Spectrum+ 48k repair

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

Slauri wrote: ↑Fri Feb 14, 2020 6:15 pm Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.
The -5V supply (pin 1 on the lower RAM sockets) should be between -4.45V and -5.1V.

Can you please post up the actual voltages you get on the +5V, +9V and +12V rails please.

Do you still have a heatsink on your 7805 voltage regulator? If yes, the machine screw holding it in place nice and tight? Or have you fitted a more modern DC/DC converter switching type?

Mark
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

Slauri wrote: ↑Mon Feb 10, 2020 12:10 pm On the ULA pin 13 there is only 3.17 volts. On the pin 14 there is almost perfect 5 volts. Is this normal?
On pin 13 of the ULA in an issue 1 board, I get 2.9V soon after power up, with the voltage rising slowly a little bit.

Mark
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

Pin 9 5.01 volts, pin 8 12.14 volts, pin 1 4.80 volts (lower ram)
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

Slauri wrote: ↑Fri Feb 14, 2020 6:15 pm Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.
Yestreday i measured the -5v rail the voltage was much lower.
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

Slauri wrote: ↑Sat Feb 15, 2020 4:23 pm Pin 9 5.01 volts, pin 8 12.14 volts, pin 1 4.80 volts (lower ram)
Those voltages are all fine :D

Mark
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

Slauri wrote: ↑Sat Feb 15, 2020 4:24 pm
Slauri wrote: ↑Fri Feb 14, 2020 6:15 pm Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.
Yestreday i measured the -5v rail the voltage was much lower.
Strange :shock:
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

@Slauri
Can you please post a photo of your Spectrum board as it is now.

Mark
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

Image
Image
Image
Image
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

Everything I have replaced:
1)electrolytic capacitors
2)the entire lower ram
3)zilog z80 cpu
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

I don't know what to do.
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

Can you please take a close up photo, under good light, of the underside area of the board where the 4116 DRAM chips are.

Mark
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“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

Image
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

I’ve enhanced your picture and marked a few areas that I suspect might be solder bridges/shorts:

Image

Perhaps Mark will be able to see more. I’d remove the lower-RAM board and inspect the traces on the logic board underneath it.
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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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri »

I've cleaned the board multiple times with alcohol. Those bridges weren't there before. Maybe they have come from my workdesk? I have not cleaned it for a while. Some of those may look like bridges but they are just reflections of the soldering mask that has melted away due to my bad soldering skills.
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