vLA81 ULA and 16/32K internal RAM

[1024MAK]

Thank for the links. I was wondering how using one of these to get 32K would work in relation to your mod process?

Presumably the same tracks would be cut. Would the part where you reconnect the two cut tracks be connected directly to a pin on the vLA?

The orange wire in the photo above is maintaining the +5V (VCC) rail, so that stays
All the track cuts are done exactly as shown in the blog entry.

It’s the uninsulated wire in this photo that goes:


Instead, connect the remains of the track that goes to pin 2 (of the SRAM chip) to address line A14 that comes from the Z80 CPU.

Mark

[1024MAK]

Top of ZX81 PCB showing temp +5V wire link to be removed.

Bottom of ZX81 PCB showing signal details including A14

Bottom of ZX81 PCB showing wiring and added A14 wire

Mark

[Erm]

I always prefer not to cut anything. Just personal preference, I'm trying to go for the record of having the most pins bent out on a ZX81

[Lardo Boffin]

Presumably the major issue with bent pins on an IC is that it makes maintenance more difficult in the future?
Would it be possible to fit a low profile socket, then fit another socket (pins bent out) and put the IC in that? Only the sandwich socket is modified and the RAM chips are easily swappable?
Or would the additional socket start to cause issues with good connectivity from IC to PCB?

[Erm]

Yes, I suppose it's harder to swap out a RAM chip if it has soldered attachments, but the way I see it, it's not like they are likely to see so much use that the new RAM will ever fail and even if it does, if you use a modded ZX81 in 2019 then it's likely a bit of soldering won't be all that daunting a task.

Not sure about space issues with 2 sockets. That would probably work and probably without any connectivity probs. On my own quite modded ZX81 I have socket strips under both sides of the Z80, with the RAM A11-14 connections on the strip, so that if needed the 32k RAM chip can be completely removed along with its connections to the Z80 and then the Z80 popped back into its normal socket etc.

[1024MAK]

Presumably the major issue with bent pins on an IC is that it makes maintenance more difficult in the future?
Would it be possible to fit a low profile socket, then fit another socket (pins bent out) and put the IC in that? Only the sandwich socket is modified and the RAM chips are easily swappable?
Or would the additional socket start to cause issues with good connectivity from IC to PCB?

It’s for each person to decide which method they prefer. My primary reason is so that I can swap ICs easily (during fault finding or when experimenting or modifying). And I have no issues with modification of PCBs. When I make PCBs, if I make a mistake, or change my mind later (original part no longer available for a reasonable price say, or in order to amend the functionality), I have no scruples about cutting tracks. The same as original equivalent manufacturers would have done, and indeed did do in the past. Both Acorn and Sinclair amongst others recommended modifications (by service agents) to existing PCBs and until a new board issue could be produced, included modifications to the current production run. Sometimes this involved bent pins, but other times it included cutting tracks. Sometimes both. Often the actual method used depended more on practically and ease rather than anything else.

If ZX81s were ultra rare, then yes, then you would maybe keep it completely original. But that means keeping it as a 1K byte machine. All of the ZX81s that I have modified arrived in a poor state, either incomplete, non-working, faulty or damaged in some way. So if not repaired/fixed, they could have just been binned. The working ZX81s that I have in good condition have not had any modifications carried out. Although I dare say I am behind compared to some members here with the number of ZX81s that I have bought . In fact, I have only modified four ZX81s so far out of the twelve that I have (some are in storage in the awaiting attention “pile”), and one of these had already been changed from a stock model (a TS1000 issue 3 board where someone had cut R30 out). How many people have ripped the guts out of the modulator so that a composite video circuit board can be fitted inside?

If there is space (in a ZX81 there is space for at least a single extra socket, that is PCB socket, ‘sandwich’ socket and IC) and if the existing socket contacts are still tightly fitting, yes you can use a couple of stamped pin sockets to act as adaptors. Or use a single socket if all the bent pins are going to be picking up signals via wiring that does not need to access the existing PCB socket contacts (unless the existing socket is a turned pin socket). Or use a single socket mounted on stripboard with suitable pins to fit in the existing PCB socket. However this later approach may expand the contacts in the existing PCB mounted socket, such that if a chip is reinserted, not all pins may make good contact .

Most sockets these days are described as low profile. If you want really really low profile, that means that the socket pins must be designed such that the part that acts as the socket for the IC pin also goes through the PCB. Hence the PCB holes have to be a bigger diameter. These won’t fit in an existing socket.

Anyway, I’ve waffled on for far too long. Do only what you feel happy about .

Mark

[Moggy]

Excellent post.

They are there to be used and abused imo. not polished or put in glass cabinets.
The first thing I did to my 80 was fit a switching reg, then ripped the guts out of the modulator to fit a composite board then finally a new ROM for ZXpand use and ended up with a much better tooter.

My only regret was fitting the replacement keyboard membrane which is as annoyingly stiff as the early ZX81 replacements were, the ZX80 original keyboard although crude never really required such a hard keypress as the replacement ones seem to do.

But as Mark says each to their own.

[Lardo Boffin]

Excellent post.

They are there to be used and abused imo. not polished or put in glass cabinets.
The first thing I did to my 80 was fit a switching reg, then ripped the guts out of the modulator to fit a composite board then finally a new ROM for ZXpand use and ended up with a much better tooter.

My only regret was fitting the replacement keyboard membrane which is as annoyingly stiff as the early ZX81 replacements were, the ZX80 original keyboard although crude never really required such a hard keypress as the replacement ones seem to do.

But as Mark says each to their own.

An excellent post indeed. And not owning 12 zeddies myself (I think maybe 8?) I feel better about my zeddy compulsion. Although I think I have bought in the region of 40 or more most have been mercilessly refurbed and sold on.

I’m not precious about cutting tracks but am looking to limit the risk of me bodging it when refurbing them. Especially this time of the year in my Dickensianly cold workshop (the garage). Can’t feel my fingers after 5 mins!

Certainly not meant to be put in glass cabinets. Erm...

To be fair this one is dead but I quite liked the Sinclair ROM bodge so decided not to fix it.

My recently rediscovered family zeddy (first computer I ever actually touched, happy memories) is untouched. I’m not sure I will ever refurb that one. It works fine even after three decades in the loft.

[Moggy]

I have said it once before and will say it again Lardo your refurbs, expertly done then passed on at a good honest price with an equally honest listing, does you credit and it 's nice to know there are at least 40 odd people who are part of our world, hopefully, with a decent tooter to play with.

As for a workshop I long ago commandeered the spare bedroom and made it my zeddy shrine.

As for the zeddy in the picture frame I don't think it is actually a bodge as such as I'm almost certain that at one point Timex couldn't get their hands on the real thing so used EPROMS instead.

[1024MAK]

I quite liked the Sinclair ROM bodge

You mean ‘mandatory modification’ surely

Mark