Resurrecting a ZX81 - Step by step help appreciated.

[rune]

Thanks Mark, there's a lot of info to digest there. I'll dig out the printer later and get all as hard copy. As you have probably guessed, Im a novice at this electronics thing, but trying to learn.

I'll stick to one board now for tests and get pics of the pins you mentioned.

Cheers

Dave

[1024MAK]

Keep in mind that most of the Z80 signals don't repeat in a regular pattern over a short duration, so your 'scope will show a different pattern of pulses depending on when it triggers. At this stage, we are more interested in knowing if the relevant pin is active (switching between logic high { 2V to 5V} and logic low { 0V and 0.8V }) rather than the actual pattern of the pulses. Hence no need to take photos of every signal on every pin.

Mark

[rune]

Hi Mark,

I did try some of the pins today and they did show signals. I never got any photos though as I didnt have a lot of time.

My plan is to get all the information here printed out plus the chips pin outs and then pick a board and work on that.

Thanks for your advice, its greatly appreciated.

Dave

[rune]

I've selected an issue 3 board and got a bit more testing done.

The oscillator left pin puts out a nice low amplitude sine wave, the right pin puts out a meandering wave.

I can get a very similar meandering wave at Pin 14 on the ULA. Tracing the circuit to the ULA side of C7/R6 I get the same wave as at P14.

Moving to the other side of C7/R6 I can confirm the track between there and the TR2 base is good. But there is no wave. Testing on a working board does show the signal here.

Ive replaced the ZTX313 with a BC238B (turning 180 degrees to get same orientation)
There are no shorts between the oscillator or TR pins

What could be causing the lose of signal/wave after the C7/R6?

[sirmorris]

A short upstream pulling the track low?

[zx-heinz]

Check R6 for the right value with a meter. It should have 2.2kOhm and it is located between the ULA and the CPU for Iss3 boards.

[1024MAK]

Check to make sure nothing is fitted in the position for R3.
Also keep in mind that the signal at the base of TR2 will be lower than ULA pin 14.

One more thing, TR2 has to be a transistor that can switch off very, very fast. Most (nearly all) easy to get plastic cased transistors may not work in this position. So if the ZTX313 tests okay, I suggest you refit it.

Mark

[RetroTechie]

TR2 acts like a buffer / 'enhancer' for the Z80 clock. So at the Z80 (pin 6) the clock signal should have much steeper edges (more like a block wave) than at the ULA output (pin 14).

Seems to me that's not the case on several of the scope pics shown earlier. Which suggests that TR2 or surrounding components (R6/C7/R5) are faulty. That or the ULA output (pin 14) is gone - which would show when ULA is plugged into a working ZX81 board I think?

If that's okay, check what happens at Z80 reset (pin 26) during power up. And/or C5 can be short circuited to produce a reset pulse. Then check control signals / address lines like /MREQ, /M1 etc as listed above.

Btw. if RAM socket wasn't modified, I'd just insert a 1K*8 SRAM (or a 2K*8, and change L1 wire bridge to L2 position), and remove RAM pack. Much higher chance to find working SRAM chips, than RAM packs that still work 100%. Especially if you have multiple SRAMs to choose from.

[rune]

Thanks Charlie, Heinz and Mark

I've checked the connection between the C7/R6 and the transistor base which is good. Checking between both and all other pins nearby, this doesn't show a shorts.

Tried without the Z80.

The emitter is attached to ground. R6 is 2.20 K Ohms.

The only thing that is different is the transistor. I should have some correct ones today or tomorrow. But would a "bad" transistor make the signal disappear after C7/R6? I will try decreasing the Y values to see if the signal is low.

Hi RT,

the RAM Pack definitely works. I tested on a working board. I cant swap ICs though as the working one has everything soldered to the PCB and I dont want 2 dead boards.

Comparing signals, both boards show the same signal up to the C7/R6 components. Then nothing on the dead board

[zx-heinz]

But would a "bad" transistor make the signal disappear after C7/R6? I will try decreasing the Y values to see if the signal is low.

Yes, it would in case the Tr2 has an inner short from emitter to basis. Replace that transistor by a new one. The amplitude at the mentioned point should be at least 0.7V, this should be good visible on the scope. After desoldering Tr2 do another measurement of R6 to check again the right value. Measure R5 with and without a CPU.