Sinclair Microvision MTV 1B: External aerial attachment

[HD_2017]

Hello,

I have recently purchased a Sinclair Microvision MTV 1B television. I want to connect it to a Sinclair ZX80 computer, I have previously been using an old TV which had a coaxial socket. However, instead of a coaxial socket the Microvision has two screws as shown in the photo.

I plan to strip a coaxial cable, my questions are:

How should I attach the cable? Does it matter whether I use the left screw, the right screw or both? Apart from the metal core, does any other part of the coaxial cable need to be attached to the TV? Does anyone have a scan of the relevant page of the Microvision manual?

Thank you,

Regards,

Huw

[Graham G3ZVT]

Welcome to the forum Huw, I wish I got Christmas presents like that TV.

Just guessing here but the telescopic looks as it is for unbalanced VHF and the two screws are for a loop UHF aerial and I would expect there to be an internal balun. As the UHF input is balanced, it won't make any difference which way round you connect the coax. It may help if you connect a wire link from the terminal carrying the braid to the case of the MTV1.

The signal from the ZX80 will be strong, and the modulated video waveform is notoriously far from broadcast standards compliant, you will probably find it will swamp the front end of the TV with a direct connection, you could try improvising the loop aerial on the TV assuming you don't have the original, and
forming a loop aerial at the end of your coax and lightly coupling them together a few inches apart.

Bottom line is just experiment till you find something that works for you.

[TonyDuell]

According to the service manual I have for the MTV1 (I found it somewhere on the web, can't remember where), those UHF screw terminals are an unbalanced input (!). One is connected to the set's ground rail, the other is capactively coupled to the RF amplifier transistor in the UHF tuner.

So one of them should show DC continuity to (e.g.) the outer of the earphone socket, the other won't. Connect the braid of the coaxial cable to the first one.

[Sideband]

From previous experience trying to connect a ZX80 to an RF input, as previously said, the signal is far from broadcast standard and some sets just will not lock. You may be lucky but don't expect great results first time.

[SiriusHardware]

I can confirm, having tried this, that the MTV1 displays the output from a ZX81 just fine.

....But I hope your eyesight is good!

Sideband's observation re: Some TVs not working well with the RF output from a ZX81 relates mainly to the fact that only the final version of the ZX81 ULA generated a video signal with a 'back porch'. The main symptom is a very dark background, whereas the background should normally be brilliant white of course.

CRT TVs which are of similar vintage to the ZX81 don't mind this at all, it's mainly more modern TVs which are more fussy.

As noted, the lack of back porch problem was fixed in the final version of the ZX81 ULA so if your ZX81 happens to have a ULA2C210, you are in luck.

Edit: Sorry, just noticed the computer under discussion was the ZX80, not the ZX81.

[HD_2017]

Hello,

Thank you for your replies.

In the end it was more straight forward than I had envisaged - touching the coaxial against the external telescopic aerial gave an adequate image. The hardest part was tuning the TV.

The text on the screen reads: ZX80 AND A SINCLAIR MICROVISION.

Regards,

Huw

[SiriusHardware]

I notice it's overscanning a little bit - I can't remember if the image from the ZX80 fills the majority of the raster, making it necessary to adjust the width and height inwards so that the raster only just fills the screen with very little overscan.

On the ZX81 I seem to remember that there was a significant amount of white space around the 'active' area of the screen so that even if the TV being used was overscanning a little bit, all of the displayed output would still be contained within the visible screen area.

If you are running the TV on an external supply, probably the easiest way to reduce the height and width of the raster is just to reduce the supply voltage a little bit.

Also, you should hopefully be aware that the TV runs on 6V, comprised of 4 * AA sized cells in a battery box connected to the TV via a PP3 style battery clip. The presence of this style of battery connector and perhaps the occasional absence of the original (easily removable) 4-battery clip sometimes leads people to attempt to run these on a 9V battery, on which they obviously won't run very well for very long.

[Courtney Louise]

The ZX80/81 and Spectrum should all have a border