Sharp PC-1211 / TRS-80 PC-1 weirdness

[GanjaTron]

Hi folks,

got a Sharp PC-1211 (a.k.a. TRS-80 PC-1), allegedly one of the first BASIC programmable pocket compies. That yellow LCD kicks ass!

Dere's sumptin' skwewy with this unit: it briefly powers up, the annunciators come on, but before the ">" prompt appears, it immediately shuts off again! I've tried this with batteries and, just make sure, with a lab power supply set to exactly 5.4V; same result. Anyone seen this weirdness before?

Thanks for any hints,

--Roland

 

[Druid6900]

Since my Radio Shack Computer Repair depot was the only one in Canada authorized to service the Tandy Pocket Computer line, yes, I've seen it, dozens of times.

Unfortunately, the procedure was to replace the entire logic affected logic board as we didn't have access to the chip level components, nor was it cost-effective to do so.

However, I recall that one of the diodes in the PS section would cause this problem when shorted, so, you might want to check those.

 

[GanjaTron]

I recall that one of the diodes in the PS section would cause this problem when shorted, so, you might want to check those.

Will do, thanks for the hint, Druid!

--Roland

 

[GanjaTron]

Hi Druid,

I checked all diodes, particularly those around the piezo on the lower board (I assume these are the ones you referred to). I measured them in circuit and they all gave a resistance of > 1MOhm when reversed, so I'd say they're ok.

I then checked the voltage on the electrolytic capacitor and compared it with my (working) PC-1212 (which incidentally lacks the diodes on the lower board). I noticed a difference there; the 1212 reads 4.8V, while the 1211 only reads 4.1V with the same set of batteries. I then removed the cap and checked it, but with 107uF it's well within tolerance.

Bummer, looks like a write off. Thanks for the info, tho. It was worth a try.

--Roland