Mike's PCjr Page

I have a Quadram expansion chassis with the additional 384K of memory, clock, parallel port and 2nd drive. The unit has been a little flakey ever since I received it and today it doesn't recognize the additional memory and gives an Error H. Removing the expansion and the system works fine. I checked the voltages on the expansion chassis power supply and the 12V line was very low and the 5 v was also on the low side. Looking at the board I see a resistor, R10 that is burned up. Does anyone have a schematic or could tell me what the correct value should be for this resistor?

Can you take a picture of the board, in particular the affected area? There were a few different revisions of the board and you want to be sure.

Also, look for other signs of damage. It's unlikely that the resistor just gave up on its own ... something else is probably wrong as well.


-Mike

Thanks for the reply. Yes you are right that it likely didn't just burn up without something else wrong. I did check for shorts and was unable to find anything however I did find a bad solder joint on the large capacitor. I touched up that solder joint and that may have been the whole issue with the power supply. Not sure if it would have caused the resistor to burn though.

Attached is a picture of the power supply.

Ok - I'm glad I asked for a picture because I just cracked a machine open and mine is different:
I need to open up a few more machines until I find one that matches yours ...

From the picture, I would say yours is a later version. The one I have has a couple of wire jumpers on the underside as well as two capacitors that were obviously added after the final PCB layout.

I looked closely at the burnt resistor and could see that the first band was orange. I measured it as well and got a reading of 359 ohms. Knowing the first band was orange and guessing that the value didn't change too drastically, I assumed that the value was probably 330 or 390 ohms. I soldered in a 390 ohm resistor and the power supply is now working fine.

Thanks for looking at yours to help out.