H786 power supply help wanted

[Charles]

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