shoestring
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Posts posted by shoestring
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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.
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So to be specific.
CIA U2 IRQ connects to NMI pin 4 on the 6510 ( should be held high )
CIA U1 IRQ connects to IRQ pin 3 on the 6510 ( should pulse )
If you get no activity on pin 3 then replace U1.
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8 minutes ago, DistantStar001 said:Yeah, it shows red on the vcc. I guess that means that every time I said low before I found out, that means high.
Yes. Red = high, green = low. So I guess reset circuit is good then.
8 minutes ago, DistantStar001 said:I get no detectible activity on the MNI OR IRQ on the CPU (with or without the cart), it just holds high. I got similar results with the CIAs, but I'm not sure if they work.
Yeah, it shows red on the vcc. I guess that means that every time I said low before I found out, that means high.
I think U1 is toast. Without those signals, especially without /IRQ your CPU will not do anything. CIAs are sensitive and won't take much punishment... even just touching the joystick pins with machine turned on can damage them.
U2 sounds like it might be OK. You can try swapping them around ( U1 and U2) and if the other CIA is working then you might have different symptoms and maybe even a booting machine!
Good luck.
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46 minutes ago, DistantStar001 said:The 74S257s in U13 and U25 seem to get a little warm, so I'm guessing that they really aren't as interchangeable with the 74LS257s as I was led to believe.
That's fine for them to get a little warm but not hot to the touch that it burns your finger.
How about the /NMI and /IRQ signals to the CPU and the address and data lines ( they should pulse ) ?
If you don't have anything, check the CIAs. I'm not familiar with the two carts but generally I'd imagine these should work without the kernal rom, in any case without the cart plugged in, I would be checking the enable lines from the PLA to the Kernal rom, Character rom and Basic rom ( pin 20 ) on each mask rom. Basic rom -> high , Kernal rom -> pulse , Character rom -> pulse. If these signals are wrong then U17 has crapped itself.
Regarding your probe. Is it showing green instead of red on pin 6 of the CPU or when you measure VCC anywhere else ? If so then I'd have to agree with you... otherwise the reset signal is inverted and is wrong.
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Me also. But it just doesn't make sense why he be getting different voltages on the same +5v rail from the DIN source.
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Other things to check are pulses on pin 38 of the 6510, that's the / read write enable. It should pulse.
What's the voltage on pin 6 of the 6510 ?
If something is still shorted then something should be getting hot ? ( with that much voltage drop your power supply should too )
At this point it sounds as though your CPU is not running at all and stuck waiting for a valid reset, hence the 2 x ram address multiplexers sitting around waiting for something to do.
The address pins of the 6510 are most likely stuck in a high state.. ( pin 7 - 20, 22 and 23 ).
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2 hours ago, DistantStar001 said:Swapping the 74S275s in U13 and U25 had no effect, and every pin other than ground reads low with a logic probe.
Hmm. Is your 6510 even getting a valid reset signal signal ? Check pin 40 of the CPU with your logic probe while the computer is off and then on again. You should see a low for about 1 - 2 seconds then toggle to high.
If you do not get a valid reset check the 556 timer ( U20 ). On the 6510, /IRQ line should pulse ( pin 3 ). On pin 4 ( /NMI ) you should have a high reading. If the reading on pin 3 of the 6510 is not pulsing then you most likely have a bad CIA chip at U1. And if pin 4 is not held high then the other CIA is stuffed ( U2 ).
Yes, a bad CIA can result in booting to junk or black screen as those provide some of the signals to the 6510 ( /IRQ and /NMI ).
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QuoteWhy C64 and not Atari XL/XE ?.
Why not both and an Apple IIe to go with it.. or whatever else it is you enjoy. I recently went to a demo party and there were people there busy coding to get their comp entries finished on time. There were guys doing interesting stuff on their c64s, PCs, Amigas, Macs, 2600s and even Amstrad CPCs. Was great to see the collaboration between all the different groups of people and share experiences.
1 hour ago, DrVenkman said:Having just gone through this recently, take my advice: if you’re using the original C64 black power brick, be very, very wary. I had one fail while it was in storage the first time I plugged it in and turned on the machine. It killed two 4164 DRAM chips, at least one of the MOS 7708 memory multiplexor chips, the PLA chip and the SID. When I unplugged the computer I realized the brick was super hot on the outside of the case. I measured it at 127 degrees F! 😮 This is, evidently, a common cause of failure in old C64’s. So get or make a replacement PSU in lieu of the original before it fails and kills your machine.
All Commodore power bricks will fail eventually and should be binned or rebuilt. I rebuilt my OEM power supply and added an overvoltage protection circuit with a fuse so it can't happen again. At least not from the power supply itself.
