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.

[zeno]

If you can post the PS schematic it would help.

Remembering a few 27" sets that shorted the zener turned out to be
the opto isolator wasnt getting B+. I do remember doing a few of these sets but
not the PS. They were great sets so worth the effort.
As for the diode also look for it under the LONG Panny part ##. Buy spares !

Zeno

[jr_tech]

Perhaps the bay?

https://www.ebay.com/sch/i.html?_fro...diode&_sacat=0

jr

[zeno]

https://www.ebay.com/itm/17476015291...Bk9SR8K-herXYg

Looks like the best bet but dont zorch it !
Others are stupid $$$

Zeno

[MIPS]

Originally Posted by zeno

If you can post the PS schematic it would help.

https://archive.org/download/Panason...c%20AG-500.pdf

Also those ebay listings are stupidly priced. I'm not paying $15 shipped for a part that's 88 cents. The plan is I'll assemble the BOM for a friend in Seattle and he'll place and have the order shipped to his door. I have to be in Seattle next weekend for a computer show anyways, so I just bring the TV along, pay him for the order, do the repair on his bench and go from there and if that pops the new diode, at 88 cents each it doesn't hurt to buy 10.

[MIPS]

Whoever said it's not just a diode is right and wins a cookie.
I did a suicide test and with the diode unhooked I ran it line voltage with a foot pedal to briefly power it.
We got high voltage, too much in fact. B+ peaked at 190v on the meter, so the power supply is not regulating and that is why the diode went.
(also after this test I checked the HoT and the spike didn't kill it)

I'm looking all over the primary side and the feedback area of the power supply. I see no drifted resistors, no shorted capacitors or diodes and the B+ adjustment seems to not be open. No obvious sign as to why it wouldn't be regulating.

[MIPS]

Okay I went over all the traces on the power supply and could find no indications that a cold joint or a cracked trace was causing the feedback circuit to not work.
I was able to locate and isolate the PSU from the rest of the set by desoldering and removing a single red wire immediately after out failed diode (not on the schematic but marked "a" on the PCB) and subbed a load of about 45w using 8 100ohm 25W resistors in series.
With that I can see the B+ adjustment pot is completely unresponsive, but the pot itself is not bad.

[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).