34A Power Trouble. I calmed down, here's the facts...

From: Daniel A. Seagraves <dseagrav_at_bsdserver.tek-star.net>
Date: Tue Nov 11 11:42:25 1997

On Tue, 11 Nov 1997, Tony Duell wrote:


> J5 carries the wires to the 2 primary winding on the mains transformer in
> the PSU. On 230V units the 2 windings are linked in series, on 120V units
> (like yours, I guess), in parallel.

Okay, that big transformer in the top of the PSU?

> With J5 unplugged, the PSU isn't going to do anything. The only bit of
> circuitry that is then powered up is the little PSU that operates the main
> relay in the control chassis.


> The fact that the breaker trips when the transformer is plugged in doesn't
> tell us much (other than there's something wrong in there). Obvious
> problems would be :

> 1) A short somewhere on the secondary side (in one of the regulators,
> say). This will obviously cause too high a current to flow in the primary
> circuit

How do I check that? I've never played with voltages over 24V before.

> 2) A defective mains transformer (unlikely, and pray that it's not)

It probably isn't, this worked fine until I broke it.

> 3) You've reversed one of the primary windings (swapped over leads on J5).
> I assume you've not fiddled with this

I may have. I had to unplug the breakers to replace them.

> 4) Did you fit a new breaker? The mains transformer takes a high surge
> current at switch-on. The original breaker was a time-delay one - is the
> new one of a similar type.

The breakers came from a VAX PS, DEC model # 70-18763-00

> What happens if you remove all the loads from the secondary of the
> transformer (pull out the 8-pin plug from the distibution pcb that carries
> 8 black wires, and unplug the edge connector from the 15V PCB in the PSU)?
> Will the breaker hold then with J5 in place?

I'll check that.

> Did you get my earlier message about section-isolating this fault? I have
> the BA11-K prints at home, and can talk you through the entire PSU if
> necessary, but at least let's try to find out which bit is faulty.

Hmm. My boss used to work on big PSUs in CB radio linears, if you could
scan the schematic or something he could troubleshoot this...

> What did you disconnect? If you think you may have swapped wires, let us
> know what you fiddled with _now_ rather than later...

I detached the 4 wires attached to the breakers.
I detached the 2 line wires last so as to not swap them, but I may have
Received on Tue Nov 11 1997 - 11:42:25 GMT

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