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I would never expect it would be a shorted power connector. Looks like you're getting somewhere. Does your diagnostic cart and dead test cart still work in the good c64 ?
Not sure about the 74LS vs S. But generally speaking, timing might be off when substituting 74LS with S so you have to be careful there.
What does the c64 do without the cart ?
Any screenshots ?
Since you had a short to ground, it would be good to check the PLA again,
But booting to junk generally means 1. bad ram / kernal rom or 2. bad pla. 3. bad ram address multiplexers ( U13, U25 ) 4. bad cpu or 5. CIA ( but a cart should normally work even with a bad CIA chip ).
This is a great guide btw.
https://retrocomputerverzamelaar.nl/commodore-64-problems/
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10 hours ago, DistantStar001 said:I checked the voltage at C19 and got roughly 12v, also the vcc on the VIC II was about 5.1v. So I don't think the 9v rail has a problem. However when I checked the power DIN connector, I got the same pulsing (1.6v to 0.3v). I'm guessing that means the port has failed, since the pulsing and low voltage exists so early in the circuit?
I doubt it but you could feed the 5v upstream of the CN7 to verify. Ideally, after the power switch and with the switch removed.
Something is either 1) dragging the voltage down [ bad connection, even the crud in the switch can cause this ] or 2 ) something is shorting to ground.
With the machine off and your power supply disconnected from the c64s power connector. What is the resistance between 5v and gnd pins on the connector ? ( pin 5 and pin 2 ).
On my rev C board I don't get a reading ( completely open ). If you measure really low resistance then you have a short somewhere.
On my cassette port of my rev C board I measure 3.2k ohms & 1.8k ohms on the user port. Yours might be different but if you get a very low resistance closer to 0 then expect you have a shorted component somewhere.
You need a good precise multimeter to identify a short as you get closer to the source, your meter will register a lower resistance as you get closer to it. If your machine isn't socketed, it will be harder to find.
If you don't have a dead short then what is the current draw on the 5v line ? ( you'll need to put your ammeter in series and set it to mA ). A healthy c64 will pull between 800ma and 1000ma. What is your power supply rated at if it exceeds this amount ? ( if it's exceeding this amount then your power supply is shutting down to protect itself ) ?
You've mentioned that you replaced Q3 when the issue started, what is the type of transistor you've replaced it with ? ( The original part is C1815 or 2SC1815 which is NPN, plenty of those available ).
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Great! I don’t have the Pi1541 so I can’t comment. Generally, any add ons for the c64 are expensive compared to Atari 8 bit stuff.
Im really happy with my 1541 Ultimate II+ cart. Does everything I need it to do and the SID emulation isn’t too bad.
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I've only ordered from him once and he was very pleasant and really helpful. Couldn't get the 9vAC power supply locally for my 800 so I ordered a brand new CO17945 from him.
I figured it was just as expensive to get a used one off of eBay + shipped so I settled for it and I already had the step down transformer.
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That’s quite normal. See attached. I believe that is supplied by VR2 ( have to open my breadbin and check )
So you should have roughly 12v at the rectifier CR4, VR1 and 5v at both coils ( L2 and 5 ).
If you have 5v at the power input pins 2 and 5 of cn7 and you’ve ruled out the switch then check the output of the coils at L5 and L2. They should both be around 4.9v.
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5 hours ago, DistantStar001 said:I'm leaning against the power switch being the culprit due to the regular nature of the pulsing and blinking. If it were the switch, then I'd expect the power to be either constantly low or absent, or the intermittent failure to be irregular and not evenly spaced at around once a second. This pulsing is consistent and regular, not random. The steady nature of the pulsing and flashing has me leaning towards an IC or other component that could be pulling the 5v low, and effect the timing of other components. The question is what can do that?
I hear ya. That's a weird issue for sure.
For clean power, the VIC-II requires a separate +5v power rail via an internal 7805 which is supplied by the 9v AC. To the bottom right of that board on a heatsink. I assume Commodore were trying to eliminate the noise / jail bars you typically see on all c64s.
It's also vital for the clock generation ( MC4044,LS629, LS74 and LS193 ) and a few transistors in the chroma/luma output need it too. The modulator is also fed with CAN +5v which is the secondary 5V supply.
Would be interesting to know if the components tied to the secondary 5V supply are flickering as well.
Good luck.
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4 hours ago, ChildOfCv said:I wonder if all of this was caused by the Power Brick of Death? That's a lot of dead chips.
On 6/30/2019 at 2:57 AM, DrVenkman said:I have a Breadbin that got zapped inadvertently (by me!) two summers ago when I pulled it out of storage after 12 - 13 years. The black brick PSU had gone bad over time and killed the machine. Both were working when stored years ago but we know how that goes. Too bad I didn’t realize the brick PSU’s were as dangerous as the Atari Ingots. Oh well.
