Sinclair ZX80 / ZX81 / Z88 Forums

Hello,

Ive recently been given my dads ZX81, its been stored in the loft for many years and had minimal use. I thought id get it out and have a little play with it, but sadly it isnt working .

The ZX81 powers on and produces a white screen on the TV, but i dont get the black cursor.

The board is getting the 9v & 5v supply ok.
All chips are getting thier 5v supply.
Im getting the 3.25mhz clock out of ULA on pin 14 and its reaching the Z80 on Pin 6.
Address lines & Data lines seem to be sending data.
Ram & Rom Clock is pulsing
Ive check continuity between chips referencing the circuit diagram.
Im getting a video signal from the ULA on pin 16 so i believe the ULA to be working?

The chips are soldered to the board so i would like to try and diagnose the problem before potentially trying to desolder chips :-/.
Im not sure what else to try or where i should look next?
I have access to an Oscilloscope.

Board is Iss 3 ULA Video Out - PIN 16 Ive done a composite video mod.

Check for a potential "missing" jumper around LK1. I clearly understand that your motherboard has its 1k (2) RAM chips. Thank you for the picture of your board.

Regards,
DesSony

No worries, Happy to take pictures

I traced the line where the jumper LK1 links, and it appears to go from pin 18 of the 2nd Ram chip to a dead end, so im guessing its only used when having a different RAM chip configuration.

Anything that stops the Z80 CPU from running the system code from ROM will cause a white screen.

One common cause of a ZX81 with a white screen is faulty RAM. But a fault with the ROM, the Z80, the ULA or any of the components that link any of these chips can cause the same symptoms.

Not applicable for your board, but if sockets are used, a poor contact of anyof the chips can cause this symptom.
As can a dry solder joint, or a broken (fractured/cut) PCB track.

Do you have a known good RAM pack available to test with?

On the Z80 CPU, can you test the control pins with your ‘scope please and list the results.

Mark

With the 2114 / 21L14 SRAM chips, there is no need for LK1 or LK2 to be fitted. 21L14 are the lower power versions of the 2114 chip. Each 2114 provides 4 bits. With two 2114 chips, that gives 1K bytes.

Either LK1 or LK2 is needed when different SRAM chips are fitted. For example if a single 4118 1K byte SRAM chip is fitted, LK1 should be joined with a copper link wire. If a single 4016 2K byte SRAM chip is fitted, LK2 should be joined with a copper link wire.

Mark

You may find this post useful.

Mark

Thanks for the help. I dont have a ram pack but i will look to see if i can get one cheap, so that i can test with this. When using the ram pack im guessing it ignores the internal memory then?

When you say control pins do you mean the pins mentioned in that post link?

16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Will test them with the scope tommorow.

ZeddyRepair wrote: ↑Mon Mar 25, 2019 11:47 pm Thanks for the help. I dont have a ram pack but i will look to see if i can get one cheap, so that i can test with this. When using the ram pack im guessing it ignores the internal memory then?

Yes. With an external RAM pack, the internal RAM chip(s) are disabled. So as long as it/they are not shorting out any address or data lines, the system no longer sees it/them.

If you can’t beg/borrow/steal a known good working RAM pack, instead of paying for a used one, instead you could either make your own using a 32K byte SRAM chip, or upgrade the on-board memory to 16K bytes by removing the existing SRAM chips and fitting a 32K byte SRAM chip on the board.

If you do decide to get a used RAM pack, the Memotech type regularly appears on the likes of a certain web auction site. These are far more reliable than a Sinclair RAM pack. But you do need to use a Sinclair “9V” PSU with the 16K version (or a PSU that outputs between 11V DC and 12V DC when loaded).

Yes, the control pins in the linked to post, plus address pin A14 and address pin A15. This will give us an idea if the Z80 CPU is alive and well.

Mark

Any reason for the Sinclair 11-12v mark?

I only ask as I have been using my Memotech packs on a regulated 7.5v supply (multi-metered today just to make sure) for some years with no problems and I'm now worried I could be causing damage.

Which one? The 16K, the 32K or the 64K? My comment above only applies to the 16K RAM packs.

The 16K version uses 4116 (or equivalent) DRAM chips, which require three different supplies, one of which is a +12V DC nominal supply (with a tolerance of +/- 10%). To get the “+12V”, they use the nominal “+9V” supply from the ZX81 edge-connector (which is just the input +9V routed straight through).

This works, because when not at full output current, the output voltage from a Sinclair 9V power supply unit is typically greater than 10V DC. Often around the 11V mark.

A Memotech 16K RAM pack may well actually work at lower voltages, but below 10.8V, the 4116 DRAM chips are working outside the specifications stated by the manufacturer.

Note that the above also applies to some other third party 16K RAM packs.

The Memotech 32K or 64K RAM packs are not affected, as they use DRAM chips that only require a single +5V supply.

@Moggy - If you are using a 16K version, I don’t think the +9V (which feeds the +12V supply to the 4116 DRAM chips) being low will damage them, as long as the inverter circuit which generates the -5V supply can still operate at the applied input voltage. I forget now the exact details of this circuit, but it uses a positive supply rail and generates a low current -5V supply for the 4116 DRAM chips. If this -5V supply is lost, or gets close to 0V, this may cause internal damage to the 4116 DRAM chips.

Incidentally, the reason that some people operate their Zeddies on a regulated 7.5V PSU is simple. The ZX81 uses a 7805 series pass voltage regulator chip to provide the +5V supply rail. It’s minimum input is between +7V and +7.5V (depending on which manufacturers version you use). By running a ZX81 at 7.5V, the amount of power the 7805 has to dissipate is considerably reduced, which in turn means less heat is produced, so the ZX81 runs cooler

Mark