At 09:13 AM 09/08/2012, you wrote:
>Thanks for the interest in the thread.
>I believe the fellow the equipment belongs to would prefer that I simply
>restore the supply to original condition.
>He had the problem and also had backup equipment to take the pressure
>off so I could take my time. I prefer switchers for my own use. Early
>switchers were noisy in amateur radio service and linear supplies are
>still popular as a result. I felt the 2.2 ohm resistors in the base
>circuit of the paralled pass transistors should be replaced due to the
>extended use in overload conditions. I am waiting for an EBAY order to
>arrive so I can take that step.

I hope you did not buy replacement transistors on eBay. There
are many grossly sub-standard counterfeit power transistors out
there, and I would deal only with a proper franchised distributor
on such a part.

Eg.
http://sound.westhost.com/fake/counterfeit-p1.htm


>I may have bought into a problem there
>because I ordered 3 watt metal film units to replace 2 watt Wire
>wound's. The failure of one of those kept the general failure from
>potentially causing a big smoke event.  The primary fuse is not enough
>(IMHO) for full safety protection due to its normal load requirements.
>
>When I get the parts all back in place I plan to verify that the Pass
>transistors are sharing the load across the operating range and tidy up
>the internal wiring before returning it to the owner. I believe the
>power supply will outlast the owner & I both. Apparently the 17 volts or
>so that occurs with a shorted pass transistor is not considered enough
>to damage connected equipment. A crowbar on this power supply would
>likely have wiped out more components in the supply. A better idea would
>be a circuit that disconnects the load when the source is out of spec.
>
>I have not approached the matter of what to use for a test load yet.
>Probably water-cooled wire.

We were using various kludges to test power supplies and batteries,
and finally broke down and bought a couple of electronic loads.

Left-over steel strapping (as on crates) can sometimes be pressed
into service, though brass has a better tempco.

Best regards,

--=20
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