11/73 (ba23) bringup after 12 years in deplorable storage conditions

Robert Jarratt robert.jarratt at ntlworld.com
Sat Nov 22 17:57:16 CST 2014

> -----Original Message-----
> From: cctech [mailto:cctech-bounces at classiccmp.org] On Behalf Of Jacob
> Ritorto
> Sent: 22 November 2014 22:33
> To: General Discussion: On-Topic Posts
> Subject: Re: 11/73 (ba23) bringup after 12 years in deplorable storage
> conditions
>   Thanks for the replies.  Hope to remember enough about electronics and get
> enough guidance to be able to use the diagrams you've made, Rob.

Please be aware that the diagram I did is only for the part where I have had a failure (Q301), and the diagram is not drawn logically (because my understanding on SMPSs is limited. I also keep finding mistakes, in fact I had to re-upload it again a short while ago.

> Okay, I opened the power supply and checked for corrosion (none at all),
> worked back and forth all of the push-on connectors thoroughly and replaced
> the PSU into the chassis.  I don't yet understand where to insert the resistors,
> but I guess anywhere there's 12v and 5v would be sensible.
>    So now, with no boards at all in the chassis, I do at least get a few lights on
> the little front panel but unfortunately not the "DC OK" light and still no fans.
> To get them to stay on, I have to kind of 'trick' the PSU by turning it off, waiting
> a second, then turning it back on.
>   While I'm not stickler for perfect authenticity, I do respect it to a degree.  So
> I'm wondering but am hesitant to ask: Would it be more sensible to just buy a
> commodity PC power supply that has lots of 5v and some 12v, find a way to bolt
> it in and rework the wiring to fit?  Or  do you think it's likely I'll be able to bring
> this power supply back to safe, reliable condition without getting in over my
> head?

Seems like yours is working better than mine....

It is very easy to get sucked in, this has happened to me, but if you want these things to work I think you just have to bite the bullet.

I am hesitant to make suggestions giving my limited knowledge, but if you can use a dummy load (use some motor vehicle light bulbs) and then use an oscilloscope to check for ripple on the outputs that might begin to help. Be careful where you connect the scope though as you can damage it if you try to trace back too far with it. Probably best to use it only on the outputs. Also I am sure you are aware that there are lethal voltages inside PSUs, so be careful and make sure any big capacitors are discharged before touching anything.

>   Suggestions on where to begin tracing the thing would be most welcome,
> please and thanks!
>   It's been some decades since I've dealt with electronics at this level of detail
> and in the interim, someone has made off with my nice fluke meter, so I don't
> even have anything to check volts with at the moment.  Going to find another
> now.  Got lots to re-learn here; thanks for bearing with..

I have found that I have had to re-learn the little I knew, and I am learning more and more. I have found that over time I have had to buy more and more equipment. One of the biggest lessons for me is to invest in reasonable equipment. I only just bought a desoldering station, I wish I had done this a long time ago!!



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