How to replace the Ceramic frequency discriminator CDA-6,5MHz with a modern equivalent/circuit?

Discussions about Sinclair ZX80 and ZX81 Hardware
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mark1791
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How to replace the Ceramic frequency discriminator CDA-6,5MHz with a modern equivalent/circuit?

Post by mark1791 »

Hi

My first post on this forum, my first computer was a ZX81 back in 82, one of two people in the class at school to have a computer in those days...

Any way trying to fix a ZX81 to get working, have progressed to a white screen on boot. I do have a Issue One which works no problem and has all but the Ram socketed.

The one i have that doesn't work is a Iss 3, I have replaced the ULA/Z80/Memory/7805/Electrolytic Caps/TR1/TR2, put in the Composite mod. ROM/Z80/vLA81 tested in the Issue one and they work. Buzzed out the connections between chips in case of breaks, not found any. Not much left to replace...

The conclusion I am coming too is its the Clock, looking at it closely looks damaged, but trying to find a replacement to try is the tricky part...

From some research and looking at the ZX81+38 seems to indicate using a 13Mhz Clock crystal and some clock dividing circuitry, which is where i come unstuck as not a Electronic guru, 3D CAD Engineer by trade.....

So looking for some advice on how to create a little circuit on breadboard or something to feed the right clock signal into the ULA.

Thanks for you help

Mark
Clock_Comparitor.JPG
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1024MAK
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Re: How to replace the Ceramic frequency discriminator CDA-6,5MHz with a modern equivalent/circuit?

Post by 1024MAK »

Hello Mark,

Welcome to our forum :D

It’s a ceramic 6.5MHz resonator. The ZX81 uses a three pin version. They are still available (although may look different) from electronic suppliers.

Do you have, or have access to an oscilloscope?

If yes, what do you get on pin 6 of the ZX81?

If you don’t have a ‘scope, but do have a logic probe, what do you get on the Z80 control lines and address lines A0 to A6 and A14 and A15?

Mark
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Paul
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Re: How to replace the Ceramic frequency discriminator CDA-6,5MHz with a modern equivalent/circuit?

Post by Paul »

The vLA81 bring his own quartz, no need for resonator in the ZX81.
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mark1791
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Re: How to replace the Ceramic frequency discriminator CDA-6,5MHz with a modern equivalent/circuit?

Post by mark1791 »

1024MAK wrote: Thu Nov 24, 2022 3:11 pm Hello Mark,

Welcome to our forum :D

It’s a ceramic 6.5MHz resonator. The ZX81 uses a three pin version. They are still available (although may look different) from electronic suppliers.

Do you have, or have access to an oscilloscope?

If yes, what do you get on pin 6 of the ZX81?

If you don’t have a ‘scope, but do have a logic probe, what do you get on the Z80 control lines and address lines A0 to A6 and A14 and A15?

Mark
Hi, thanks for the welcome.

I do have a scope, got to figure out how to use it yet...., i do have a probe and done some tests:-

Pin 6 Z80 - High
A0 - High
A1 - Low
A2 - High
A3 - Low
A4 - High
A5 - Low
A6 - Low
A14 - High
A15 - High

I have found some Resonators at Farnell 6.5Mhz i think should be ok, shall order some:-
https://uk.farnell.com/murata/cstls6m50 ... dp/2913670

Thanks for the help.

Mark
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1024MAK
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Re: How to replace the Ceramic frequency discriminator CDA-6,5MHz with a modern equivalent/circuit?

Post by 1024MAK »

If pin 6 of the Z80 is stuck either high or low, then a fault in the clock circuitry is suspected.

It’s best to use a ‘scope to test.

This is supposed to be a 3.25MHz square wave (although the edges are far from being square).

The next step is to test on ULA pin 14.

Is your ZX81 using an original ULA chip or are you using a replacement module such as a vLA81?

Mark
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