ZX Spectrum Issue 3 repair

[Tezz]

Are you sure that it is the Traco Power DC/DC converter module that is making the noise?

Only, the coil (transformer) that together with TR4 and TR5 on the Spectrum PCB (that produces the +12V and -5V supplies) often buzz. Some rather loudly.

Mark

That's a possibility with them both very close together, I'll listen closer to see if I can make out the source

[Tezz]

On an unrelated topic, did all of the issue 6 Spectrums come with 6C001E-7 ULAs? I bought one a couple of years ago non-functioning which had a 6C001E-6. I asumed that someone had swapped it out but it didn't appear to have been taken apart before. I've read that Saga produced some of the later ULAs, were they exactly the same design?

[1024MAK]

Throughout the life of Sinclair Research, computers were built with available stock. So no, it's possible that not all issue 6 machines came with the later ULA.

Throughout Sinclair made the minimum number of changes to the design of the ULA, so even though it has a different name, internally it is logically the same. How exact or not compared to the Ferranti ULAs is not known. See https://en.wikipedia.org/wiki/Ferranti#Semiconductors

Mark

[gozzo]

supposedly a -6 'shouldnt' work in an issue 6 , only -7, due to timing change, but might have been a 'selected' one that did !! -7 's are supposed to be used in issue 4 and above, except apparently samsung issue 4s which can handle -6 's .. ?!

[gozzo]

..that could've been why it didnt work..???

[Tezz]

Throughout the life of Sinclair Research, computers were built with available stock. So no, it's possible that not all issue 6 machines came with the later ULA.

Thanks, after some Google searches I've read that some earlier ULAs have worked reliably in later Spectrum issues and visa versa.

Throughout Sinclair made the minimum number of changes to the design of the ULA, so even though it has a different name, internally it is logically the same. How exact or not compared to the Ferranti ULAs is not known. See https://en.wikipedia.org/wiki/Ferranti#Semiconductors

Thanks, it would be interesting to understand the revisions/changes if they are ever documented.

supposedly a -6 'shouldnt' work in an issue 6 , only -7, due to timing change, but might have been a 'selected' one that did !! -7 's are supposed to be used in issue 4 and above, except apparently samsung issue 4s which can handle -6 's .. ?!

That's what I thought to be the case. I'll be interested to see if the -6 does function in the issue 6

..that could've been why it didnt work..???

I bought a 6C001E-7 ULA recently to try with it but it showed the same issue so I still need to find the fault.

[Tezz]

I'm just getting back onto this now, I was missing some sockets for the upper RAM. I had a look at another issue 3 board this morning, it's a 16k board so all the main ICs are socketed except 4 of the lower RAM. See photo attached of the output.. does this look like probably just a lower RAM fault? I first swapped out each socketed RAM chip at a time with a good one and the garbage changed which was a good sign, it also changed with a forced reset. I then swapped all the 4 socketed lower RAM chips with 4 soak tested good ones and the same problem persists so I guess one or more of the other 4 soldered chips are faulty. I've not tried swapping the ULA.

issue_3_fault.png (484.56 KiB) Viewed 5677 times

[1024MAK]

Well, the border is white, that normally indicates that the Z80 CPU has executed the first handful of instructions in the ROM.

The photo you posted is not typical of a lower RAM fault, so it may be worthwhile trying a good ULA in it. But keep in mind, what you get on screen, even with a RAM fault, is a combination of possibly wrong data due to RAM fault, plus garbage data that either is preset in the memory at power up (because it never gets cleared by the ROM code) and random data written to screen ram by the CPU which is executing garbage program instructions.

If your ROM is in a socket, power down, remove it, and test again (with the ROM not fitted).

Mark

[Tezz]

Thanks Mark, I've now tried a -7 ULA and no change so with the original ULA back in place I tried swapping the ROM with a replacement I bought recently, again no change. Neither the -7 ULA or new ROM has been tested in a known working board however so that's not conclusive. Could it be a bad capacitor? I think I remember reading a similar looking fault which was traced to a bad capacitor but if I remember correctly that one was c44 causing 12v issues in the circuit so perhaps not relavant.

[1024MAK]

If you have a colour picture, there is at least 9V on the +12V rail. Still worthwhile testing all three DC supply rails (+5V, +12V and -5V) with a meter. In fact, that's the first thing I recommend when powering up any Spectrum where it's condition is unknown.

It could still be one or two faulty 4116 DRAM chips.

Mark