MFA Lumi suddenly whistles (on bench) - diyAudio
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Old 12th July 2012, 06:10 PM   #1
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Default MFA Lumi suddenly whistles (on bench)

In struggling with a problem in my MFA Lumi preamp, I've come up with another problem. I did some rewiring in the power supply (psu is regulated). Mainly 1) moving the regulator ground from after the choke to the first filter cap before it and 2) setting up the feed from the umbilical connecting the psu to the preamp so that a separate B+ lead would go to each channel. Formerly there was one lead that ran through one channel in series to the other. I also replaced a small electrolytic on a SS regulator for the filaments, and put in a new 12AX7 reference tube in the regulator. I did not replace the 8417 series pass tube, which has been in there for a long time.

I put the thing on a Variac and started it up. All looked fine. Both leadouts had what looked like normal voltage and both sides seemed to be receiving power, including the decouplers. Suddenly, with the Variac set at 106VAC and the regulator up to about 260 VDC when last checked (out of 330 when powered), I heard a whistling start up, like a tea kettle, but more machine like, increasing in pitch. It was not loud. My first thought was a drill outside the house, but when I shut the preamp off, the noise stopped. The noise was coming from the psu.

A minute or so later, I checked the voltages on the raw side and the B+, and they were 0. Usually, this preamp takes quite a while to discharge on turnoff (bleeder resistors across power caps in series, 100K each). I realize the preamp wasn't fully charged, but this seemed very quick. I later tested for shorts from the B+ to ground but didn't find any. The 12AX7 tested as strong with no leaks. I tested the 8417 as a 6550, lowering the recommended bias from 23 to 17 (on a Triplett 3444),. It tested at about 500, which I believe is low, but low enough to whistle?

I'm guessing something is shorted and an electrolytic was getting ready to blow. (They're almost all fairly new.) None look swollen or damaged, though. Or perhaps the new ground wire was not soldered well.

If I can't find any short, bad ground, or signs that a particular electrolytic is damaged, I'm thinking I may have to replace them all (not a big deal).

Any thoughts before I plunge in?
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Old 12th July 2012, 06:22 PM   #2
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you need to put everything back to original, then retest to see if the oscillation stops.
If the oscillation stops, make 1 change at a time and test each time, until you find what specific change is causing it.
If the oscillation does not stop, you need to narrow that down. Separate your modifications from the original, before making a conclusion.
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