PDP 11/24 - A Step Backwards

Rob Jarratt robert.jarratt at ntlworld.com
Thu Mar 31 01:16:56 CDT 2022

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Tony Duell via
> cctalk
> Sent: 31 March 2022 04:26
> To: Noel Chiappa <jnc at mercury.lcs.mit.edu>; General Discussion: On-Topic
> and Off-Topic Posts <cctalk at classiccmp.org>
> Subject: Re: PDP 11/24 - A Step Backwards
> On Thu, Mar 31, 2022 at 3:52 AM Noel Chiappa via cctalk
> <cctalk at classiccmp.org> wrote:
> > If the machine then runs, it's up to you as to whether you get the P/S
> > repaired so that ACLO work properly - your call. (I wonder how the
> > -15V got to ACLO - I suspect a solder bridge from the prior repair -
> > but knowing the answer is not important to getting the machine
> > running.)
> A short in FET Q15 on the bias/interface board in the PSU could do it.
> The gate of that FET is driven from an LM339 comparator the -ve supply of
> which is -15V.

Yes I found that on the schematic and surmise that this is what has failed.

> Of course _why_ Q15 failed, if it has, is another matter.

Yes, a bit concerning, and as Noel says maybe this has damaged something else, or that something else on the CPU caused Q15 to fail (if indeed it did). Perhaps I should follow Noel Chiappa's suggestion and disconnect ACLO, DCLO and LTC, they are all on the same connector and see how far it gets. If that works then I think repair ACLO and see if anything on the CPU is bad or anything else that might cause a short on the ACLO signal of the bus.

> -tony

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