H786 power supply help wanted

Aaron Jackson aaron at aaronsplace.co.uk
Wed Apr 3 17:20:49 CDT 2019

Glad the 23 PSU seems to be working again.

My H7861 had a fault a couple of years ago where the 12V rail stopped
working. It turned out to be a dead 555 timer, totally
flat-lined. Perhaps it would be worth checking that in your H7861 if it
is the MJE15030 that is blowing. At least it's a nice power supply to
work on. :)


Charles via cctalk writes:

> Update: I removed the H786 from the chassis, and set it up on the workbench with loads on the +5 and +12.
> No output. 320V across the half-bridge, but no +12 Startup. Found I had forgotten to put a cliplead to the primary of the startup transformer.
> Turned it on and it works... 5 and 12 volts into 1 ohm and 4.7 ohm respectively. WTF.
> So I disconnected the PC supply, put the H786 back in, and it fired right up (including the real-time clock) and I ran it for half an hour.
> Go figure. Ain’t classic computers fun sometimes...
> From: Paul Anderson
> Sent: Saturday, March 30, 2019 3:26 PM
> To: Charles ; General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: H786 power supply help wanted
> Hi Charles,
> The H786 was standard for the 11/23, BA11-N. The H7861 was for the 11/23+, BA11-S and had afrw more amps of +5 volts.
> I would start by looking at the electronic caps.
> I have a few extras here but am pretty busy for the next week or so.
> Paul
> On Sat, Mar 30, 2019 at 2:25 PM Charles via cctalk <cctalk at classiccmp.org> wrote:
>   I have a PDP-11/23+ and the power supply (H786) "last ran when parked" a
>   year or so ago. But there's no DC output at all today, and the fans are
>   running so there is AC power...
>   I also have the original H7861 that came with it, which had a blown chopper
>   transistor. I couldn't find anything else bad, so I replaced the transistor
>   and within a few seconds of running, it blew again. :(
>   So I need some help - I've never been good at fixing switching supplies, not
>   to mention the high-side hazards.
>   The simplest solution would be just to replace it with a working unit.
>   Anyone got one to sell, hopefully cheap? :)
>   If not, can anyone fix one or both of mine?
>   thanks!
>   Charles

Aaron Jackson - M6PIU

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