C128DCR with hot 318047 and Logic IC

[maynyrd150]

As the subject says I have a 128DCR with some problems. U102, a 318047 that controls the floppy, and U17, a D latch (SN47ALA373N), get really hot within about 15 seconds of power up. The floppy runs continuously and I can't get into C64 mode. Before all this happened I had diagonal lines that would occasionally barber pole. Possibly the first caused the second. Problem one is video related though. I don't know. I guess what I need is a plan of action. Should I replace U102 and U17 and see if they pop again? Does someone make EPROM replacements for the DCR's 318047? I have already done some basic troubleshooting looking for obvious shorts and broken components. I have the proper voltages on the proper pins on both chips and my power supply seems to outputting the correct voltages although 5v tests at about 4.8v according to my $12 digital volt-ohm meter. I refuse to part it out!

Thanks

Wayland

[RobertB]

The floppy runs continuously and I can't get into C64 mode.

Hmm, yeah, that sounds like a PLA problem.

                 Truly,
                 Robert Bernardo
                 Fresno Commodore User Group
                 http://videocam.net.au/fcug
                 Catch the Fatman and Circuit Girl at
                 http://vimeo.com/jeri

[RobertB]

I have the proper voltages on the proper pins on both chips and my power supply seems to outputting the correct voltages although 5v tests at about 4.8v according to my $12 digital volt-ohm meter.

Hmm, you shouldn't go below 4.95 volts under load.

I refuse to part it out!

That's the spirit! 

                   Truly,
                   Robert Bernardo
                   Fresno Commodore User Group
                   http://videocam.net.au/fcug
                   Catch the Fatman and Circuit Girl at
                   http://vimeo.com/jeri

[Andrew Wiskow]

Hello, Wayland!  I'm glad you found your way here.  

I'm not too savvy on this sort of thing, which is why I suggested you try asking here.  I'm sure that with all the knowledge here, we'll be able to get your problem fixed.  Even Bil Herd, the primary hardware designer of the C128, visits here from time to time.  

[SmallCleverDinosaur]

Should I replace U102 and U17 and see if they pop again? Does someone make EPROM replacements for the DCR's 318047?

Did the circuits pop? In what way? Do you think they are completely broken? Then there is probably a big risk that they will pop again, but still well worth trying. Are any of the circuits socketed or are they soldered directly to the mainboard? If they are soldered to the mainboard and you decide to desolder them, I highly recommend that you solder a socket for each of them to the mainboard so you can replace them again if necessary.

Burning an EPROM replacement for the C128DCR is no problem, just let me know if you need help

[maynyrd150]

Ok I will see if I can borrow a better meter and double check the supply voltages.

Did the circuits pop? In what way? Do you think they are completely broken?

Nothing popped. I guessed that was just an expression. The ROM and IC get hot enough to burn your finger within about 15 seconds. Could an internal short cause them to get this hot? If there was an external short, I supposed that as soon as I replaced them, they would be damaged. Could an internal short in one of these chips carry enough current to destroy another chip? The D latch is soldered but it's a 20 pin package so it should be terribly difficult to remove. I will order the D latch and socket later. I have a 318047-01 coming from eslapion.

Burning an EPROM replacement for the C128DCR is no problem, just let me know if you need help

Whats your price for a 318047-01 delivered to the states?

Wayland

[SmallCleverDinosaur]

Could an internal short cause them to get this hot?

That is unlikely, but not impossible of course. Have you measured the voltage on the supply voltage pins of these circuits?

If you look at this picture (once linked, click to enlarge), you'll se that there is a capacitor (C102) between +5v and ground. Have you checked that one? It is the same for the D-latch, look at this picture. There is also a capacitor between +5v and GND (C17). If they are blown, you might get this strange result.

If there was an external short, I supposed that as soon as I replaced them, they would be damaged.

Unfortunately, that is probably true.

Could an internal short in one of these chips carry enough current to destroy another chip?

It is possible, but very unlikely.

The D latch is soldered but it's a 20 pin package so it should be terribly difficult to remove.

When you do this, it should be with the intention of not salvaging that particular circuit. Just cut the legs of the IC from the top, remove the IC and then desolder the legs one by one. Then get a new circuit and a socket.

Whats your price for a 318047-01 delivered to the states?

I have localized and downloaded that ROM-file from the internet. It is a 23256 eprom. It wouldn't be expensive to get one to you. Provided the whole package weighs under 100g the price to the US from Sweden is SEK 26 (USD - little more than 3). And there is also the eprom. I think I have a few (used ones) lying around So let's say USD 5 in total.

[maynyrd150]

Thanks for the tip on removing the logic IC. That should be much easier. I will check the capacitors as well. Thanks for all the information. It is helpful.

Wayland

[maynyrd150]

As of now I am afraid to power it on again to test those things. I think there is something wrong with the PSU. I had already replaced the bridge so I know its good (well its new anyway, will come back to this) but I am still getting AC on DC outputs. 26 volts AC on 12v DC and 15 volts AC on 5v DC. As well as 8 volts DC on 5v DC. I noticed that a sinked transistor was getting really hot as well. I replaced it (Hitachi 2SC2613) with a FJPF5321 which doesn't get as hot but has not affected the outputs at all. As for the bridge (D3SB80), I am getting 80 volts DC out of it and over 300 volts AC. I'm not an expert but this doesn't seem right. Does it need a load? Is this whats called an unregulated power supply (since there isn't a voltage regulating IC)?

I've seen ordered the logic and ROM now all I need is the time to replace them...


Here is a pic. Those always help, right?

http://i306.photobucket.com/albums/nn280/maynyrd150/psu.jpg


Wayland

[RobertB]

     Wayland, your C128DCR sounds as if it has multiple problems which you cannot diagnose.  It may be time to send it off to a C= technician, like Ray Carlsen.

                      Truly,
                      Robert Bernardo
                      Fresno Commodore User Group
                      http://videocam.net.au/fcug
                      CommVEx v5 info - http://www.commodore.ca/forum and click on ComVEX

[maynyrd150]

Hey now! I need ideas and positive reinforcement. I can always resort to making a 9V AC power supply and using it in combination with an off the shelf 12/5 v supply.

Wayland

[SmallCleverDinosaur]

Or you can replace the power supply With one like this for example.

[maynyrd150]

That could be done but wouldn't it upset some 128 users? I suppose I could source the female connector and make an adapter...

[Blacklord]

That could be done but wouldn't it upset some 128 users? I suppose I could source the female connector and make an adapter...

Upset ? how so ?

[maynyrd150]

Just saying I wouldn't hack it up