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Author Topic: Nu67 PSU troubleshooting  (Read 345 times)

afterlifestudios

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Nu67 PSU troubleshooting
« on: July 10, 2020, 11:15:38 am »

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 itís good. Opened up the PSU to drain the caps but when I measured them, they werenít holding any stored charge.  Powered it on and Iím only seeing like 7VDC on the B+.

Am I to suspect the power transformer?
Also, what is the function of the large ďdoubleĒ capacitor mounted on top of the xformer?  I donít 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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panman

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Re: Nu67 PSU troubleshooting
« Reply #1 on: July 10, 2020, 04:33:12 pm »

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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Esa Tervala

RadarDoug2

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Re: Nu67 PSU troubleshooting
« Reply #2 on: July 10, 2020, 08:31:27 pm »

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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afterlifestudios

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Re: Nu67 PSU troubleshooting
« Reply #3 on: July 11, 2020, 11:06:33 am »

Thanks gentlemen.  I measured 20VAC at the transformer output to GR2.  Then as I was probing for the DC voltages I heard a ďclickĒ 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 itís blue casing, but Iíve read thatís 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 wasnít coming on either, and isnít the lamp just connected to line voltage (through this filter)?

Thanks again.
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