Graham ,

I get a stream of them in with PSUs that shut down, blow fuses, etc (current
a pair of '142s'). They all seem to suffer from extreme temperture from the
2 x 3T regulators on that little U-shaped heatsink rod. No osciallation,
and load seems normal per channel-strip (those little shorted yellow caps
+17 to -17 is a different story).

Is it 'normal' that the 3Ts run the heatsink almost too hot to touch, or is
there something more obscure causing this, AC secondary comes out 22v as it
should, so it's not that. Surely they don't all run that hot ?

geoff

Geoff Wood wrote:

Graham ,

I get a stream of them in with PSUs that shut down, blow fuses, etc (current
a pair of '142s'). They all seem to suffer from extreme temperture from the
2 x 3T regulators on that little U-shaped heatsink rod. No osciallation,
and load seems normal per channel-strip (those little shorted yellow caps
+17 to -17 is a different story).

Is it 'normal' that the 3Ts run the heatsink almost too hot to touch, or is
there something more obscure causing this, AC secondary comes out 22v as it
should, so it's not that. Surely they don't all run that hot ?

Curious that you mention this so long after they went on the market. The
heatsink does run fairly hot but never heard of a 'stream' of them causing
trouble.

Possibly might be a bad batch of voltage regulators shutting down prematurely.
That wouldn't explain blown fuses normally though.

Technically, the regulators will operate OK at heatsink temps of around 100C
would you believe ? Ahhh - but there are insulating washers are there not ?
Maybe our Indian friends skimped on the thermapath ? That would explain it ! As
would poor metalwork ( burrs - flatness ), preventing a decent thermal
interface.

The company that made that product for us gave us endless headaches. They still
make some of those models ( original Studiomaster designs ) for other companies
now too.

The psu caps you refer to are the 0.1uF ceramic multi layer types I guess. Turns
out that some don't like their rated voltage in that application. I would
suggest replacing with 100V parts when required.


Cheers, Graham

Pooh Bear wrote:
Geoff Wood wrote:

Graham ,

I get a stream of them in with PSUs that shut down, blow fuses, etc
(current a pair of '142s'). They all seem to suffer from extreme
temperture from the 2 x 3T regulators on that little U-shaped

Curious that you mention this so long after they went on the market.
The heatsink does run fairly hot but never heard of a 'stream' of
them causing trouble.

3 in 2 weeks. And intermittantly but regularly over the last few years. May
be a component aging thing, or maybe they used crap thermal compound that
loses something. Washers appear to be mica.


geoff

"Geoff Wood" -nospam wrote in message

Graham ,

I get a stream of them in with PSUs that shut down, blow fuses, etc
(current a pair of '142s'). They all seem to suffer from extreme
temperture from the 2 x 3T regulators on that little U-shaped
heatsink rod. No osciallation, and load seems normal per
channel-strip (those little shorted yellow caps +17 to -17 is a
different story).

Is it 'normal' that the 3Ts run the heatsink almost too hot to touch,

Too hot to touch for very long is only about 125 degrees F, which is nothing
at all for a 78xx or 79xx chip to handle. You can make crude but sometimes
helpful measurements of the temps of electronic parts with those little
dial thermometers that air conditioning technicans use. They're under $10 in
car parts stores in the US, don't know about the UK.

or is there something more obscure causing this, AC secondary comes
out 22v as it should, so it's not that. Surely they don't all run
that hot ?

For reference purposes, Rane runs the power supplies in most of their signal
processing gear from a 18 V transformer via a voltage doubler running into
the regulator. Last time I measured the DC going into one it was over 30
volts with 120 vac line voltage going in. The chips run hot, but IME they
typically last a few decades, at the least.

http://www.rane.com/pdf/old/pe15sch.pdf page 2.

Later production uses 317/337 chips instead of 7815/1915, but for this
question, same diff.