ZX81 rebuild

[1024MAK]

Yes, with no on board RAM, you can run a Zeddy with an external RAM pack.

Don’t despair, you’ll get there in the end

Mark

[Lardo Boffin]

Fingers crossed.

I tried it without RAM and with a ZXpand attached to a Chroma 81 and got nothing.

I have reflowed all of the pins on the sockets and that made no difference either. I recently got a very strong magnifying glass so will go over the pcb at some point looking for anything dodgy. I may have to remove all of the sockets first though I guess.

[1024MAK]

Okay, so to sum up, excluding the SRAM chip, the Z80, ULA and ROM have been tested in another Zeddy and work fine.
Different known good Z80, ULA and ROM chip have been tested in in this board, and it makes no difference.

You are using a 16K SRAM chip. You have tried at least three chips. Which type / make by the way, only actual 16K SRAMs are rather rare? Further, you have tried the board with no on-board RAM, but with a known good external RAM pack (or equivalent).

Can you please put the chips back in and try again. Test both the /RESET pin of the Z80. And the clock pin on the Z80 (pin 6). Do this with a logic probe or better, an oscilloscope.

Mark

[Lardo Boffin]

I don’t have a ‘scope so used a logic probe.

Reset (pin 26) on Z80 goes low / high quite quickly.
Pin 6 on Z80 has a relativley slow steady pulse.

I’m not sure of the make of the RAM. It says ALLIANCE on the chip.

[1024MAK]

Ahh! Those SRAM chips in your photo are modern 32K x 8 bit SRAM. Giving 256K bits, hence the last three digits of the numeric part of the part number. But in a typical minimal ZX81 internal RAM configuration, only half is used, giving 16K bytes of useable RAM. According to the date code they were made late in 2016.

The clock on pin 6 should be so fast that you don’t see any pulses, but the logic high and logic low LEDs should show even levels of brightness (both at about half brightness). If your logic probe has a pulse detector, this however should continuously detect and retrigger.

Mark

[Lardo Boffin]

After much prodding.

Key
HLMP - High / Low / Medium Pulse
LFP - Low / Fast Pulse
HSP - High / Slow Pulse
HLFP - High / Low / Fast Pulse
HLVP - High / Low / Variable pulse (slow through to fast in a cycle)
HFP - High / Fast Pulse
HMP - High / Medium Pulse
H - High / No pulse
L - Low / No pulse

Z80 results

Code: Select all

Pin	Expected	Actual
=======================================
1	HLMP		L
2	HLMP		L
3	LFP 		L
4	HLMP		LSP
5	LFP		L
6	HSP		SP
7	HLFP		HLMP
8	HLFP		HLMP
9	HLFP		HLMP
10	HLVP		HLMP
11	H		H
12	HLMP		HLMP
13	HLFP		HLMP
14	HLMP		HLMP
15	HLMP		HLMP
16	HLSP		LMP
17	HLFP		H
18	HLFP		L
19	HLSP		LSP
20	HFP		H
21	HLMP		LSP
22	HMP		H
23	H		H
24	HLFP		H
25	H		H
26	Low to High	Low to High
27	HLMP		LSP
28	HLMP		LSP
29	L		L
30	HLMP		HMP
31	HLMP		HLMP
32	HLMP		LMP
33	HLMP		HLMP
34	HLMP		HLMP
35	HLMP		HLMP
36	HLMP		LMP
37	HLMP		L
38	HLMP		L
39	HLMP		L
40	HLMP		L

ULA results: -

Code: Select all

Pin	Expected	Actual
=======================================
1	HLMP		L
2	HLMP		L
3	HLMP		HLMP
4	HLVP		HL
5	HL		H
6	HLMP		LSP
7	HFP		H
8	HSP		H
9	HLSP		LSP
10	HLSP		H
11	HLSP		L
12	HLSP		H
13	HLSP		HLMP
14	LSP		SP
15	HLFP		HLSP
16	H		HLMP
17	HLFP		H
18	LFP		L
19	HLFP		H
20	L		L
21	HLMP		H
22	H		H
23	HLMP		L
24	HLMP		L
25	H		H
26	HLMP		H
27	H		H
28	HLFP		L
29	H		H
30	HLMP		L
31	H		H
32	HLMP		L
33	H		H
34	L		L
35	H		H
36	HLMP		H
37	HLMP		H
38	HLMP		H
39	HLMP		L
40	H		H

So not looking too healthy! The expected results were taken from a healthy zeddy.

Slow / medium and fast pusle is a little subjective. Slow is a couple of times a second while fast is so many per second that it is not possible to count but it is still flashing.

I was using an LP-2800 logic probe as shown below. Set to TTL and Pulse.

[Lardo Boffin]

Having swapped all of the chips and replaced the resonator I figured that the only things left that could be dodgy were the transistors. So I changed them.

And there we go. A K again!

Now I just need to fit a video out without killing it...

[TMAOne]

Congratulations on a very neat job Lardo Boffin. I just love a happy ending.

Now that tape scrubber prototype needs to find its way back to you so you can try loading your problematic cassettes with this machine.

[Lardo Boffin]

Yes. I keep forgetting about the tape scrubber.

I originally socketed the new 6.5 MHz crystal so decided to remove the socket and solder it in.
Did this and switched it on and got nothing. Arggghhh!!!!
So I removed it and soldered in a reclaimed one from a nearly bare board and it worked straight away.

I don’t have a plug in keyboard to test the connectors / diodes etc so I plugged in my memotech. It works well (so far anyway).

Sorry about the heat sink Moggy. Thats how it arrived to me.

I will however be putting it in a custom case with plenty of air flow.

Things I have learned from this experience:-

How to de-solder!
A bit more about how the zeddy works.
New 6.5 crystals do not like me.
If you connect the outputs of a ZX8 the wrong way round you will need to replace your transistors.
After a year of pain I can definitely say this feels like MY ZEDDY!

I am definitely going to try and do this again. At some point. When I have recovered from this one.
And finished it for that matter - still a couple of sockets to add and the question of output - do I bother or just use it with my Chrome 81?

Maybe an issue three next time? Anyone have a parts list?....

[Lardo Boffin]

I hooked up a keyboard borrowed from elsewhere and that works fine. Presumably I got the diodes the roght way round!

Shortly after taking the photo of boulder crush the screen started jumping a bit - horizontally which is strange.

It does this fairly consistently after a while. But only with hi-res games. I played Gauntlet and 3D Moster maze for a while without it doing this once.

I will experiment with different chips and TVs to see if it is specific to this TV. None of my other zeddies do this on this TV but it has historically been the most picky with signal quality.