Bright White screen .No cursor

[povvercrazy]

I've recently got hold of a nice clean Issue 3 ZX81, unfortunately I just got a white screen no cursor, I've recapped it, did a composite mod (transistor version), still getting a bright white screen no cursor, it has a sinclair labeled rom, a zilog z80 and a single 24 pin toshiba ram chip, I've noticed that both the cpu and the rom get quite warm, ram stays cool. I've just got a logic probe and ran through the various pins, I got some strange results off some pins, Pin 16 of the ULA is giving high/low and pulse which I believe means the ULA is functioning okay?? as far as display goes?
Im waiting for a scope to arrive so I can check further but any ideas what could be wrong or what I should check next, im concerned the rom is running quite warm I wouldn't say hot but very warm. I've also check continuity from each pin (on the shoulder of the chip) through to the relevant pin according to the schematic, cant find any shorts or open circuits.
I've socketed out the ram and done an internal 16k upgrade no change, thinking og getting a new cmos z80 as there quite cheap and swapping that to see if that helps, oh and the transistors appear ok...

any ideas?

[1024MAK]

So which chips are now socketed?

The best way forward, if you have spare chips, or better, a working ZX81 with sockets for all the chips, is to swap the chips between the faulty board and a working board, one at a time (suspect chip goes in working board - does it still work? Known good chip goes in fault board, does it clear the fault?).

The Z80 CPU should run slightly warm, but not hot. I forget how warm the ROM runs, but it should not run hot. I think mine run cool. But Sinclair used various different chips from different suppliers.

Which ULA and CPU pins did you test? Can you post your results. The most important pin on the CPU is the clock input (pin 6). If this is not pulsing at 3.25MHz (approx), the CPU will do nothing.

Mark

[povvercrazy]

Thanks for your reply Mark, unfortunatly I only have one zx81 so swapsies are out, I tested continuity on all pins all chips, and tested all pins with a logic prob, i'll post my results in the morning when ive written them up nicely, pin 6 on the cpu was high and pulsing (ph) so I dont think that is correct, i've ordered a diy DSO kit so hopefully when that arrives I can check the frequencies.
I did notice that on some points I was getting all leds pulsing I don't think that should happen.
I also noticed that some of the results changed after a while when I went back to check some results....
oh and only the ram is socketed it was a 24 pin chip i've now fitted a 28pin socket and a 28pin 62256 ram chip mod.

[povvercrazy]

Hope I've made my results clear, tried to colour code the errors to make it easier to look between chips, results were take with no tv or keyboard connected not sure if that will change anything.

I notice HALT is high, so as that is linked to WAIT and NMI, via TR1 could TR1 be faulty? or something ? also CLK has PL is that a problem ?
I know very little of z80 or the proper ULA behaviour

[Zxkim81]

Hi matey
Have you tried booting your zx81 with the 16k ram pack on
I think it could be the internal memory at fault at a guess.

If it does boot to the K with the ram pack attached then you have your answer......

Zxkim81

[povvercrazy]

Shes only a 1k model no ram pack, I had a thought about RAM possibly being an issue so I did the 16k internal mod as it was a very cheap solution and the same result :'(
I've just ordered a new Z80 and a couple of transistors so I'll replace those which just leaves the ULA and ROM.

[amigaman07]

If your ULA is a known Good one, and you install the New CPU and still get No Cursor (K) it could be the ROM. (Smaller Chip between ULA and CPU)

As this stores all the ZX81 Machine Language and working Memory.

I do have 1 Working Spare ZX81 ROM I could part with, if needed.

It is currently in a ISS 3 TX 1000 Mainboard. Dont think this makes any difference?

Darren in OZ (amigaman07)

[1024MAK]

Oops, I forgot about this thread

Sorry if you were waiting on me...

I'll take another look this weekend.

Mark

[povvercrazy]

Thats ok Mark, I've been sitting here staring at the screen waiting for your response for the last few weeks... made quite a mess of myself :p
No seriously, I had put it on the back burner as I had been also trying to sort out a 48k spectrum plus various other projects.
I now have a 'toy' digital oscilloscope so I can get some rough data, and today I had another ZX81 I found on ebay turn up so
I'm back looking at the damn thing.
I was hoping to use this recent acquisition to play swapsies and fault find but the new one is an issue 1 with the daughterboard on top of the z80
so I'll have to decide what to do with that, its another fauly unit, but I haven't tested yet.
I have removed all chips and installed sockets on the Issue 3 I'm trying to get working, also installed a murata 5v 7805 replacement.
In the second bundle of kit I have a 16k ram expansion (unsure if that works though) so might be able to use that to test.
I was also thinking of burning a new rom for the issue3 as I've just got an eprom programmer and been messing with that too.

[povvercrazy]

Well good and bad..
The issue 1 with the rom patch appears to work fine, its just the typical lack of back porch problem, so as its in pretty good condition
should I leave as is, or add a back porch mod to it?
The issue 3 has developed a further problem, 'if' i get a white screen it goes black after a few minutes then its had to get a white screen back.
I'm pretty sure I tested the voltage on the power supply before I used it but cant remember anyway now its outputting 16v....
So I have switched to my bench power supply set to 9v and 500mA and its only drawing about 0.14-0.2 mA.
I am still getting signals on pin 6 z80 so has the ula been fried ?
Rom still gets warm.
Should I risk swapping the 184 ula in the working issue 1 to the temperamental issue 3 which has a 210? will it work?