Trying to fix my dead QL help needed

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FrancoisLanciault
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Trying to fix my dead QL help needed

Postby FrancoisLanciault » Mon Mar 23, 2020 4:42 pm

Hi,

In this thread https://www.qlforum.co.uk/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


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Pr0f
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Re: Trying to fix my dead QL help needed

Postby Pr0f » Mon Mar 23, 2020 5:11 pm

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?


FrancoisLanciault
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Re: Trying to fix my dead QL help needed

Postby FrancoisLanciault » Mon Mar 23, 2020 6:54 pm

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.


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bwinkel67
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Re: Trying to fix my dead QL help needed

Postby bwinkel67 » Mon Mar 23, 2020 7:58 pm

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.


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Pr0f
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Re: Trying to fix my dead QL help needed

Postby Pr0f » Mon Mar 23, 2020 8:21 pm

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


FrancoisLanciault
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Posts: 108
Joined: Mon Aug 08, 2011 11:08 pm

Re: Trying to fix my dead QL help needed

Postby FrancoisLanciault » Mon May 04, 2020 3:46 pm

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


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bwinkel67
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Re: Trying to fix my dead QL help needed

Postby bwinkel67 » Thu May 07, 2020 9:57 am

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.



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