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PostPosted: Thu Sep 03, 2015 12:57 am 
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It's pretty impossible to get 460VDC coming out of the diodes if you only have 64VAC at the fuse. I suspect there was something wrong with the way that 64VAC measurement at the fuse was made. Either bad connections to the meter or you didn't measure from the fuse terminals to the neutral of the AC supply (white wire on the power cord).

If you measured from the fuse terminals to ground, you might have a bad ground connection in your wall outlet or a bad connection from the green wire in the power cord to the chassis. Not too safe, but it wouldn't affect your amp's power supply.

Again, if you have a fairly steady 460VDC at C13 (the second 50uF on the board) most of the power supply is working. If you only get 47VDC at C12 (the first 50uF on the board) with the VRM at max there could be a problem with the mosfet or the parts and wiring associated with it.

To avoid confusion it's best to refer to the parts by their designation on the schematic. Especially if there is more than one part with the same value and what's first in the circuit is not first on the board, like C12 and C13.


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PostPosted: Sat Sep 05, 2015 9:37 am 
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Thanks Mitch.
The measurement at the fuse was wrong. It's 123VAC.

My friend took a look and found the zener diode was reversed so that's been fixed. He thinks it and/or the MOSFET might be damaged so is recommending they be replaced.

Current measurements
C12 idle: 43VDC
C12 max: 413VDC
C13 idle: 455VDC
C13 max: 421VDC


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PostPosted: Sat Sep 05, 2015 11:43 am 
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Test the MOSFET this way.

Attachment:
MOSFET test.jpg
MOSFET test.jpg [ 1.53 MiB | Viewed 16529 times ]

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PostPosted: Sat Sep 05, 2015 1:53 pm 
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coco wrote:
Test the MOSFET this way.


Should I remove the MOSFET for this test?


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PostPosted: Sat Sep 05, 2015 5:24 pm 
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Removed the zener diode, tested it and it's good.

Removed the MOSFET, tested it and doesn't seem to be good. The measurement never changes from infinite (OL) on the multi-meter.

I also watched a few youtube videos for multi-meter usage including the following for MOSFET testing.
https://www.youtube.com/watch?v=RBJGOOTEwfU


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PostPosted: Sat Sep 05, 2015 6:08 pm 
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On a positive my cabinet is ready for the tweed


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PostPosted: Sat Sep 05, 2015 6:28 pm 
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If you cant get a replacement mosfet locally, email me. You could bypass this section amd test the amp out?

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PostPosted: Sat Sep 05, 2015 6:49 pm 
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Yes, I'll bypass and test it out.

I did look over a thread describing the bypass so I'll try it.

Thanks, I'll let you know Tuesday if I can find one locally. Is my only choice the specific part (IRFPE50) on the wiring diagram or just a MOSFET that matches the BOM (Single-gate N-chan 800V 7.8 amp)?


Last edited by mmcginn on Sun Sep 06, 2015 7:10 am, edited 1 time in total.

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PostPosted: Sat Sep 05, 2015 11:34 pm 
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Glad you are making progress. You can bypass the VRM by running a jumper from the end of R23 that normally connects to the drain of the MOSFET and the VRM pot over to the + end of C12. All the better if the MOSFET is still removed.

Hope that gets you some sound!

Your cabinet looks good. Did you build it yourself?


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PostPosted: Sun Sep 06, 2015 7:05 am 
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mitch m wrote:
Your cabinet looks good. Did you build it yourself?
Yes. There are plenty of plans online and it didn't require any troubleshooting like the wiring! :oops:


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PostPosted: Sun Sep 06, 2015 9:02 am 
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Still no sound, so I took the board out to check layout, reflow all the solder joints, remeasure all resistors. The good thing of course is solving the problems (eventually) will help me to understand what's going on. Not in a rush, I have other amps.


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PostPosted: Sun Sep 06, 2015 12:23 pm 
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The bottom and top of the board after:
1. Confirmed top and bottom parts layout against specs. Nothing was out of place.
2. Reflow solder (my attempt anyway).
3. Measured all resistors and diodes, which are either right on or very close.
4. Wait for some comments to make sure I'm on track. (Please and thank you).
5. Put the board back in.


