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I'm finally getting around to installing the VRM I ordered back in the fall....

Does the MOSFET have to be screwed to the chassis? I'm not sure from the documentation on whether it needs it or not. With the MOSFET mounted onto the circuit board, and a ground wire from the circuit board to the chassis ground, it makes me wonder at the need. It certainly is easier if it isn't needed.

Thoughts?

Needs to be bolted to the chassis with the mica insulator pad in between.

The MOSFET can dissipate a lot of energy and needs to use the chassis as a heat sink.

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http://pdfelectronics.com

Thanks...never thought about it in terms of a heat sink.

Well, I thought I had it all sorted out....

With the VRM installed, I seem to have a steady hum. Changing the power know or volume knobs doesn't change the volume of the hum. When first turned on, no hum, it seems to come as the tubes heat up.

Just as I was about to do some voltage tests, I heard a good electrical snap and got to see some smoke coming off the circuit board from the zener diode, on the input side of the diode.

Thoughts?

All right, ripped everything apart more or less and redid it all. Think my problem was caused by the pot being wired backwards.. once I googled the pot and saw how the which feet did what, it allowed me to redo things.

So...fired it up...no hiss...no smoke...no sparks... no tapering of volume...sigh. A lot of work to make a knob act as an on/off switch.

How likely a cooked zener is the problem?

Sure sounds like it is. But it could have take out the MOSFET. It's a 6.1V 5W Zener as per the instructions.
Funny how solid state stuff is less forgiving than tube technology.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

6V 5W zener or a 6V 1W zener? The build sheet in the kit says 1W.....

Well, replaced the old zener with a new 6V/1W one and still aren't getting any variable B+...

With the power off...

Checking out the MOSFET the best I can..

No continuity between any leg and ground....
There is continuity between any 2 legs

The Zener has no continuity across it.

All three resistor and the zener diode have resistance

With the power off there is no continuity between the standby switch and the filter cap, with the power switch turned on, there is.

Anything else I can check to try and troubleshoot this?

viewtopic.php?f=16&t=3088&p=24508&hilit=testing+mosfet#p24508

Try this



I always replace the Zener if I'm having issues. They are cheap and I usually have a few on hand. I've had a couple go bad on me.

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http://pdfelectronics.com

Mosfet seems to pass....

Replaced the MOSFET and the VRM seems to be working other than a hum I need to track down...

So here's a question regarding the schematic and the build diagram.... I'm trying to reconcile my limited knowledge, please feel free to correct me...

The schematic for the VRM shows pin 3 of the pot going thru R3 and into ground....The build diagram shows the left most pin on the pot (when viewed from the front) going to R3 and ground... isn't the left most pin normally considered pin 1?

Just trying to sort out the hum, which I assume is a ground issue and yet everything else seems grounded OK...as I've mounted the Mosfet/PCB separately from the pot, just wondering if 1 and 3 are crossed....

Here's a good picture. Remember when the pot is upside down, the numbers go from right to left - 1-2-3

It's not uncommon for me to get here numbers reversed.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I think I'm viewing this right... which is why I'm confused... looking at the schematic..

Drain from the MOSFET to #1
3 to R3 and ground....

The little build diagram shows the pot sitting on top of the PCB so the pins would be ordered 1, 2, 3 as you look down and have the switch over the PCB. This leaves pin 1 on the ground via R3 and pin 3 is on the MOSFET drain

Or am I viewing something upside down? or really..will I harm something if I try to reverse what I think are 1 and 3... I suspect my ground hum would go away... but I'm still thinking I'm viewing the build drawing correctly....

The hum your looking for might not be coming from the vrm it self could be preamp tube causing the issue (pull one at a time)
Ive even had a rectifier tube cause a hum issue( course this was after i wired the power supply to do more than its capable of doing)

And if possible could you take a picture of the vrm wiring and post it

I checked and updated the information. I wasnt too worried since I expected most people to use the board attached to the PCB, but this should clarify things.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I've attached a scan of the schematic I have... which shows the 1 and 3 pins reversed.

So, I suspect what you posted is what I figured is should be. I guess I'll rewire my 1 and 3 and see where it gets me.

And...success?

I did NOT change the wiring, I did have it wired as if the pot were mounted on the board. I realized that you'd updated the schematic to reflect the layout drawing and not the other way around.

The hum seems to have been a bad solder joint on one the 1M resistors on V1 that was installed as part of this. All good.

Amp seems to be running dead quiet... B+ between 80 and 369 V DC (was 377 before VRM installed).

I'll put it back in a cabinet and see if we got music happening.

Good news. Good Solder joints -Can't live with out em!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps