Sinclair ZX80 / ZX81 / Z88 Forums

Ok gents,

Looking for a little advice as I am quite a novice to fault finding on the 128K.

I had a 128K, issue 6K Toastrack that originally had a back screen with white lines and blocks on the paper. I found that the ROM (EEPROM) was faulty and replaced it with a new OTP 27C256.

Now I get the fault where, the machine boots to the main screen and has either a Black or Pink border with the menu on the white paper. This is where it stops and can go no further.





If I run with a diagnostic card attached and Brandan's latest firmware, it will boot (but without the diagnostic menu (Soak, Ula etc.) and go will through the ULA and RAM testing and PASS! Once the interrupt test is complete it does not go back to the normal Sinclair menu but reboots the diagnostic card which starts testing again, with the menu this time but cannot select anything!



Getting a little confused with this one. ULA has been tested and also a new one installed just to make sure. I assume the CPU is OK as the diagnostic ROM is running..

Any pointers appreciated.

Russ

Is the keyboard using an original membrane or a replacement?

Do you have another ZX Spectrum (ideally a 16k/48k or plus) to test the keyboard on?

What happens when the 128k is run with no keyboard attached? Are the symptoms the same?

Mark

I initially thought it was the keyboard (I can see it's a replacement keyboard) but the black or pink border around the start up screen is perplexing (almost as though it is trying to load something)..

The one thing I did notice is that the PSU is an original Speccy one (UK 1400) - the Spectrum+128 uses a UK 1750 - could the PSU just not be beefy enough?

For short runs with no interfaces attached the UK1400 PSU is more than sufficient.

Certainly try and see if it does the same without the keyboard - check for debris in the keyboard connectors too - I wonder if it thinks a key is stuck down?

Thanks for the replies gents,

I get the same results with the new membrane, a 48k membrane and nothing attached.

I did notice the keyboard connector is loose but I buzzed it out with reference to the ULA and
all seemed fine. I will check both keyboard connectors again...

Russ

Just checked the 5 connections to the ULA on the small KB1 and their associated pull up resistors and all seems OK.

No shorts between each other or earth either..

Think I have found the problem!

A cap next to the ULA looks like it has been leaking!!!



I think this is C129, can anyone confirm( I think the mask above is incorrect)? can any 100pf cap do?

Looks like it has killed the track between TR6 emitter and ULA pin 40 for the IORQL...

Russ

OK gents,
Had to put a hard wire in between pin 40 of the ULA and the emitter of TR6. Replaced the 100pf cap.

System now boots correctly and is happily running the diagnostic as well as the keyboard is fine....

Phew, sorted.... Never seen a leaking cap of that type before!

Russ

The components in your photo don't leak - that damage has been caused by something else...

In "modern" electronic items (yes, a 30 year old Sinclair ZX Spectrum counts a modern in this context), it's only electrolytic capacitors (and in other computers / devices, cells and batteries) that leak.

Resistors, inductors / chokes, ceramic and other capacitors with solid dialectics don't have any liquid in them to leak...

Mark