Spectrum+ 48k repair

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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llopis
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Re: Spectrum+ 48k repair

Post by llopis » Thu Feb 13, 2020 3:55 pm

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.
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Thu Feb 13, 2020 7:08 pm

Recapping was the first thing i did.
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Feb 14, 2020 6:15 pm

Hi everyone, i got 2,98 volts on the pin 1 of the lower ram. The other voltages were good.
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Firefox
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Re: Spectrum+ 48k repair

Post by Firefox » Fri Feb 14, 2020 6:41 pm

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.
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Feb 14, 2020 6:46 pm

I already have the lower ram replacement board soldered in :P
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Ast A. Moore
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Re: Spectrum+ 48k repair

Post by Ast A. Moore » Fri Feb 14, 2020 6:46 pm

Slauri wrote:
Fri Feb 14, 2020 6:15 pm
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?
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Feb 14, 2020 6:47 pm

The ULA warms up so much that it's painful to press your finger against it for over 5 seconds.
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Feb 14, 2020 6:50 pm

My probes are too big to get in there. But i'll try.
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Slauri
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Re: Spectrum+ 48k repair

Post by Slauri » Fri Feb 14, 2020 7:01 pm

Almost 9 volts
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1024MAK
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Re: Spectrum+ 48k repair

Post by 1024MAK » Sat Feb 15, 2020 3:34 pm

Slauri wrote:
Fri Feb 14, 2020 6:47 pm
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
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