28 Mar
2020
28 Mar
'20
6:28 p.m.
Does anybody have a maintenance print or service manual for the DEC H7874?
This is the power supply used in the BA4xx and R400x cabinets. As you might
guess, I have one that tries to power up but shuts down after a second.
Probably a bad capacitor (or several), but this thing is ridiculously
complicated and not all that easy to disassemble, either. I'd like to be
able to trouble shoot it rather than just firing the proverbial parts cannon
at it. FWIW, none of the electrolytic (of which there are many) have
obviously failed - no leaking, no bulges, etc. Of course, that proves
fairly little.
Thanks,
Bob Armstrong
28 Mar
28 Mar
11:39 p.m.
-----Original Message-----
From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
Armstrong via cctech
Sent: 29 March 2020 02:29
To: cctech at classiccmp.org
Subject: H7874 power supply
Does anybody have a maintenance print or service manual for the DEC
H7874?
This is the power supply used in the BA4xx and R400x cabinets. As you
might
guess, I have one that tries to power up but shuts
down after a second.
Probably a bad capacitor (or several), but this thing is ridiculously
complicated
and not all that easy to disassemble, either. I'd
like to be able to
trouble shoot
it rather than just firing the proverbial parts cannon
at it. FWIW, none
of the
electrolytic (of which there are many) have obviously
failed - no leaking,
no
bulges, etc. Of course, that proves fairly little.
This PSU is a real problem, as you say, complicated and hard to disassemble.
I had one that didn't work, if I remember correctly in the end I had to get
another one. I have a second machine with this PSU which has an intermittent
problem and shuts itself down after a few minutes if it hasn't been used for
a while, but a subsequent power up sees it work for as long as I need it.
It looks like I reverse engineered the schematic of the 12V output board. I
don't know if that would be of any use? How correct it is I don't know.
Regards
Rob
Thanks,
Bob Armstrong
29 Mar
29 Mar
3:30 a.m.
Rob Jarratt via cctech wrote:
I would be interested. I have an VAX400/300 with such an working again
PSU, Changed Capacitors two years before..now it works. A friend gave me
another PSU with a similar fault, changed capacitors as before on the
other one..but no luck at all. It still powers down shortly after
switching on. Yes..thats the most ugly PSU Design I ever saw..and I'm
interested in schematics or parts of them..
Regards,
Holm
--
Technik Service u. Handel Tiffe, www.tsht.de, Holm Tiffe,
Freiberger Stra?e 42, 09600 Obersch?na, USt-Id: DE253710583
info at tsht.de Fax +49 3731 74200 Tel +49 3731 74222 Mobil: 0172 8790 741
-----Original Message-----
From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
Armstrong via cctech
Sent: 29 March 2020 02:29
To: cctech at classiccmp.org
Subject: H7874 power supply
Does anybody have a maintenance print or service manual for the DEC
H7874?
This is the power supply used in the BA4xx and R400x cabinets. As you
might
guess, I have one that tries to power up but
shuts down after a second.
Probably a bad capacitor (or several), but this thing is ridiculously
complicated
and not all that easy to disassemble, either.
I'd like to be able to
trouble shoot
it rather than just firing the proverbial parts
cannon at it. FWIW, none
of the
electrolytic (of which there are many) have
obviously failed - no leaking,
no
bulges, etc. Of course, that proves fairly
little.
This PSU is a real problem, as you say, complicated and hard to disassemble.
I had one that didn't work, if I remember correctly in the end I had to get
another one. I have a second machine with this PSU which has an intermittent
problem and shuts itself down after a few minutes if it hasn't been used for
a while, but a subsequent power up sees it work for as long as I need it.
It looks like I reverse engineered the schematic of the 12V output board. I
don't know if that would be of any use? How correct it is I don't know.
Regards
Rob
7:43 a.m.
Rob Jarratt >robert.jarratt at ntlworld.com>
wrote:
I reverse engineered the schematic of the 12V output board. I
don't know if that would be of any use? How correct it is I don't know.
Sure, if you don't mind sharing. Everything helps.
