White screen on a ZX81

[Quantum]

Hi to all,
I am waiting on a vLA81 ULA from NZ to fix my TS1000, while waiting I purchased a ZX81 for parts hoping to swap out the ULA.

When I got around to open up the ZX81 I was dissappointed to see all the IC were soldered in, it even has the 2C210 ULA dated 8240. Just what the TS1000 needs.

I decided to keep it all original, but did do a composite mod so that I could use it as is. I did the mod, and all I get is a very nice white screen with no
K cursor

I have tried a ram pack, checked voltage readings all seem OK, I even checked for a clock signal on pin 6 of the Z80 and got a reading of 3.238 MHz.
What other tests can I do? Is there a step by step testing method? Any help is muchly appreciated.

Cheers.
RJ

[1024MAK]

I don’t know of a step by step guide, other than spending some time browsing and reading through the hardware sub forums here…

The ULA appears to be working if you have a white screen and the correct clock signal at pin 6 of the Z80.

Because EVERYTHING (apart from the keyboard) has to be working in order to get to having a “K”, it could be the Z80, the ROM or the RAM.

Did you clean the edge-connector contacts on the board before trying the RAM pack? And is the RAM pack a known good working unit?

If the answers to both are yes, then that tends to indicate the internal 1K bytes RAM chip(s) are not the cause. Leaving the ROM and the Z80 as prime suspects.

So, I suggest the next step, is to see what the Z80 is doing. The following assumes you are using a logic probe or oscilloscope. What signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
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)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark

[Quantum]

Hi Mark,
Thanks for the reply and information. I did clean the pcb contacts for the rampack, it is a Memotech 64K unit but not sure if it's good.
I do not have a scope or a logic probe, although I have ordered a logic probe and will test the pins as per your last post.
I will let you know the results.
Cheers.

Richard.

[Quantum]

Hi Mark,
I got a logic probe and tested the pins, below are my findings.

Pin Function
16 / INT is pulsing, showing H & L
17 / NMI is pulsing, but rapidly, showing H only
18 / HALT is pulsing, but is rapidly, showing H & L
19 / MREQ is pulsing rapidly, showing H & L
21 / RD is pulsing but not rapidly, showing H & L
22 / WR is pulsing, showing H only
24 / WAIT is pulsing, but rapidly, showing H only
25 / BUSRQ is HIGH
26 / RESET is HIGH, LOW for a short time after power is applied
27 / M1 is pulsing, showing H & L
28 / RFSH is pulsing, showisng H & L

A0 is pulsing, showing H & L
TO
A12 is pulsing, showing H & L
A13 is pulsing rapidly, showing L
A14 is pulsing, showing H & L
A15 is pulsing rapidly, showing H & L

D0 to D7 all have activity.

Let me know what you think.
Cheers.

Richard.

[1024MAK]

The only one of these that concerns me is A13.

Can you please test this again.

Assuming address line A13 is working, then that indicates that the Z80 is likely okay.

Can you test pin 20 on the ROM.

And can you either take a photo of your board (make sure it’s possible to see the details on the ROM) and post the picture here. Or describe the details on the ROM.

Mark

[Quantum]

Hi Mark,

A13 is pulsing very rapidly, showing L only

ROM pin 20 is pulsing showing H & L The ROM is a Motorola ZCM38818P date 82 40

My old camera is only 5MP but works great for Ebay, I hope the pictures come out OK for you.

Cheers.

Richard.

[Quantum]

Hi all, I finally got back to working on this problem, and I can not understand why it only showing a white screen with no K prompt. I done the same video mod that I used on my other ZX81, and my TS1000 and they both work on my LCD TV. I got to thinking that the mod is for a composite monitor, I got out my old green screen composite monitor and plugged in the ZX81 and up came the K prompt , I also tried the other two and all worked perfectly on this green screen monitor? Can anyone tell my why this one ZX81 will not display the K prompt, on the LCD TV through the Video IN RCA Plug? The other two work perfectly, even my TS2068 works?
Cheers.

Richard.

[1024MAK]

The circuitry differs widely between different TVs. Back in the days of CRT TVs, a monitor was essentially a TV without a tuner, but with ‘baseband’ (composite video) inputs or RGB inputs.

Since the advent of specialised computer video standards (such as VGA), the definition of a monitor has changed somewhat.

Which composite mod did you do? There are a number of variations. So it would be helpful if you could point to the type you used.

There are also slight differences with the output voltages from the ULA chip, even the same types.

The video output pin on the ULA actually outputs three voltage levels. A low for the sync low. A high for peak ‘white’ video. And a intermediate voltage for the black video level. If the output stage of the ULA has degraded or been damaged, it may not produce the correct voltages. Some TVs/monitors are more tolerant of this than others.

Sorry about not replying to your earlier post. I was seconded to carry out another role, and I was extremely busy.

Mark

[Quantum]

Yes I agree, my LCD TV is fairly old and most likely won't work with this ZX81

I just did the one NPN transistor a 2222A, with one 100 ohm resistor off the E of the transistor connected to the center of the RCA and then to ground, worked very well on my other two, but not this bone stock one.

The ULA is a 210 E date 8240 as seen in the pictures I posted a while back, and seems to be functioning correctly. A friend suggested that I try the composite MOD that uses a 555 IC and see if that does the job. I look simple enough to build, and will try it when time permits.

Thanks for all your help.

Rich

[1024MAK]

Try my ZX81 Video Transistor Buffer Amp version before your try the 555 version. The main benefit of the 555 version, is to add the back porch. This should not be needed for a 210E ULA.

Mark