Restaurating a very special 48k and needing help

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Eratosthenes
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Restaurating a very special 48k and needing help

Post by Eratosthenes » Thu Aug 17, 2017 7:30 pm

Moin, moin! - (==Hi there)

At the moment I'm restaurating a very special Sinclair ZX Spectrum. It's an unused 48k I've got which was sold from the german distributor of the late 1970th/early 1980th.

The device is "as new" including the package and the welded accessories. All that I know is that the device was owned by the distributor and that it may have been exhibited at fairs or for some other reasons.

Here some pictures:
48kNew_case.jpg
48kNew_board.jpg
As I met the distributor personally, I made him sign the device which - may be - makes it some more special.
48kNew_Signature.jpg
My problem is that the device is inoperable. It's an Issue 2 48k. The 1st Problem was, that TR4, the power supply transistor was gone. After exchanging it, the device still doesn't work. :(

It shows a picture with a some characters on it and a black line every 8 pixels. => I think that a RAM chip is gone because -5V was missing... Is there anybody out there who can help me repairing this Speccy without replacing more parts then neccessary?

THNX 4 info.

der Eratosthenes

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Andy P
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Location: Lancashire

Re: Restaurating a very special 48k and needing help

Post by Andy P » Sat Aug 26, 2017 10:27 pm

Eratosthenes wrote:
Thu Aug 17, 2017 7:30 pm
Moin, moin! - (==Hi there)

.......

My problem is that the device is inoperable. It's an Issue 2 48k. The 1st Problem was, that TR4, the power supply transistor was gone. After exchanging it, the device still doesn't work. :(

It shows a picture with a some characters on it and a black line every 8 pixels. => I think that a RAM chip is gone because -5V was missing... Is there anybody out there who can help me repairing this Speccy without replacing more parts then neccessary?

THNX 4 info.

der Eratosthenes
At least one of the lower RAM chips (the row of eight at bottom left) will more than likely have failed if TR4 goes down.

Powering the board with faulty lower RAM can take out TR4 again (and possibly TR5). If not then you’re lucky.
If the screen has no colour then the +12V is missing. Check pin 8 of lower RAM for +12V and pin 1 for -5V. If any are missing then check TR4 and TR5 again.
However, if you are getting colour on your distorted screen and you have -5V at pin one of your lower RAM IC's then the power supply circuitry could be working fine.

Let me know the results

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Andy P
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Re: Restaurating a very special 48k and needing help

Post by Andy P » Sat Aug 26, 2017 11:36 pm

Eratosthenes wrote:
Thu Aug 17, 2017 7:30 pm

It shows a picture with a some characters on it and a black line every 8 pixels.
I missed that bit. That could mean that IC6 has gone down as that is the RAM responsible for that line (bit position 0). IC13 for bit 7 etc.

I am only saying it could be.

Eratosthenes
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Re: Restaurating a very special 48k and needing help

Post by Eratosthenes » Sun Aug 27, 2017 8:12 pm

Hi Andy P,

Thanks for the info.

After putting a new Transistor in Place, the power supply is up and running.

There is a "Spectrum Fair" (==meeting of about 20 People) here in Germany. I hope to get the Spectrum fixed there or at least to get information about what's going wrong... :D

I hope to be able to exchange as less es possible of the parts..

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Andy P
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Location: Lancashire

Re: Restaurating a very special 48k and needing help

Post by Andy P » Tue Aug 29, 2017 9:12 pm

Eratosthenes wrote:
Sun Aug 27, 2017 8:12 pm
Hi Andy P,

Thanks for the info.

After putting a new Transistor in Place, the power supply is up and running.

There is a "Spectrum Fair" (==meeting of about 20 People) here in Germany. I hope to get the Spectrum fixed there or at least to get information about what's going wrong... :D

I hope to be able to exchange as less es possible of the parts..
There are diagnostic interfaces that can be plugged into the expansion port on spectrum's to check and report which RAM has failed.

I have one and they are invaluable. If any guys at the meeting has one of these then it will save much time and guesswork.

Eratosthenes
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Re: Restaurating a very special 48k and needing help

Post by Eratosthenes » Mon Sep 04, 2017 10:47 pm

At the Meeting (Spectrumania 2017) a ROM has been provided which does all the memory tests. (1000(+1) THNX to Paul... :D )

This is somehow similar to the diagnostic interface but I don't need to use my expansion port.

Using this I will check if more than just the RAM is gone...

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x24b
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Re: Restaurating a very special 48k and needing help

Post by x24b » Wed Sep 06, 2017 1:15 am

I'd love to see it in action once you have it fixed and running. Maybe a short video clip. Good Luck!
Backer №2590 ZX Spectrum Next
Backer №372 Crash!
Order №544 Accelerated ZX Spectrum Next

Eratosthenes
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Re: Restaurating a very special 48k and needing help

Post by Eratosthenes » Fri Sep 08, 2017 2:55 pm

Hi x24b,

today I tried to fix "the new 48k" another time...

Once again it has put me back. It seems as if it doesn't want to be fixed... :D

Result: Adding Pauls ROM to the 48k at first it works fine but after some minutes of operation the power supply once again broke down. So now another ZTX 651 is gone... :(

As I was disappointed because of this, I fixed "Luzies 48k" (Hi@Luzie you also are registered here, aren't you?) which has had several issues because it has been fixed by a real specialist before I got it... ;->

Now, after being recaped and after several strange repairs have been fixed it seems to be working well.

The result: Klick here to see...

Looking at the video Luzies 48k operates the same way "my brandnew 48k" should do. But maybe it will never...

Have a nice day.

P.S.: Here's a Photo of Luzies 48k
Luzies48k01.jpg
P^2.S.: Thanks to the creator of the divMMC Future.
This is one of the best devices I ever bought...

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x24b
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Re: Restaurating a very special 48k and needing help

Post by x24b » Fri Sep 08, 2017 3:11 pm

Sorry for your trouble. But, thanks for sharing the other as well. Manic Miner is a Classic. Good luck!
Backer №2590 ZX Spectrum Next
Backer №372 Crash!
Order №544 Accelerated ZX Spectrum Next

Eratosthenes
Posts: 231
Joined: Mon May 29, 2017 8:17 pm

Re: Restaurating a very special 48k and needing help

Post by Eratosthenes » Fri Sep 08, 2017 7:10 pm

No problem.

You're not responsible for the problems. It's the F*CKING 48k which doesn't wants to be repaired.

As written: Because of this I made Luzies 48k run. It makes much more fun than my "very special 48k"... :D

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