Help! Anyone in Seattle area willing to take on a challenging F5T issue? - diyAudio
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Old 13th November 2013, 11:40 PM   #1
SirByrd is offline SirByrd  United States
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Default Help! Anyone in Seattle area willing to take on a challenging F5T issue?

I have a pair of F5T V2 monoblocks that I'm building, they are all assembled, but I am having issues with both amps in the amplifier section, power supply is working fine. Everything is wired up, and I've triple checked all component values and locations, polarities, all wiring arrangements.
Problem is that I'm getting .4V across the source resistors with only 20VAC going into the amp... P1,P2, and P3 do not change anything, DC on the output is about 1V with the same voltage in, power supply is about half the voltage it should be at 20VAC, but is fine when unloaded.

I don't think I have the right kind of brain for amplifier work and trouble shooting, so I want this to be the last amp I do, but that means it has to be finished first...

Anyway, I know some of you are more sadistic than I and would probably enjoy trouble shooting these, since it should be a pretty obvious solution that I'm just overlooking because I'm too dense.

Here's what we are dealing with.
Try not to give me too much **** about my wiring, it's only my second solid state amp
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Don't have any money, but would be willing to help with a speaker project if there is anyone who can conquer this beast.

Last edited by SirByrd; 13th November 2013 at 11:44 PM.
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Old 14th November 2013, 12:27 AM   #2
6L6 is offline 6L6  United States
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The wiring is actually very beautiful!

Where did you get the PCB?
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Old 14th November 2013, 12:33 AM   #3
6L6 is offline 6L6  United States
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I see that the bias thermistors (the 4.7K ones on the PCB) are mounted under the board. Are you sure they are not shorted?
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Old 14th November 2013, 02:22 AM   #4
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Its possible the mosfets are too far from the PCB. Not saying that's the exact problem, but usually lead lengths go 3-5" max.

This might sound bad, but you are having the same exact problem on both sides, so its consistent.
Also, nothing burned up. That's a plus as well. You are half way there. Don't give up.

It probably a reversed component somewhere or a resistor value is too high.

Checking your wiring and component placement 2 or 3 time will cost nothing but time.
Check everything again with no power to amp.
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Old 14th November 2013, 03:38 AM   #5
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Carefully check your rectifier and its connections.
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Old 14th November 2013, 04:14 AM   #6
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by 6L6 View Post
I see that the bias thermistors (the 4.7K ones on the PCB) are mounted under the board. Are you sure they are not shorted?
I am sure they are not shorted unless the painted case is electrically conductive, I could space them off of the heat sink just in case, right now the leads are bent so that when I install the board there is a bit of spring tension from the leads to keep it in contact with the heatsink. What would happen if they were shorted?

Quote:
Originally Posted by vdi_nenna View Post
Its possible the mosfets are too far from the PCB. Not saying that's the exact problem, but usually lead lengths go 3-5" max.

This might sound bad, but you are having the same exact problem on both sides, so its consistent.
Also, nothing burned up. That's a plus as well. You are half way there. Don't give up.

It probably a reversed component somewhere or a resistor value is too high.

Checking your wiring and component placement 2 or 3 time will cost nothing but time.
Check everything again with no power to amp.
I really hope that's not the issue. If I really have to, I could mount all devices on the left heatsink using a heat spreader, no new holes needing to be drilled, but I would be wasting the right heatsink.

The leads are not as long as they look, the chassis is really tiny, they are at most 6", I knew I was pushing it when I did that, but with my boards there was no other option since all devices were on one board. Didn't know there were boards with separate N and P channel PCBs until it was too late.

I checked all of the component values and orientations against the silkscreening AND the schematic, everything is in the right place, is the right value, and is the right orientation. The output devices are all in the right place, and are the right polarity. None are shorted to ground from any pins.


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Originally Posted by KatieandDad View Post
Carefully check your rectifier and its connections.
I'll check them again.
I have operated the power supply independently from the amplifier board, and have a bit over 32V unloaded, don't remember exactly how much.
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Old 14th November 2013, 04:26 AM   #7
SirByrd is offline SirByrd  United States
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I think I figured it out!!!
I'm using the IRF9240/IR240 (I think, I'll double check those transistors) same transistors as many use in the F5, but the IRF 9240 is a P type and the FQA19N20 is an N type.
So it could be that the output transistors are in the wrong place, piece of cake, just need to resolder a couple of them, and remount two on the opposite side.
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Old 14th November 2013, 05:04 AM   #8
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I think I figured it out!!!
I'm using the IRF9240/IR240 (I think, I'll double check those transistors) same transistors as many use in the F5, but the IRF 9240 is a P type and the FQA19N20 is an N type.
So it could be that the output transistors are in the wrong place, piece of cake, just need to resolder a couple of them, and remount two on the opposite side.

Just on other thing (which probably might not solve your issues yet): Solder the gate stopper resistors directly to the gates of the output MOSFETs to prevent the amp from HF oscillation. As far as visible on your photographs, you didn't.
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Old 14th November 2013, 05:11 AM   #9
SirByrd is offline SirByrd  United States
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Quote:
Originally Posted by permaneder View Post
Just on other thing (which probably might not solve your issues yet): Solder the gate stopper resistors directly to the gates of the output MOSFETs to prevent the amp from HF oscillation. As far as visible on your photographs, you didn't.
R13 through 16?
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Old 14th November 2013, 05:17 AM   #10
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R13 through 16?
Yes
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