no clock

[dimebag]

Hi

i try to resurect a zx81 .

First of all,, after have checked the voltage regulateor ansd the distribution of the voltage over the chips, il have discover i do not have a clock signal, like 6.4Mhz as i remember, i have crap....

So i think it's a RC resonator, like the Condensator must be dead...

Yout mind ??

Laurent

[mrtinb]

By the looks of this post viewtopic.php?f=7&t=1249 clock is generated on pin 14 of the ULA. If the clock is missing here, most likely you ULA IC is broken. It also looks like the oscillator is input to the ULA on pin 35. So if there is an oscillating signal in but none out, the ULA is broken.

Mark please correct me if I'm wrong.

[dimebag]

Hi

thanks for the reply.

So this is what i have at ULA's pin 14

not a real clock signal

So i guess you are right, ULA must be dead.

I like to know what is the role of the 6.5Mhz filter ? (mean X1)

i tought it was producing the system clk

Cheers

Laurent

[1024MAK]

Hi, sorry, not been here much today, too busy with work stuff

So, the ULA uses a ceramic filter known as a resonator (X1 on the schematic) to form part of an oscillator. This part was used, as it was cheaper than a quartz crystal when the ZX81 was designed.

The resonator, R4 and C6 are what you see connected to ULA pin 35. These parts and the circuitry in the ULA form the 6.5MHz oscillator or "clock". However, pin 35 is not a good place to test with a 'scope, or indeed any other test gear. As the "loading" caused by the test gear may upset the oscillator. If you do wish to test, use a "X10" 'scope probe.

The two best places to test are the 3.25MHz output from the ULA on pin 14. And the clock input on the Z80A (or equivalent) on pin 6.

If you get a black picture, and, or, the clock output on ULA pin 14 is static (always low or always high), and the +5V (pin 40) and the 0V/GND (pin 34) are okay, in most cases, it's the ULA that is faulty.

6.5MHz resonators are still available from some electronic suppliers. The ZX81 uses the three pin version.

Mark

[1024MAK]

Oh, and Welcome to our forum Laurent

Mark

[dimebag]

Hi Mark

thank you for the explanations and the Welcome !

i have check the clock signl at the oscilator X1, and get nothing except crap like picture below (taken on Pin4 ULA)

so i have ordered one brand new....

can we found ULA replacement for a nice price ?

Cheers

Laurent

[1024MAK]

New ULA chips are unavailable. The company that made them no longer exists.

Occasionally new old stock ULA chips can be found (but are far from cheap).

Sometimes, working used ULA chips can be found, either for sale from a member here, or on trading sites like eBay.

Otherwise, you may decide to get a ULA replacement module, such as the vLA81. Do browse this web site, they sell via a number of different sources depending on where you are in the world. So read this page.

Mark