Spectrum+ 128k repair help (No CPU clock signal from working ULA)

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handsfelloff
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Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by handsfelloff » Sat Dec 30, 2017 11:56 am

Hi Everyone,

I am trying to coax a Spectrum+ 128 (toastrack) back to life.

The symptoms:
Power on I get a Black border (likely indicating cpu was faulty or a clock issue) and black and white bars inside. Sometimes clean black / white 'bars' occasionally with odd bits.

What I checked:
I checked for 3.5Mhz clock on the CPU clock pin 6 and nothing there. So I moved back and checked the clock output from the SLAM (pin 39) and found also no signal. I checked the following ULA pins VCC 1 & 2 on pins 13,14, 17Mhz pin 1, 8.8Mhz on pins 46,47 and ground on 48. IORQL pin 40 @ 3.8v.

I also did resistance checks from the z80 data / address lines to 5v and ground (high 8/9M)
I did low resistance checks along the data / address lines to from the ULA <470> CPU <low> ROM and these looked reasonable.
I checked the ram pin 2 to other pins for shorts and found none.

I checked TR3 / TR6 ands these are ok.

So i am not seeing any clock from pin 39 of the ULA (as I am comparing to a working spectrum)

The known good:
ULA is a new SLAM module fitted in the socket (continuity checks to from the module to the board ok)
New EPROM fitted
TR3 & TR6 removed, checked and put back, both matched

I have:
a DMM
mini Oscilloscope 203
a working toastrack to compare
no much time but the will ;)

I'd really appreciate any ideas / help with tracking this one.

Many thanks,

Wilson

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Re: Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by RWAP » Sat Dec 30, 2017 2:08 pm

Check that the 1K pullup resistor on the ula's clock pin output is present on the Speccy motherboard - SLAM needs it but it has been known to be omitted on some toastracks.

handsfelloff
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Joined: Sat Dec 30, 2017 11:30 am

Re: Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by handsfelloff » Sat Dec 30, 2017 10:14 pm

Thanks Rich,

Yep the pull up was omitted and now in place its heart yet beats! Thank you! Now its White border with black paper plugged into SCART I see occasional screen twitch flicker.

On SLAM an impressive job indeed, only that I found no documentation. This would have saved a lot of head-scratching 'debugging'

This is a very promising road to recovery. Perhaps this could be a faulty RAM IC...? I will do some checks.

handsfelloff
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Re: Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by handsfelloff » Sun Dec 31, 2017 8:35 pm

So I have a white border with a black square that remains (most powers ons, although occasionally its all white with the odd coloured block also every 3-5secs the picture 'twitches' RGB out SCART)

I suspected a RAM fail and I tried to run a RAM tester from divMMC https://github.com/brendanalford/zx-dia ... Tape-Tests
I could boot into ExDOS (which is very promising), but I when I tried to load the RAM test program it immediately hang / crashed. That could I suppose be because it trying to load the program into faulty RAM.

I tried the piggy back RAM trick, which made no difference I also note that the RAM itself does not get warm or hot. Aside from pulling RAM chips, possibly what I need is a plugin RAM tester that runs independently of the system RAM. I have heard Paul Farrows ROM works well.

Any recommended RAM tester or other tests / checks I can make to hone in on the problem?

Many thanks and happy New Year!

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1024MAK
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Re: Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by 1024MAK » Sun Dec 31, 2017 9:21 pm

Any ROM / EPROM / EEPROM / flash based RAM tester will work, but you need either an expansion device that has EPROM / EEPROM / flash memory, or be able to burn a EPROM / EEPROM / flash chip and put it in place of the machines ROM chip.

See this site ;-)
Note that the diagnostics boards are currently not available.

Mark

handsfelloff
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Re: Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by handsfelloff » Mon Jan 01, 2018 10:24 pm

Thanks Mark,

I'll probably try to pick up an Ram Test burned EPROM.
I wondering if it could possibly be a fail with the RAM address decoder? on the toastrack this a ZX8401

See 2 of the white border black centre fails on this page (two +'s IC3 & IC4)
http://hardware.speccy.org/reparar-i.html

Also I may look at the PAL IC29 (I have a spare one about)

Given there is a socket I'll buy this replacement board (if it turns out to be ok I'll at least have the back up)
https://www.sellmyretro.com/offer/details/4241

Wilson

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1024MAK
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Re: Spectrum+ 128k repair help (No CPU clock signal from working ULA)

Post by 1024MAK » Tue Jan 02, 2018 1:09 am

When a 16k or 48k ZX Spectrum is reset or powered on, one of the first things that the code in the Sinclair BASIC ROM does is to set the border white. It then does a memory check (not a full memory test) to see how much RAM is fitted. During this test, values are written to all the 48k RAM area, this makes the non-border area of the screen go black. Shortly after this, if your eyes are quick, you should see thin vertical red lines, then the screen goes white and the normal copyright message gets displayed.

In a U.K. ZX Spectrum 128k machine, there is an extra area of ROM, so the sequence is not the same as I describe above.

I'll have to check what actually happens. The ROM in the Spanish version of the ZX Spectrum 128k is different again...

But any failure of any of the RAM chips that are part of the screen RAM (which also has the system variables) or the RAM where the Z80 machine stack is located, will result in a frozen machine...

Mark

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