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ok... I've had it!

channelmaniac said:
Sweet!

It feels great to be able to bring an old system back to life. :D

RAM chips can be piggybacked but I haven't had a lot of luck in doing that to the C64.

If you have a logic probe it'll help you "look" at the signals on the chips and can help you determine which RAMs are bad based on those signals.

It can also help you find bad mulitplexers as well. (The address lines are multiplexed on the DRAM ICs - That's how you can get 16 address lines on a 16 pin chip.)

Also, Commodore used several 74xx logic chips they made themselves. They were stamped with their own part numbers.

65245 = 74LS245
7707 = 7406
7708 = 74LS257
7709 = 74LS258
7711 = 74LS139
7712/8712 = 74LS08

They also had a 7714 which I haven't figured out a cross for yet.

Raymond
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I'm not ashamed to say that I was grinning from ear to ear after repairing those systems - kinda like this: :-D

I do have a logic probe or three :rolleyes: but I would have no idea what I was looking at. The last time I used a logic probe was probably 20 years ago and that was on circuits that I had built, so I knew what to look for and where....


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Actually, I was just reading through the service manual and I may be able to figure this out after all. And I found a SAMS Troubleshooting and Repair Guide that should help.
 
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It's rather nice that the programmer's manual for the C64 includes full schematics. This should help you a lot if you have it. It should also be available somewhere online if you don't. I know there's one web site which has nice large PDF scans of the SX-64 schematics.
 
Yeah, I've got the Service Manual and like I said, I found the Troubleshooting and Repair Manual. Both were indispensable in repairing these things.
 
rebeltaz said:
That is a lot of unsoldering to be done on a double sided board. I seem to recall seeing something about testing RAM chips by piggybacking known good chips on top of the suspect chips, but I can't find it now. Can someone tell me more about how to do this, if it can in fact be done this way?
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This is what I wrote about it and it seems to work as well on a 64 as anything else.

Shorted RAM chips, especially 16K ones will be HOT, hot enough to burn the chip info into your fingertip in less than a second.

On a chip with an open circuit, in most cases, piggy-packing will complete the circuit through the known good chip.

If you have MORE than one open chip (and I have seen it happen), you will probably notice a change in the "garbage" on the screen. You should leave a chip piggy-backed on that chip and use another to continue.

Ideally, if you have enough chips, each one should have a chip PB'd on it and, if the garbage clears up, remove one at a time until you get garbage, then replace it and continue, the chips that are still PB'd at the end is/are the bad one(s).

It doesn't work 100% of the time, but, damn close.

If having a PB'd chip on all the RAMs doesn't clear up the garbage, you should be looking somewhere else.
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Druid6900 said:
This is what I wrote about it and it seems to work as well on a 64 as anything else.
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Hmm... that mentions garbage on the screen. I was under the impression (from the service manual) that a faulty RAM chip could cause a blank black screen. Would that require that one or more of the RAM chips be shorted instead of open? If so, then that probably isn't my problem, since none of the chips are running warm, much less hot.

channelmaniac said:
Sweet!
It feels great to be able to bring an old system back to life. :D
Raymond
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tezza said:
Good one. As others have said, it's very satisfying bringing a system back from the dead.
Tez
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Very satisfying indeed! And I'm on a roll... In addition to the C64s, I also resurrected a ColecoVision with cold solder joints and an Atari 5200 with a defective flip-flop this weekend. I guess I better work on the TRS-80 while I'm on a roll before this streak ends!
 
rebeltaz said:
Hmm... that mentions garbage on the screen. I was under the impression (from the service manual) that a faulty RAM chip could cause a blank black screen. Would that require that one or more of the RAM chips be shorted instead of open? If so, then that probably isn't my problem, since none of the chips are running warm, much less hot.
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A defective RAM chip can cause, oh, about a dozen different problems, depending where in the memory map the defect resides.

Believe me, if a RAM is shorted, the finger test would let you know PDQ. If the RAM IS the problem, it has to be an open.

As a long time electronics repair tech, I can almost guarantee that any problem you have is NOT in the trouble shooting guide.
 
Even if the problem is covered in the service manual, the repair procedure will read "Replace the MainBoard"...with detailed diagrams and instructions how to remove the two screws holding it in!

