Spectrum+ 48k repair

[1024MAK]

I tested the voltage on the ROM and it was good. I'll replace the tr4 too. Is there any equivalents to the ZTX 650 transistor?

ZTX651, or ZTX653

Mark

[Slauri]

Can i replace the ZTX213 transistor with ZTX750 or ZTX753?

[1024MAK]

Before you change anything else, can you please post up what voltages you get on the +9V, +5V, +12V and -5V rails. I posted details on how to do this here.

And yes, TR5 (ZTX213) can be replaced with a ZTX750 or ZTX753 (as well as many other small signal PNP types although the lead out may not be the same as the ZTX213 that was originally fitted).

Mark

[Slauri]

On the ULA pin 13 there is only 3.17 volts. On the pin 14 there is almost perfect 5 volts. Is this normal?

[Slauri]

The picture changes when the ULA heats up.

[1024MAK]

The voltage on pin 13 of the ULA will be lower than the voltage on the +5V rail due to resistor R34 (15 ohms).

Internally the ULA operates at a much lower supply voltage (less than 1.8V).

I can’t remember what the normal voltage is on pin 13, I would have to test one of mine. But I am not in a position to do that today.

The picture changes when the ULA heats up.

In what way?

Mark

[Slauri]

The machine had been on for a while already. I pressed the reset button multiple times and this happened: https://youtu.be/PVr2weWOylE

[1024MAK]

The ULA is displaying the random content (garbage data) of the lower (4116) DRAM. But although the CPU looks to be running, it is not able to write new data to the lower (4116) DRAM. Hence the program in the ROM is unable to clear the memory and generate the normal screen display.

You can tell that the CPU is running, by the border mostly being white (one of the first things the program code in the ROM does after power up / reset is to turn the border white (although it sometimes get described as light grey) and later on, for some random reason (likely due to the random data in RAM) it shows moving “loading” bars (stripes) in the border area.

You also do not appear to have any colours on screen.

I’ve ran out of time for now. If I remember, I post some more tomorrow.

Mark

[Slauri]

>>You also do not appear to have any colours on screen.
I was just adjusting the trims before I recorded the video.

>>You can tell that the CPU is running, by the border mostly being white.
Yes. Before i replaced the CPU or the lower ram the border would stay black and it wouldn't do anything when I pressed the reset button.
Before: https://youtu.be/mGKvQZV_2P4
After: https://youtu.be/PVr2weWOylE

[Slauri]

I noticed an another problem. If the machine had been on for a while the top section of the screen would start to flicker a bit, and after a few minutes the top section would go nuts. It just went out of sync and flickered even more. At that point I disconnected the power cable and cooled down the voltage regulator and the ULA with some compressed air. I turned the machine on again. Now the bottom section was going crazy too. I let the machine cool down for half an hour and the flickering was gone.

[llopis]

I haven't read the whole thread, but did you change the capacitors? With problems like these, you might as well start there. That way you can rule that out as a potential problem.

[Slauri]

Recapping was the first thing i did.

[Slauri]

Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.

[Firefox]

That should be -5V, so that's obviously not good! No wonder your lower RAM isn't working.

I was looking at how the voltage generation circuity functions just the other day, but it's already evaporated out of my head...

The alternative to fixing the -5V generator is to fit a lower RAM replacement board, which doesn't need -5V.

[Slauri]

I already have the lower ram replacement board soldered in

[Ast A. Moore]

Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.

And what do you get on the collector TR4?

[Slauri]

The ULA warms up so much that it's painful to press your finger against it for over 5 seconds.

[Slauri]

My probes are too big to get in there. But i'll try.

[Slauri]

Almost 9 volts

[1024MAK]

The ULA warms up so much that it's painful to press your finger against it for over 5 seconds.

I don’t have a working issue 2 board to hand at the moment, but the ULA in an issue 1 board (soldered in) is similar. From being left switched off at a room temperature of about 20°C, and then turning it on, I can hold my finger on the central area of the ULA chip for only 37 seconds before it gets uncomfortable, then a few seconds later I have to remove my finger.

Once it has been on for a while, I can only hold my finger on the centre section for about five seconds.

It’s normal for these ULA chips to run hot. The internal temperature of a silicon chip can rise to around 100°C to 125°C and they will still work.

Part of the reason that this design of chip gets so hot, is that internally it operates at a voltage much lower than +5V. Around all four edges of the silicon chip inside the plastic package, there are lots of series pass regulators. Each of which produces lots of heat on their own. Plus there are lots of bipolar transistors switching at high speeds, these form the bulk of the digital circuitry, all of which produce a fair amount of heat as well.

For comparison, most modern chips use CMOS transistors, which run much cooler. In fact, a CMOS chip that is NOT running at a very high clock speed is cool to the touch.

So don’t worry about the temperature of the ULA.

Mark

[1024MAK]

Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.

The -5V supply (pin 1 on the lower RAM sockets) should be between -4.45V and -5.1V.

Can you please post up the actual voltages you get on the +5V, +9V and +12V rails please.

Do you still have a heatsink on your 7805 voltage regulator? If yes, the machine screw holding it in place nice and tight? Or have you fitted a more modern DC/DC converter switching type?

Mark

[1024MAK]

On the ULA pin 13 there is only 3.17 volts. On the pin 14 there is almost perfect 5 volts. Is this normal?

On pin 13 of the ULA in an issue 1 board, I get 2.9V soon after power up, with the voltage rising slowly a little bit.

Mark

[Slauri]

Pin 9 5.01 volts, pin 8 12.14 volts, pin 1 4.80 volts (lower ram)

[Slauri]

Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.

Yestreday i measured the -5v rail the voltage was much lower.

[1024MAK]

Pin 9 5.01 volts, pin 8 12.14 volts, pin 1 4.80 volts (lower ram)

Those voltages are all fine

Mark