26 Nov
2015
26 Nov
'15
1:01 p.m.
Hi folks,
I've begun repairing my PDP 11/40 and have tracked down the first major issue to the
H745 (-15v) regulator.
Before I dig into it, I just wanted to see if there are any common known failure cases...
Otherwise I'll just dive into troubleshooting it. :)
Thanks as always-
- Ian
Sent from my iPhone
26 Nov
26 Nov
9:13 p.m.
I can only say that they fail often. We have replaced lots in
our 11/70. Luckily we have had spares.
Sometimes the failure has been rather catastrofic with blown
components. I think these modules run quite hot since
discoloured pcb:s are common.
/P
On Thu, Nov 26, 2015 at 01:01:07PM -0800, Ian Finder wrote:
Hi folks,
I've begun repairing my PDP 11/40 and have tracked down the first major issue to the
H745 (-15v) regulator.
Before I dig into it, I just wanted to see if there are any common known failure cases...
Otherwise I'll just dive into troubleshooting it. :)
Thanks as always-
- Ian
Sent from my iPhone
27 Nov
27 Nov
2:29 a.m.
On 2015-11-27 06:13, Pontus Pihlgren wrote:
I can only say that they fail often. We have replaced
lots in
our 11/70. Luckily we have had spares.
Sometimes the failure has been rather catastrofic with blown
components. I think these modules run quite hot since
discoloured pcb:s are common.
I think we more often see failures in the H744(0) modules, but we've had
failures in both, yes.
Hot is a good guess, I think.
Johnny
/P
On Thu, Nov 26, 2015 at 01:01:07PM -0800, Ian Finder wrote:
> Hi folks,
>
> I've begun repairing my PDP 11/40 and have tracked down the first major issue to
the H745 (-15v) regulator.
>
> Before I dig into it, I just wanted to see if there are any common known failure
cases... Otherwise I'll just dive into troubleshooting it. :)
>
> Thanks as always-
>
> - Ian
>
> Sent from my iPhone
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
2:59 a.m.
On Fri, Nov 27, 2015 at 11:29:32AM +0100, Johnny Billquist wrote:
On 2015-11-27 06:13, Pontus Pihlgren wrote:
My memory is fussy, I recall failures in both 744(0) and 745, but not
which one is more common. You are probably correct.
/P
I can only say that they fail often. We have
replaced lots in
our 11/70. Luckily we have had spares.
Sometimes the failure has been rather catastrofic with blown
components. I think these modules run quite hot since
discoloured pcb:s are common.
I think we more often see failures in the H744(0) modules, but we've had
failures in both, yes. 
8:48 a.m.
On 11/26/2015 11:13 PM, Pontus Pihlgren wrote:
I can only say that they fail often. We have replaced
lots in
our 11/70. Luckily we have had spares.
Sometimes the failure has been rather catastrofic with blown
components. I think these modules run quite hot since
discoloured pcb:s are common.
My experience with an 11/45 that ran 24/7 was the AMP
Mate-n-Lok connectors would get a poor contact, heat up and
go into thermal runaway, melting the plastic housing and
totally frying the contacts. I had to repair these at least
5 times while we had that machine. I never actually had a
regulator itself go bad, it was always the contacts. Maybe
we just got lucky. (This was also about 1977 - 1980, so
parts have probably deteriorated a lot more since then.)
Jon
5:01 a.m.
Hi folks,
I've begun repairing my PDP 11/40 and have tracked down the first major issue to the
H745 (-15v) regulator.
Before you dive into the brick (sounds painful!) check that the +15V DC input is present
and correct. Unlike
the +5V units (H744, etc) the H745 needs both the 20V AC from the transformer and a 15V
supply which
comes from either the PCB on top of the transformer (in an H742 supply used with the
BA11-F) or the PCB
under the transformer in the supply used with the BA11-K
If that's OK, then it's the usual suspects in the brick. The capacitors can dry
up. There is a fuse, but it
never fails for no good reason, so if it is blown you need to find out why. Power
semiconductors on
the heatsink bracket.
Whatever you do do not disable the overcurrent trip (e.g. by desoldering the collector of
the sense
transistor). If you do, and the crowbar fires then you will be cleaning bits of transistor
off the
ceiling. Do not ask how I found that out!
-tony
6 a.m.
On Nov 27, 2015 8:04 AM, "tony duell" <ard at p850ug1.demon.co.uk> wrote:
>
> Hi folks,
>
> I've begun repairing my PDP 11/40 and have tracked down the first major
issue to the H745 (-15v) regulator.
