TRS-80 Model 200 power supply

[Dustym]

Please would someone advise me. Does the M200 have the same polarity on the external power supply, as the M100? i.e. 6V negative pin.
Thanks

 

[Hans01]

@dustym.

This is the part of the schematic with the power connector.

 

[Dustym]

Thanks Hans

 

[EkriirkE]

To actually answer your question: yes it does.

 

[Dustym]

Thanks for that. Just in case you're interested, there's a short article about m100/200's power supply, with photos, in the June edition of TRS8BIT (available, free, from www.trs-80.org.uk)

 

[RetroHospital]

I'm repairing an M200 that I just got today

One fot he problem is that ZNR3 (that you can clearly see in the above schematic) has exploded.
It seems to be a zener diode, but I can't find a value. It has a weird shape, it's also referred as a "surge absorber" which doesn't help me much

ANy help appreciated !

 

[Koolstar42]

The print on ZNR3 reads SK D18L 57. (Green part in the center of the picture)
It measures as a diode Anode on center tab (minus)
Safe to assume it is a zener diode of unknown value. Maybe 18 v? .
It protects against over- and reverse voltage. As long as you trust your power supply it should be safe to ignore this part.
A blown part would indicate that reverse voltage (or way too high, or both) was applied. Also that possibly the power supply survived the short and kept going after the zener gave up, which might indicate further damage to your system :-(

 

[RetroHospital]

After further investigation and help from someone on Facebook ; this part is actually not a zener diode ( despite the name and schematic ) but a 10V varistor

Which also explains it’s weird shape ; it’s not a diode at all .

A modern equivalent would be a 7mm , 10V varistor

Looking at the specs ; varistors seem to behave roughly like zener diodes but with much less steep curves . Interesting

Replacement part ordered . The rest of the M200 seems to work ; this doesn’t prevent it from booting but I want it « mint « and also the fried component made the entire motherboard smell awfull so all that gets a cleaning

 

[RetroHospital]

For this poor ZNR3 to look like charcoal like it does on my unit someone must have applied a ridiculous voltage for a ridiculously long time ; ignoring the smell and smoke that must have gotten out of it
I’ll post a picture just for fun

 

[Koolstar42]

Hmmm. a 19 v notebook supply comes to mind.
You do have the service manual I assume?