Spectrum+ 48k repair

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

Post by Slauri » Tue Sep 24, 2019 4:45 am

There was modifications on the joystick/sound unit too.
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Oct 25, 2019 3:13 pm

Hi,
I have now removed the old cpu from the pcb. I don't remember, that is the resistor on the left side of the cpu supposed to be connected to the +5v leg of the cpu. I'll put a picture here so you can get better understanding of what i mean. I just want to be sure.
Image
Image
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore » Fri Oct 25, 2019 7:01 pm

Yes, the resistor is supposed to be there. Here’s one of the pictures I referenced to earlier:

Image
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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.

Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Oct 25, 2019 7:35 pm

Of course the resistor is supposed to be there, i know it, but is it supposed to be connected to the +5v leg through a solder blob?
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK » Fri Oct 25, 2019 8:15 pm

Yes

Mark
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK » Fri Oct 25, 2019 8:44 pm

ZX Spectrum Issue 1 and 2 boards
Extra resistor near the CPU.
This is R73, a 1kΩ resistor connected between +5V and ULA pin 32 / R24 / C67 (D14).

From the service manual:
service manual wrote:This modification is required for efficient operation of certain machine code software and should be implemented on all units. It has been implemented in manufacture for all ULAs 5C112-2 and later.
D14 replaced by C67 (100pF)
R24 changed from 3k3 to 1k
R27 changed from 680R to 470R
R73 (1k) added between IC1/32 and +5V
Mark
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Sat Oct 26, 2019 6:53 am

Thanks.
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