[Kenwood] PS-53 on strike

[Avi on 7510]

Hi Peter,
There must be a power supply virus going around - yesterday I posted about my PS50 quitting.

I'm assuming the PS53 is also a linear supply.
The 19v you mentioned sounds like the output from the bridge rectifier, before the pass transistor(s? - I'm not familiar with the PS53 circuit diagram), and the slow decrease in voltage sounds like normal behavior for smoothing capacitors with a bleed resistor across them for safety - they can pack quite wallop for quite a while without a bleed resistor. It sounds like the regulator circuitry may be faulty, or perhaps the pass transistors themselves? 

Just a semi-ignorant guess....

Avi 4X6UA on htc advantage 7510

-----Original Message-----
From: Peter Ravn <fasan5 at privat.dk>
Sent: יום שני 03 מאי 2010 13:21
To: Kenwood at mailman.qth.net
Subject: [Kenwood] PS-53 on strike

Without warning my otherwise reliable PS-53 died the other day: no 13.8 V on the output terminals. The fuse was ok, so the board was inspected. Q5 (2SC2458) sitting close to thermistor TH1 seemed to have melted and fused with TH1, which was covered with sticky white powder. Further inspection and internet research proved that Q5 and TH1 are covered with a piece of shrink tubing to keep them in close contact with one another. I replaced the transistor and cleaned and checked the thermistor, which measured 2.4 KOHM with falling resistance by increasing temperature.

Still no voltage on the output terminals. But when the power switch is turned and off,  there are 19 V on the terminals falling slowly to zero, which might give the experts on this thread an idea where the fault is.

Thanks in advance
OZ8CTH
Peter
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