Looks like there are five separate PCBs in there. There's one long,
skinny one that's obviously the AC input and rectifier board, which probably
produces something like 300VDC for the regulator boards. There are two
similar but not identical boards with huge heat sinks that are obviously the
output regulators. I think this PS has +5, +3.3, +12 and -12 volt outputs,
but I've no idea which board does which.
And then there's a giant PCB, almost 11" square, covered with opamps, 74LS
logic and discrete parts. There are filter caps, a transformer, and
switching transistors on this board too so it obviously produces yet another
power output for something. Oh, and I think this cabinet has variable speed
fans, right? SO there's probably a fan speed controller on here as well.
You'd think that will all this logic it could at least have some LEDs or
something to tell me which output has failed, but it doesn't seem like it.
Thanks
Bob
8:43 a.m.
OK, I posted it here:
https://robs-old-computers.com/2020/03/29/h7874-power-supply/
-----Original Message-----
From: Robert Armstrong <bob at jfcl.com>
Sent: 29 March 2020 15:44
To: rob at jarratt.me.uk; 'General Discussion: On-Topic Posts'
<cctech at classiccmp.org>
Subject: RE: H7874 power supply
skinny
one
Rob Jarratt >robert.jarratt at
ntlworld.com> wrote:
I reverse engineered the schematic of the 12V output board. I don't
know if that would be of any use? How correct it is I don't know.
Sure, if you don't mind sharing. Everything helps.
Looks like there are five separate PCBs in there. There's one long, that's obviously the AC input and rectifier board,
which probably produces
something like 300VDC for the regulator boards. There are two similar but
not
identical boards with huge heat sinks that are
obviously the output
regulators. I
think this PS has +5, +3.3, +12 and -12 volt outputs,
but I've no idea
which board
does which.
And then there's a giant PCB, almost 11" square, covered with opamps,
74LS
logic and discrete parts. There are filter caps, a
transformer, and
switching
transistors on this board too so it obviously produces
yet another power
output
for something. Oh, and I think this cabinet has
variable speed fans,
right? SO
there's probably a fan speed controller on here as
well.
You'd think that will all this logic it could at least have some LEDs or
something to tell me which output has failed, but it doesn't seem like it.
Thanks
Bob
9:17 a.m.
On Sun, 29 Mar 2020, Robert Armstrong via cctech wrote:
And then there's a giant PCB, almost 11"
square, covered with opamps, 74LS
logic and discrete parts. There are filter caps, a transformer, and
switching transistors on this board too so it obviously produces yet another
power output for something. Oh, and I think this cabinet has variable speed
fans, right? SO there's probably a fan speed controller on here as well.
There's logic included there for synchronised control of multiple H7874
PSUs at once in a master-slave configuration via a set of auxiliary
connectors so as to power up or down all pieces simultaneously in a
complex machine setup with storage and/or Q-bus expansion boxes. No
documentation for the PSU has been chased so far as far as I know.
Note that also with this PSU there were capacitors used that suffered
from the issue with the quaternary ammonium salt system
used in the
electrolyte. All mine had numerous Chemi-Con SXF parts scattered
across
all boards, some leaking. They are prone to leaks even if unused and have
to be replaced. Especially those next to the 5V circuit are hard to get
to to desolder and replace. Also Chemi-Con SXE parts present in this PSU
are reportedly affected although I haven't seen them leak myself.
This PSU has been my worst nightmare as far as keeping old equipment
alive has been concerned. I was able to make just one working PSU out of
three broken ones by mixing and matching individual pieces with caps
replaced until I found one set that worked. I consider replacing the
large cuboid 6800?F Sprague part as well as I have seen its capacitance
drop down to like half of its nominal.
Maciej
9:50 a.m.
I forgot to mention that I replaced a fair number of capacitors, even ones that had not
leaked, because their ESR was a bit suspect. I have some notes, but it is hard to
reconstruct exactly what I did.
Regards
Rob
-----Original Message-----
From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Maciej W.
