Sinclair ZX-81 -Inspection, Cleaning, Possible Repair

[+DrVenkman]

So I finished the composite mod for my TS1000 this afternoon and - once I realized I had installed the BC548 transistor backwards! - I get a nice sharp image. Unfortunately I’m also getting jailbars in the video. I gather this is not terribly uncommon. So I need to figure out where the noise is coming from and see if I can install some decoupling caps to reduce or eliminate it. On the plus side, the composite mod is reducing the power draw by almost 80 milliamps.

 

 

 

 



 



 


 

 

[+DrVenkman]

I still need to repaint the logo and install some new rubber feet, but still and all, it's turned out pretty well all things considered.  

 

 



 

[Ed in SoDak]

Almost there! Looks like it got to come inside from the garage too.

[+DrVenkman]

Messing around with my TS1000 again this week. First thing I did was clear out the through hole for the second positive pin on the 3.5mm plug, then bent it back down relatively flush to board and soldered it back. That adds nothing to basic functionality but it fixes a 4 decades old factory mistake, and that is good enough for me! ?

 

 

The next thing I did was take steps to try to clean up the noisy “jailbars” in the composite video mod. This is digital switching noise on the bus, the first thing I tried as installing a small (33uF) electrolytic cap between the +5V and GND rails near the voltage regulator. Unfortunately that made no discernible difference. So next I used one of the Traco high-efficiency VR’s I bought a few weeks ago and replaced the vintage regulator (see photo above). Also no difference. So next I added .47uF ceramic caps under the board between the Vcc and Vss pins of the ULA socket (which generates the video signal) and the under the location of an existing ceramic disc capacitor at the C2 location. This did wonders to clean up the signal! There’s still a little bit of residual noise - I might add additional ceramics under the board between the Vcc and Vss of the Z80 and SRAM sockets, and maybe the OS ROM socket as well. 

 



 



(ignore those fine vertical lines - they’re artifacts of my camera sensor and the LCD array of the TV)

 

Although replacing the old VR didn’t improve the video, it dropped the power draw by about 100-110 mA and allows me to dump the giant heat sink, so major win there.

 

 

 

[+DrVenkman]

Even after filtering caps to major components on my TS1000, I was still getting minor jailbars from noise in the system, plus the older ULA chip doesn't produce a proper composite "back porch" in the signal, so most older TVs and CRT monitors don't produce a good image when used with the computer (the "back porch" serves as a black-level reference for the TV's demodulation circuits).  So a few weeks ago I ordered a couple "ZXvid" boards from SellMyRetro in the UK - these little PCBs use an SMD 555 timer chip to produce a pulse that provides the "back porch" in the video signal produced by the ULA. Two little potentiometers adjust  the output contrast and the timing of the 555, and thus the width of the "back porch" in the signal. This lets you basically fine tune the output of your particular ULA chip to your particular display. It's an easy install, with only 3 wires in (5V, GND and VIDEO) and VIDEO OUT to the center of your RCA jack. It's nice and tiny, fitting neatly into the existing RF modulator can.

 

 

 

 



 


 

 

 

I have a second one for my ZX81 as well. Although the modern vLA81 in that system produces a proper back porch in the video, something is not quite right with my homemade video mod board - video out occasionally dips in signal level and goes dark. There might be an intermittent connection in one of the leads somewhere, or possibly a bad transistor. Rather than rebuilding it, I will replace it with another ZXvid, then set the vLA81 installed in that system to stop creating the "back porch" itself. It's nice that the vLA81 has so much built-in flexibility! 
 

When I install that second mod board into the ZX81, it will be a good excuse to finally repaint the logo, repair that remaining screw post that is stripped out, and then install some new feet on the case. At that point, it will be as "done" as it can be, and ready for another few decades of use and admiration.