Power supply voltage adjustment

[gfmoore]

Hi all, new here!

I have an old 5150 that I'm putting together from parts. The power supply has three pcbs in it.. I replaced the rifa caps and all seems good, at least I'm getting long, two short beeps out of motherboard.

When I checked the voltages I noted that instead of +12 -12 volts I seem to be getting just +9 and -9 volts which seems low. Is there any way of adjusting the voltages. I noted at least what seemed to be three trimmers/trimpots. Making slight adjustments to each did not seem to make any difference. I was using an untested floppy drive as a load.

As I said the mobo seems fine anyway, but just thought I'd check. If you want pics let me know. I'm hoping to get some video output later today

Thanks

 

[gfmoore]

Update: The mobo is working well and I can get output via the rgbtohtml adapter. (Though the first 3 columns do not display, but another issue )

 

[modem7]

Hi all, new here!

Welcome to these forums.

... at least I'm getting long, two short beeps out of motherboard.

When I checked the voltages I noted that instead of +12 -12 volts I seem to be getting just +9 and -9 volts which seems low.

Suggesting that the version of motherboard that you have is a '64KB-256KB' one. (That version does not rely on +/- 12V for operation; the +/- 12V are simply routed through to the ISA slots.)

I was using an untested floppy drive as a load.

That is not always a good enough load. The motherboard plus floppy drive is better.

Update: The mobo is working well and I can get output via the rgbtohtml adapter.

With motherboard plus floppy drive as the load, is the +/- 12V still at +/- 9V ?

 

[paul]

Load it up properly and focus on the 5V rail which needs to be within 0.1 V of 5.0.

It wouldn't hurt to specify the input voltage standard because the 110 and 230 VAC versions of this PS are not the same.

 

[modem7]

Load it up properly and focus on the 5V rail which needs to be within 0.1 V of 5.0.

According to the IBM 5150 technical reference, the tolerance is larger than that.
If we take the more restrictive of the two sets below, the tolerance range for the +5V rail is, +4.8V to +5.25V

 

[modem7]

It wouldn't hurt to specify the input voltage standard because the 110 and 230 VAC versions of this PS are not the same.

And because of the OP's, "I noted at least what seemed to be three trimmers/trimpots.", the power supply is not one the two circuit-diagram-available ones at [here].

 

[gfmoore]

Spot on. When motherboard is connected (no floppy drive though) the rails are now 5, -5 +12 and -12V which will do me! - Sorry I guess I should be more accurate in recording voltages

It is indeed a 230/240V power supply (UK) and 64k-256k motherboard.

Thanks for all your help and for helping me learn something. Now to the floppy...

(Oh I adjusted the RGBtoHDMI adapter to recover the missing 3 columns so all very excellent now.)

I'm now almost back where I started in 1984 (on a Sirius 1 clone) and will go on to relearn dBase II

Thanks, Gordon