Rozycki via cctech
Sent: 29 March 2020 17:18
To: bob at jfcl.com; General Discussion: On-Topic Posts
<cctech at classiccmp.org>
Cc: rob at jarratt.me.uk
Subject: RE: H7874 power supply
On Sun, 29 Mar 2020, Robert Armstrong via cctech wrote:
And then there's a giant PCB, almost
11" square, covered with
opamps, 74LS logic and discrete parts. There are filter caps, a
transformer, and switching transistors on this board too so it
obviously produces yet another power output for something. Oh, and I
think this cabinet has variable speed fans, right? SO there's probably a fan
speed controller on here as well.
There's logic included there for synchronised control of multiple H7874 PSUs at
once in a master-slave configuration via a set of auxiliary connectors so as to
power up or down all pieces simultaneously in a complex machine setup with
storage and/or Q-bus expansion boxes. No documentation for the PSU has
been chased so far as far as I know.
Note that also with this PSU there were capacitors used that suffered from the
issue with the quaternary ammonium salt system used in the electrolyte. All
mine had numerous Chemi-Con SXF parts scattered across all boards, some
leaking. They are prone to leaks even if unused and have to be replaced.
Especially those next to the 5V circuit are hard to get to to desolder and
replace. Also Chemi-Con SXE parts present in this PSU are reportedly affected
although I haven't seen them leak myself.
This PSU has been my worst nightmare as far as keeping old equipment alive
has been concerned. I was able to make just one working PSU out of three
broken ones by mixing and matching individual pieces with caps replaced until I
found one set that worked. I consider replacing the large cuboid 6800?F
Sprague part as well as I have seen its capacitance drop down to like half of its
nominal.
Maciej
11:14 a.m.
Maciej W. Rozycki <macro at linux-mips.org>
wrote:
numerous Chemi-Con SXF parts scattered across
all boards, some leaking
Yeah, mine is full of "Nippon Chemi-Con" electrolytic too.
What about just gutting the PS chassis and sticking in an ATX power supply? I don't
know how the maximum current per output breaks down, but as I understand it the H7874 is
only a 600W supply. That's not a problem for an ATX supply, and it would have all the
required output voltages. You'd have to run the cabinet fans at a fixed speed and
you'd lose the power control bus feature, but that's not a show stopper.
Actually my unit is in an R400x chassis full of RF drives, so I believe I only need +12
and +5. I don't think the 3.3v and -12 are used in that case. I wonder how much
current I really need for the drives? I should look that up and I might be able to find a
surplus dual output supply that would do the job.
Thanks again,
Bob
11:24 a.m.
On Mar 29, 2020, at 2:14 PM, Robert Armstrong via
cctech <cctech at classiccmp.org> wrote:
I had a similar thought for a PRO supply that blew its input filter (of all things). It
does +5, +12, and -12. The -12 is needed at least for RS232.
paul
Maciej W. Rozycki <macro at linux-mips.org>
wrote:
numerous Chemi-Con SXF parts scattered across
all boards, some leaking
Yeah, mine is full of "Nippon Chemi-Con" electrolytic too.
What about just gutting the PS chassis and sticking in an ATX power supply? I don't
know how the maximum current per output breaks down, but as I understand it the H7874 is
only a 600W supply. That's not a problem for an ATX supply, and it would have all the
required output voltages. You'd have to run the cabinet fans at a fixed speed and
you'd lose the power control bus feature, but that's not a show stopper.
Actually my unit is in an R400x chassis full of RF drives, so I believe I only need +12
and +5. I don't think the 3.3v and -12 are used in that case. I wonder how much
current I really need for the drives? I should look that up and I might be able to find a
surplus dual output supply that would do the job.
2:04 p.m.
I have some notes I made about trying a PC PSU, I don't think it really worked, there
are signals to indicate power OK and perhaps others. But perhaps a more concerted effort
might yield something that could work.
Regards
Rob
-----Original Message-----
From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
Armstrong via cctech
Sent: 29 March 2020 19:15
To: 'Maciej W. Rozycki' <macro at linux-mips.org>; 'General Discussion:
On-
Topic Posts' <cctech at classiccmp.org>
Cc: rob at jarratt.me.uk
Subject: RE: H7874 power supply
Maciej W. Rozycki <macro at linux-mips.org>
wrote:
numerous Chemi-Con SXF parts scattered across all boards, some leaking
Yeah, mine is full of "Nippon Chemi-Con" electrolytic too.
