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Re: Broken interesting Spectrum

Posted: Tue Nov 07, 2017 11:08 pm
by Lardo Boffin
I replaced the two transistors (still one cap to go but I am an optimistic kind of guy), did some voltage tests and look what we have here!

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I think the screen says it all!

To do:-
One last cap
New regulator
Composite mod

There is a bit of a strange start up so not sure if anything else is amiss?

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This appears immediately before the screen goes black and the (C) 1982 message. It is only visible for a split second and took a number of attempts to photo!

Lardo

Re: Broken interesting Spectrum

Posted: Wed Nov 08, 2017 12:10 am
by 1024MAK
Keep in mind that on power on, the RAM may well contain random data. Or if you have just quickly powered down/up, it may still contain some old bits of data. The ULA will immediately display whatever it finds in the display RAM. Next, the CPU reads the code from the ROM. One of the first things it does, is to set the border to white. It then proceeds to do a memory check. The purpose of this check is to see how much RAM the system has. It is not a proper memory test. It is this test that causes the screen to go black, then to have red vertical stripes.
If the CPU makes it past this part of the ROM code, it sets up the system, the end result being the copyright message.

There is a video on YouTube showing a ZX Spectrum starting up, click here

Mark

Re: Broken interesting Spectrum

Posted: Wed Nov 08, 2017 7:15 am
by Lardo Boffin
Good to hear it’s probably ok thanks.

Are there any Spectrum games with a good ‘attract’ mode? That way I can switch it on and let it go for a few hours for a good soak test.

I ordered one of these:-

http://blog.retroleum.co.uk/smart-card-for-zx-spectrum/

It has a RAM test so will run that as well when it arrives.

Lardo

Re: Broken interesting Spectrum

Posted: Fri Nov 10, 2017 9:43 am
by Lardo Boffin
The RAM tests in the retroleum smart card passed ok. That card is awesome for £26!

Any thoughts on the polarity of the remaining cap?

Still RF but Manic Miner doesn’t look too bad!
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Lardo

Re: Broken interesting Spectrum

Posted: Fri Nov 10, 2017 11:15 am
by Paul
Lardo Boffin wrote:
Fri Nov 10, 2017 9:43 am
Any thoughts on the polarity of the remaining cap?
You could try with a volt meter.
I guess the Ring indicates minus. If you unsolder it and have a look at its bottom side. If the ring would indicate plus there should be a plus sign somewhere.

Re: Broken interesting Spectrum

Posted: Fri Nov 10, 2017 11:07 pm
by 1024MAK
Your board has the Sinclair recommended upgraded DC/DC converter (inverter) circuitry.
The screen print under the 1uF capacitor will likely have a diode number.
If it is the one that I think it is, it smooths the -5V supply. So the positive lead should connect to the 0V rail and the negative lead should connect to the -V rail.

But I will have to check, as life is rather busy, I won't have time until Tuesday...

Mark

Re: Broken interesting Spectrum

Posted: Fri Nov 10, 2017 11:31 pm
by Lardo Boffin
1024MAK wrote:
Fri Nov 10, 2017 11:07 pm
Your board has the Sinclair recommended upgraded DC/DC converter (inverter) circuitry.
The screen print under the 1uF capacitor will likely have a diode number.
If it is the one that I think it is, it smooths the -5V supply. So the positive lead should connect to the 0V rail and the negative lead should connect to the -V rail.

But I will have to check, as life is rather busy, I won't have time until Tuesday...

Mark
No rush thanks. I remember seeing a D number underneath the capacitor while looking for identifying features. Not near it now to check what the number was but will have a look.

Re: Broken interesting Spectrum

Posted: Sat Nov 11, 2017 11:36 am
by Lardo Boffin
It’s nearly fully refurbed so I decided to replace the face plate and put a new keyboard on - I much prefer the original grey ones to the later dark blue (even though they do work better in artificial light). It looks pretty well!

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Re: Broken interesting Spectrum

Posted: Sat Nov 11, 2017 7:52 pm
by sirmorris
Nice work (as always ; )