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Monopix
Tinkerer
Username: Monopix
Post Number: 176
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 01:51 am: | 
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Has anyone experience in repairing deposited carbon resistors?
I have one in a Demi EE17 which is used for the ISO dial and the carbon is coming away from the substrate. I was thinking of mixing carbon powder with something and trying to patch it up.
Any ideas? |
Gez
Tinkerer
Username: Gez
Post Number: 239
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 03:31 am: |
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This isn't much help but the Demi EE17 I'm working has the asa resistor track made from discrete metal pads. It is probably an early or late version, depending on Canon's design or cost policy at the time.
You could try aluminium paint or better still the 'silver' paint used to restore broken PCB tracks. Calibrating in terms of Ohms/mm would be difficult and the surface wouldn't stand much abrasion as the contact finger moves over it.
A better solution is to get the track out of a new or old full sized control potentiometer and cut it to size and resistance requirements.
PS if you are delving deeper into the EE17 please give me a shout as I'm contemplating removing the shutter unit and its backing plate, but unlike the big Canonet 25 there are no obvious locking rings visible. |
Sevo
Tinkerer
Username: Sevo
Post Number: 72
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 04:59 am: |
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There are (or were) carbon/binder compounds for that purpose, but they are best suited to rather crude applications (like volume control on audio equipment), where you can interactively dial in a perceptively "nice" value rather than attempt to set accurate values by numeric scale. It will be really hard to cut/file/whatever their scaling to exposure meter precision, and even if you manage to do so they might soon drift off beyond acceptable tolerances.
The best bet is to find some other pot whose track fits or can be dremelled to fit - Super Ikonta pots can for example be culled from broken Kievs. |
Monopix
Tinkerer
Username: Monopix
Post Number: 177
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 05:27 am: |
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The board does have the silvered pads. Each pad has a short stub extending from it and the carbon is deposited across the top of these stubs. So the wiper only passes over the pads and not the carbon itself. So sounds the same as yours Gez.
Calibration is an issue. Originally, the calibration was done by scraping away some of the carbon. There are signs where this was done.
I have another, working, EE17 so I guess I could measure the resistance between each segment and calibrate the new one to that - but I still need some way of depositing the carbon onto the board first.
Sevo, do you have any other info on those carbon binder compounds?
Gez, This EE17 has been stripped down to a bare chassis as it had some corrosion due to a leaking battery I wanted to treat. So I've removed everything. The lens panel is held on with four screws. To get at the top ones you'll need to remove the viewfinder and meter. The bottom ones are accessible. I have photographs of some of the process. Let me know if you need more help. |
Sevo
Tinkerer
Username: Sevo
Post Number: 73
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 06:20 am: |
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I got some about fifteen years ago to patch up some audio equipment - IIRC from one of the major parts suppliers, probably RS or Farnell. As I said the quality of the tracks was marginally acceptable for uncritical audio applications - they positively were not suitable for devices that did not provide instant feedback, needed an accurate value or any degree of linearity. |
Charlie
Tinkerer
Username: Charlie
Post Number: 246
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 06:56 am: |
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Most auto repair shops sell kits for repairing rear window defroster circuit lines. The kits contain a conductive epoxy which can be used for many kinds of electrical repairs including acting as a substitute for solder or for repairing breaks in printed circuitry. |
Monopix
Tinkerer
Username: Monopix
Post Number: 178
Registered: 11-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 08:21 am: |
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Thanks Charlie. What I need is a resistive substance. From the measurements I took, the resistance of the track (or what's left of it) is several Kohms per millimetre. I presume the repair stuff has a low resistance. |
Mareklew
Tinkerer
Username: Mareklew
Post Number: 209
Registered: 03-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 03:10 pm: |
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Monopix, forget it.
Re-creating the track is not possible in home workshop, period.
The best you can do is figure out, if the stubs are solderable and if yes, replace the track of carbon with discrete SMD resistors of appropriate value. If the stubs aren't solderable, you still can do the trick by cautious use of conductive epoxy instead of solder.
You have to know the resistance between stubs of course.
The printed carbon film resistors that were used in these designs are tricky to design to say the least and, even if you could lay your hands on the chemistry necessary to apply such a resistive layer on an arbitraty substrate, it would be extremely hard to get the resistance right. And by 'right' I mean getting it in the ballpark of +400%/-80% range of nominal value. It's not only about track width, it's much more about its thickness and method of application...
Marek |
Sevo
Tinkerer
Username: Sevo
Post Number: 74
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 03:56 pm: |
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Another snag might be that almost all carbon print resistors I have come across were factory trimmed. That is, they were deposited with some uniform value (or two or three, if they needed more range), and ground down to specific values for calibration.
As a consequence they often have odd values that are critical to the function of the board - if such a trimmed resistor is gone to the degree that you cannot meter its working value, you need the original service docs or must reverse engineer the PCB pretty thoroughly to find the proper replacement value. |
Papco_instruments
Tinkerer
Username: Papco_instruments
Post Number: 3
Registered: 10-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 10, 2010 - 09:36 pm: |
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My suggestion is to measure all of the resistance values and plot them on a graph paper. Along the horizontal axis mark the ASA values. Along the vertical axis mark resistance values.
The most important values will be at the ASA25 and ASA400 points. These will give you the minimum and maximum resistance values.
Now plot all the other midpoints. Look at the plot line on your graph, is it a straight line increasing or decreasing in value? This represents a linear resistance value. If the plot line has a definite curve, then the resistance has a log resistance value. Knowing the type of resistance curve helps you choose values that are worn away.
Now... you can purchase the SMT resistors from www.mouser.com or www.digikey.com. SMT size 0402 or 0603 will have you working with tweezers and a loupe. These are available in 1% value increments, so you will be able to recreate the resistor pattern and plot.
This is a doable project but I don't envy you for all the work involved.
Good luck!
Paul |
