Spectrum 48k Boot pattern

A place to discuss the ZX80's and ZX81's younger brother - the Sinclair ZX Spectrum
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Erm
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Joined: Thu Jun 20, 2019 7:07 pm

Spectrum 48k Boot pattern

Post by Erm »

I know this is predominantly ZX81 here, but I'm currently not a member of a Spectrum forum and perhaps someone here can assist.

This is what happens to my Spectrum 48k (issue 3b board) when I turn it on (sometimes the border can also be black or yellow, as well as red, shown):

https://youtu.be/abHhpDffsqQ

At first I noticed that the voltage reg output was too high so changed that and it's 5v now. No idea if it ran for long with the high (8) voltage as I've just got it. 2 or 3 of the lower RAM chips get very hot and none of them show -5v on pin 8. So I imagine changing them is probably the first thing to do?

Thanks,
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
Lardo Boffin
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Re: Spectrum 48k Boot pattern

Post by Lardo Boffin »

Hi Chris

This site has some good info:

https://spectrumforeveryone.com/2017/04 ... nja-style/

Lardo
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a
Lardo Boffin
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Re: Spectrum 48k Boot pattern

Post by Lardo Boffin »

Also the Smartcard from Retroleum has a diagnostic mode which may help:

https://www.retroleum.co.uk/spectrum-peripherals

And it is a very cheap SD card reader!
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +2a
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1024MAK
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Re: Spectrum 48k Boot pattern

Post by 1024MAK »

If the -5V supply is not present, the ‘lower’ 4116 DRAM chips have a tendency to self destruct. So the first order of business is to sort out the DC/DC converter, TR4 & TR5.

Also the over voltage on the +5V line may have damaged some of the other chips.

More later...

Mark
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1024MAK
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Re: Spectrum 48k Boot pattern

Post by 1024MAK »

I’m mobile in a poor signal area, so can’t watch you video at the moment.

With the exception of the ULA, and the 7805 voltage regulator, if any of chips are too hold to touch, then they should be treated as suspect.

If TR4 or TR5 have been damaged due to an overload on the +12V or -5V lines, then the cause may be either a defective 4116 DRAM chip or an interface has been connected or disconnected with the power to the machine on.

The +12V supply feeds the 4116 DRAM chips and the colour encoder for the TV signal (RF/UHF or composite if the machine has been modified).

One of the first things that the ROM code does after the Z80 is powered up or reset, is to set the border colour to white. If the border is not being set to white, and you get a video picture,then either the Z80 is not running, or the ROM is damaged, or something (possibly DRAM chip(s)) are affecting the address, control or data busses.

Are you good at desoldering?

Mark
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1024MAK
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Re: Spectrum 48k Boot pattern

Post by 1024MAK »

Okay, I’ve watched the video. I would say that it looks like there is definitely a problem with the RAM.

Mark
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Erm
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Re: Spectrum 48k Boot pattern

Post by Erm »

Thanks for the replies.

The border isn't white, it seems to be mostly red, but I do remember it being yellow or black too on various re-power-ups, but never white. I de-soldered (I'm ok at desoldering) the Z80, popped a socket on and put a known good Z80 on it - same thing, so put the old one back and also, obviously, same thing. So it's not the Z80.

I checked the TR4 and TR5 for shorts between the legs (there were none) but haven't de-soldered them.

I now have another non-working 16k (that makes a different pattern but does absolutely always have a white border) so I suppose I could swap the ROM.

De-solder ROM and try that before de-soldering the RAM you think? The ULA does get very very hot and as that was already socketed I put that in the other non working (but with white border) 16k and it made completely different patterns and had a coloured border so I suppose that probably indicates that the ULA is fried.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
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XavSnap
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Re: Spectrum 48k Boot pattern

Post by XavSnap »

HI,
Yes, is a memory case error.
have a look to this tread: (fr) select the langage at the right top...
https://forum.system-cfg.com/viewtopic.php?f=1&t=10106
Xavier
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Erm
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Re: Spectrum 48k Boot pattern

Post by Erm »

I also ordered the Retroleum SmartCard, perhaps that will help with one or the other.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).
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1024MAK
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Re: Spectrum 48k Boot pattern

Post by 1024MAK »

First, please test the supply voltages to the 4116 (or equivalent) DRAM chips, IC6 to IC13). Report your results here please. See this post for a pin-out.

Although most of the circuitry runs off just the +5V rail, the 4116 DRAM actually runs off the +12V rail. But the 4116 also needs a -5V supply, plus the +5V for the bus interface. If the -5V is missing, or significantly out of tolerance, the machine should not be left powered up until this problem is corrected.

Some board versions had a DC/DC converter circuit that could not cope with input supply voltages (from the external PSU) of greater than 11.9V. With this circuit design, the nominal +9V (which can be between 8V and 12V) from the external PSU can feed through the transformer (called the ‘coil’ by Sinclair) and power the +12V line. This misleads people into thinking that it’s only the -5V rail that is wrong.

ROM failure is rare. There can be various reasons why the Z80 is not running. Oh, and by running, I mean, running the ROM code. Apart from a multimeter, what other test gear do you have? Do you have a logic probe? A logic analyser? An oscilloscope?

Mark
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