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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Spectrum+ 48k repair

Post by Slauri »

My old spectrum+ 48k won't work. I have replaced the electrolytic capacitors, and it didn't fix it. Here's a video of what it does when i switch it on:

https://youtu.be/mGKvQZV_2P4

Help, please.
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

Does the image change when you press the reset button? What happens if you press and hold 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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Re: Spectrum+ 48k repair

Post by Slauri »

Nothing happens
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Re: Spectrum+ 48k repair

Post by Slauri »

And the manufacturer of the power supply is unknown.
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

First, check the voltages on the RAM, ROM, and the CPU.

It’s likely the CPU or (less likely) the ROM that is fried. If you have an oscilloscope, test the clock on the ULA (Pin 39) and the CPU (Pin 6). Check TR3 for the presence of the clock pulse.

The usual procedures (i.e. checking for broken traces, solder splashes, etc.) also apply, naturally. Was the machine working before recapping?
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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Re: Spectrum+ 48k repair

Post by Slauri »

I don't have an oscilloscope, and it didn't work before the recapping, but the video quality is much better after recap.
I have a multimeter.
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

Slauri wrote: Sun Sep 22, 2019 2:27 pm I don't have an oscilloscope . . .
I have a multimeter.
Proceed to checking the voltages as I stated above. On the Z80, Pin 29 is ground and Pin 11 is Vcc. On the ROM, it’s pins 14 and 28, respectively.

The lower RAM chips (ICs 6–13) have three voltages: –5V on Pin 1, +5V on Pin 9, and +12V on Pin 8. Pin 16 is the ground.

Incidentally, what’s your logic board revision? Is it an Issue 4B?
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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Re: Spectrum+ 48k repair

Post by Slauri »

In my opinion, it looks like lower ram fault. But i'm still a beginner in these things.
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Re: Spectrum+ 48k repair

Post by Slauri »

Issue 2
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Re: Spectrum+ 48k repair

Post by Slauri »

And btw i should get a new power supply for this thing
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

Slauri wrote: Sun Sep 22, 2019 2:43 pm In my opinion, it looks like lower ram fault. But i'm still a beginner in these things.
No. In that case, the border would have still turned white (unless one of the lower RAM ICs was hogging the data/address lines).
Slauri wrote: Sun Sep 22, 2019 2:44 pmIssue 2
Wait, so you have an Issue 2 board in a Spectrum+ case?
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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Re: Spectrum+ 48k repair

Post by Slauri »

Yes
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Re: Spectrum+ 48k repair

Post by Slauri »

How do i check the voltages?
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Re: Spectrum+ 48k repair

Post by Slauri »

It's actually my uncle's old speccy. He bought it used. That would explain the mods.
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

Slauri wrote: Sun Sep 22, 2019 5:31 pm How do i check the voltages?
Set the multimeter in the DC voltage mode, select the 20V range, and touch the black (common) probe to the pin marked as GND in each picture and the red probe to the other pins as indicated. The indicated voltage values are nominal; your actual measurements might deviate slightly from them.

Lower RAM:

Image

CPU:

Image

ROM:

Image
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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Re: Spectrum+ 48k repair

Post by 1024MAK »

Please note that it is very important that you don’t accidentally short out any of the pins on the chips. So be careful with your meter probes.

Can you post a photo of the power supply that you are using.

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

Post by Slauri »

Image
Image
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK »

Going on the rating plate/label, that power supply looks fine.

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

Post by Guesser »

Ast A. Moore wrote: Sun Sep 22, 2019 2:50 pm Wait, so you have an Issue 2 board in a Spectrum+ case?
I have an issue 2 Spectrum+, it's an Upgrade case.
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore »

Guesser wrote: Mon Sep 23, 2019 12:58 pm
Ast A. Moore wrote: Sun Sep 22, 2019 2:50 pm Wait, so you have an Issue 2 board in a Spectrum+ case?
I have an issue 2 Spectrum+, it's an Upgrade case.
Sure, I know they existed. It’s just they don’t pop up all that often; so, whenever I see someone mention a Spectrum+, I assume it’s an Issue 4A or later.
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 »

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

Post by Slauri »

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 »

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

Image
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 »

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 »

Yes

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
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