Help please for a newbie with a 48K

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pauljohn
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Joined: Sun Apr 07, 2019 3:21 am

Help please for a newbie with a 48K

Post by pauljohn » Sun Apr 07, 2019 2:16 pm

Hello all,

First of all, I'm new here so apologies if this post has been answered previously. I used to love my Speccy back in the 80's so thought it would be fun to remember the good old days of typing BASIC.

I've recently been handed an old Spectrum 16K (Upgraded to 48K many, many moons ago). I've had to order a new keyboard membrane as the old one was degraded and the keyboard no longer worked.

When I opened up the case I noticed a component (Think it's a transistor??) with a wire detached. Can anybody help me identify it please and what it does?? Why does it look like it was added as a bit of an afterthought?

I really appreciate any help anybody can give.

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

Re: Help please for a newbie with a 48K

Post by 1024MAK » Sun Apr 07, 2019 6:37 pm

Is this an issue two board?
Can you please take an overhead photo of the whole board please.

During the early production runs, two problems with the ULA were discovered.

The first problem discovered was corrected in the next version of the ULA. So most issue two boards have a later version of the ULA (5C112E) compared to the issue one boards which used ULA type 5C102E.

However, soon afterwards the second problem was found. Sinclair therefore added a single extra transistor to all boards that were produced from that point on (and modified and boards that were returned for servicing). This transistor is often found either mounted on top of the Z80 CPU (where it is known as the ‘spider’), or on the PCB between capacitor C47 and IC19. Although it is occasionally found in other locations.

This transistor was permanently incorporated into the design, so on issue 3 and onwards boards, it appears as TR6 on both the schematic and the PCB layout.

The connections for the transistor are:
Transistor collector to +5V supply rail (Z80 pin 11 or any other convenient point),
Transistor base to address line A0 (from the Z80) (Z80 pin 30),
Transistor enitter to IOREQ to the ULA (resistor R27 on the leg that connects to ULA pin 33, ULA pin 33 or the track that links these points).

The resistor on the left hand side of the Z80 CPU may be part of a previous version of this fix which used a resistor and a diode instead of a transistor.

Mark

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

Re: Help please for a newbie with a 48K

Post by 1024MAK » Sun Apr 07, 2019 7:57 pm

Some photos of the ‘spider’ on one of my issue two boards here.

Mark

pauljohn
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Joined: Sun Apr 07, 2019 3:21 am

Re: Help please for a newbie with a 48K

Post by pauljohn » Sun Apr 07, 2019 10:05 pm

Thanks Mark for that detailed response. That is very helpful, so thank you very much.

It is indeed an issue two board, but the ULA is a 6C001E-6 so I guess it's a later edition of the board.

I've attached an overhead shot as you requested.

Thanks again and kind regards,

PJ
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Muttley Black
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Re: Help please for a newbie with a 48K

Post by Muttley Black » Thu Apr 18, 2019 12:31 pm

Here in my issue two board with ULA5C112E-2 when it came to me, before the resurrection. :lol:
A ZTX313 is soldered on top of the Z80 CPU as 1024MAK already mention.

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After the resurrection, i decide to move the ZTX313 on solder side, to be able to swap CPU's as a make this one a components tester board.

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