Spectrum+ 128k repair help (No CPU clock signal from working ULA)
Posted: Sat Dec 30, 2017 12:56 pm
Hi Everyone,
I am trying to coax a Spectrum+ 128 (toastrack) back to life.
The symptoms:
Power on I get a Black border (likely indicating cpu was faulty or a clock issue) and black and white bars inside. Sometimes clean black / white 'bars' occasionally with odd bits.
What I checked:
I checked for 3.5Mhz clock on the CPU clock pin 6 and nothing there. So I moved back and checked the clock output from the SLAM (pin 39) and found also no signal. I checked the following ULA pins VCC 1 & 2 on pins 13,14, 17Mhz pin 1, 8.8Mhz on pins 46,47 and ground on 48. IORQL pin 40 @ 3.8v.
I also did resistance checks from the z80 data / address lines to 5v and ground (high 8/9M)
I did low resistance checks along the data / address lines to from the ULA <470> CPU <low> ROM and these looked reasonable.
I checked the ram pin 2 to other pins for shorts and found none.
I checked TR3 / TR6 ands these are ok.
So i am not seeing any clock from pin 39 of the ULA (as I am comparing to a working spectrum)
The known good:
ULA is a new SLAM module fitted in the socket (continuity checks to from the module to the board ok)
New EPROM fitted
TR3 & TR6 removed, checked and put back, both matched
I have:
a DMM
mini Oscilloscope 203
a working toastrack to compare
no much time but the will
I'd really appreciate any ideas / help with tracking this one.
Many thanks,
Wilson
I am trying to coax a Spectrum+ 128 (toastrack) back to life.
The symptoms:
Power on I get a Black border (likely indicating cpu was faulty or a clock issue) and black and white bars inside. Sometimes clean black / white 'bars' occasionally with odd bits.
What I checked:
I checked for 3.5Mhz clock on the CPU clock pin 6 and nothing there. So I moved back and checked the clock output from the SLAM (pin 39) and found also no signal. I checked the following ULA pins VCC 1 & 2 on pins 13,14, 17Mhz pin 1, 8.8Mhz on pins 46,47 and ground on 48. IORQL pin 40 @ 3.8v.
I also did resistance checks from the z80 data / address lines to 5v and ground (high 8/9M)
I did low resistance checks along the data / address lines to from the ULA <470> CPU <low> ROM and these looked reasonable.
I checked the ram pin 2 to other pins for shorts and found none.
I checked TR3 / TR6 ands these are ok.
So i am not seeing any clock from pin 39 of the ULA (as I am comparing to a working spectrum)
The known good:
ULA is a new SLAM module fitted in the socket (continuity checks to from the module to the board ok)
New EPROM fitted
TR3 & TR6 removed, checked and put back, both matched
I have:
a DMM
mini Oscilloscope 203
a working toastrack to compare
no much time but the will
I'd really appreciate any ideas / help with tracking this one.
Many thanks,
Wilson