Issue 3 zeddy no K on screen output

[Lardo Boffin]

I have just acquired a zeddy issue three which has no K displayed when switched on.

I can see that it appears to be trying to produce an output - when it is off the screen is just fuzz but when on the screen goes a solid light grey / white depending on tuning.

Is this most likely to be the ULA? It is not socketed so have not been able to chip swap yet. Need to order some sockets...

Lardo

[Lardo Boffin]

I have reseated the ROM and RAM chips and tried a working external 16k RAM pack.

[sirmorris]

Try different PSU.
Does it 'work' - type (blind) SAVE "P" and listen for any save noises out of MIC socket.
Try a composite output on it to eliminate the modulator.
If none of the above then likely ULA.

[Lardo Boffin]

Try different PSU.
Does it 'work' - type (blind) SAVE "P" and listen for any save noises out of MIC socket.
Try a composite output on it to eliminate the modulator.
If none of the above then likely ULA.

Thanks. The PSU is a working one so I will try the save trick and then the composite mod. Cunning plan.

[Lardo Boffin]

I have tried saving and get nothing recorded (tried on a working zeddy and that certainly left some sounds on the tape!). But then the keyboard ribbon was cracked so I cut that down and reattached it and so can't be 100% sure that is working until I get output! There's a hole in my bucket...

Will composite mod it and see if that helps.

[sirmorris]

Will composite mod it and see if that helps.

Yeah, just tack it on underneath. I now have a 'quick test' mod that has an RCA on one end - it gets tacked on to the mobo and voila! Robert is your father's brother.

[1024MAK]

White screen means part of the ULA is working, and the modulator is at least partly working (or fully working).
Any of the chips could be the cause. Or a bad joint / damaged track / short circuit.

You have tried an external RAM pack, so as long as the internal RAM does not have a bus-interface fault, it's not the internal RAM.
EPROMs sometimes fail (forget some data), but Mask ROM chips are normally reliable. If it is socketed, check by swapping it to a working Zeddy.
That leaves the CPU and the ULA.
Z80 CPUs are fairly reliable, but do sometimes die. If it is socketed, check by swapping it to a working Zeddy.
ULA chips are the most difficult, because (a) replacements are difficult to find, and (b) they fail in many different ways. Again, if it is socketed, check by swapping it to a working Zeddy.

If most (or all) of the chips are soldered, then it's time to attack the Zeddy with test gear. At the simplest level, use a logic probe, or a 'scope to check the clock (pin 6), control lines (/RESET, /MREQ, /IORQ, /RD, /WR, /RFSH etc) and the A14 & A15 address lines on the CPU (see the link below for pin-outs).

One failure that I have had, was the ULA was failing to provide a clock to the CPU, so the CPU just sat there doing nothing.

Mark

[Lardo Boffin]

White screen means part of the ULA is working, and the modulator is at least partly working (or fully working).
Any of the chips could be the cause. Or a bad joint / damaged track / short circuit.

You have tried an external RAM pack, so as long as the internal RAM does not have a bus-interface fault, it's not the internal RAM.
EPROMs sometimes fail (forget some data), but Mask ROM chips are normally reliable. If it is socketed, check by swapping it to a working Zeddy.
That leaves the CPU and the ULA.
Z80 CPUs are fairly reliable, but do sometimes die. If it is socketed, check by swapping it to a working Zeddy.
ULA chips are the most difficult, because (a) replacements are difficult to find, and (b) they fail in many different ways. Again, if it is socketed, check by swapping it to a working Zeddy.

If most (or all) of the chips are soldered, then it's time to attack the Zeddy with test gear. At the simplest level, use a logic probe, or a 'scope to check the clock (pin 6), control lines (/RESET, /MREQ, /IORQ, /RD, /WR, /RFSH etc) and the A14 & A15 address lines on the CPU (see the link below for pin-outs).

One failure that I have had, was the ULA was failing to provide a clock to the CPU, so the CPU just sat there doing nothing.

Mark

Thanks Mark. The Z80 and ULA are both socketed and I lack a logic probe so limited options at this stage! I will check for dodgy joints and tracks.

I can see quite a few logic probes at around the tenner mark on eBay - any recommendations for a cheapie? It won't be getting a lot of use to be fair...

[Lardo Boffin]

Will composite mod it and see if that helps.

Yeah, just tack it on underneath. I now have a 'quick test' mod that has an RCA on one end - it gets tacked on to the mobo and voila! Robert is your father's brother.

Hmmm. Get an RCA plug and connect it to the ZX8-CCB and maybe connect flying leads to allow it to be clipped to the ULA? Instant composite mod with no soldering if it works.

[1024MAK]

Will composite mod it and see if that helps.

Yeah, just tack it on underneath. I now have a 'quick test' mod that has an RCA on one end - it gets tacked on to the mobo and voila! Robert is your father's brother.

Hmmm. Get an RCA plug and connect it to the ZX8-CCB and maybe connect flying leads to allow it to be clipped to the ULA? Instant composite mod with no soldering if it works.

Ahh, I must get around to taking a photo of mine...

Mark