QL wont work

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Traindriver69
ROM Dongle
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Joined: Mon Feb 16, 2015 8:42 pm

Re: QL wont work

Postby Traindriver69 » Sun Sep 23, 2018 3:42 pm

I have replaced the regulator which is now showing correct voltages and it now tuned in but only to a black screen.


Traindriver69
ROM Dongle
Posts: 37
Joined: Mon Feb 16, 2015 8:42 pm

Re: QL wont work

Postby Traindriver69 » Sun Sep 23, 2018 5:15 pm

Hi all
I have tried what it says in the ql service manual about no sinclair screen and tested D26,C41,C38 and C42 which all checked out apart from D26 which instead of showing 12v is showing 21.5v.
many thanks


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belg4rion67
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Location: Milan - Italy

Re: QL wont work

Postby belg4rion67 » Sun Sep 23, 2018 7:08 pm

D26 is the diode that bring negative rail to -12 volt regulator ic so is it possible that you have checked the ac voltage that feed power supply; try check the c40 condensator if it has 0 and -12 volt, but in effect that voltage is needed only for rs232 ports so is not the main problem ....


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Traindriver69
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Re: QL wont work

Postby Traindriver69 » Mon Sep 24, 2018 9:44 am

hi C40 is showing 12.31v and 0v so that looks ok.


Traindriver69
ROM Dongle
Posts: 37
Joined: Mon Feb 16, 2015 8:42 pm

Re: QL wont work

Postby Traindriver69 » Mon Sep 24, 2018 10:27 am

hi
would like to check out the power supply just in case it is that causing the problems I did find the other day someone who showed a picture of the inside of the power supply showing what voltages they should get off the numbered points of the transformer but now I need it I cant find it has anyone got such a thing please.
many thanks


martyn_hill
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Re: QL wont work

Postby martyn_hill » Mon Sep 24, 2018 10:52 am

Hi Traindriver

[EDIT: You'll find the QL PSU schematic with voltages annotated here: http://www.dilwyn.me.uk/docs/hardware/QLperip.zip]

You'll find quite a lot of relevant info already discussed here on the forum across many good articles, but here's some thoughts gathered together...

Before going to the trouble of replacing ICs that are soldered to the board, the following ICs that rely on the +5v rail are socketed and could be replaced with known-good units more easily:

1. ZX8301 (IC22) - the display and DRAM controller (the so-called 'Master Chip')
2. ZX8302 (IC23) - the peripheral controller - 'PC'
3. 8049 (IC24) - the secondary processor or 'IPC' - managing Serial input, Keyboard and Sound *
4. 2x ROM (ICs 33 & 34)**
5. MC68008 (IC18) - CPU

Items 1..4 are fairly easy to come by (e.g. from Rich's 'SellMyRetro' auction site), although the ZX8301 is getting more scarce...

The CPU is trickier to find - at least in the required DIP form-factor.
[EDIT: Found quite a few 48-DIP form-factor 68008s on eBay at a very reasonable price - make sure you go for the -L or -P suffix - and not the 'FN' suffix which is in PLCC or similar form.]

* Instead of replacing item 3 with another 8049, you'd want to take the chance to upgrade it to the 'Hermes' equivalent.
** Instead of replacing the originally masked-ROM ICs directly, consider a Minerva EPROM upgrade or better-still, Marcel's updated Minerva+QL-SD solution (but only once you've got the rest of the system running again.)

These items are not especially costly individually, but you'd still be looking at 60 - 120 GBP for all five items, assuming you can still find them - and even then, there's no guarantee that other components wouldn't also need replacing (though typically cheaper.)

Except the TV modulator (which wouldn't itself stop the system running), the other ICs relying on the +5v rail are readily available 74-series logic/buffers - though not socketed (there's a couple of RS-232 line drivers, but the system would still function even if they were dead.)

Aside from the ICs, my guess is that the other passive components are probably still OK - most of the smoothing caps in the QL for example are rated at 16V or more, so may well have survived.

Whilst I wouldn't wish to discourage you from pursuing the potential satisfaction of fixing-up this unit, bear in mind that you can pick-up another working QL for less than that from time to time on eBay, etc and even a fabulous Q68 might cost less!

[EDIT: How about this replacement QL for auction on eBay? (nothing to do with me...):

https://www.ebay.co.uk/itm/Sinclair-ZX- ... 0282.m3503
]

Good luck!



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