H7861 PSU issues

Rob Jarratt robert.jarratt at ntlworld.com
Thu Nov 2 10:14:10 CDT 2017

In my case the PSU where I had this still does not work. It is a H7140 from
a PDP-11/24. I found the failed 555 after help from a list member, but when
I replaced it and powered on there was a big bang (I should have told you
that!). I have never been able to repair this PSU and I need someone who
really knows what they are doing to repair it as I think it is beyond me.



> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Aaron
> Jackson via cctalk
> Sent: 02 November 2017 14:23
> To: cctalk at classiccmp.org
> Cc: jnc at mercury.lcs.mit.edu
> Subject: Re: H7861 PSU issues
> Thanks! It was very satisfying and not the worst thing to go wrong for a
> beginner.
> I was also quite surprised that such a simple component would die, and
what I
> find more confusing is that it died while I was using the machine.
> Aaron.
> Noel Chiappa via cctalk <cctalk at classiccmp.org> wrote:
> >     > From: Aaron Jackson
> >
> >     > Picked up a few 555s and sockets and now it works!
> >
> > Congratulations!
> >
> > It's odd that a 555 failed, but sometimes there's no rhyme or reason
> > to what fails. E.g. I was fixing some broken M7859's (KY11-LB
> > Programmer's Console), and on one of them a 7493 (4-bit counter) had
> > died. That's not one of the 'problem' 74xx chips, like ISTR the 7474
> >
> > 	Noel

More information about the cctalk mailing list