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Common Power
PSU Models
Remove a 56 / 76 PSU
Repairing a 35 / 56 /76 PSU (Magnetek, I bet!)
PSU Models
Model 56: 06H3668 Power Supply (118 Watt 79F3443)
Remove a 56 / 76 PSU
Open the case. Pull out the planar power cable (P1) and
the signals cable (P2). Unplug all drives from power cables. Remove the
barbed fastener at the top rear of the PSU (black or white). I used a standard
screwdriver. At the front of the case you will see the control assembly
(power switch). Notice the vertical catch at the bottom center of it? Lift
up and out on this latch and the entire control assembly will come out.
Now slide the PSU forward until it stops on the front
of the case. Pull up. Congratulations.
Remove Control Assembly
Remove PSU
Repairing a 35 / 56 /76 PSU (Magnetek, I bet!)
From Peter:
Does anyone know how to fix these things? I have a whole bunch
of them, none of them work...Any help would be appreciated.
I bet they are all Italian made Magnetek ...
they are known to fail after two years in average. It is darn hard to fix
them. If you opened them you will have noticed that parts of the printboard
are dark brown from something growing hot.
Now, in most cases there are:
- 3 mini resistors 1/8 Watts 1% (R41, R52, R55)
- 1 large resistor 5 Watts (R19)
- 1 Zener diode (VR9)
- 1 220 uF electrolytic capacitor (C46)
- BUX85 (Q10)
- maybe the BTA06 (Q5) blown.
While there, check 1N4007 (D16) as well.
All these parts belong to the "standby circuit"
that keeps the +5V sensing voltage up as long as the PSU is connected to
AC mains. Obviously the parts are under-dimensioned and get toasted over
the years. When the first resistor goes south it takes the other components
with him ... and sometimes the large Hybrid chip below the coils as well,
which is the over / undervoltage detection circuitry as far as I know.
The above mentioned parts can be obtained
in good electronic stores ... the hybrid module cannot. It is a special
part. "Issue 4" and above models contain an additional 1N4007 Si-Diode
in series with the resistors.
A similar malfunction is known from several Apple
power supplies... accidentally made by Magnetek too...
The *cooling design* was okay - but the Italian manufacturer
loused up the innards by using inappropriate parts in the 35 / 56 / 76 PSU.
Same on some Apple power-supplies: there is one electrolytic
condenser specified for lower temperature ranges. When this thing dies
after some years it takes some resistors, a diode and a VMOS transistor
with it. Result: dead PSU.
The UK-made Plessey PSUs are not known to blow up. Sadly
these are rare.
Recommendation: try to get some spare PSUs. The IBM models
35, 56 and 76 small desktops use the same PSU (and Mod. 40, 57 and 77 large
desktops use the same larger PSU as well).
And use a common line breaker to fully separate the machine
from the AC-power when not in use for a longer period of time.
> Thanks for the info. It appears that the PSU is made by Delta.
The IBM P/N is 79F3367 and the FRU is 79F3443. So hopefully it isn't
he PSU if you say that the Delta PSU's aren't known to have a significantly
high failure rate.
The FRU is 79F3443 for all sorts of "Mod. 35/56/76" 118 Watt power supplies,
the P/N might differ. The objection on the failure rate however is my own
- and derived from a decade with these units from which 90% had the Italian
Magnetek PSU - and all of them were Greenock/UK productions (serial number
starts with 55-). So the statistical number might be invalid, because I
did not have too many units under my hands with Non-Magnetek PSUs. But
these *are* know for significant high long-term failure rates.
While being on it: I have seen a few 9556 with ASTEC / Taiwan PSUs.
> As for the delay after power off, I was aware of that, but this problem
makes the machine unusable for a couple of hours afterwards. [snipped]
Then if I come back and power it on a couple of hours later, it runs like
normal for only a few minutes.
Hmmm. That sounds a lot after a shot / dying PSU. But can be the planar
as well. Maybe it is a coincidence, but I have a Mod. 95 with a Delta-PSU.
Once you switch it on it takes about 10 - 15 seconds before any light comes
on and the fans start to turn.
> Out of curiosity, if the PSU was failing, would it get hot or anything
like that?
Not necessarily. On the Magnetek-units the failure is basically caused
by a too high thermal loss on one or two resistors, which are dimensioned
for a loss of 1/8th Watt ... over the years they are "cooked" - get brittle,
until they are so much worn that they burn off. A short "pop" - and they're
gone. Now the standby-part of the PSU is non-functional (does not supply
the sense voltage for the electronic switch any longer) and the main power
supply cannot be turned on. Often the failing resistor causes an overvoltage
in a Z-Diode - which damages a Thyristor ... which fries some other components
... a little firework. The PSUs used to have a certain "electronic smell"
after burned resistors and capacitors. When you disassemble the PSU you
will found some parts that turned into coal and a printboard having turned
to a dark brownish color in some areas.
Sometimes the PSU still works - partially. You can turn it on with unplugging
/ plugging back the AC-plug ... but the on/off switch is no longer functional.
I think yours might be partially damaged, so that the "Power Good" line
from the PSU to the planar hangs in "undefined" condition, which holds
the Reset line of the CPU low. Therefore the HDs and fans start to spin
- but the board does not start up the power on self test (POST) and memory
count during that. This -however- could be a symptom of a shot board with
a thermal malfunction as well.
Therefore my recommendation: try to get a spare PSU to test the unit
with it. It is good to have a spare anyway. Maybe there is someone out
who can lend out one for a test...
---
Dave Johnson observed:
I noticed that a 9556 with its' case removed, plugged in, power
switch off (no fan or drives whirring), felt very warm to touch on top of
the power supply.
Peter has a flashback and says:
You had just experienced the "standby power warming effect" :-)
The power supplies on various PS/2 (33, 35/40, all 56/57, all 76/77,
85, all 90/95) do not really "switch off" - a part of the PSU is always active
and the frontside power button in fact switches only a "sense voltage" from
the standby part of the PSU against GND... which starts up the main power
supply.
Especially the PS/2 Mod. 9556 PSUs are known for a
high failure rate. Once having opened the PSU you will find parts of the
PCB having gotten dark-brown from the heat emitted by components.
Particularly know for "sudden death by aging" are the Italian Magnetek PSUs for the 56. They cannot be repaired
! I have tried that various times. They use a sort of "hybrid circuit" for
the PSU-internal failure detection, which seemed to be fried after some components
died by thermal overstress. I have replaced various diodes, resistors and
the main switching hi-voltage VMOS transistor (all were defective) - and
the PSU did not work but fried some larger resistor I'd replaced some minutes
before.
Recommendation: try to get some spare PSUs. The IBM models 35,
56 and 76 small desktops use the same PSU (and Mod. 40, 57 and 77 large desktops
use the same larger PSU as well) And: use a common line breaker to fully
separate the machine from the AC-power when not in use for a longer period
of time.
The machine itself is a nice little thing with reasonable performance.
The case might be a little tight and does not offer much room for expansions,
but you could replace the harddisk against a more modern and much faster
2.16GB DCAS-32160 from IBM (or anything up to 3.94GB), install a CD-ROM drive
and expand the memory up to 16MB ... XGA-2 video comes standard and gives
acceptable results even with Win95.
Btw, the machine might not run with the cover removed. There's a
little blue security switch at the front side, which shuts down the PSU when
released (= when the cover is removed). You need to push the security switch
inside/up to start the machine with the cover removed. Just for
completeness.
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