The Sinclair QL Forum

Hi,

In this thread https://www.theqlforum.com/viewtopic.php?f=2&t=3133 I was exlpaining that, like the original author, my QL was dead i.e. no image on screen. I also explain that the only thing that works are: the power led in "on", due to my hermes chip, the keyboard produce click sound but only until the hermes buffer fills up (i.e. 7 keystrocks) and the reset switch does also produce a click sound when pressed. Other than that, the machine is dead. No image, no microdrive, my QL-SD leds don't light up, floppy drive don't spin, etc. I had already tried to resettle all chip with no change in behavior.

So I have opened the QL and did a few measurements:

I have a Teroid voltage regulator and when measure directly at the output of the regulator, the voltage is 4.96 volts so this is fine. However, when measure at the +5V pin of the ZX8301 and the 68008, the voltage is 4.78 at best. Is this normal ? Could a damaged chip creates this voltage drop ? I have also measure using a scope the output of the crystal on pin #35 of the zx8301 and the result is a correct wave of 15mhz. When I measure the CPUclock line, I get some garbage and the scope try find a recurring frequency but shift between many values, with 15Mhz being the one seen most often.

That is the extent of my test so far. To be honest I don't know what to look for so any suggestion is welcome.

If anyone can guide me in my investigation, it would be appreciate.

François

That does sound like your ZX8301 chip may be faulty - it is responsible for video output, address decoding, ram refresh and access, and providing the cpuclock.

Do you have another one you could swap it with?

Pr0f wrote:That does sound like your ZX8301 chip may be faulty - it is responsible for video output, address decoding, ram refresh and access, and providing the cpuclock.

Do you have another one you could swap it with?

No I don't have any other 8301. Will order one from SellMyRetro ASAP

Is there anything else that can be checked with a scope on the ZX8301 ? Maybe the ram refresh signal ?

F.

Ugh, sorry you haven't been able to fix yours yet. Mine ended up just needing re-seating of all the chips. Has been normal since I did that over six weeks ago now.

The cpu clock signal is a good indicator - it should be a 7.5 MHz square ish wave - it does show some noise, but you should be able to set trigger threshold to get a half decent lock on a scope, you would also see 50hz frame pulses, as these get sent to video connector and the ZX8302, as well as the big edge connector

Hi,

To conclude on this, I have received a new ZX8301 ULA and, as expected, it solved the problem.

Next on the list is trying to fix my Golcard that behave strangly at boot time. This is a complexe problem so I will start a new thread.

François

Happy to hear you got it fixed. Mine hasn't glitched since I re-seeded the chips, so I'm happy I didn't have to open it up again.