Reviving a dead Z80 (TIMEX-Sinclair 2068) on a Sunday…

Posted by on September 17, 2012

 

A Sunday wouldn’t be complete without it’s quote of nostalgia!

That’s when the Z80 comes in, but alas it displayed no signs of life… A quick check with the DMM showed an open conductor on the coax output, so I scrambled for another cable, didn’t have one so I put something together for testing purposes (RF losses didn’t matter at this point) … Still no signs of life, is it dead?

We opened up the power brick, there are 3 screws underneath the rubber feet. Caps seem to be decaying – a bit dry – but they still have a respectable capacitance. I noticed there’s no suppression for high frequencies though but that’s a fix for later. The bridge rectifier tested OK, the transformer was OK, voltage and ripple were OK under load. Time to close it up and discard this as the primary fault.

 

It’s all down to the unit itself now, a few screws out and the lid pops up, pull-out the keyboard ribbon cable on the mainboard and you’ve got full access to the top of the board now!

This Timex had a custom joystick “expansion” board installed at one time… Notice the gold permanent marker used to hide the IC numbers, weasels…

First things first, I probed the 5v linear regulator. Some surface oxide at first prevented me from getting a good reading, this is common in such old equipment — Make sure you use sharp probes! — On a second test, with good probes, I got a reading and it was up to spec, back to fault-finding mode!

Probed for CLK at the main processor, I got a fairly stable frequency (although it was jittering quite a bit) — So the unit is alive, we just don’t have any video…

Since I didn’t have a RCA cable to test the non RF output with — Which was fitted in case you bought the special monitor that went with the Spectrum. I couldn’t test it this way.

 

Pointing out at the culprit…

 

With the RF connected to an old TV, all we got was interference (seems to be putting out a broad spectrum, lot’s of noise throughout the band) but no video at all. So all eyes went to the RF output box, And right out of the gate I noticed a cold joint on the output connector, where the series resistor goes… Bingo!

This was most likely caused by mechanical stress due to lack of physical decoupling of the parts.

 

But, still no video. Now all I got was a black screen, I wonder what’s wrong with it… A closer look revealed an inductor had been also disconnected at the output, parallel to the series resistor… That’s it!

Now the z80 is alive once again!

 

Man… those games really did suck heaps…

 

Sorry, no action shots at this moment, we were too busy fiddling with the audio level on the laptop to get the roms to load (they loaded!)

 

However, here are a few extra pics:

 

 

That’s all for now!