[Kenwood] PS-40 has me at wits end!

[Jeff Kershaw]

Hi all,

I been fooling with this thing for a couple of weeks off and on.

The supply will make about 15v with no load, meaning that I have removed =
the internal load resistor.  With just that load, I get 8 volts and with =
any type of external load, 5.1v +/- .05 or so.  The voltage fluctuates =
on my 'scope by .2v or so every 2 seconds, going from 5.1v to 4.8v every =
2 seconds.

As a possible additional clue, the supply will not start faithfully =
every time. There is a barely audible buzz, perhaps 150hz or so =
(guessing by ear). =20

So far, I have changed the switching transistors, the diodes across the =
emitter/collector junction at those transistors, the output diodes, the =
680uf e-caps after the bridge, the bridge itself, the tl494c. I have =
tested all the transistors with a transistor checker, all the diodes =
with the Fluke meter in the diode check position, removed the caps on =
the output one at a time (no local subs avail). =20

It is acting like an over current situation is existing so I =
disconnected the output circuit from the junction of the output diodes, =
bypassing the output filtering and measured the current flow, thru my =
"Dummy Load" (a muffin fan) and found about 30 mills or so when the fan =
was running and 0 without.  I have played with inputs on both of the =
error amps at the chip using both my resistor sub box and a variable =
power supply. All I can manage to do there is cause a protection fault.

I don't need this supply so trashing it would not be an option, I bought =
it cause I was sure that I could fix it.  Besides after countless hours =
troubleshooting and over $50 in parts, I coulda bought two of em by now =
hihi.

I have posted the schematic @ http://kj6fd.freeservers.com/PS-40.pdf =
(actually the whole manual as provided by Kenwood).
If someone would be willing to look at it.  I sure could use the help =
before I begin mumbling and drooling because it has driven me over the =
edge.

Thanks

Jeff KJ6FD


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