Models 35, 56, and 76 - Power Supply

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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