the 0.1uf DC blocking caps can be located right on the board .

viewtopic.php?f=12&t=1176&hilit=vrm

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 Post subject: Re: Tweed VRM pictures
PostPosted: Sat Jan 07, 2017 5:03 pm 
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Hi Stephen,
Thanks for the pictures. Very helpful. I just finished the install of the VRM. The amp works and sounds good. There is a more linear control on the volume controls...the amp used to hit "loud" by the time it was on 2. Both channels work and High and Low functions seem intact. I modified the inputs with the blocking capacitors and moved the 1M resistors directly to the tube grid. The VRM, however, doesn't seem to do anything. It turns the amp on, then there is no difference in volume or sound throughout the range of the adjustment. I have done what I can to check my connections but can't find where I've gone wrong. I'll attach a few pictures in case there's something visibly and obvious that I've messed up. Thanks for any help you can provide.
George


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IMG_0695.JPG
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 Post subject: Re: Tweed VRM pictures
PostPosted: Sat Jan 07, 2017 11:09 pm 
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Does the voltage change when you turn the VRM?
Check the connections to the little PCB.

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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 9:04 am 
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Hello,

Thanks for your quick reply.
The voltage at each rear lug on the VRM reads 33.5 volts and does not change as the pot is turned. I haven't taken things apart yet but I believe I made solid connections from the pot to the pcb. I carefully trimmed the lugs to make good mechanical contact before soldering and I checked continuity of these after making the connections. I've double checked my wire colours from the mosfet to the PCB and they seem to be correct.
George


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 9:29 am 
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Something definitely wrong. You should have full voltage IN and VARIABLE voltage out. Check continuity to MOSFET and the wires are correctly connected. Do you have full B+ in?

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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 2:49 pm 
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Hi Stephen. Thanks again for helping me track this down.

I have 355 v at the standby switch and at "in" on the PCB. I get 355 out regardless of where the pot is turned. The continuity from each MOSFET prong is good. Wires are correctly connected at the PCB. Ground from the board to chassis checks out . Next steps?

George


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 6:08 pm 
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Make sure the pot works. You will probably have to remove it and check for variable resistance between the wiper and the two ends as the shaft is rotated.

Check for shorts between traces on the VRM board.

The MOSFET might be blown. Check the resistance from drain to source. It shouldn't read as a dead short.
Here's a short video on how to test a MOSFET

https://youtu.be/RkWy1EirEu8


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 7:56 pm 
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Hi again,

Thanks for the ideas. The pot seems to be working fine. It reads 1000k and gives variable resistance as wiper is rotated. Continuity from pot lugs to pcb is good. For the MOSFET, there are no shorts. In following the video, I'm unable to get any continuity among any of the prongs. My meter has a diode test function but I have to switch it into resistance mode to get continuity. None of the testing sequences result in any closed circuit. I'm not positive I'm doing this correctly though. I've attached a pic of the pcb back.
GB


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 8:25 pm 
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Test the MOSFET with it removed from the circuit. Here's another video that shows a test procedure. Pretty much the same as the first one but there's a bit more detail.

https://youtu.be/RBJGOOTEwfU


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 8:53 pm 
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Thanks for reply. I tested with the MOSFET removed from the circuit. I had found this video as well.

GB


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 9:15 pm 
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Well, if all connections on the VRM board are good, there aren't any shorts, the connection to ground is good and the pot works, then the only thing left to cause problems is the MOSFET.


Last edited by mitch m on Sun Jan 08, 2017 9:21 pm, edited 2 times in total.

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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 9:18 pm 
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OK, I've gone over the MOSFET again using this video. I've followed his procedures closely. I've found the following:
Negative to Source: Postive to Drain: OL
After touching the positive to gate there is no change between Drain and Source. Remains at OL

Reversing the leads I read 470 with Black to Drain and Red to Source.
I don't understand this stuff well enough to interpret but to me it would seem that the MOSFET doesn't turn on.

Correct?

Bad MOSFET?

GB


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 08, 2017 9:22 pm 
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These tests are just basic ones. They don't show how the MOSFET will work with high voltage applied. I would say the MOSFET is bad.


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 Post subject: Re: Tweed VRM pictures
PostPosted: Mon Jan 09, 2017 10:02 am 
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Can you test the diode as well? Tough i know

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 Post subject: Re: Tweed VRM pictures
PostPosted: Mon Jan 09, 2017 12:32 pm 
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Diode reads 690 one direction OL other direction. Seems ok.


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 Post subject: Re: Tweed VRM pictures
PostPosted: Tue Jan 10, 2017 8:47 pm 
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George B wrote:
Diode reads 690 one direction OL other direction. Seems ok.


Yes it should be fine. Will send one anyway.

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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 15, 2017 6:28 pm 
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Hey tone searchers,
A quick update on my VRM situation. Stephen sent me a replacement MOSFET and diode. Installed yesterday, tested today. I'll just say to anyone who's sitting on the fence about doing this mod... run, don't walk to the order page and do it! The VRM enables you go get great tube distortion sound quite literally at bedroom volumes. It's kind of like plugging in the best overdrive pedal ever made into your front end and getting a tiny bit of warm "hair" on single notes and "hairier" sounds on multiple string sounds but with no colouration coming from the innards of the pedal. You can of course dial in much more overdrive at will.

I detect absolutely no increase in noise from the amp...it was pretty quiet to begin with.

Thanks again to those of you who responded and huge thanks for Stephen for following up with his always terrific customer service.

George


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 Post subject: Re: Tweed VRM pictures
PostPosted: Sun Jan 15, 2017 8:54 pm 
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Happy to. As we say "Made to be Played"

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 Post subject: Re: Tweed VRM pictures
PostPosted: Mon Jul 23, 2018 5:33 am 
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