Spectrum+ 48k repair

[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

[1024MAK]

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.

Strange

[1024MAK]

@Slauri
Can you please post a photo of your Spectrum board as it is now.

Mark

[Slauri]

[Slauri]

Everything I have replaced:
1)electrolytic capacitors
2)the entire lower ram
3)zilog z80 cpu

[Slauri]

I don't know what to do.

[1024MAK]

Can you please take a close up photo, under good light, of the underside area of the board where the 4116 DRAM chips are.

Mark

[Slauri]

[Ast A. Moore]

I’ve enhanced your picture and marked a few areas that I suspect might be solder bridges/shorts:



Perhaps Mark will be able to see more. I’d remove the lower-RAM board and inspect the traces on the logic board underneath it.

[Slauri]

I've cleaned the board multiple times with alcohol. Those bridges weren't there before. Maybe they have come from my workdesk? I have not cleaned it for a while. Some of those may look like bridges but they are just reflections of the soldering mask that has melted away due to my bad soldering skills.