Topic: Commodore 128DCR blown resistor - is there a service manual?

[Kremlar]

Hi -

Trying to go through my "new" Commodore 128DCR, having an intermittent issue booting into 128 mode.  I noticed a resistor on the board that appears to be blown/leaking.  See attached pic:

Not sure if it's related to my issue, but it does look like it should be replaced.

Trying to find out the value of this resistor, but I only seem to be able to find C128 and C128D service manuals, and the board layout does not seem to match.

Can anyone point me in the right direction?

Thanks!

[RobertB]

Trying to find out the value of this resistor, but I only seem to be able to find C128 and C128D service manuals, and the board layout does not seem to match.

Can anyone point me in the right direction?

     Try the schematic at Ray Carlsen's website at

          http://personalpages.tds.net/~rcarlsen/cbm/c128dcr/

               Truly,
               Robert Bernardo
               Fresno Commodore User Group
               http://videocam.net.au/fcug

[Hydrophilic]

That type of resistor does not leak.  In fact, I am not aware of any kind that would.  From my experience, I it looks like discoloration of the protective film on the circuit board due to high temperature of the resistor.  Based on the color bands, it appears to be 110 ohm resistor.
 
I think you are on the right track (some problem related to the power supply circuit), although I doubt replacing the resistor would help.  I guess it couldn't hurt if you are good enough at soldering to not damage the board.  I imagine the problem is a bad chip somewhere in the machine that is drawing too much current and eventually (after heating up) causing the power supply to shut down as a safety feature... could also be a problem with the power supply itself.
 
Anyway, good luck!

[Kremlar]

Thanks for the replies.  The service manuals I found showed a different layout board, apparently the 128 and 128D - but none looked like my 128DCR.  I saw those schematics, but I might as well be trying to read Japanese.  I did locate R5 on there, but did not see anything indicating a value for the resistor.

A buddy of mine (Mech from www.a4000t.com - great guy!) was able to locate a scanned copy of SAMS Computerfacts for the 128D, which does match the layout of my board.  According to that document R5 is a 27 OHM 3W 5% Carbon Film resistor.  Looks right to me, though the bands on the original resistor seem to have lost their color due to heat/age so I'm going to have to trust the document.

Maybe it's not leakage, but it does look like it's damaged or has somehow popped open.  I'll have to take and post up an image from another angle where the resistor looks worse.  Whatever it is it doesn't look good to me.  Perhaps it's not the cause of the issue, but at this point it's my best guess and probably should be replace either way. 

As for troubleshooting my issue further, I do have a couple other standard 128s I could try swapping some chips with I guess, wherever they match up - but no other 128DCRs to try swapping power supplies with.  I don't see any leaking capacitors.

I do think I can replace it without ruining it - I'm an amateur solderer for sure, but getting better.  I have replaced some blown resistors on my recently acquired Amigas, motherboard power socket, leaking battery, etc.... successfully.

[Kremlar]

Here are a couple more pics of the resistor, sorry for the bad quality - couldn't get my phone camera to focus too well.

As you can see, whether it's "leaking" or not it does seem damaged - especially on the end closest to the R5 marking - it appears busted open.

I'm no electronics guy, so don't understand the function of the resistor or what effects a damaged one could have - but common sense is telling me I should replace it if it looks like that.

[Hydrophilic]

I think it still looks like damaged coating on the circuit board due to heating by the resistor, and not the resistor itself leaking.  And if you want to replace it, then it should not hurt as long as you don't burn up the circuit board in the process.  When a resistor burns up, it usually turns the entire body black (or dark brown), but because the photo still shows the normal color of light blue it does not appear burnt up (but that does not mean it is good).
 
So if you replace it, be sure to replace it with the correct value.  The color in your photos seems pretty good, and the colors on the resistor indicate 1 (brown) 1 (brown) x10 (brown) = 110 ohms (the gold band indicates 5% tolerance).  A 27 ohm resistor would be colored 2 (red) 7 (violet) x1 (black) and your photo does not look anything like that to me.  Of course the camera could be playing tricks on me, so use your best judgement.
 
 

[Kremlar]

I agree, the bands do look brown.  However, the service manual I have clearly shows this location as having a 27 ohm resistor.

So that's the quandry - replace with what the service manual says it SHOULD be?  Or replace with what looks like is currently there?

I'm leaning on the 27 ohm resistor which I have on order...

[Hydrophilic]

Don't take my word for it.  When you pull out the old resistor, measure it.  I suspect you would get a reading between 105 and 115 ohms.
 
If the resistor should be 110 ohms and you replace it with 27 ohms, then it will pass about 4x more current than it should.  Who knows what would happen in that case...
 
Anyway, good luck and let us know how it goes.

[Kremlar]

Right... But even if it is 110, is it possible that it's wrong and the service manual is right?  I've seen Amiga capacitors in backwards or worse from the factory....

Ugh!

[maraud]

The problem is going from 110 to 27 could conceivably either alter the state of the signal (on/off within TTL spec) or even worse damage a chip by allowing too much voltage through.  I agree with the other responders though, these things dont leak they do burn up and turn black if too much current is pulled through which isn't the case here.  I think you're barking at the wrong component in my opinion.