Before you dive into the brick (sounds painful!) check that the +15V DC
input is
present and correct. Unlike
the +5V units (H744, etc) the H745 needs both the 20V
AC from the
transformer and a 15V supply which
comes from either the PCB on top of the transformer
(in an H742 supply
used with the BA11-F) or the PCB
under the transformer in the supply used with the
BA11-K
If that's OK, then it's the usual suspects in the brick. The capacitors
can
dry up. There is a fuse, but it
never fails for no good reason, so if it is blown you
need to find out
why. Power semiconductors on
the heatsink bracket.
Whatever you do do not disable the overcurrent trip (e.g. by desoldering
the
collector of the sense
transistor). If you do, and the crowbar fires then you
will be cleaning
bits of transistor off the
ceiling. Do not ask how I found that out!
-tony
There is a pdp 11 troubleshooting guide on Bitsavers that covers the h742
powersupply and regulators 744. 745, 754. Assuming that is what you have.
They're all tough to work on especially the regulators but I agree with
Tony that you need to lock down the main suppl first.
That troubleshooting guide very useful just be prepared to completely
disassemble the parts.
Bill Degnan
twitter: billdeg
vintagecomputer.net
10:09 a.m.
There is a pdp 11 troubleshooting guide on Bitsavers
that covers the h742
powersupply and regulators 744. 745, 754. Assuming that is what you have.
They're all tough to work on especially the regulators but I agree with
Tony that you need to lock down the main suppl first.
I am not sure what you mean by 'tough to work on'. If it's a physical access
problem then what I did was to obtain a spare 8 pin mate-n-lock plug and
socket and make up a pin-pin wired extension lead about 1m long. Then I
can take the brick out of the PSU, connected it via the extension lead and be
able to probe it. I normally sit it on top of the CPU box of my 11/45, you get
easily enough air flow from the fans to run it for testing.
If it's that the circuits are complicated, well, remember these are swtiching
regulators (not linear) but are a lot more repairer-friendly than most SMPSUs
in that nothing in the brick is directly connected to the mains.
-tony
1:14 p.m.
On Fri, Nov 27, 2015 at 1:09 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:
Tough-sh to physically work on/test. These are not made for field service
IMHO, just swap out part.
--
Bill
There is a pdp
11 troubleshooting guide on Bitsavers that covers the h742
powersupply and regulators 744. 745, 754. Assuming that is what you
have.
They're all tough to work on especially the regulators but I agree with
Tony that you need to lock down the main suppl first.
I am not sure what you mean by 'tough to work on'. If it's a physical
access
problem then what I did was to obtain a spare 8 pin mate-n-lock plug and
socket and make up a pin-pin wired extension lead about 1m long. Then I
can take the brick out of the PSU, connected it via the extension lead and
be
able to probe it. I normally sit it on top of the CPU box of my 11/45, you
get
easily enough air flow from the fans to run it for testing.
If it's that the circuits are complicated, well, remember these are
swtiching
regulators (not linear) but are a lot more repairer-friendly than most
SMPSUs
in that nothing in the brick is directly connected to the mains.
-tony
28 Nov
28 Nov
3:40 a.m.
[H745 regulator]
Tough-sh to physically work on/test. These are not
made for field service
IMHO, just swap out part.
If you make up the extension lead so you can run it outside the PSU chassis then
it is very easy to work on. The PCB is exposed on one side of the brick and as
I said it's all isolated from the mains.
-tony
27 Nov
27 Nov
1:44 p.m.
Yep, I've already checked out the rest of the voltages thru the machine. The AC inputs
are correct, +15 is good, +5 is a little high at 5.5v but workable and I'll tweak it
later, -5 is good, etc.
Just the darn 745 module. Fuse and connector are okay as well, no obvious signs of
catastrophic failure.
Sent from my iPhone
On Nov 27, 2015, at 05:01, tony duell <ard at p850ug1.demon.co.uk> wrote:
Hi folks,
I've begun repairing my PDP 11/40 and have tracked down the first major issue to the
H745 (-15v) regulator.
Before you dive into the brick (sounds painful!) check that the +15V DC input is present
and correct. Unlike
the +5V units (H744, etc) the H745 needs both the 20V AC from the transformer and a 15V
supply which
comes from either the PCB on top of the transformer (in an H742 supply used with the
BA11-F) or the PCB
under the transformer in the supply used with the BA11-K
If that's OK, then it's the usual suspects in the brick. The capacitors can dry
up. There is a fuse, but it
never fails for no good reason, so if it is blown you need to find out why. Power
semiconductors on
the heatsink bracket.
Whatever you do do not disable the overcurrent trip (e.g. by desoldering the collector of
the sense
transistor). If you do, and the crowbar fires then you will be cleaning bits of
transistor off the
ceiling. Do not ask how I found that out!
-tony
3312
days inactive
3314
days old
10 comments
6 participants
participants (6)
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