Need help understanding some Issue 2 Mods

[MrClump]

I've had an Issue 2 on the bench for some time that works, but has some mods I don't understand!

First there is a 1KΩ resistor near the CPU that bridges 5V to Pin 32 on the ULA. I've lifted one leg of it and it doesn't seem to affect anything.

Next the legs of IC26's socket on pins 10 & 11 are shorted by a solder bridge between the pins beneath the socket, but then the leg of pin 11 is lifted and connected to GND by a wire. As an experiment I removed this mod and then the Spectrum only starts as a 16k machine. One other clue is that none of the upper ram select links have been jumpered.

There is no spider mod present.

[1024MAK]

First there is a 1KΩ resistor near the CPU that bridges 5V to Pin 32 on the ULA. I've lifted one leg of it and it doesn't seem to affect anything.

The 1kΩ resistor near the Z80 that goes between the +5V line and pin 32 on the ULA is R73 on later board issues. It is part of the Z80 clock circuitry. It’s a modification detailed in the service manual (see section 4.1). It’s fitted to improve the clock signal and therefore improve the system reliability.

Next the legs of IC26's socket on pins 10 & 11 are shorted by a solder bridge between the pins beneath the socket, but then the leg of pin 11 is lifted and connected to GND by a wire. As an experiment I removed this mod and then the Spectrum only starts as a 16k machine. One other clue is that none of the upper ram select links have been jumpered.

This has been done because the issue two board is only compatible with some types of DRAM chip. From the issue three boards onwards, the PCB and the circuitry was altered to allow DRAM chips from a different manufacturers to be used.

Not all issue two boards got the spider modification, or it may have been lost if the Z80 was replaced. Looking at the date code on the Z80 that’s fitted (made in 1987), I would say it’s the latter reason.

Mark

[MrClump]

The 1kΩ resistor near the Z80 that goes between the +5V line and pin 32 on the ULA is R73 on later board issues. It is part of the Z80 clock circuitry. It’s a modification detailed in the service manual (see section 4.1). It’s fitted to improve the clock signal and therefore improve the system reliability.

Thanks for the info, I've found that in the service manual now. You wouldn't believe I've read it from start to finish, but obviously didn't absorb/retain that.

This has been done because the issue two board is only compatible with some types of DRAM chip. From the issue three boards onwards, the PCB and the circuitry was altered to allow DRAM chips from a different manufacturers to be used.

Aha! As you can see on the pic below this Speccy has OKI ram fitted. I'm assuming this mod has been carried out by a third party at some point, not sure the factory would have completed it in such a messy fashion?

I'm going to see if I can re-instate it in a tidier way, the chip having its leg cocked up like it's doing the cancan at the Moulin Rouge offends my sense of order. If I removed that pin on the socket, then attach a thin wire to the leg of the chip that can be passed down through the socket and the via to make a connection underneath the board. This would allow me to have the leg of the chip back in the socket, but not actually making electrical contact with the socket or the via underneath. It might work!

Not all issue two boards got the spider modification, or it may have been lost if the Z80 was replaced. Looking at the date code on the Z80 that’s fitted (made in 1987), I would say it’s the latter reason.

I seem to recall the mod was half hanging off, so I removed it with the intention of putting it back once the Speccy was working again.

Thanks for the advice, top notch guidance as always!

[1024MAK]

...the chip having its leg cocked up like it's doing the cancan at the Moulin Rouge offends my sense of order.

Sorry, I lost it at that point. Once I’ve recovered from a lot, I’ll continue...

Mark

[1024MAK]

There is another way to do the spider modification (adding TR6).

Some photos of the ‘spider’ on one of my issue two boards here.

Mark

[MrClump]

There is another way to do the spider modification (adding TR6).

Some photos of the ‘spider’ on one of my issue two boards here.

Mark

Thanks for the link, very useful! I've completed the spider mod using that as a guide, much tidier than having it straddling the CPU, my diag cartridge confirms the floating bus is present so I guess that means it's working. As the CPU is socketed I wanted it to be possible to remove it without interfering with the mod, so I soldered the wire from the base of the transistor to a via on the same trace as PIN30 rather than to the CPU leg itself.



I was able to re-implement the mod to allow OKI ram on the underside of the PCB by removing from the socket of PIN11 on IC26, soldering a wire to the chip and passing it down through the via, then used a jumper on the underside. All that is visible from above now is a small blob of solder on the leg of the chip which is no longer legs akimbo.







Now to figure out why it has no colour signal! I thought at first it was because someone had been fiddling with TC2, but there seems to be more to it than that. After much fiddling around and swapping out X2 I've noticed there is no 4.432Mhz signal on PIN17 of the LM1889 when the Speccy is cold. I can replicate it my spraying a cold spray in the area, when it warms up the signal re-appears.


PIN17 when cold


PIN17 when warm

The whole hot/cold thing has made me think there is a bad connection somewhere, I've started poking around with the multimeter but haven't found it yet (having to have the board cold hampers this a bit). But, when PIN17 is outputting the correct signal it is also not steady, shaking up and down. I have swapped out the LM1889 but it's the same.

[Hedge1970]

Very nice work. So many mods get applied to these boards both by factory, service centres and people like us. Personally I find it fascinating

[MrClump]

I've now got the colour correct, the trimmers were waaaaaay out of whack! Someone has clearly been fiddling. My xmas pressie (oscilloscope) that I'm still learning how to use made it super easy. Still got to fiddle with the dot crawl though, which I think I just have to do by eye.

[MrClump]

But, when PIN17 is outputting the correct signal it is also not steady, shaking up and down. I have swapped out the LM1889 but it's the same.

Could this jittering be there because I have a modern Murata switching voltage regulator fitted rather than a 7805?