Repairing ZX Spectrum issue two

[Pygmalion]

I recently dusted off my ZX Spectrum, which my father bought me and smuggled to Eastern Europe in the first half of the 1980s. What a story.

I made my first programming steps in Basic and Assembler on this computer. Those were the days.

But it does not work. I checked the -5V, +5V and +12V lines and they are fine. But when I switch it on, all I see is a flashing square (https://www.youtube.com/watch?v=9GzntY5dAAM). The pattern of the flashing square changes between resets.

Do you have any idea how I can make this thing work?

I have basic knowledge of electronics and soldering.

There are few peculiarities on the board: big cooler on ULA, unusual spider mod and something that looks like extra audio output.

[1024MAK]

Someone has worked on this board before. It's unusual for the 'lower' 4116 DRAM to be in sockets. The 4116 chips also have a very late date code for an issue two board. So they may have been replaced during 1983 or later.

There are a number of variations of the spider mod. I don't remember if I've seen this version before.

I'll have a look at your video later.

Mark

[redgatemodels]

unusual capacitors too, look like axial have been used instead of radial. That red and black wire is soldered to the location where the Plus case reset button usually connects to ....

Very early Issue 2 as well with the ceramic disc caps

[1024MAK]

As the ‘upper’ DRAM is in sockets, the first thing to do is to remove these, if only to eliminate them as the cause.

Note: the poster has also opened a topic on Spectrum Computing here.

Mark

[Pygmalion]

Someone has worked on this board before. It's unusual for the 'lower' 4116 DRAM to be in sockets. The 4116 chips also have a very late date code for an issue two board. So they may have been replaced during 1983 or later.

Judging from the back side, sockets were there from the very start.

There are a number of variations of the spider mod. I don't remember if I've seen this version before.

This spider mod includes MPS2369 with collector connected left of R9, base to Z80A pin 11 and emitter to left of R27; and 1k between Z80A pin 11 and ULA pin 13.

R57 seems to be missing.

unusual capacitors too, look like axial have been used instead of radial. That red and black wire is soldered to the location where the Plus case reset button usually connects to ....

Red and white wire is for reset button; the board was put into non-original case built in Yugoslavia. This was done by my father.

From what I've seen on the web, this board is pretty unique.

TR4 is replaced with BFJ50 RIZ. I suspect RIZ stands for Radio Industrija Zagreb, which means it is from Yugoslavia.

Fortunately, ULA is socketed and labelled from below. It is 6C0001E (4307)

[Pygmalion]

As the ‘upper’ DRAM is in sockets, the first thing to do is to remove these, if only to eliminate them as the cause.

I did. Almost nothing changed. Shall I will leave them out until the fault is found/corrected (or the fault is not found and ZX Spectrum gets thrown away)?

[1024MAK]

Okay, can you remove all the 4116 DRAM chips as well, then post up a video of the resulting screen display.

Mark

[1024MAK]

If the +12V and -5V are both within the tolerances shown in the service manual (ZX Spectrum Service Manual, available from here), with the 4116 DRAMs fitted, then TR4 and TR5 are working okay.

+12V +/-1.2V (10%).
-5V should between -5.5V to -4V (this is due to Sinclair using a strange design of circuitry).

Mark

[1024MAK]

As the ‘upper’ DRAM is in sockets, the first thing to do is to remove these, if only to eliminate them as the cause.

I did. Almost nothing changed. Shall I will leave them out until the fault is found/corrected (or the fault is not found and ZX Spectrum gets thrown away)?

Ha! No, it's reparable, you just have a more challenging fault than typical.

Leave them out for now, it's easier to fault find a 16K Spectrum. Which is what you have if IC15 to IC22 ('upper') DRAM is not fitted.

Mark

[Pygmalion]

Okay, can you remove all the 4116 DRAM chips as well, then post up a video of the resulting screen display.

Here it is from the reset on.

https://youtu.be/sN2nRGOs_vY