Help repairing my ZX Spectrum+

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
Olipix
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Help repairing my ZX Spectrum+

Post by Olipix » Sun Nov 10, 2019 7:44 pm

Hi guys

Can someone help me identifying the problem my ZX Spectrum+ has ?
The motherboard is an issue 4B
This is the actual display right now :

Image

So far here is what I've done on this speccy :
Capcitors C34, C50, C44, and C45 changed
Transistor TR4 changed
Voltage regulator changed

All the 4116 RAMs have been tested and are ok.
+5, -5 and 12v voltages are ok

Could you help me identifying what may be wrong ?
ULA ? Z80 ? ROM ?

This type of garbaged display might be symptomatic of a specific problem?

Thanks for any help
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1024MAK
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Re: Help repairing my ZX Spectrum+

Post by 1024MAK » Sun Nov 10, 2019 8:19 pm

Does the border always turn to white when you power down, then up?

If yes, that’s a good indication that the Z80 CPU is able to read the first part of the ROM okay.

What happens when you press the reset switch/button?

Can you record a video of it as you power it up, or reset it please (upload to YouTube or somewhere else that lets you upload videos.

Is this a 48K model (although the ZX Spectrum+ was only available as a 48K version, ZX Spectrum+ cases & keyboards were available for users to upgrade their rubber key models) ?

And can you please report back which, if any chips are in sockets.

Also, can you please tell us how the 4116 DRAM chips were tested.

Mark
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Olipix
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Re: Help repairing my ZX Spectrum+

Post by Olipix » Mon Nov 11, 2019 8:43 am

Thanks Mark for your quick reply !

> Does the border always turn to white when you power down, then up?

yes

> What happens when you press the reset switch/button?

Well, the reset wires are desoldered and I don't know where exactly to resolder them.
Can you help with that ?

I will record a video when the reset button will be reinstalled

> Is this a 48K model

Yes
This is the mother board :

Image

> And can you please report back which, if any chips are in sockets.

The ULA and the low RAM 4116s are on socket

> Also, can you please tell us how the 4116 DRAM chips were tested.

I have tested all the RAMs on a working Colecovision.
1 RAM chip was broken, and I have replaced it with a "new old stock" working one.
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Ast A. Moore
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Re: Help repairing my ZX Spectrum+

Post by Ast A. Moore » Mon Nov 11, 2019 9:45 am

Olipix wrote:
Mon Nov 11, 2019 8:43 am
Well, the reset wires are desoldered and I don't know where exactly to resolder them.
On a Spectrum+, they are typically soldered directly to the leads of C27, which sits across the RESET and GND pins of the CPU.
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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.

Olipix
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Re: Help repairing my ZX Spectrum+

Post by Olipix » Mon Nov 11, 2019 9:48 am

Yes, I found the information too this morning...
I soldered the reset button on both sides of the C27, bit it seems to do nothing on the computer when I press the reset button...

Here is the video :

https://www.youtube.com/watch?v=ai-S3MqJbuU

I noticed that if I let the computer powered on for a few minutes, borders start to change colour from white to blue or red, then black...
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Ast A. Moore
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Re: Help repairing my ZX Spectrum+

Post by Ast A. Moore » Mon Nov 11, 2019 10:35 am

Thanks for making the video.
Olipix wrote:
Mon Nov 11, 2019 9:48 am
I noticed that if I let the computer powered on for a few minutes, borders start to change colour from white to blue or red, then black...
This is curious. Wait until the border changes to something other than white, then press the reset button momentarily. Does the border become white?

If it does, than it looks like the CPU can execute a few ROM instructions, including a jump. I’d check the address lines for continuity/shorts. Possibly a broken trace or a solder splash.
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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.

Olipix
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Re: Help repairing my ZX Spectrum+

Post by Olipix » Mon Nov 11, 2019 11:35 am

I've made a second video :

https://youtu.be/nYZ58mBKXqw

After a few minutes (at first Reset does nothing), pressing the reset button change the border color !!

Does that help to narrow the problem ?
Do you have a guideline to test the address lines for continuity ?

Thanks for your help
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Ast A. Moore
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Re: Help repairing my ZX Spectrum+

Post by Ast A. Moore » Mon Nov 11, 2019 11:58 am

Olipix wrote:
Mon Nov 11, 2019 11:35 am
After a few minutes (at first Reset does nothing), pressing the reset button change the border color !!
Yes, but not to white.
Olipix wrote:
Mon Nov 11, 2019 11:35 am
Does that help to narrow the problem ?
I’m kind of suspecting the CPU/ROM now. At this point, I’d replace them with known good parts beginning with the CPU. If you have an oscilloscope, check the clock at Pin 6 of the CPU.
Olipix wrote:
Mon Nov 11, 2019 11:35 am
Do you have a guideline to test the address lines for continuity ?
Nothing specific other than using a multimeter in continuity mode with the computer unplugged from the power source. Test each address line for continuity from the CPU to ROM, RAM, and multiplexor ICs (IC3, IC4, and IC 24). Test each address line for shorts with each other at a few points (CPU/ROM).
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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.

Olipix
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Joined: Sun Nov 10, 2019 7:06 pm

Re: Help repairing my ZX Spectrum+

Post by Olipix » Mon Nov 11, 2019 2:38 pm

Sadly, I don't have any Oscilloscope right now

I will change the Z80 first and let you know what happens

Thanks guys for your help
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1024MAK
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Re: Help repairing my ZX Spectrum+

Post by 1024MAK » Mon Nov 11, 2019 6:18 pm

Ast A. Moore wrote:
Mon Nov 11, 2019 9:45 am
Olipix wrote:
Mon Nov 11, 2019 8:43 am
Well, the reset wires are desoldered and I don't know where exactly to resolder them.
On a Spectrum+, they are typically soldered directly to the leads of C27, which sits across the RESET and GND pins of the CPU.
See this photo:

Image

Before trying to change any soldered in chips, do you have experience of desoldering?

Mark
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