--T
 
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For the shorted ram test, I am thinking "Best to lick finger first?" Likely, I still remember my 2nd degree burn from "testing" a cyrix doubler chip I had installed 90 degress from where it belonged. Nice lymph bubble on my finger.
 
Terry Yager said:
Even if the problem is covered in the service manual, the repair procedure will read "Replace the MainBoard"...with detailed diagrams and instructions how to remove the two screws holding it in!

--T
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LOL! Aint that the truth! Especially IBM manuals.

Tez
 
chuckcmagee said:
For the shorted ram test, I am thinking "Best to lick finger first?" Likely, I still remember my 2nd degree burn from "testing" a cyrix doubler chip I had installed 90 degress from where it belonged. Nice lymph bubble on my finger.
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I still remember the AMD 486/80 I installed 90 degrees off in the socket (hey it fit), melted the chip into the socket.
 
Terry Yager said:
I just realized I'm thinking of old-skool manuals. Modern ones are dumbed down for today's techies. They simply read, "Jack up the computer and drive a new one in under it."

--T
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Old school manuals were written by english majors in ENGLISH, todays manuals are written by Indians who are too dumb to make the cut at IBM Bangalore headquarters so they write manuals in Engrish to pass the time.
 
Yes thats ture IBM manuals are bad that way when thay tell you replace it, but you gotta stop and think IBM worte those with the mind frame that replacment parts were everywere and that was the case back then, and when the next model of the comptuer came out you were expected to throw out the old one and start again.

There's not a part of the manul that's labeled "20 plus years later do this" :-D


tezza said:
LOL! Aint that the truth! Especially IBM manuals.

Tez
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frozenfire75i said:
Yes thats ture IBM manuals are bad that way when thay tell you replace it, but you gotta stop and think IBM worte those with the mind frame that replacment parts were everywere and that was the case back then, and when the next model of the comptuer came out you were expected to throw out the old one and start again.
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And don't forget that it is standard operating procedure for service technicians to not diagnose individual components on circuit boards. It is more cost effective and time effective for the service tech to just replace the entire board rather than try to trace the fault any further than socketed components. And eventually the manufacturing process improved enough that the only socketed chips left were the RAM and CPU. Anything beyond that means "Replace system board" is the official bottom line.
 
vwestlife said:
And don't forget that it is standard operating procedure for service technicians to not diagnose individual components on circuit boards. It is more cost effective and time effective for the service tech to just replace the entire board rather than try to trace the fault any further than socketed components. And eventually the manufacturing process improved enough that the only socketed chips left were the RAM and CPU. Anything beyond that means "Replace system board" is the official bottom line.
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When I ran a Radio Shack Computer Repair Depot, we worked at component level for two basic reasons; we trained to REPAIR things, not replace them and, the price to the owner was much lower even with the board exchange program which got you a new Model I logic board for $220.00 plus labour. Not good for some poor kid that saved every penny to buy himself a computer.

The only time we replaced boards was for customers that had on-site service contract to get them up and going. Then the board was brought back, repaired, tested and put back in inventory. I'd be damned if I was going to send something back to head office to let someone else repair it.

Now-a-days, I doubt that, except that one end is hot, most "technicians" (read: board swapping chimps) know which end of a soldering iron to grab.

I'll still replace LSI surface mount chips when the need arises and even tackle unsoldering pinned chip on a six-layer board. It's how I was trained to fix electronics.
 
I don't get to work with very many service manuals on TVs anymore, but I just got a new laptop and I swear to you the QuickStart manual that came with it did not have a single word on it! All it has were four pictures. The first one depicted putting the battery in, the second one showed opening the lid, the third showed plugging the charger into the laptop and the last one was of a finger pressing the power button. I'm just curious - When did we all go back to hieroglyphics?

And as for component level repair, I agree with Druid - I have customers call all the time who have already called the other two repairs shops in the county (did I mention consumer electronic repair is a dying business?) and the techs there told them that you could no longer get replacement parts for their particular televisions. Of course, the repair parts that are 'unavailable' are complete boards. Needless to say, I have had numerous satsified customers when I was able to repair their units despite the fact that 'parts were no longer available.' It seems like todays so-called technicians either only know how or only want to replace entire PCBs rather than take the time to troubleshoot the board down to the component level.
 
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