What about just gutting the PS chassis and sticking in an ATX power supply? I
don't know how the maximum current per output breaks down, but as I
understand it the H7874 is only a 600W supply. That's not a problem for an
ATX supply, and it would have all the required output voltages. You'd have to
run the cabinet fans at a fixed speed and you'd lose the power control bus
feature, but that's not a show stopper.
Actually my unit is in an R400x chassis full of RF drives, so I believe I only need
+12 and +5. I don't think the 3.3v and -12 are used in that case. I wonder how
much current I really need for the drives? I should look that up and I might be
able to find a surplus dual output supply that would do the job.
Thanks again,
Bob
3:15 p.m.
Remembering that I have the R400x enclosure and I'm only interested in powering disk
drives - I looked up the specs for the RF7x drives, and they all use about 1.5A at 5V and
2.5-3A operating at 12V. There's a 5-6A spinup surge on the 12V supply, but as long
as you spin them up in sequence you can probably gloss over that. Anyway, I have 4xRF7x
drives and two SCSI drives, so figure maybe 20A at 12V and 10A at 5V. That's entirely
doable with an ATX supply.
The fans, annoyingly, are 24VDC so those would either have to be replaced with 12V fans
or I'd have to kludge up an extra 24V supply.
My next step was to figure out the pinout of the connectors on the back of the H7874.
The big, high current, pins aren't marked anywhere inside the H7874 that I could find
but, conveniently, they ARE marked on the R400x backplane. There are also a couple of
high density connectors sandwiched in between the power pins; the lower one doesn't
appear to be used but the upper one has a few connections. From looking at the backplane
it appears that only four pins in the upper connector are actually connected. That's
a guess though, because I can't see that back side of the backplane.
I needed to see more of the backplane to figure out where these four connections went,
so I removed all the drives from the R400x. Then, on a whim, I stuck the H7874 back in
there and tried to power it up. It works! It appears that I don't have a bad power
supply after all; I have a bad drive somewhere. Don't I feel silly :)
So I don't need to fool with the ATX supply after all, or at least not until my PS
really does die. Thanks for the help, and sorry for the false alarm.
Bob
-----Original Message-----
From: Rob Jarratt [mailto:robert.jarratt at ntlworld.com]
Sent: Sunday, March 29, 2020 2:05 PM
To: bob at jfcl.com; 'General Discussion: On-Topic Posts'; 'Maciej W.
Rozycki'
Cc: rob at jarratt.me.uk
Subject: RE: H7874 power supply
I have some notes I made about trying a PC PSU, I don't think it really worked, there
are signals to indicate power OK and perhaps others. But perhaps a more concerted effort
might yield something that could work.
Regards
Rob
-----Original Message-----
From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
Armstrong via cctech
Sent: 29 March 2020 19:15
To: 'Maciej W. Rozycki' <macro at linux-mips.org>; 'General Discussion:
On-
Topic Posts' <cctech at classiccmp.org>
Cc: rob at jarratt.me.uk
Subject: RE: H7874 power supply
Maciej W. Rozycki <macro at linux-mips.org>
wrote:
numerous Chemi-Con SXF parts scattered across all boards, some leaking
Yeah, mine is full of "Nippon Chemi-Con" electrolytic too.
What about just gutting the PS chassis and sticking in an ATX power supply? I
don't know how the maximum current per output breaks down, but as I
understand it the H7874 is only a 600W supply. That's not a problem for an
ATX supply, and it would have all the required output voltages. You'd have to
run the cabinet fans at a fixed speed and you'd lose the power control bus
feature, but that's not a show stopper.
Actually my unit is in an R400x chassis full of RF drives, so I believe I only need
+12 and +5. I don't think the 3.3v and -12 are used in that case. I wonder how
much current I really need for the drives? I should look that up and I might be
able to find a surplus dual output supply that would do the job.
Thanks again,
Bob
11:39 p.m.
