Druid6900
Veteran Member
Make sure none of the pins missed the socket wipers and got bent underneath the chips when you put them in.
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RAM repair for a C64
- Thread starter CommodoreZ
- Start date
CommodoreZ
Experienced Member
I *carefully* put each new RAM chip into the sockets, and only bent the pins once (don't worry, I straightened it, and reseated it). But I checked again simply because you mentioned it. The pins are perfectly socketed.
I also tried pulling some of the ROMs to see if I could narrow it down. For obvious reasons, when the character ROM was removed, an entirely different image was present much simpler, and more colorful. No change was present when the Kernel or BASIC interpreter were pulled.
Any other suggestions?
I also tried pulling some of the ROMs to see if I could narrow it down. For obvious reasons, when the character ROM was removed, an entirely different image was present much simpler, and more colorful. No change was present when the Kernel or BASIC interpreter were pulled.
Any other suggestions?
cosam
Veteran Member
I'd try beeping out all the connections to the new RAM. 30 or 40 watts is a lot of juice for those narrow PCB traces, so maybe there's one lifted of broken somewhere you didn't notice. The way these things are connected up, a break can cut a signal to a whole bunch of RAM chips. I'd even go out on a limb and say start looking around A7 ;-)
CommodoreZ
Experienced Member
What do you mean by A7? I looked on the board, and I spot nothing marked with an A.
And by beeping out, Im assuming you mean using the continuity tester function on a multimeter (mine is always silent). I betcha another trace got destroyed that I missed. I will test as many as I see.
Unfortunately, 30 watts is as small as I can get, and even that has a hard time melting my solder. Thank you for the advice.
And by beeping out, Im assuming you mean using the continuity tester function on a multimeter (mine is always silent). I betcha another trace got destroyed that I missed. I will test as many as I see.
Unfortunately, 30 watts is as small as I can get, and even that has a hard time melting my solder. Thank you for the advice.
cosam
Veteran Member
Sorry, yes - I mean checking for continuity. A7 (i.e. address line 7) would be pin 9 (with the notch at the top, far bottom right corner) but it's worth checking the lot to be sure.
Good luck!
Good luck!
CommodoreZ
Experienced Member
Ah, now I understand which pin you mean. So what would I be comparing A7 to? Am I checking its continuity with something specific? Or am I looking for something happening at that pin on each of the chips?
If by each of the pin 9's has continuity to all the other pin 9's of the RAM, yes they do.
BTW, I pulled all the RAM out, and doubled checked my solder points (touched up one or two) then tested continuity on the traces with some wear on them. Based on the ones I tested (all underside ones, and at least half of the topside, mainly damaged looking ones) and they all checked out fine. I also did a test in which the RAM was completely out, and the computer gets fired up- different graphics appear, mainly with alphabetic characters, but still the same garble or colors and other graphic bits intertwined.
I will keep trying. *sigh* I will get this thing working yet.
If by each of the pin 9's has continuity to all the other pin 9's of the RAM, yes they do.
BTW, I pulled all the RAM out, and doubled checked my solder points (touched up one or two) then tested continuity on the traces with some wear on them. Based on the ones I tested (all underside ones, and at least half of the topside, mainly damaged looking ones) and they all checked out fine. I also did a test in which the RAM was completely out, and the computer gets fired up- different graphics appear, mainly with alphabetic characters, but still the same garble or colors and other graphic bits intertwined.
I will keep trying. *sigh* I will get this thing working yet.
Last edited:
cosam
Veteran Member
Continuity between all the RAM chips would be the first thing to check. That's probably easiest done with the chips out, measuring on the sockets. Then you need to check each pin is also connected to the rest of the circuit, either by using a schematic or simply visually checking which traces go where. I've skipped that last step before and it turned out there was a break (practically invisible, of course!) between the first chip in the chain and the CPU...
Druid6900
Veteran Member
The other thing I'd check would be the pins on the RAM chips you removed.
What you are checking for is a little metal cylinder around one of them to see if you pulled out a via when you were removing the chips.
What you are checking for is a little metal cylinder around one of them to see if you pulled out a via when you were removing the chips.
CommodoreZ
Experienced Member
By that, do you mean the trace popping up with the chips? I watched carefully with each one: only one chip did that, and I followed the traces, and put a little wire in to replace it (not visible in the picture, because its attached to the socket from the underside).
Alright, I will check all 128 connections... crap, where am I going to get a good schematic that is board shaped? I will start looking...
Druid6900
Veteran Member
No, a via is the plated hole that goes through the board from the component side to the solder side and, if you yanked one out, you're not quite screwed, but close. The are several ways of fixing it.
The way to determine this is to check the continuity from the pad (not the pin) on one side of the board to the corresponding pad on the other side of the board for all the RAM.
The way to determine this is to check the continuity from the pad (not the pin) on one side of the board to the corresponding pad on the other side of the board for all the RAM.
CommodoreZ
Experienced Member
Oh great.... that sounds like fun. But Ill do that next.
I just spent the past hour observing schematics, and checking where each pin should lead:
Pins 3-13 & 15-16 all properly connect from one to another, no matter the chip selected. 5,6,7,12,& 15 were all easy to test their continuity to some other component on the board, and they all checked out. Pin 1 is supposedly not connected, according to schematics I dug up.
Its pins 2 & 14 that are giving me trouble. Some of them have continuity to others, and some seem isolated. Also note, that 2 & 14 are connected to each other, which makes sense, if BOTH are giving trouble. U9, U10, U21, U22, & U23 all are properly connected to each other, but U11, U12, & U24 are the isolated ones. They have no contact with each other, or the other chips.
The other 5 chips have continuity at pins 2/14 all the way to the 6510 on pin 37 (D0) but those three outcasts are not connected. *sigh*
I will try checking the continuity from the pin to the pad. At least Im getting somewhere.
I just spent the past hour observing schematics, and checking where each pin should lead:
Pins 3-13 & 15-16 all properly connect from one to another, no matter the chip selected. 5,6,7,12,& 15 were all easy to test their continuity to some other component on the board, and they all checked out. Pin 1 is supposedly not connected, according to schematics I dug up.
Its pins 2 & 14 that are giving me trouble. Some of them have continuity to others, and some seem isolated. Also note, that 2 & 14 are connected to each other, which makes sense, if BOTH are giving trouble. U9, U10, U21, U22, & U23 all are properly connected to each other, but U11, U12, & U24 are the isolated ones. They have no contact with each other, or the other chips.
The other 5 chips have continuity at pins 2/14 all the way to the 6510 on pin 37 (D0) but those three outcasts are not connected. *sigh*
I will try checking the continuity from the pin to the pad. At least Im getting somewhere.
cosam
Veteran Member
So far, so good ;-)
As yes, I was forgetting each of those chips connect to a single bit of the data bus. So while 2 and 14 on each individual chip should be connected, they shouldn't be connected from one chip to the next.
From the schematic you will see that each RAM chip has a D<number> line going to it. These should connect up with the D<same number> pin on the CPU. If you really are measuring close to zero ohms between pin 2 on different chips, you have a short somewhere. Hopefully you were just measuring the resistance of some part further down the line.
Speaking of shorts, it's worth checking you didn't inadvertently create any between adjacent pins when soldering. Thankfully that's easy to check by "walking" your test leads up each side of the socket and checking for continuity.