Stuck with repair of Spectrum+

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speccyplus
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Stuck with repair of Spectrum+

Post by speccyplus »

Hi All, never owned a spectrum, just read a lot about them back in the day when I was using C64s and Ataris.

So I had a chance to pick one up for free! Looked to be in good shape, was in storage for the last 20ish years. Do no know if it was working or not, looks like it may have been opened in the past.

The keyboard large ribbon was broken at the fold, so that is shot unless I wire a connector all the way up and extend it from the motherboard.

Image

I did all the usual checks, the barrel connector re-soldered just in case. Coils looks good on the transformer, the specs from the regulator looked good, did some test on the lower ram bank.

TR4 and TR5 looked good from what I read on online doing diode test.

I then did the RF to Composite mod with 100uf cap.

So was feeling pretty lucky, and powered it on and got this:

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I then checked all the voltages coming in at the regulator, checked the first lower bank ram chip, got all around the specs for 0v,-5v, 5v and 12v.

I started feeling the chips with the ''finger" test and the 2nd from the left ram chipped burned my finger, I shut everything down, and felt all the other chips, nothing felt hot.

So after trying to solder suck, decided to cut the chip out, I then pulled the pins, solder sucked the holes clean and I didn't see any trace damage, so I tested with a multimeter, looked good.

Image

I powered it up again to see if I got a different screen, since I read you get different patterns for lower ram being bad and got this which implies that ram chip was bad(missing in this case):

Image

I then put a socket on.

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And got this screen.

Image

Checked all the continuity on the socket, power on and off, got the right voltages, etc.

With a scope I see I am getting clocks, the 3 and 14mhz on the ULA. CPU is showing activity, which I assumed anyways since I get the white border.

Not sure what changed, or what to look at next. I assume something was acting a bit flaky. The screen is in motion, flashing blocks and bars.

I did the 4164 to 4116 mod, cutting pin 1 and looping pin 8 over to pin9, installed that but no difference. I am not sure of the speed of the 4164 chips I had laying around.

I am thinking TR4/TR5 issues, but having a hard time finding what I should check for exactly, as the readings I get are close but slightly off I think.

I had a very similar looking issue on a C64 with the 7lS chip being defective, I am still reading up on the motherboard for these systems and how they work.

I cant seem to find any posts with a similar screen. Any help appreciated.
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Ast A. Moore
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Re: Stuck with repair of Spectrum+

Post by Ast A. Moore »

Hmm. Looks like your second screen is the best result you’ve gotten so far, which is curious. Just for kicks and giggles, try reseating the ULA and the CPU (if you haven’t already).

In each case it looks like the CPU can execute at least some code from the ROM, which is a good sign. I’d try disabling the upper RAM and, if that doesn’t help things, I’d remove all the upper RAM chips and retest.

Also, even though it’s pretty tedious, I’d check of all the data lines between the CPU, ROM, and RAM for continuity/shorts.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

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and zasm Z80 Assembler syntax highlighter.

speccyplus
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Re: Stuck with repair of Spectrum+

Post by speccyplus »

Thanks! I did reseat the CPU and ULA a few times.

How do I disable the upper ram bank?
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Ast A. Moore
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Re: Stuck with repair of Spectrum+

Post by Ast A. Moore »

speccyplus wrote:
Wed Apr 15, 2020 8:34 pm
How do I disable the upper ram bank?
You pull one of the inputs (pin 5) of one of the OR gates of the 74LS32 (IC23) up to the +5V rail. That will disable the CAS signal for the upper RAM chips (well, prevent it from reach them). Note, that if one of the upper RAM ICs is permanently pulling one or more of the data lines high or low, this procedure won’t clear the fault. But it’s easy to do and reverse, so it wouldn’t harm to try.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

speccyplus
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Re: Stuck with repair of Spectrum+

Post by speccyplus »

Cool thanks, btw this is an issue 4B board.

Looking closer at the back, I see old flux on a few places, it looks like maybe the CPU was not socketed before and a socket was added, and not done well, I am going to reflow that.

I see that IC14 may have been replaced, looks like TR1, TR2 and TR4 too. Also see IC15 as well as the mini phono jacks.
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Ast A. Moore
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Re: Stuck with repair of Spectrum+

Post by Ast A. Moore »

speccyplus wrote:
Wed Apr 15, 2020 9:29 pm
Cool thanks, btw this is an issue 4B board.
I know. First thing I looked at when I read your post. ;)
speccyplus wrote:
Wed Apr 15, 2020 9:29 pm
Looking closer at the back, I see old flux on a few places, it looks like maybe the CPU was not socketed before and a socket was added, and not done well, I am going to reflow that.

