Panasonic AG-500 No Power

[MIPS]

Brought it out to clean and prep for a demo and I had no power to the VCR half or B+ to the CRT half. It did work when it was put away. Audibly though I can hear a high speed buzzing, so something was alive.

Schematics are here. Page 58, lower left corner.
https://archive.org/details/PanasonicAG500/mode/2up

I can see AC in, the fuse is good, the power switch is responsive, the degauss thermistor gets warm, I get DC rectification and there's around 160v on C9805 (there's a 220uf 200v on the board but the schematic shows 220uf 450v) and the buzzing seems to originate from Q9801, which means it's possibly running and receiving feedback from primary taps F1 and B1. I don't have a scope available at the moment.

I am however seeing nothing between S1 and S2, S3 and S4. The fuse is not open.
To me that screams an open winding on the secondary side. I went across the caps on the rest of the PSU and the rest of the chassis and found nothing too far marginal.

[zeno]

Cant work with manual SO.....
If its a SMPS there is usually a short. Often the Hoz out or a 150 V
zener diode from main B+ to ground.
To keep from grenading when the SMPS sees over current it drops in
freq & screams or ticks. If the zener is shorted often the SMPS is
running wild & putting out too much V.

73 Zeno
LFOD !

[Alex KL-1]

Also, in more rare cases, a secondary diode shorted (secondary of SMPS trafo).

[MIPS]

That looks like it might be it.
D9812 (R2KN) is reading a dead short. The other two voltages generated by the PSU head off-board to the VCR and I'm not finding anything else downstream that's shorted. I would of expected that to blow the fuse. I don't have one on hand but I can try to source one in short order.

[damen]

That R2KN is a protection diode I believe. It sacrifices itself if the B+ is too high. Check the caps in that SMPS circuit, especially the low value ones.

[MIPS]

I've gone over the entire board and found two .01's in the audio circuit that tested poor and C514 (1uf 350v) also didn't test great but otherwise could find no other issues. I was going to replace those and the main secondary filter caps as preventative maintenance when I placed he DigiKey order but it turns out I can't easily source the diode. An exact replacement is not available, the NTE 570 equivalent is available, but DigiKey will not ship it to Canada.

[JohnCT]

I'd first check C9812 and C9522 (particularly 9522)- they're the bypass filters for the B+ but also bypasses the high freq signal generated by the flyback when the sweep is running.

A way to check this is to disconnect the horiz output transistor and sub in a resistive load. If the B+ now regulates, the bypass from the sweep is causing the loss of regulation.

John

[Alex KL-1]

After checking that, is good also to check the smaller reg. transistors; one can be inoperative (open/short).