VIC 20 ASSY 324003 Black Screen

[beamrider]

Just looked at your scope pics again.

6560 PIN 4 (R/W) - looks a bit strange, is this a floating input? Do you get the same pic on your good Vic (I don't have a scope so can't tell).

http://archive.6502.org/datasheets/mos_ ... 61_vic.pdf

Read/Write— Pin 4
This signal is an input only on the 6560 and controls the flow of data between VIC and
the microprocessor. When the R/W signal is low and the VIC chip select conditions have been
satisfied, the microprocessor can write data into the selected VIC Control Register. If the R/W
signal is high and the chip select conditions have been met, the microprocessor can read
data from the selected VIC Control Register.
It is important to note that all VIC/microprocessor data transfers can only occur when
PΦ2=1. During PΦ2=0 the VIC will be fetching data from memory for display and the R/W
signal must be held high to insure that VIC will not write into any memory location.

(mod: repaired botched OCR from PDF file and put in quotes. "RA/V" in fact is R/W and it's the PΦ2 signal mentioned here.)

[SmakyTakyOne]

I don't have any more ideas I'm afraid.

I will go back and do a re-check of all the traces on the board... I guess it will take me a few days to come back again with any findings.

[SmakyTakyOne]

6560 PIN 4 (R/W) - looks a bit strange, is this a floating input? Do you get the same pic on your good Vic (I don't have a scope so can't tell).

Bingo... seems you've got something here. This is the output of PIN 4 of the VIC:



To rule out the IC itself. I took it from the non-working VIC 20 and put it on the working one. It booted normally... however, output of PIN 4 is very different:



Compared to the one of the very same IC on the non-working one... Now, what could be causing this.. will look at the schematics and follow that trace to try to find something off.

[SmakyTakyOne]

Ok.. I have gone through most of the traces that include the ICs that seem to be related to PIN 4 (R/W) of the VIC. However, I am not really sure what to look for... thee are no broken traces nor grounded ones. Not sure where to take it from here.



As PIN 4 is fed from UE8, I reviewed traces from that IC... All 74LS245 ICs were replaced with new ones that were tested on a TI866 programmer. Validated traces are highlighted in yellow in the image above.

However, I was unable to find out what might be the issue. Removing any of these two ICS (VIC or UE8) would not help as these two seem to be generating most of the clock signals, and without them nothing is really "active".

[beamrider]

Something must be causing the erratic signal on Pin 4.

My skills are limited to de-soldering and replacing ICs so probably don't have much more to offer.

You have replaced/tested most ICs in another machine (although your new RAM could theoretically still be faulty) - what untested ICs listed in Ray Carlsons guide that can produce a blank screen remain to be verified?

What happens if you put the Vic chip from the working unit in your faulty motherboard?

[beamrider]

Compared to the one of the very same IC on the non-working one... Now, what could be causing this.. will look at the schematics and follow that trace to try to find something off.

This looks different to the original capture of VIC PIN 4 in your original post?

[SmakyTakyOne]

This looks different to the original capture of VIC PIN 4 in your original post?

If I take the VIC from the working computer into the faulty motherboard I get a similar signal on PIN 4. The difference in the screenshots may be related to the configuration of the scope (probably I zoomed more/less into the signal).

[beamrider]

Okay, sure looks weird. Positive no bridged connection between pins 3&4 ?

[SmakyTakyOne]

Okay, sure looks weird. Positive no bridged connection between pins 3&4 ?

Well... I decided to go on the safe route and ended up replacing the VIC socket. Also socketed the 7402 IC next to it. This to rule out any issue with the socket (and bridges or anything like that) on the VIC Pins. However, after this, I am experiencing the same output on Pin 4.





Something I noticed while comparing the signals of the working computer vs this motherboard is that most address and data lines seem to be floating or hold high. In addition, when looking at Pin 20 of the ROMs, the only one that is getting enabled at times is UE12.

Additionally, I went through the oscillator circuit and the VIC is being fed with adequate signales on Pins 38 and 39. Clocks to the CPU are also fine (Pins 37, 39, and 3).

[SmakyTakyOne]

Ok.. I ordered a PCB to build a 2114 SRAM tester.. will have to wait to get these and test the memory to rule that out. Other than this.. I have no more ideas.

[beamrider]

so after the SRAM has been checked are you saying all other components have been verified?

Also, what is the voltage coming from the regulator - I notice you scope is showing 5.37 which I think is a bit high I think, the 5V rail should be between 4.95 to 5.10.

[SmakyTakyOne]

Yesterday I got the PCBs for the SRAM tested delivered at my door. So I went and assembled the device to test all 2114 SRAM ICs on the VIC 20. None of them were faulty.



+5V DC measured at the datasette port is 5.17V... this is what is being fed to the ICs. However, voltage between the two terminales of the regulator is 6.13 VDC. Not sure if this is normal, though.

I am getting 6.13 VDC here...



and 5.17 VDC here...

[beamrider]

What is the voltage on your working Vic?

I'll check mine tonight if I get chance.

[SmakyTakyOne]

Ok, I've got 4.89 VDC on the +5V rail of the working VIC-20 and 6.70 VDC between the two pins of the regulator.

Now I just noticed the working VIC-20 has a cap underneath close to the VIC.. I recall that when I cleaned I put some capton tape there to prevent any shorts. No idea if this was some kind of mod or if this is from the factory.



Point is, this is something that is in the Video/Audio circuitry and the non-working VIC-20 does not have it... any ideas what this could be for?

Upon inspection of the schematics this is related to PIN 17 which is related -with PIN 18- to the POT X,Y inputs.. so not really related to the video circuitry.

[SmakyTakyOne]

As I am petty much out of ideas, I went and checked all elements of the circutry close to the VIC.

The image below, shows in Yellow all the elements that where verified for shorts, broken traces, or any other issue. I even removed a few elements that were suspicious when measured in circuit (the variable resistors for example). I was unable to find any real issue.



During element removal, I ended up breaking one leg of thermistor TCR1, so I had to order some replacements, adding more time to continue troubleshooting this board.

While checking this, the only "problem" I faced was with Caps 53-56, which I was unable to locate... I looked for a board layout, however the only one I found comes from the Technical Manual for the VIC-20 and the scan quality is not good to identify them there.



I am going to recap the board as soon as I get them (already ordered them from Console5).