I see that IC14 may have been replaced, looks like TR1, TR2 and TR4 too. Also see IC15 as well as the mini phono jacks.
Sinclair usually didn’t socket the CPU (as far as I know). I wouldn’t worry about TR1, TR2, or IC14 for the time being, since you’re video works just fine.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

speccyplus
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Re: Stuck with repair of Spectrum+

Post by speccyplus »

I reflowed the CPU socket, no change, then tried disabling the upper memory, no change.

Any other thoughts? Maybe still a lower ram problem? I double checked all the pins on the new socket, they all have continuity.

I guess I will check the traces for CPU/ROM/RAM next.

I can start replacing parts, but its more fun to try to find the exact problem.

Will pulling all the upper ram and still leaving the support chips for it truly not leave any issues to cause problems?
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Ast A. Moore
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Re: Stuck with repair of Spectrum+

Post by Ast A. Moore »

speccyplus wrote:
Thu Apr 16, 2020 4:32 am
Maybe still a lower ram problem?
Well, it’s clear that the CPU is not executing the boot code from the ROM. Try resetting the CPU by momentarily pressing the reset switch and watch the screen. Does anything change? (Note that the flashing of attributes is performed by the ULA, not the CPU, so flashing in and of itself does not indicate “life.”) What happens if you hold the reset button for a few seconds?

One of the first instructions that the CPU executes upon reset is to set the border to white, which seems to be the case, judging from all the pictures you’ve supplied us so far. If this is a hundred-percent consistent behavior, chances are the CPU can talk to the ROM fine.
speccyplus wrote:
Thu Apr 16, 2020 4:32 am
Will pulling all the upper ram and still leaving the support chips for it truly not leave any issues to cause problems?
It depends. IC3 and part of IC24 do the multiplexing and select between the lower and upper RAM. This is controlled by the ULA. If one of them went tits up, you’re going to see garbage on the screen as soon as the ROM boot process starts writing and reading to the RAM. So, it’s not an open and shut case.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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1024MAK
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Re: Stuck with repair of Spectrum+

Post by 1024MAK »

If the +12V and -5V supply rails are okay, then TR4 and TR5 are fine.

Yes, you can remove all the upper DRAM chips (IC15 to IC22) plus the two multiplexer chips for this DRAM (IC25 and 26). Leave all the others alone. However as all these appear to be directly soldered to the board, I would do the videoing detailed below first. Then check that you disabled the upper DRAM chips correctly.

As said above, the white border indicates that the Z80 CPU is able to run the code from the ROM, but is getting stuck at some point further on.

Some “upper” RAM faults can also cause problems. Although a lot of the time, a Spectrum will fail the “upper” RAM and it will start up and operate as a 16k byte machine.

To disable the "upper" 32k of RAM, (so attempt to eliminate this as the problem) take a piece of wire and connect pin 5 on IC23 (a 74LS32) to +5V. (link).

I suggest you make a temporary solder connection with an SPST on/off switch in series. Start with the switch set to on. Then if you get to the copyright screen and BASIC works, after that, you can turn the switch off so that some simple BASIC commands can access and therefore test the upper RAM.

This is done because the ROM code tries to see how much RAM there is at start-up. After this, the BASIC ROM will ignore any RAM that is not present (or in this case, is disabled)(well, unless you reset the limits using CLEAR n). But the BASIC POKE and PEEK commands have no limits and work across the whole Z80 address range.

Double and triple check that you have the correct pin on the correct chip, or you WILL damage something. This temporary modification will disable the "upper" RAM (when the switch is ON), as the CAS signal won't reach the RAM chips.

Note that if a faulty DRAM IC is jamming one of the data lines, this won't really help.

Disabling or removing the upper DRAM won’t help if the problem is elsewhere, for example, another faulty lower DRAM chip, a cracked/broken track or via, or a poor IC/socket connection of dry solder joint.

Can you do a video of what happens after power up. This is best done with the TV left on, and with a TV that does not take any length of time before it gives you a picture. Upload the video to You Tube.

Then do the same, but this time, press and hold the reset button, then turn on the power. Then after the TV displays a screen of random coloured blocks, let go of the reset switch. Indicate when you do this so we know.

Mark
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speccyplus
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Re: Stuck with repair of Spectrum+

Post by speccyplus »

Thanks Mark!

I will do the video tonight, sneak peek, this is what happens if I hold the reset button and power up, this is constant and nothing changes, when I release goes back to the previous scrambled screen.

Doing the pin 5 to +5v shows no difference btw.
Image
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