[RESOLVED] VC20 (no serial number)

[mrr19121970]

So these are now my observations:

VIC1

CLK IN

CLK OUT

6502

Address Bus

STARTUP (non working machine)

[mrr19121970]

Please also check the logic level at the /IRQ and /NMI pins of the CPU (IRQ = pin 4, NMI = pin 6) - a stuck-on-0 on the IRQ, or a flakey signal on the NMI (NMI is edge-sensitive) could put the CPU into an eternal interrupt loop.

At startup

WORKING

NON WORKING

[mrr19121970]

Data lines 6502 PINS 26-33

Working

Not Working

[mrr19121970]

Following the schematic trying to find out why the address is not going through UC5&UC6 74LS138N. Hooked up the logic analyzer to one of the glue chips that I didn't socket or replace UC3 74LS02N and differences are visible (PIN5)

Working

Not Working

[mrr19121970]

Also UC2 74LS04N looks different too

Working

Non Working

[mrr19121970]

I replaced UC2 74LS04N and UC3 74LS02N with no improvement

[mrr19121970]

I am lost, I don't know how to diagnose why the address is not being populated. CC12 doesn't say what to do, only says test it.

Address lines at startup

Address lines after startup

[mrr19121970]

So it seems that the KERNAL is not being loaded

Working

Address

Data

NOT Working

Address

Data

[mrr19121970]

I tested all traces around the memory and corresponding logic chips and found no breaks. I've examined all joints with a magnifying glass, and also held the board upto the light, and examined. I see no cold solder points or light.

Slowly I am thinking that eslapion is right, this is a tedious task. I will give it abit longer, before finally casting the REVC board and it's components to the 'spare part' box.

[mrr19121970]

My tedious quest continues. I bought an IC tester and found a faultly logic gates:

Code: Select all

UD9		74LS133		13 INPUT LOGIC GATE
  Blank screen.

was thrilled, and replaced it and turned on the VIC20. Still black. 2 more to test, and sure enough my Master Oscillator is showing up as 'not found' when I search for it.

Code: Select all

UB9		7402 (M53202P)	OSC	MASTER CLOCK OSC
  White screen... no RF interference in AM radio (see text). Partial 
failure can produce missing color or incorrect colors, slight or severe 
tearing of picture or diagonal lines, sounds play at incorrect pitch. 
Symptoms may only appear after warmup.

I replaced it with the 7402 from my supply. Still a black screen. Maybe I have the wrong IC ?? Texas Instruments 74HC02N ???

[srowe]

I replaced it with the 7402 from my supply. Still a black screen. Maybe I have the wrong IC ?? Texas Instruments 74HC02N ???

I don't think you are supposed to mix HC and LS devices, they have different voltage levels and propagation delays.

[mrr19121970]

So I bought the wrong chip? I didn't even know it was a HC it was my IC tester that told me I just bought 7402.

[srowe]

Someone more knowledgeable than me will be able to say whether the differences are significant in this case. In general I would try to use an exact match for any replacements. I accidentally used a 74HC00 instead of a 74LS00 in my VIC-1112 board, it worked fine but depending on what else is in the circuit you could end up with something that doesn't quite work right.

[mrr19121970]

Well I always thought (after being educated by a youtube video - ROLF) that the letters made no difference. Now after reading the spec sheets, I believe the LS is TTL and HC is CMOS. Not really sure why this makes any difference.

I ordered some Hitachi 74LS02 now from Hong Hong. 10 pieces for 2.50$ incl postage. I guess I can wait 4 weeks or more now.

[srowe]

I believe the LS is TTL and HC is CMOS. Not really sure why this makes any difference.

Here's a page that explains the differences in input & output levels of TTL and CMOS

http://www.learnabout-electronics.org/Digital/dig33.php

If you have an input voltage of say 2.2V TTL would see that as a 1, CMOS as a 0.