On Wed, 2008-11-05 at 22:20 +0000, n2ouv@optonline.net wrote:
> Hello to the list,
>
> My 962 died the other day while transmitting on 80. Unit just shut right off.
> Found 5 amp MDL blown and replaced it. Upon power up it blew again.
> Opened the unit up and tested the Bridge Rectifier and found it to have
> shorted junctions.
>
> I got the replacement mouser p/n from a previous post on the list from
> someone who had a blown bridge.
>
> My question is what would take the bridge out like this? I am wondering if
> the pass transistors could be shorted or a filter cap shorted out.
>
Could have been a pass transistor shorted, then the crowbar tripped to
protect the radio which shorted the rest of the supply and the rectifier
was faster than the fuse. An MDL fuse has time delay, overloaded
semiconductors don't, so the blown fuse is a reliable indicator of a
solid state problem, not protection. Unless the filter capacitor turn on
charge current is too much, AGC-5 fuses might protect the rectifier
marginally better. But AGCs are not all that fast either.
As has been noted, filter capacitors are also suspect, especially after
having been fed AC from the shorted rectifier. Or it could be that the
rectifier was shorted by a line voltage transient. Using a bridge with a
higher voltage rating can make that less likely and wiring a paper or
disc ceramic capacitor from AC to AC at the bridge can help absorb
transients.
> Any advice on troubleshooting this supply further would be greatly
> appreciated. Its a heavy sucker and I'd hate to send it back to TT for repair.
>
You need a VOM or VTVM or digital meter to check for shorted parts, like
pass transistors, driver transistors and rectifiers as well as filter
capacitors and binding posts if any.
> 73,
> Joe N2OUV
73, Jerry, K0CQ
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