-----Original Message-----
From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
Armstrong via cctech
Sent: 29 March 2020 23:15
To: rob at jarratt.me.uk; 'General Discussion: On-Topic Posts'
<cctech at classiccmp.org>; 'Maciej W. Rozycki' <macro at
linux-mips.org>
Subject: RE: H7874 power supply
Remembering that I have the R400x enclosure and I'm only interested in
powering disk drives - I looked up the specs for the RF7x drives, and they all use
about 1.5A at 5V and 2.5-3A operating at 12V. There's a 5-6A spinup surge on
the 12V supply, but as long as you spin them up in sequence you can probably
gloss over that. Anyway, I have 4xRF7x drives and two SCSI drives, so figure
maybe 20A at 12V and 10A at 5V. That's entirely doable with an ATX supply.
The fans, annoyingly, are 24VDC so those would either have to be replaced
with 12V fans or I'd have to kludge up an extra 24V supply.
My next step was to figure out the pinout of the connectors on the back of the
H7874. The big, high current, pins aren't marked anywhere inside the H7874
that I could find but, conveniently, they ARE marked on the R400x backplane.
There are also a couple of high density connectors sandwiched in between the
power pins; the lower one doesn't appear to be used but the upper one has a
few connections. From looking at the backplane it appears that only four pins
in the upper connector are actually connected. That's a guess though, because
I can't see that back side of the backplane.
I needed to see more of the backplane to figure out where these four
connections went, so I removed all the drives from the R400x. Then, on a
whim, I stuck the H7874 back in there and tried to power it up. It works! It
appears that I don't have a bad power supply after all; I have a bad drive
somewhere. Don't I feel silly :)
I would just say that you must be hugely relieved! Looks like for this enclosure an ATX
supply could well work. For my VAX my notes say it didn't. It would still be worth
checking the ESR on the electrolytic capacitors and replacing any suspect ones while you
are at it.
So I don't need to fool with the ATX supply
after all, or at least not until my PS
really does die. Thanks for the help, and sorry for the false alarm.
Bob
-----Original Message-----
From: Rob Jarratt [mailto:robert.jarratt at ntlworld.com]
Sent: Sunday, March 29, 2020 2:05 PM
To: bob at jfcl.com; 'General Discussion: On-Topic Posts'; 'Maciej W.
Rozycki'
Cc: rob at jarratt.me.uk
Subject: RE: H7874 power supply
I have some notes I made about trying a PC PSU, I don't think it really worked,
there are signals to indicate power OK and perhaps others. But perhaps a more
concerted effort might yield something that could work.
Regards
Rob
> -----Original Message-----
> From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
> Armstrong via cctech
> Sent: 29 March 2020 19:15
> To: 'Maciej W. Rozycki' <macro at linux-mips.org>; 'General
Discussion:
> On- Topic Posts' <cctech at classiccmp.org>
> Cc: rob at jarratt.me.uk
> Subject: RE: H7874 power supply
>
> >Maciej W. Rozycki <macro at linux-mips.org> wrote:
> >numerous Chemi-Con SXF parts scattered across all boards, some
> >leaking
>
> Yeah, mine is full of "Nippon Chemi-Con" electrolytic too.
>
> What about just gutting the PS chassis and sticking in an ATX power
> supply? I don't know how the maximum current per output breaks down,
> but as I understand it the H7874 is only a 600W supply. That's not a
> problem for an ATX supply, and it would have all the required output
> voltages. You'd have to run the cabinet fans at a fixed speed and
> you'd lose the power control bus feature, but that's not a show stopper.
>
> Actually my unit is in an R400x chassis full of RF drives, so I
> believe I only need
> +12 and +5. I don't think the 3.3v and -12 are used in that case. I
> +wonder how
> much current I really need for the drives? I should look that up and
> I might be able to find a surplus dual output supply that would do the job.
>
> Thanks again,
> Bob
30 Mar
30 Mar
7:17 a.m.
Looks like for this enclosure an ATX supply could well
work.
For my VAX my notes say it didn't.
A VAX would certainly be harder. You'd have to kludge up the ACOK and DCOK signals
for one thing, which I don't think the R400x uses. It'd be really handy to find
even a schematic for the backplane so we could see which signals are actually connected,
but I did some searching and came up with nothing.
