Sinclair ZX80 / ZX81 / Z88 Forums

Hi all i have spent two days modifying my zx81 and gone through some highs and lows.
I have done a recommended composite mod on my issue 3 Zx81 and the picture quality looks great. One problem now exists after the device is on for 5 mins the video turns off and on no data loss but disruptive. It will stah steady as rock for another 5 minutes and same happens again. This is the 2nd power supply abd still the same

Help

Don't let Moggy know but try a heatsink on the ULA...

BTW which composite mod did you do?

Andy

Andy Rea wrote: ↑Sun Apr 21, 2019 1:36 pm Don't let Moggy know but try a heatsink on the ULA...

BTW which composite mod did you do?

Andy

What TV / monitor / display are you using? Have you tried a different TV / monitor / display?

Before sticking a heatsink on the ULA, can you please take a photo of the lettering/numbering on the ULA.

Is the ULA in a socket?

Do you just own one ZX81?

And as Andy says, which version of the composite mod. are you using? We would prefer to link to the web page of the circuit you are using.

If you are using a design that is different to a published design, please describe it in detail.

Mark

I went for a transister to cap and resister mod.. see pics re the mod and the ULA. Overheating ULA really requires a heat sink? I have replaced the regulator already to cut down the heat.

Hope you are able to make out the pics.. and advise.

Note i see Sinclair decided not to fit two resistors next to the ula see the markings on the board.. im no expert at D9

Both the ZX81 issue 1 PCB, and the ZX81/TS1000 issue 3 PCB have component positions for a variety of different video configurations. So you will see some positions where components have not been fitted.

Mark

That light-colored band on the electrolytic capacitor (facing resistor), is that the band indicating "-" (minus)? If so, you've got cap reversed. Most positive voltage will be on the transistor/resistor side, lower voltage on RCA center pin (monitor input). So + of capacitor goes onto resistor/transistor, - onto RCA connector pin. If in doubt: measure actual capacitor voltage (polarity) on low voltage DC scale.

In this situation capacitor will probably behave very leaky, but still hold a bit of charge, and -possibly- from time to time 'short circuit', after which the cycle repeats. With exactly the symptoms as described.

Also dump that red wire in between! It's just an extra / unnecessary connection that can fail. Capacitor leg directly onto resistor, done (preferably not on the transistor leg as transistors are much more sensitive to the heat from a soldering iron).

Edit: consider just grabbing a new capacitor, as this one may have been damaged by the reverse charge. They cost pennies anyway...

Great that makes a lot of sence , gonna try it and ket you know thw outcome

Thanks

unless you really need the capacitor i'd ditch it... most modern tv's don't have any DC bias on the composite inputs.

regards Andy

@RS232

Okay, I’ve studied your photos.

The following points concerns me:

• The polarity of the electrolytic capacitor appears to be incorrect. As the existing capacitor may have been damaged, I suggest removing the existing capacitor and fitting a new one.
• What value of capacitor did you use? For this application either a 47uF or a 100uF electrolytic capacitor with a voltage rating of at least 16V is normally used. I strongly recommend you use a part from a well known supplier.
• What is the red wire doing. It’s not clear from the photos where the other end of this wire is connected.
• The transistor you used. In one of your other threads, I can see that it is marked BC108B. However, the transistor you used is in a plastic TO92 package. Whereas BC108 transistors (from the large semiconductor companies) are supplied in metal TO18 cans. NOT plastic TO92 packages. Hence, despite searching, I could not find any data sheets showing the pin out / lead out of a plastic TO92 BC108 transistor. The direct modern version of a BC108 in a plastic TO92 package is the BC548 transistor. But looking at your photos, the connections do not appear to be correct for the pin out / lead out of a BC548 transistor. Now, the transistor you have used may be okay (as any reasonable small signal NPN type transistor should work okay in this circuit). But the connections have to be correct.

May I ask where you got your parts from?

Also, where in the world do you live? Knowing this will help us to provide further help.

I still strongly recommend trying a different TV / monitor/ display. This is because the video output from the ZX81 is not fully compliant to broadcast TV standards and it is possible that the TV / monitor/ display you are using just does not like the video signal from your ZX81.

Mark