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Why the difference in TV out port?

Posted: Thu Mar 29, 2018 1:32 am
by Bill H
I have a USA version of the ZX81 (not the TS1000) and it has the TV out like the picture below but it also has a recess where you could remove the cut out for TV out closer to the EAR & MIC like the 2nd picture - why the difference?

Re: Why the difference in TV out port?

Posted: Thu Mar 29, 2018 2:51 am
by RetroTechie
At least for the TS1000, RF (TV) output is on VHF channel 2 or 3. Versus UHF channel 36 on a regular (UK) ZX81. Also note the channel select switch on the bottom, which is not present on a UK ZX81. And the somewhat-conductive silvery paint + metal strip inside the case on US models (for EMI shielding).

VHF output uses a different type modulator: the one in my TS1000 reads "UM 1082 LA2/3" on the label. Versus UM1233 in the UHF output ZX81's. And these modulator types happen to have their RF output at a different location.

I'd assume that it was easier/cheaper for Sinclair (or Timex) to provide for cutouts in different locations in the case, than have custom RF modulators produced. In terms of injection molding a case like this, it is done using some insert(s) that are placed or removed in the injection mold before producing a batch. Not sure if the same method was used for the Sinclair logo on top vs. the recess where "Timex Sinclair 1000" sits on the TS1000's. Surrounding area seems too smooth - maybe there were 2 different injection molds used for the upper half of the case? :?:

Anyway I'd guess your US model ZX81 looks pretty much like a TS1000 inside. Maybe it was an early production model before Timex-specific customizations were done?

Re: Why the difference in TV out port?

Posted: Fri Mar 30, 2018 2:20 am
by gammaray
UHF has the RCA jack on the left side of the modulator, I think. ... Swh6xTtqov

VHF has the jack on the right side of the modulator case. ... 00x800.jpg

I suspect this is to prevent mistakes in assembly. The location seems to make a "key".