29 Aug
2014
29 Aug
'14
12:40 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Chuck McManis
Sent: 28 August 2014 23:09
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
If it actually made a 'bang' sound the most likely culprit is an electrolytic
capacitor. A visual inspection should find it, although I've known them to fail
pretty amazingly leaving the case intact except for the circle on the top where
the plates came flying out and landed somewhere remotely.
The system is waiting for P_OK (power OK) before it will fire up, and the fans
are on the non-mains side of that PSU as I recall. Is it the BA23 case? (most
common case) there was an issue with the power distribution harness on those
where DEC used several wires in parallel to carry the 5V current. Except they
were not all the same length and the short one would some times vaporize.
The 'fixed' harness you can see all the wires are the same length which makes
it harder to tuck in behind the back plane but it doesn't blow up.
Yes, it is a BA23 and I do have the "bad" wire harness. In fact I think all my
BA23 machines have the bad one, but I don't run them that often, and they are not
particularly loaded, so hopefully that will be OK.
The fans are powered from the half of the power supply that contains the input filter and
the rectifier board. The bulk of the power supply is in the other half and does not appear
to have any involvement with the fans. As the fans don't work, this would suggest that
the fault is either on the input filter board or the rectifier board.
Is it safe to connect up the first half of the PSU to the mains, switch it on and try to
probe around to see if I can see where the fault lies?
Regards
Rob
--Chuck
On Thu, Aug 28, 2014 at 1:19 PM, Robert Jarratt <robert.jarratt at ntlworld.com
wrote:
>
>
> > -----Original Message-----
> > From: cctalk-bounces at classiccmp.org [mailto:
> cctalk-bounces at classiccmp.org]
> > On Behalf Of Matt Burke
> > Sent: 28 August 2014 20:07
> > To: cctalk at classiccmp.org
> > Subject: Re: Mysterious rtVAX 1000 Failure
> >
> > On 28/08/2014 18:53, Robert Jarratt wrote:
> > > There are two outputs from the PSU that I don't fully understand,
> > > one is P OK, I am guessing that means *something* is OK, but not
> > > sure what, power perhaps, but what power?. The other is called
> > > LTC, I have no idea what that one means. Can anyone enlighten me?
> >
> > 'P OK' is indeed power OK. This signal will be asserted once all the
> power
> > supply outputs have reached their normal operating voltage. The Qbus
> devices
> > will be held in reset until this signal is asserted.
> > 'LTC' is the mains derived line time clock (50hz here in the UK).
> >
> > Have any of the internal fuses (or circuit breakers) blown? Don't
> > replace
> then
> > until you find the real problem. I would probably start by checking
> > the
> bridge
> > rectifier and main switching transistor for shorts.
> >
> > Matt
>
> I can't find any fuses. There is a circuit breaker, but it doesn't
> seem to have tripped (it would be sticking out, right?). I notice that
> the light in power switch comes on when I turn it on, if it was the
> fuse/circuit breaker, I expect even the switch's light would not come
> on.
>
> There are a number of candidates for the main switching transistor, I
> think.
> Is there a printset somewhere? I have never been able to find one, and
> it would help in identifying components.
>
> Regards
>
> Rob
>
>