Oh, and I did try to power up the H7874 on the bench for testing, but it wouldn't
turn on there either. It probably has a minimum load requirement, or it needs some signal
from the backplane to turn on. FWIW, the R400x does not have a power switch - the only
way to turn it on or off is thru the H7874, either by the circuit breaker or the power
control bus. The R400x also has several large power resistors on the M7493 SCSI connector
module; those may be there to provide a minimum load for the power supply.
Bob
9:49 a.m.
On Mar 30, 2020, at 10:17 AM, Robert Armstrong via
cctalk <cctalk at classiccmp.org> wrote:
Thise signals don't seem like a problem: just hardwire them to the desired logic
level. PDP-11s need a real functioning DCOK signal iff you want to do power fail
interrupt handling, otherwise they don't. Do VAXen have power fail interrupt? If
yes, does anyone actually use it?
paul
Looks like for this enclosure an ATX supply could
well work.
For my VAX my notes say it didn't.
A VAX would certainly be harder. You'd have to kludge up the ACOK and DCOK signals
for one thing, which I don't think the R400x uses. 
9:52 a.m.
Vectoring through 24 was very important back in the core memory days
when the memory would still be there after a power failure and you
wnated the system to keep running, as was our Ontario Bellboy paging
controller.
cheers,
Nigel
On 30/03/2020 12:49, Paul Koning via cctalk wrote:
--
Nigel Johnson
MSc., MIEEE
VE3ID/G4AJQ/VA3MCU
Amateur Radio, the origin of the open-source concept!
You can reach me by voice on Skype: TILBURY2591
If time travel ever will be possible, it already is. Ask me again yesterday
This e-mail is not and cannot, by its nature, be confidential. En route from me to you, it
will pass across the public Internet, easily readable by any number of system
administrators along the way.
Nigel Johnson <nw.johnson at ieee.org>
Please consider the environment when deciding if you really need to print this message
On Mar 30, 2020, at 10:17 AM, Robert Armstrong
via cctalk <cctalk at classiccmp.org> wrote:
Thise signals don't seem like a problem: just hardwire
them to the desired logic level. PDP-11s need a real functioning DCOK signal iff you want
to do power fail interrupt handling, otherwise they don't. Do VAXen have power fail
interrupt? If yes, does anyone actually use it?
paul
Looks like for this enclosure an ATX supply could
well work.
For my VAX my notes say it didn't.
A VAX would certainly be harder.
You'd have to kludge up the ACOK and DCOK signals for one thing, which I don't
think the R400x uses. 
11:37 a.m.
On 03/30/2020 11:49 AM, Paul Koning via cctalk wrote:
Thise signals don't seem like a
problem: just hardwire them to the desired logic level. PDP-11s need a real functioning
DCOK signal iff you want to do power fail interrupt handling, otherwise they don't.
Do VAXen have power fail interrupt? If yes, does anyone actually use it?
The VAX 11/780 absolutely had power fail, and it DID work.
You could just walk up to the CPU and turn the key to
standby, and it would power down everything but the memory.
When you turned the key back to on, it would power up, load
microcode and resume the paused system image VERY smoothly.
You could also power down the Unibus box, and it would
recover all the devices and resume
when you turned it back on.
Anything tasks that ran off Massbus peripherals could keep
running while the Unibus was off.
I did this quite often while swapping I/O devices and such
after device drivers became loadable.
Jon
31 Mar
31 Mar
10:19 a.m.
Jon Elson <elson at pico-systems.com> wrote:
The VAX 11/780 absolutely had power fail, and it DID work.
So did the 11/730 and I imagine the 750 as well. No core, but there was a
battery backup option to keep main memory alive and refreshed even in the
event of power failure.
I did this quite often while swapping I/O devices and
such
after device drivers became loadable.
It was (is!) quite handy on the 730 as well. If your configuration has a
separate BA11-K for the UNIBUS peripherals, then you can power down that box
without turning off the CPU. It took 20-25 minutes to boot up the 730 from
a dead start, so this was a great time saver.
Bob
1726
days inactive
1728
days old
16 comments
7 participants
participants (7)
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bob＠jfcl.com -
elson＠pico-systems.com -
holm＠freibergnet.de -
macro＠linux-mips.org -
nw.johnson＠ieee.org -
paulkoning＠comcast.net -
robert.jarratt＠ntlworld.com