C-64 Black Screen and 5v Blinking?

[shoestring]

Yeah you can solder a leg from a doner chip and it will hold, have done it many times. Trim the remaining leg from the good CIA but leave enough metal exposed at the base of the chip so you can get a doner leg on to it. Get your dead, doner chip and cut the leg right at the base..  this will give you enough metal to solder on to the CIA. You'll need some needle nose or long nose pliers or similar and a steady hand whilst you solder it to the base of the chip..  a clamp to gold the chip definitely helps too. ( Practise on the dead chip ).  Then trim the end of the leg so it's flush with the other pins once you're sure the pin is durable for insertion.

 

 

CPU is crashing so U13 & U25 are still a possibilities but in most cases a dead test cart should still work, something to keep in mind for later.

 

I can't think of anything else that could cause this issue other than a logic fault on the board but it seems you have shotgunned everything else that would cause this problem.

 

You've done a lot of work so I think it's time to take out the schematic and check your work, set your DMM to continuity test and verify no floating connections on the the data & address bus between all the chips where they interconnect. 

 

 

 

 

 

 

[+DrVenkman]

32 minutes ago, shoestring said:

 

... so I think it's time to take out the schematic and check your work, set your DMM to continuity test and verify no floating connections on the the data & address bus between all the chips where they interconnect. 

 

This, 100%. Just go brute-force and figure out WTF is going on.

[DistantStar001]

I've found what I think is a short between pins 4 and 5 on the RAM, however, I'm having trouble localizing exactly where the short is.  The sort reads on all 8 RAM sockets, and my multimeter doesn't have enough resolution to detect any variances in resistance between them.  The board is almost completely plucked now with only U6, U16, and U28 remaining (as they are still soldered in), as such I'm relatively certain that the fault isn't in any of the chips.  Any ideas on how to locate the short beyond replacing every socket?  Really don't have enough to replace them.

[shoestring]

/RAS and A0 should not be bridged.  Most likely happened when you installed the sockets. If they were not installed flush against the board, then it's likely solder has built up in between pins 4 and 5 on the component side and the two connections have been bridged.

 

 

 

 

 

Edited August 23, 2019 by shoestring

[+OLD CS1]

I am way late to this discussion but I wanted to interject regarding a common problem: USB chargers.

 

These are seen as a great panacea for powering everything from LED lamps and fans to mini-computerized gadgets.  They are not all, however, suitable for the purpose of sensitive computerized stuff.  Voltage will often fall as current draw rises, with some I have seen fall to below 4V when running at rated amperage.  Others introduce a terrible and often measurable amount of ripple.  Worst of all, some reference the USB outpt to mains input and you can actually run a light bulb from the charger.

 

In short: when running into issues with any USB-powered gadget, suspect the USB power supply first and always try to use quality USB supplies.  Once that is out of the way, then start digging into the hardware

[DistantStar001]

I managed find the bridge.  There was a spot on U9 where the trace from pin 5 is really close to the pad on pin 4 with the tiniest bit of solder between the two.  I needed a magnifying glass to find it.

[shoestring]

Plenty of light is important too. I have a USB scope which helps locating these types of things.

 

Is your c64 working now ?

 

 

[+DrVenkman]

I use a nice, bright LED-illuminated magnifying headset when I'm soldering or examining old electronics for diagnosis. At five decades in and counting, they're certainly not as sharp (or able to focus up close!) as they used to be. My wife surprised me on my birthday this year with a $35 upgrade from my previous cheap ($8) set from 5 years ago. This headset has 4 or 5 interchangeable optical-quality lenses ranging 1.5X to 3X power and an articulable LED.  It's really quite fantastic. 

[+OLD CS1]

@DrVenkman Nice!  I have a magnifying desk lamp with a good old fluorescent ring.  Not nearly as portable and your head gear, and I think I need to consider that for my take-along soldering kit.

[DistantStar001]

3 hours ago, shoestring said:

Is your c64 working now ?

Sadly no.  All I'm getting is a black screen, no flashing, no nothing.

[shoestring]

A bridge between pins 5 and 4 shouldn't damage the RAM.

 

But you might want to check all of them again individually to make sure.