Help with Commodore 128 Repair: Black Screen in Both Modes, Even with Diagnostic Cartridge

[Desperado]

Sorry...not U30 but U37!

 

[daver2]

>>> so I checked all the pins of U30...

With WHAT results? And (more importantly) WHY?

EDIT: Ah, wrong IC... So my previous (long) post still stands, if you are not getting what you expect, work your way backwards on the schematic...

This is a bit like a child's maze game. You have to find the logical path between the RESET signal and the Z80 pin that is not behaving as the troubleshooting guide says it should. You can only do this by studying the schematic and knowing how the logic devices operate.

Dave

 

[Desperado]

i suspect that the culprit it's the cpu Z80!

 

[daver2]

Why?

If a signal is not behaving as expected INTO a part, why blame the part itself?

Probabilities say not...

Dave

 

[Desperado]

Why?

If a signal is not behaving as expected INTO a part, why blame the part itself?

Probabilities say not...

Dave

because i have bad signal directly on pin 23 of the Z80...

 

[daver2]

But as I stated many, many many posts ago...

Pin 23 is an OUTPUT from the Z80. If it is permanently HIGH you need to check pin 25 ALSO.

If pin 25 is permanently HIGH also, it means that the Z80 is responding CORRECTLY and is NOT AT FAULT.

You can absolutely choose to ignore me if you wish, but I am giving you 35+ years of wisdom here designing Z80 computers. Replace the Z80 if you want to, but I am 99.99% expect that nothing will change...

Dave

 

[Desperado]

But as I stated many, many many posts ago...

Pin 23 is an OUTPUT from the Z80. If it is permanently HIGH you need to check pin 25 ALSO.

If pin 25 is permanently HIGH also, it means that the Z80 is responding CORRECTLY and is NOT AT FAULT.

You can absolutely choose to ignore me if you wish, but I am giving you 35+ years of wisdom here designing Z80 computers. Replace the Z80 if you want to, but I am 99.99% expect that nothing will change...

Dave

Yes Sir, Z80 pin 25 it's also High...so I trust your experience blindly, so I'll take the Z80 for granted!
I need to understand if U37 is faulty as the guide says, or if there are other chips involved....
Although for some time now I have started trying to study the basics of electronics and some logic ICs, I always end up getting stuck somewhere if it weren't for your help....

 

[daver2]

Read the Z80 technical manual and look at the page describing the bus request and bus acknowledge functionality.

So, my post a while ago, has already identified the parts of U37 (2 gates) and U60 (1 gate) driving Z80 pin 25.

Go back and read it...

Dave

 

[Desperado]

U37 pin 8 driving pin25 of Z80, isnt'it ?

 

[Desperado]

Also U60 Pin2 driving Z80 pin 25...

 

[daver2]

And U37 pin 6.

Dave

 

[daver2]

All as posted in #55...

Dave

 

[Desperado]

So, i have:
U37
PIN 8= H
PIN 9= L

PIN 6= H
PIN 5= L

U60
PIN 1 & 2= H

 

[daver2]

Are the logic levels consistent with the types of gate (inverter or buffer) the pins are connected to?

Dave

 

[Desperado]

Are the logic levels consistent with the types of gate (inverter or buffer) the pins are connected to?

Dave

Yes!

 

[daver2]

So, now you have three points (the inputs to the three gates we have just looked at) to work back from...

One of the three signals will carry the reset pulse. The trouble is, we don't know which one - so we have to work backwards either one path at a time, or look at all three...

Dave

 

[Desperado]

So, now you have three points (the inputs to the three gates we have just looked at) to work back from...

One of the three signals will carry the reset pulse. The trouble is, we don't know which one - so we have to work backwards either one path at a time, or look at all three...

Dave

Ok thanks, so for example now need i check U8...correct?

 

[Desperado]

Hello everyone, after some time I’m updating this post because my friend, Dr. Antonio Caradonna ( @Xchip on this forum), and I haven’t been able to identify the cause of this fault. We got stuck in a loop with the checks, and even though almost all the ICs have been tested, we still get a black screen or sometimes a yellow one with vertical lines… Could someone kindly help us, please? I really thank you very much!

 

[Istarian]

What do you expect someone else to do for for you?

Unless they have had exactly the same problem before (on exactly the same revision of the C128), correctly diagnosed the fault, and successfully fixed it..... they can probably only advise you to continue troubleshooting.

If you don't mind throwing money at the problem you can replace whatever you suspect is faulty (U37?) and pray that it will fix this.

Supposing you have a defective logic gate and you replace it with an identical, working part (or an appropriate substitute), the worst that is likely to happen is you kill the replacement chip.

 

[daver2]

You still haven't completed the measurements I suggested back in post #76...

Dave