I think that answers your question
Just like the Atari XL/XE power supplies, the 7805 is used in most of the c64 linear power supplies so the same failure modes.
I did have an old c64 supply that was getting really hot in summer, so I replaced the 5v circuit with an LM2956 DC-DC buck converter and added a 5.7v clamp circuit to it. I did the same to my XL/XE power supply.
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Regarding the 5v 'blinking' and assuming the power supply is working OK on the other motherboard. Have you a ruled out a dodgy power switch on the c64 ?
The power switch in the 64 is known to oxidise internally. Have repaired at least one c64 that either gave me a blank screen, booted to junk or failed to a blank screen randomly during use.
Each time I had a blank screen, the LED went off. So that was a hint that I was dealing with a power issue. After power supply, I'd be looking for bad capacitors and closely examining the power switch.
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Good choice. I think I'll be doing the same if the SID stops working in my breadbin and it's pretty close to the real thing, besides I don't have any use for paddles and mice.
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Looks like you're on to it.
Other parts you can search for are. 6526A, 6526B or 8521 ( hard to find ).
For troubleshooting, you could use some 8520s from an A500 ( if you have one ) temporarily and those will work for the most part. Not recommended though as a perm fix.
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It could be the non inverting buffer ( cd4050 ) after the GTIA at the output.
I had something similar to what you've described.
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My bread bin has one from 88 ( 2488 21 ). Obviously, this was completely rebuilt with sockets at some point before I bought it in 2007.
I recently added a bunch of heatsinks to all the major chips ( 6510, PLA and SID ), it should help but I'm not counting on it to save the PLA from certain doom.
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On 7/7/2019 at 2:00 PM, ChildOfCv said:Yep, the PLA is a common problem. According to someone (maybe even the designer of C64 himself), Commodore didn't do the passivation on the PLA chips correctly, so it's not a matter of "if" the chip fails, but "when".
That was Bil Herd, the C128 designer. The TED machines had the same issues. He's done a lot of talks about the process they used to manufacturer those chips.
The other issue was high power dissipation on a small die area which sped up the aging of the PLA.
I believe it was the early PLAs that had this problem from 82-83. Not sure about later batches.
http://skoe.de/docs/c64-dissected/pla/c64_pla_dissected_a4ds.pdf
Edit:
You can also use a PLS100N which run much cooler and more reliable. Commodore originally used 82S100 before going to the mass produced mask version ( 906114-01 ).
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On 7/7/2019 at 7:02 AM, DrVenkman said:As it happens, I have an UNO, a breaboard, a ZIF I can use for testing, and a whole mess of jumper wires. So that looks like a fun project. If the modern PLA replacement doesn't get it done, I'll likely build that tester. Of course, the Dead Test Cart is already paid for and en route, so that'll help.
I did do some more testing today while I wait. I think it's pretty clear my PLA is dead. There's lots of activity on the address bus inputs to the chip, but the CHAROM, KERNAL and BASIC outputs are nowhere near where they should be, per the schematics in SAMS COMPUTERFACTS.
Expected signals on these three outputs:
What I actually see on pin 16 for the Kernal ROM (measured with the probe right on the chip shoulder - note the amplitudes of all signals):
Pin 15, the Character ROM:
And pin 17, the BASIC signal (should be held high):
Those are the /CE ( chip enables ) lines to the three roms.
Guarantee the other signals to the VIC-II will be funky as well.
Definitely a bad PLA.
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There are some additional things you could try using Ray Carlsen's page as a reference / guide.
http://personalpages.tds.net/~rcarlsen/cbm/c64/c64blank.txt
As you can see a lot of things can cause a black screen.. even a bad SID or shorted CIA.
Good luck, but I am sure you'll get it working soon.
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Check the other ram chips as well. There's also two ram address multiplexers at U13 and U25 ( MOS 7708 ) which are identical to the 74LS257.
Other things to check.
1. PLA
2. Kernal rom.
3. Try a cartridge if you have any
4. Signals to the 6510 ( NMI and IRQ ).
5. VIC-II
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Why C64 and not Atari XL/XE?
in Atari 8-Bit Computers
Posted
Wish we had those back in 86. I do clearly remember my next door neighbour exchanging their c64c model about 5 or 6 times before they got one that did not die.
It was in the middle of summer. In hindsight, I think it was the power supply that failed and most likely took the machines out in the process. Power supply was one of those white bricks with the C= logo embossed on the top of the casing. Pure evil.
Of course, I had the bread bin which came with the more durable power supplies made by Futuretronics. ( C= on top , Atari XL/XE below ).