Fixable IBM 5155?

[ibmapc]

Are you 100% certain about this?
I've never used an original IBM PSU, but all others PSUs I've ever seen spins the fan when you power on, even with nothing connected. I will try to find a 5150 PSU schematic to take a look, I'm very curious to understand this circuit now.

Yep, when I thought I had blown the PSU in my 5155, I disconnected it from the motherboard, then turned it on and nothing. I then posted HERE about it in a panic and I believe it was Chuck(G) who informed me that some power supplys would not come on without a load. So I then connected it to a known good motherboard and it came to life and I was both embarassed and relieved.

 

[pearce_jj]

5150 PSU is different as it uses an AC fan. The 5155 PSU is powered from a separate 12v connector comming out of the unit, P13 according to my notes.

 

[Denniske1976]

Maybe this won't help at all, but here's something else you might try: Just remove all the cards and insert them again.

Might sound stupid, but i've had this happen to an IBM 5155. It just sat in the closet for about 4 years, I took it out, turned it one and nothing happened. Opened up the case and all seemed fine (no blown up caps). Then an ex IBM employee told me I should "reseat" everything since it was the first thing they did when a machine seemed dead without any reason. So, took everything out, disconnected the floppy drives and put everything in again... and prompto! it worked again.

But since it's been in the closet again for about 8 years now there's a good chance it won't boot again when I take it out But I'll know what to do ;-)

Maybe it won't fix your problem, but it did mine and my 5155 didn't do anything either when turned on. Although mine only has the CGA adapter and a memory board (to get to 448K memory), nothing with a battery on it (I'd leave that one out anyway).

 

[CannedBread]

I'll get to the rest over the weekend, but here are some more pictures of the boards.

Board 2




This is the board with the leaked battery. It is a Varta SafeTronic, 2.4V, 100 mA.



I don't know if it's really needed, but here is a close up of the battery. Other than that, this board seems fine.

Board 3



As far as I can tell, this board seems fine. I think it is the floppy controller.


That's as far as I've got in taking it apart, but here is a photo of the motherboard anyway:



I'll probably have it apart on Sunday, although I've got Monday as well because it's a bank holiday :D

 

[Chuck(G)]

Board 2 is a Quadram Quadboard--a serial, parallel, game, clock and memory expansion board. For purposes of testing, it's not needed, so you can put that in the "later" pile.

Board 3 is indeed a floppy controller. Those rarely go out of whack.

 

[pearce_jj]

Have a look at that monitor shield - it looks like it's not clipped in at the bottom and could short across the back of the CGA card.

Mobo removal is easy enough (1 screw) but you need to remove both the FDDs to get at it IIRC.

 

[modem7]

5150 PSU is different as it uses an AC fan.

Early ones, yes. Later ones, no. On this LINK is is a circuit diagram for a later 5150 PSU.

 

[modem7]

I've placed some information on the 5155 power supply at
http://www.minuszerodegrees.net/5155/psu/5155_psu.htm

 

[modem7]

I placed my 'CRT shield removal' procedure at
http://www.minuszerodegrees.net/5155/crt/5155_crt_shield_removal.htm

 

[ibmapc]

I'll probably have it apart on Sunday, although I've got Monday as well because it's a bank holiday :D

@CannedBread
Have you made any progress with your 5155? I have a soft spot in my heart for the old 5155 and would like to see you succede in getting her up and running again.
I have two of them myself. One is the first computer I owned back in 1986 when bought new by my father who is a retired long time IBM employee, and the other was purchased from EBAY as a non working unit. The latter one had a shorted CGA board, and after replacing it, and doing some maintanance, the unit works perfectly.

Please update us on your progress.

Greg

 

[CannedBread]

I got the motherboard and power supply out, and from what I can tell the PSU is broken. The fan doesn't come on at all, and the motherboard looks to be in good shape. There's also a cable hanging out the back of the CRT, I don't know if it's supposed to be there... I'll take some pictures soon. If it is the PSU, does anyone know where I could find a working one?

