Spectrum Computing

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?

Yes

How do i check the voltages?

It's actually my uncle's old speccy. He bought it used. That would explain the mods.

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:



CPU:



ROM:

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

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

Mark

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.

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.

There was modifications on the joystick/sound unit too.

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.

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

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?

Yes

Mark

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

Thanks.

This diode snapped in half at some point when desoldering the lower ram chips. Can somebody tell the specs?

Slauri wrote: ↑Sat Dec 28, 2019 10:26 am This diode snapped in half at some point when desoldering the lower ram chips. Can somebody tell the specs?

Looks like a 47nf capacitor used on the Speccy and not a diode if it came from C4?

Never seen a capacitor like that.

Slauri wrote: ↑Sat Dec 28, 2019 12:29 pm Never seen a capacitor like that.

They were pretty common back in the day. A ceramic cap will do just fine in its place, though. Don’t sweat it.

I picked up some 100nF (I think) capacitors like that from a surplus supplier (Greenweld) many years ago.

Anyhoo, the machine should still work with it missing. I do however recommend you fit a modern 47nF or 100nF multilayer ceramic as a replacement.

Mark

And it doesn’t hurt to combine several capacitors of different values in parallel, say a 47nF and a 1µF one. Each capacitor will operate at a different frequency, because their efficiency is very frequency dependent. In some cases, however, it’s mostly gilding the lily.

Did anyone ever fit the 220nf capacitors on the lower RAM to help with the “jailbars” ?

Morpheus wrote: ↑Sat Dec 28, 2019 6:56 pm Did anyone ever fit the 220nf capacitors on the lower RAM to help with the “jailbars” ?

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