Help debugging Harlequin 48k rev g kit

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lmsilva
Posts: 1
Joined: Mon Sep 23, 2019 7:17 am

Help debugging Harlequin 48k rev g kit

Post by lmsilva »

I was wondering if anyone can give me some tips on debugging an Harlequin 48k rev g I just assembled?

Image
Image

Here’s what I already tried:
- double checked the jumpers
- measured the voltage on all ICs (they’re all getting 5v)
- double checked all ICs are in the right spot, reattaching each one and cleaning all the sockets individually
- changed all ICs (including the CPU and ROM)
— I have a spare Harlequin 128k I tried to assemble a couple of years ago and failed (heheh) so I found one of each ICs that could be swapped and changed them one by one, booting upon every change to see if it made any difference (nothing changed)
- cleaned all contacts, made sure everything is soldered “properly” and, to the best of my knowledge, there are no shorcuts anywhere
- after cleaning everything, I did manage to get a cleaner picture with all vertical black lines (SOMETIMES), so cleaning seemed to have some effect
Image
— also tried booting with a zx hd adapter (which attaches on the back and interprets the memory to recreate the screen) and that results in the exact same screen (which I interpret as meaning the computer is displaying exactly what is on ram as the zx hd reads the exact same, so the ram should be fine, the problem is probably between the cpu and the ram?)

Any thoughts?? I’m a total noob when it comes to hardware so any help is greatly appreciated!
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1024MAK
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Joined: Mon Sep 26, 2011 10:56 am
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Re: Help debugging Harlequin 48k rev g kit

Post by 1024MAK »

Hello and welcome to our forum :D

The links to your images don’t work as displayable images. They appear to be URL web page addresses.
You may want to see if the image hosting site you used has direct links to the image files. Or if not, convert the links you have provided into clickable links by using the URL tags.

In regards to your Harlequin board problem. Does the border always change to non bright white (sometimes described as light grey) every time you power it up?

If yes, then the CPU is running the first part of the machine code program from the ROM. Is this a real Sinclair ROM chip, or is it an EPROM programmed with the ROM code?

The machine appears to be stalling during the test to see how much memory there is. I suspect there is a fault with the address, control or data busses. Have you tested each address and data line from the relevant pin on the Z80 to the relevant pins on the RAM chips using the continuity or resistance range on your multimeter?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Help debugging Harlequin 48k rev g kit

Post by Moggy »

Right clicking on the word IMAGE and then view image works for me but to save time I took the liberty of downloading them so others could diagnose.
Attachments
eI6RJo9.jpg
bpZH4MB.jpg
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