Date   : Mon, 17 Aug 2009 17:05:40 +0200
From   : anders.carlsson@... (Anders Carlsson)
Subject: Power supply +5V problem revisited

Hi,

Today I had some spare time to look into one of my two BBC B's, the one
with a broken power supply. Based on previous observations and advice,
the Q2 BU208A transistor was to suspect: power supply silent, no smoke,
reads 5V unloaded but only 0.8V with load.

I replaced the case transistor and mounted it back together. Now the
cassette + caps LEDs slowly light up, but nothing else happens - no beep,
no video signal. I still read 5V when not connected but about 3.2V when
leads are connected.

To ensure the motherboard is not dead, I borrowed the PSU from my other
Beeb. The computer boots up immediately (although with a Language? error,
but I'm sure that is easier to solve once the PSU is working).

On the power supply I was working on, a few diodes and resistors seem
missing, even looks like they were cut off once. But since that supply
used to work nicely for a while, I suppose those are parts not required.
I didn't check in detail how the other supply looks like, if it also has
what seems like parts missing.

What can I have done wrong? Is there any way to test a BU208A outside of
its operational environment? I bought a two pack so I might be able to
solder in the other one instead but since they came from some Chinese
seller on eBay, for all I know they could be dead or fakes. It says
Toshiba Japan but that is very easy to fake.

Best regards

-- 
Anders Carlsson