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PostPosted: Sun Sep 06, 2015 1:46 pm 
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The board looks OK for the most part. There might still be some iffy solder connections. Use your meter to check for continuity between everything that should be connected. Probe on the components' leads and not the eyelets. That way you can find bad solder connections.

You have a 100K resistor on the board below C12 and C13. This is R22 in the VRM circuit. It's shown as a 220K on the schematic and layout posted here on the forum. It sets the lower limit for the VRM. 100K just gives you more range. Everything else looks to be the right part in the right place.

I would say any N channel MOSFET that is close to the voltage and current specs will work in the VRM.


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PostPosted: Mon Sep 07, 2015 8:36 am 
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Once you put it back in, and turn it on, measure the voltages and compare to the layout and manual. Voltages give a good status, electrically. Post voltages here.

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PostPosted: Mon Sep 07, 2015 9:23 am 
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Rechecked the solder joints and then did the continuity testing between connections, all seems good. Aside from the jumpers and the 2@220K resistors, I'm moving all other connections to topside.

I switched R22 to 220K to match the current specs, instead a staying with the 100K.

Now, i'll put the board back in and re-connect. Tomorrow, hopefully pick up the MOSFET locally.


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PostPosted: Tue Sep 08, 2015 8:16 pm 
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MOSFET not in stock but I was able to order locally ($15.00 Gervais Electronics), same model IRFPE50, it arrives Thursday. I'll be carefully installing, rechecking, powering on, and then running a complete set of measurements.


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PostPosted: Tue Sep 08, 2015 9:44 pm 
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Too bad the MOSFET isn't in stock locally. While you wait for it to arrive you could bypass the VRM again and make sure the amp works. I'd get the amp working normally anyway before I installed anything that messes with the B+. Also there might be a problem somewhere that blew the MOSFET.


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PostPosted: Sat Sep 12, 2015 1:58 pm 
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Unfortunately there was no time during work the week to work on the amp, but back to it now. Received the MOSFET Friday and decided to just finish installing it and do some measurements.

The overall measurements seem reasonable, although some are a little low.

VRM B+ at C12
Idle = 82 VDC
Max = 407 VDC

cathode voltage (29V baseline)
Idle = 5.29 VDC
Max = 29 VDC

R10/C7 junction (340V baseline)
Idle = 72 VDC
Max = 333 VDC

R9/R6 junction (191V baseline)
Idle = 50 VDC
Max = 196 VDC

R3/C3/C4 junction (1.7V baseline)
Idle = .26 VDC
Max = 1.65 VDC

6V6
pin 2 = 3.4 VAC
pin 3 = 404 VDC
pin 4 = 402 VDC
pin 7 = 3.4 VAC
pin 8 = 30 VDC

12AX7
pin 1 = 196 VDC
pin 3 = 1.7 VDC
pins 4 and 5 = 3.4 VAC
pin 6 = 152 VDC
pin 8 = 1.1 VDC
pin 9 = 3.4 VAC

Still no sound though, not a peep, nor a hum. But I'm sure I'm getting closer and at least I'm more comfortable measuring and reading the wiring diagram.

Would bypassing the VRM be of any use if the measurements are reasonable at this point? Can it be back to proper grounding of the shielded cable?


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PostPosted: Sat Sep 12, 2015 3:14 pm 
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Turn amp off. Plug in a cable and test continuity from tip to 0.1 uf input cap. If that's OK, you better check your OT and speaker jack wiring.
No hum at all? Not even a pop?

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PostPosted: Sat Sep 12, 2015 3:34 pm 
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Great! You got the power supply working including the VRM. No need to bypass it.

The voltages are all OK, except V1 pin 6 is low. I would expect something more in the 180 to 190 volt there. That could just be the tube, though.

But you should have sound now. Since you aren't even hearing a bit of hum or hiss check the connections to the output transformer. The primary side is OK since you are reading 400 volts on pin 3 of V2. Check the secondary connections to the impedance selector and the speaker jacks.

I guess you know your speaker patch cord works?


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