At 01:24 AM 21/12/2009, you wrote:

The Branson devices are ultrasonic plastic welders. There's a high
frequency power circuit that operates a piezo driver which gets coupled
through a custom fixture to the plastic to be welded.

I suspect those transistors from ca. 1983 are just power BJTs- probably
high voltage/current and reasonably fast (compared to 2N3055s anyhew).

These guys sell replacement transistors (no doubt at a handsome price
since those welders are worth thousands of dollars, even used)
http://www.bransonultrasonicwelders.com/showPage.php?page=buw500
I'd be curious to know how proud they are of their stock, and if the
parts if/when you order them have markings that indicate a different
part number.

But I agree with the person who suggested looking elsewhere first..
perhaps an electrolytic or perhaps the temperature sensor itself
(thermistors in particular can be quite unreliable). A flakey power
supply (due to an old 'lytic) could be causing it to shut down even
if there is no heat problem. Can you confirm whether it appears to
actually be getting excessively hot?

Use an ESR meter to check all the electrolytics (particularly any
big aluminum can units mounted in hot areas) and replace any dubious
ones.

I would *not* shotgun replace semis. Aside from stressed ones (eg. the
power output stage) they are unlikely to be bad, and they are unlikely
to cause the symptoms you describe in any case. Have the welding
characteristics changed recently (eg. requiring more time?).. that might
indicate a closer look at the output stage (could be half fried perhaps).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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