Hello Roy,

 RM> We had a gem last week. Converter blowing its fuse. Easy, I
 RM> thought, leaky 3055s. No load checks came up with nothing, it
 RM> behaved fine even when heated up. It only tripped when a load of a
 RM> few amps was applied. Odd.

Yair, an unusual one...

 RM> Puzzlement. Scratched collective heads. Then someone noticed that a
 RM> whistling note zipped across the workshop AM radio as the load came
 RM> on. Grabbed the cro, lo and behold there's about 5 volts p-p rf on
 RM> the output, and 12 volts on the input. The thing was a better
 RM> bloody transmitter than the radio it was feeding.

I've seen a few regulators "take off" like that, but never one
of those 24-12 things.

One gizmo I built years ago gets used whenever I suspect that
a DC supply is taking off - it's a little box with four wires
that goes between the supply and a multimeter. Contains basically
a small series cap and a "voltage doubler" in series with the
red wires, the two blacks are ground.  Shows any oscillation
on a DC line very well...

 RM> Final cure was a teensy little .01uF across the collector-base
 RM> of the TIP driver.

Was there one fitted in that place originally that had maybe
gone open?  If not, it sounds like a "modify the circuit to
conceal the real fault" type senario, something I try and avoid
here if practically possible.

 RM> Total time spent exceeded the cost of a new one. Customer got
 RM> charged a lot less, of course (we'd like to keep him...)

Always the way, isn't it? :-)

 RM> Slight topic change - I'm looking at stocking some transistors to
 RM> replace the 3055s in these things, but the good looking ones are
 RM> pricier enough to be noticeable in the bill when four have to be
 RM> replaced.

I haven't had to replace any in ages - it's simpler and often
cheaper to replace the whole converter.

There are some nice Motorola (from memory) 20A TO3's that can be
had from a few suppliers (and even DSE at a vastly inflated price),
but after replacing four '3055's with them, and adding labor costs,
you're above the price of a new converter...

Regards,
Chris.


--- GoldED