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View Full Version : What is the construction of the load resistor needed for load on 5170/5162 startup?



Trixter
August 22nd, 2017, 12:35 PM
The MFM drive has failed in my IBM 5162 (IBM PC/XT-286, it's nearly identical to a 6 MHz AT except has 0-wait-state RAM). In trying to remove and replace it with a 16-bit IDE card, I ran across two annoying issues: First is the already documented 601 POST error (http://www.minuszerodegrees.net/5162/problems/5162_known_problems_issues.htm) when you remove the IBM fixed disk adapter. Secondly, the power supply will not completely start without sufficient load. IBM supplied a load resistor (http://www.minuszerodegrees.net/5170/misc/5170_load_resistor.htm) to provide dummy load, but I don't have that.

To generate load, I've just plugged the failing ST-251 back in so that the system will start, but this seems really stupid to have a dead/failing drive in the system just to provide startup load. Is it possible to somewhat easily construct a new load resistor? What would it consist of?

glitch
August 22nd, 2017, 12:38 PM
Probably just a wirewound "sand" resistor from 12V to ground, mounted on something convenient. Depending on sizing it may get quite warm.

Chuck(G)
August 22nd, 2017, 12:54 PM
I'd probably pinch-hit and use a automobile lamp. I've got some 12V 10W festoon bulbs for just that purpose.

SpidersWeb
August 22nd, 2017, 03:44 PM
I notice the IBM supplied resistor seems to only be on the 5V line and looking at the ST251 specs, it apparently only draws around 1A on the 5V line when idling. So a single ~5 ohm resistor, rated at say 10W, should cover it I would've thought (5V @ 1A - dissipating 5W of heat).

I usually have some laying around and just stick them in to PSU connectors when needed.
For a permanent solution I'd be tempted to just solder it inside the PSU.

ibmapc
August 22nd, 2017, 08:19 PM
I'm surprised that it won't run without the Hard drive. My 5162 runs fine with just the Mother Board as a load. I have noticed that the power supply take 1 or 2 seconds to start after the switch is flipped.

Trixter
August 22nd, 2017, 08:28 PM
You might have a different power supply than mine, which is IBM P.N. 83X1557.

modem7
August 22nd, 2017, 11:31 PM
I notice the IBM supplied resistor seems to only be on the 5V line ...
The bottom photo at [here (http://www.minuszerodegrees.net/5170/misc/5170_load_resistor.htm)] shows that the red wire from the load resistor is connected to the yellow wire of the power cable.