Broken interesting Spectrum

Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Broken interesting Spectrum

Post by Lardo Boffin » Sat Nov 04, 2017 11:59 am

I bought an interesting looking Spectrum recently but sadly it is not working too well...

EBC8589E-9D3A-478E-92B1-B69F948A8CB0.jpeg
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Inside the black TS1500 style case it is a standard issue 3B.
B23C2C32-9CA4-44EA-ADEC-D3B10E34E558.jpeg
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Assembled by Timex:-
474FE3C8-9DA0-46B9-A99A-23ADA7652941.jpeg
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But a bit poorly - this is the boot screen in RF.
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I may do a quick composite mod tonight so I can see the image a bit better.

As far as I can tell it is getting stuck on the initial black screen?

Any suggestions for someone with no ‘scope or logic probe? Apart from to get a ‘scope and probe... :shock:

Thanks!

Lardo
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Broken interesting Spectrum

Post by 1024MAK » Sat Nov 04, 2017 6:34 pm

Yep.

First off, check the three supply rails, +5V, +12V and -5V. If the -5V is not within spec. (-5.5V to -4V) OR the +12V is the same as, or lower than the +9V input voltage (and/or not in spec. 12V +/- 10%) switch off and investigate TR4, TR5 and the 'coil'.

If the all the supply rails are okay, I recommend a test ROM which contains a RAM testing program. I use a diagnostic board. But there are other options.

More later...

Mark

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1024MAK
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Re: Broken interesting Spectrum

Post by 1024MAK » Sat Nov 04, 2017 7:53 pm

Diagnostic ROM

Brendan has improved and expanded on some existing ideas and code for testing ZX Spectrum memory. See here.

There are some Spectrum fault finding tips in this recent thread ;-)

Mark

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: Broken interesting Spectrum

Post by 1024MAK » Sat Nov 04, 2017 7:57 pm

Lardo Boffin wrote:
Sat Nov 04, 2017 11:59 am
I bought an interesting looking Spectrum recently but sadly it is not working too well...


Image
Yeah, Timex of Portugal made various ZX Spectrum 'models' that were sold in Portugal and Poland and some other nearby countries that were not within the area controlled by Sinclair Research Ltd.

Mark

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1024MAK
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Re: Broken interesting Spectrum

Post by 1024MAK » Sat Nov 04, 2017 9:37 pm

Here is a link to a discussion on WoS forums about the Timex ZX Spectrum computers... :mrgreen:

Mark

Lardo Boffin
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Re: Broken interesting Spectrum

Post by Lardo Boffin » Sat Nov 04, 2017 11:15 pm

1024MAK wrote:
Sat Nov 04, 2017 9:37 pm
Here is a link to a discussion on WoS forums about the Timex ZX Spectrum computers... :mrgreen:

Mark
Interesting stuff! So having had a quick read no one actually knows what was going on?
Mine is a normal (I assume) 48K Spectrum inside a black TS1500 case.

I will give the voltages a go tomorrow. Ran out of time packing stuff to send tonight.
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

User avatar
1024MAK
Posts: 1913
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Broken interesting Spectrum

Post by 1024MAK » Sat Nov 04, 2017 11:26 pm

Oh, re-reading what I posted, I forgot to say...
If the -5V is missing or out of specification, don't leave the machine powered up for very long, as this is a required bias voltage for the 4116 DRAM chips. Without the -5V, the chips may be damaged.

And of course, sometimes a damaged 4116 DRAM chip will drag down either or both of the +12V and -5V supplies.
The 4116 DRAM chips are actually powered by the +12V, the +5V is just to Interface to the rest of the computer.

Mark

axtelle
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Re: Broken interesting Spectrum

Post by axtelle » Sat Nov 04, 2017 11:40 pm


axtelle
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Joined: Sun Sep 10, 2017 6:37 pm

Re: Broken interesting Spectrum

Post by axtelle » Sat Nov 04, 2017 11:50 pm

I wonder could you be trying to display a PAL-N (Argentinian) signal on a PAL-I (UK) TV set ?

Here's some info on the PAL variants - https://en.wikipedia.org/wiki/PAL

Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Re: Broken interesting Spectrum

Post by Lardo Boffin » Sat Nov 04, 2017 11:57 pm

Thanks for the links!

I have a Spectra SCART which produces a picture by listening to RAM writes which presumably would ignore the tv output type? My only concern is that if this is just a dodgy computer it could damage my Spectra (which was rather expensive).
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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