Yet Another Black Screen (YABS)

[Bryce]

I've ever seen an 8255 fail except in very extreme circumstances.

Why dont read LK1-LK3 and Vsync...

There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.

Bryce.

[namaman]

Hi all,
I've taken the PIA and the AY-3-8912 chips out. Socketed the PIA.
PIA and AY-3-8912 both removed cleanly so can use if not dead.

Waiting for a new PIA, YM2149F, and the adaptor board for the AY-3-8910 (YM2149F) to AY-3-8912. Should all be here in a couple of days I hope.

...then it should be super easy to see what works and what doesn't.

[McArti0]

PIA

PPI , PIO but why PIA?

[namaman]

Ooops...because I collect and work on a variety of different systems and it's hard to remember all the chip acronyms.

[robcfg]

PIA

PPI , PIO but why PIA?

Peripheral Interface Adapter 

[Bryce]

Ooops...because I collect and work on a variety of different systems and it's hard to remember all the chip acronyms.

Outed as a Commodore fan! ?

Bryce.

[McArti0]

I've ever seen an 8255 fail except in very extreme circumstances.

Why dont read LK1-LK3 and Vsync...

There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.

Bryce.

Ok. This is after replacing IC112 and IC110, i.e. control for /RD /WR is new. With a correctly loading system /CS, i.e. A11 must be functional. What is broken?
What other pins are required to operate the 8255?

[namaman]

The sound chip and the PIO have arrived from china.
(The sound adaptor PCB is still in transit but should be here soon)

Installing the new PIO now allows the Diag test to get past the CRTC test.
So some progress. Just waiting on the PCB to arrive and hopefully it'll all just work out

[McArti0]

Installing the new PIO now allows the Diag test to get past the CRTC test.

I've ever seen an 8255 fail except in very extreme circumstances.

Why dont read LK1-LK3 and Vsync...

There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.

Bryce.

Or 8255 is damaged, which is clearly shown by the logic of the diagnostics like This time.

So stop posting nonsens Bryce...

[namaman]

Ooops...because I collect and work on a variety of different systems and it's hard to remember all the chip acronyms.

Outed as a Commodore fan! ?

Bryce.

I've been found out!!!!

[Bryce]

Installing the new PIO now allows the Diag test to get past the CRTC test.

I've ever seen an 8255 fail except in very extreme circumstances.

Why dont read LK1-LK3 and Vsync...

There could be many reasons, /CS, /RD, /WR or some other required pin not being set correctly.

Bryce.

Or 8255 is damaged, which is clearly shown by the logic of the diagnostics like This time.

So stop posting nonsens Bryce...

Since when is suggesting the most likely part to have failed nonsense?

Bryce.

[McArti0]

I don't think I've ever seen an 8255 fail except in very extreme circumstances.

Bryce.

What exactly were you suggesting with this sentence namaman, who had a broken 8255?

[Bryce]

I don't think I've ever seen an 8255 fail except in very extreme circumstances.

Bryce.

What exactly were you suggesting with this sentence namaman, who had a broken 8255?

Exactly what I said: I've only seen 8255's fail in situations where there was an external cause for the part to fail: Over-voltage, reverse-voltage, static discharge etc. But I don't think I've ever seen an 8255 that failed in normal working circumstances, something that often happens with RAM IC's, CPU's and the AY.

Bryce.

[namaman]

the PCB sound adaptor board arrived today and I promptly built it up...

IT'S ALIVE

[namaman]

So that was a mission. Thanks for everyone who stuck with me through it all.

I'm guessing it must have been an over voltage, or reverse voltage issue:

[namaman]

The only thing to fix is the DELETE key not working...for now I'm just going to pretend I didn't notice...lol

[McArti0]

Quick denture.

Key def 10,1,55,127   Makeing SHIFT+F7 as DEL.

10 is F7 key
55 is 7 char
127 is DEL char

Check 50Hz pulses between +5v and pin11 74LS145.
 

[Bryce]

The only thing to fix is the DELETE key not working...for now I'm just going to pretend I didn't notice...lol

The DEL key is on a row and column of its own. I'd try cleaning and reseating the membrane first. Then try shorting the appropriate pins between the two keyboard connectors. If you still don't get a DEL, then it can only be IC101 (74LS145) that's faulty. Every other associated IC has already been swapped. If shorting at the connector gives a DEL signal, then the membrane has a problem. Separating and (carefully) cleaning both sides of the membrane might solve this. 

Bryce. 

[dodogildo]

I'd be extra careful when cleaning the membranes. Sometimes the lanes get deleted. Thankfully, the PCB repair pens are handy in such cases.

[Bryce]

I'd be extra careful when cleaning the membranes. Sometimes the lanes get deleted. Thankfully, the PCB repair pens are handy in such cases.

That's why you should only use water, no chemicals and only clean very lightly with a soft cloth.

Bryce.

[namaman]

So the final fix for the non working delete is in!!!!!
Corrosion on the keyboard PCB traces in and around the delete key.
(As you can see I don't have the membrane keyboard.)

I scraped off the old solder mask, and ran some jumper wires to bridge the corroded area... and then sealed it all up with nail polish.

Phew...all done!

Thanks again for all your help!

Philip

[Bryce]

Hey, didn't know it was a PCB keyboard. In that case it's always repairable.

Bryce.

[FruityFrank]

quicky: no intention to hijack this thread but since im fighting with a similar problem: got my spare 6128 and it shows no sign of life (apart from pwr led). I only checked for hotspots on the board so far and the floppy controller is the most toasty one by a good margin. That normal? or should i try to desolder it? would the cpc start up with a missing fdc?

[McArti0]

FDC is clearly warm. CRTC too. Write what tools you have and what you know about electronics.

[Bryce]

quicky: no intention to hijack this thread but since im fighting with a similar problem: got my spare 6128 and it shows no sign of life (apart from pwr led). I only checked for hotspots on the board so far and the floppy controller is the most toasty one by a good margin. That normal? or should i try to desolder it? would the cpc start up with a missing fdc?

The FDC does tend to get quite warm, so I wouldn't automatically suspect it. If you have a spare CPU and GA, then I would try swapping them first. The CPC won't boot without the FDC, so don't touch it until you have confirmed that everything else is ok (voltages, clock signal, reset signal etc).
You probably should start a new thread for this repair though.

Bryce.