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Old 6th February 2013, 09:16 PM   #1
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Join Date: Sep 2012
Question Need help with MF Citation 12

Guys,

I need a little help with a build of a MF Citation 12 I'm working at. A P2P build, for what that's worth, and I built only one channel at the moment.

I followed papa's schematics, with some slight mods. I used MPSA42 and MPSA92 transistors, IRFP240 mosfets, eliminated the diodes between the source and drain of the latter, removed R8 at the input.

high-current.jpg

The PSU is nothing special, Nelson's schematics with a bit more of capacitance and some bleeding resistors. Two 300VA toroids.

Papa's instructions say:
Using only a variac, slowly raise the line voltage to the channel while watching the output. The output voltage should exhibit DC offset at first and then settle into a +/- 2V sine wave; if it does, continue to slowly raise the voltage until the full line voltage is achieved.
I gave it a 1V peak-to-peak sine at the input and no load. It's not clear to me how much of DC offset I should see before I get the since wave at the output. I raised the variac until half of the input voltage (that's 110V for me) and I get something around 30V of DC at the output.

Looked wrong to me and I didn't dare to go all in with the variac. Before I blew the output mosfets, I disconnected them and gave a full 220V to the PSU, which means +-42VDC to the amp. I looked at Q1's collector where I expected to see the input sine wave but amplified. Here's what I got (not including the PSU DC offset of course) - PLEASE ENLARGE THE THUMBNAILS AS THEY SOMEHOW GOT REVERSED:

q1-collector.jpeg

I'm still a noob but this doesn't look correct to me. I also looked at Q2. I expected the base to show nothing as the output mosfets are disconnected but here's what I saw:

q2-base.jpeg

As a bonus, here's Q2's collector:

q2-collector.jpeg

I think I got something wrong in the wiring as all the transistors and mosfets are tested OK. Any suggestions on what, if any, could be wrong?

TIA
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Old 7th February 2013, 07:37 AM   #2
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Join Date: Sep 2012
BTW this is the layout I made:

Screen Shot 2013-02-07 at 09.35.33.png

Thanks!
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