R/E/P Community
R/E/P => Klaus Heyne's Mic Lab => Topic started by: afterlifestudios on July 10, 2020, 11:15:38 AM
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After a normal day of use I powered down my u67 and went home. I came back in the morning, flipped the switch and nothing. Swapped out the PSU with one from my other 67 and all is fine with the mic. (Yes, heater and b+ are nominal with the substitute PSU.)
Checked the fuse on the problem PSU, and it’s good. Opened up the PSU to drain the caps but when I measured them, they weren’t holding any stored charge. Powered it on and I’m only seeing like 7VDC on the B+.
Am I to suspect the power transformer?
Also, what is the function of the large “double” capacitor mounted on top of the xformer? I don’t have that on my schematic. (Between AC neutral and switch?)
Also, what is Si1 on my schematic?
Thanks for any thoughts!
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It is easy to check the trafo. Just measure the AC voltage from the secondary side(c.210VAC). If ok, then proceed to measure the DC voltages after the diode(256V), then after R1(235V), R3(210V). Those numbers are somewhat what you should get. That way you`ll find out where the faulty component is. I would guess one of the filter-caps is leaking. Si1 iws the fuse.
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The thing on top of the tranny is an inline filter. Not really necessary in my opinion, but if it aint broke dont fix it. Do you have filament voltage?
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Thanks gentlemen. I measured 20VAC at the transformer output to GR2. Then as I was probing for the DC voltages I heard a “click” and the lamp came on and DC voltages were looking normal.
Possible reasons:
Bad solder joint somewhere?
Perhaps running it with no load for a while had an effect? I know the voltages are much higher with no load, so maybe broke through a cold joint?
The rectifier Gr2 does have a big crack in the top of it’s blue casing, but I’ve read that’s fairly common.
Thanks for the info about the filter on top of the xformer. (F1. Makes sense!) Any chance that could be the problem? Like I said, the lamp wasn’t coming on either, and isn’t the lamp just connected to line voltage (through this filter)?
Thanks again.
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I am sorry to be late to the party, as I had not been monitoring "Page Two" of my own damn forum, because I thought Page One indicated no new posts (that issue has now been addressed!)
Don't worry about the crack in NU67 rectifiers. They all have it, more or less, and all work just fine.
As to suddenly no juice from an otherwise healthy looking and functioning NU67:
The power switch can get intermittent through the years, and sometimes does not make proper connection between stationary and moving contact points.
I have more than once took the switch apart, cleaned its contacts from oxidization and lubricated it before reassembly. Everything will work fine again.
An interim solution: unplug the supply form the wall, and activate the power switch 20 times.
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Thanks Klaus! Must have been something like that. Hasn’t happened again since. I’ll exercise the power switch for good measure.
(Nice to have you back and to be on page 1 again!)