Trying to repair a Tandy PC-3 Printer

[MerrickD]

I recently got a PC-3 with a printer/cassette interface that I've been trying to repair. Inside my particular unit I found a burned resistor right next to the dc power jack. I can't identify the resistor (it's too far damaged to see the stripes on it) and have been unable to find a schematic of the printer. What I'm looking for is a schematic or even a picture of the circuit board, just something I can use to figure out what the proper replacement part would be. I can post a picture of the components location if that helps.

Thanks

 

[geoffm3]

Any idea what the resistor is wired up to?

 

[MerrickD]

The dc jack on the printer has three pins, a positive and two negative pins (common). The resistor is connected between the two negative pins. It is also in parallel with a diode. I'm not certain what the purpose is or how there would be any current flowing over it but I'd like to replace it just in case. I replaced the printers internal battery with a power supply and got the printer to work but I haven't tried plugging into the dc jack to power it.

 

[MikeS]

The dc jack on the printer has three pins, a positive and two negative pins (common). The resistor is connected between the two negative pins. It is also in parallel with a diode. I'm not certain what the purpose is or how there would be any current flowing over it but I'd like to replace it just in case. I replaced the printers internal battery with a power supply and got the printer to work but I haven't tried plugging into the dc jack to power it.

It's probably to charge internal NiCd batteries; if they had shorted (as they often do) that could certainly burn out the resistor. The two pins are probably only connected together as long as there's nothing plugged in.

 

[MerrickD]

Took another look at the printer today and it looks like the resistor is related to the charging circuit like Mike suggested. Currently the printer is working off of an AC adapter (battery removed) but it doesn't seem to work very well. The low battery light comes on during printing and it prints very faintly. I don't have the original ac adapter but I'm reading that it should be an 8.5 volt unit. Can anyone confirm this? Right now I'm using a variable supply set to 8.5 volts.