 

[CannedBread]

I took the cover off the PSU and found a blown fuse:


I tested it, and it is definitely blown. It is a 250V 3A fuse.

That's the only problem I could see in the power supply.



I noticed a cable dangling out the back of the CRT:



Here's the end:



Would it be an earth cable, and should it be plugged into something? The Motherboard looked fine, so I didn't take any pictures of it. If I changed the fuse and turned it on with just the motherboard plugged in, would that do anything?

 

[Chuck(G)]

Note that you usually cannot run a switch-mode PSU without a load. Hook a hard disk to the output and give it another shot before declaring it bad (and after replacing the fuse).

The lead that you show looks as if it might be a chassis ground. Such leads usually attach to a clip or "ear" attached to the system frame or case. I suspect this is more for RFI shielding than anything.

 

[CannedBread]

I don't have an old hard drive, would two floppies do it, although I assume they would have to be running?

 

[pearce_jj]

Modem7 posted already that the 5155 PSU needs quite a lot of load on it to function:

"Some loads that you think may be sufficient are not.

* An IDE drive (eighties/nineties vintage) is not enough of a load. My experiments with two 5155 power supplies show that approximately four to five IDE drives are required.
* Even some full height multi-platter MFM drives fail to enable this power supply to start.
* The 5155's CRT unit is not enough of a load.

* The 5155 motherboard is enough of a load to start/operate the power supply. This is the case even if P8 is disconnected (but note that the motherboard requires both P8/P9 to operate)."

 

[modem7]

I noticed a cable dangling out the back of the CRT:

Would it be an earth cable, and should it be plugged into something?

Take a look at the photo at http://www.minuszerodegrees.net/5155/crt/5155_crt_unit - black wire.jpg
I think the black wire you pointed out is the one that attaches to the point I have circled in yellow (top-left).

It is possible that your 'floating' wire was the cause of the fuse blowing (i.e. the end touched something it shouldn't have).

 

[CannedBread]

I took the cover off the PSU (despite the yellow warning sticker), and found a blown fuse. I replaced it with a 2.5 A (the blown one was a 3 A), and unplugged everything except the motherboard. When I turned it on, the fan spun for a second, and then cut out. I follow the instructions on the minuszero site, and unplugged the P8 connector. The fan the spun continuously. That means that there is a short in either the +12 or -12 volt rails. It says to check the capacitors, but I can't see that any are broken. What should I do now?

 

[ibmapc]

.... That means that there is a short in either the +12 or -12 volt rails. It says to check the capacitors, but I can't see that any are broken. What should I do now?

First, you should be able to isolate which line the short is on using an ohm meter. Then You need to figure out which caps are on the +12v or -12v traces depending on where the short is. (someone here should be able to help with that. ie schematic which I don't have immedediate access to) Then you'll need to start clipping off caps untill the short goes away.

 

[modem7]

First, confirm that all expansion cards (including video card) have been removed from the motherboard.

On the assumption that you did, yes, there is a short on either the +12V line or the -12V line on the 5155/5160 motherboard. Or, it be both.
As you may have seen, we are daling with that siuation now with the 5155 thread started by latvija13.

On the 5155/5160 motherboard, the only devices on the +/-12V lines are capacitors.
The capacitors are:
+12V line: Capacitors C56 (10 uF tantalum) and C55.
-12V line: Capacitors C58 (10 uF tantalum) and C57.

It is the tantalums that often fail.

As ibmapc wrote, a resistance check will inform you as to which line is short. You would then remove the tantalum cap on that line, then confirm that that action removed the short.

Alternatively, you could simply cut off C56 first, see if that corrects the problem, and if not, then cut off C58.

The 5155/5160 motherboard will operate without C56/C58, but ideally, you should fit a replacement/s later for the shorted capacitor/s.

 

[CannedBread]

I checked all the capacitors, and all of them gave a reading on the outside legs, but C58 did on all 3 legs. Does this mean it's shorted?