Time to break out the multimeter then, using the diode test mode check that there's continuity between the relevant pins of the ULA/CPU and either side of R26, also check that R26 itself is measuring 470 ohms as per the schematic.
You could try wiring point to point wires to fix the problem temporarily, then tidy it up by fixing up the traces properly later?
Time to break out the multimeter then, using the diode test mode check that there's continuity between the relevant pins of the ULA/CPU and either side of R26, also check that R26 itself is measuring 470 ohms as per the schematic.
You could try wiring point to point wires to fix the problem temporarily, then tidy it up by fixing up the traces properly later?
B
Thanks again!
Doing as you suggested pointed the issue straight back to the lifted track. Putting the red positive probe on the INF pin on the ULA and the black probe on the various points I got the below values. I resued the old picture so it doesn't look like this anymore, however I suspect the effect is the same. The track above the INT track appears to have current through it and the INT line itself appear cut right at the point of the lifted track.
I suppose I can't fix this on the top side? Would the following fix the issue?
Take a scalpel and cut the INT line good and proper near the C4 resistor
For good measure also cut close to the R26 resistor
On the reverse of the board, solder a wire to the R26 resistor junction and solder the other end to the board hole just below the CPU near C4.
Putting the negative black probe on pin 5 (INT) on the CPU and doing the tests with the red positive probe I got:
On the track above the INT line (the D15 line) I got 1.636 either side of the affected area
On the INT track on the left side of the affected area I got 1.746
On the INT track on the right side of the affected area I got 0.345
On the INT track to the right of the R26 resistor I got 0.001
Ok, I'm confused - that trace with the bend in it, left side of the bend being 0.001 and the same trace, but after the bend is 1.872......
Assuming your black test lead did not move, the reading should be the same all along the trace - since it is not, I suspect that it's broken.
For this, and the repair you mention, when I get a problem like this I take a short length of mains cable, strip it down so I have a selection of strands - much the same guage as 2 - 3A fuse wire. Hold both ends in a pair of pliers and pull on the wire gently - this will straighten it and make it easier (and neater) to work with. A tiny hook like bend at one end, which gets hooked around and soldered to the component lead, the wire is then laid flat on the board and it' route made to follow the original trace. Once the break is bridged, solder and snip - job done. Hold it down with a light smear of superglue, just to stop it lifting off and that's job done!
It's far easier to do than it is to explain, honestly :D
On a hairline break like this probably is, you can usually just bridge over the gap with a bit of solder. The solder will follow the wire trace anyway, so it's easy enough to do.
I spent this evening on it ��
It is indeed now resolved, big huge thanks to all, couldn't have done it without the help!
I lifted almost the entire INT track and took it off, flipped the board over and used a twisted pair wire from a CAT5 cable to rejoin the two points. I tided up the track above the INT track with a scalpel. Tested the voltages, resoldered the reset switch wire for the millionth time (sigh)... And it worked... Tested with the interrupt tests (thankyou!) and it was nice and slow again! I haven't finished all the tests but it does appear to be fully functional, it has not worked since 1991!
1 toast rack scart cable fixed, one 48k+ fixed, now back to the 16k Spectrum and a busted microdrive!
! Standby alert !
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb!
Looking forward to summer in Somerset later in the year :)
Comments
You could try wiring point to point wires to fix the problem temporarily, then tidy it up by fixing up the traces properly later?
B
zx-diagnostics - Fixing ZX Spectrums in the 21st Century (wiki)
Sinclair FAQ Wiki
Thanks again!
Doing as you suggested pointed the issue straight back to the lifted track. Putting the red positive probe on the INF pin on the ULA and the black probe on the various points I got the below values. I resued the old picture so it doesn't look like this anymore, however I suspect the effect is the same. The track above the INT track appears to have current through it and the INT line itself appear cut right at the point of the lifted track.
I suppose I can't fix this on the top side? Would the following fix the issue?
Putting the negative black probe on pin 5 (INT) on the CPU and doing the tests with the red positive probe I got:
Assuming your black test lead did not move, the reading should be the same all along the trace - since it is not, I suspect that it's broken.
For this, and the repair you mention, when I get a problem like this I take a short length of mains cable, strip it down so I have a selection of strands - much the same guage as 2 - 3A fuse wire. Hold both ends in a pair of pliers and pull on the wire gently - this will straighten it and make it easier (and neater) to work with. A tiny hook like bend at one end, which gets hooked around and soldered to the component lead, the wire is then laid flat on the board and it' route made to follow the original trace. Once the break is bridged, solder and snip - job done. Hold it down with a light smear of superglue, just to stop it lifting off and that's job done!
It's far easier to do than it is to explain, honestly :D
There are many ways of fixing this... :)
It is indeed now resolved, big huge thanks to all, couldn't have done it without the help!
I lifted almost the entire INT track and took it off, flipped the board over and used a twisted pair wire from a CAT5 cable to rejoin the two points. I tided up the track above the INT track with a scalpel. Tested the voltages, resoldered the reset switch wire for the millionth time (sigh)... And it worked... Tested with the interrupt tests (thankyou!) and it was nice and slow again! I haven't finished all the tests but it does appear to be fully functional, it has not worked since 1991!
1 toast rack scart cable fixed, one 48k+ fixed, now back to the 16k Spectrum and a busted microdrive!
Again thanks all!
B
zx-diagnostics - Fixing ZX Spectrums in the 21st Century (wiki)
Sinclair FAQ Wiki
Mark
Repair Guides. Spanish Hardware site.
WoS - can't download? Info here...
former Meulie Spectrum Archive but no longer available :-(
Spectranet: the TNFS directory thread
! Standby alert !
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb!
Looking forward to summer in Somerset later in the year :)