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Aaron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, July 04, 2004 - 10:07 pm: | 
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I am repairing a Metrawatt lightmeter (CdS) which gives a reading even the CdS photocell is disconnected. I have located the problem to be the PCB. The insulator board behaves like a resistor of about 20kohm, and the conductor part has some oxide. Is there any way to save this circuit board? Thanks! |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 05, 2004 - 04:49 am: |
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There are reagents available for pcb cleaning. First you can try with plain household alcohol. If it does not remove the conductive residues, try a special pcb cleaner, like LR3 made by (german) Kontakt Chemie, similar stuff should be available from other suppliers.
Which model do you have? I have a Metrastar which is working reliably for more than 20 years (in my ownership). Only flaw is that its readings are approx. 1/3 f-stop too low for slide film. |
Shel Sherman
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 05, 2004 - 11:18 am: |
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How did you measure the 20K resistance? It is unlikely that the board itself is the culprit. If you connected the ohmmeter across where the photocell was connected you are most likely reading other components in the circuit. |
Aaron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 05, 2004 - 08:35 pm: |
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Pals, thanks for your response. It is a Leica-meter MR4. I have cleaned the circuit board with alcohol to remove the oxide but the fault is still there. Do you think the conductive matter is acidic? If I wash the board with an alkaline will it help? I think the conductive matter is absorbed inside the insulator.
The reistance of 20k is obtained between two points on the circuit diagram bypassing the CdS cell and the meter switch on a plain circuit board with all the components unsoldered. |
Jim Hickinbotham
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 06, 2004 - 04:05 pm: |
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Try cleaning the board with either solder flux remover or with a non-residue solvent. I have seen the old phenolic boards abosorb things such as moisture, oil, etc. |
Shel Sherman
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 07, 2004 - 07:34 am: |
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Back to my original suspicion, are there any copper traces going to the two points you are measuring the 20k across? If there are traces to BOTH points then you are still likely to be reading other components in the circuit. The only way to prove the issue is to then cut one of the traces. If you still read the same resistance then the board is the culprit. |
Aaron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 07, 2004 - 08:55 pm: |
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Thanks Shel, there are no copper traces. I guess if there were copper traces the resistance would be considerably smaller. I have soaked the tiny PCB in a mild solution of caustic soda overnight and the trick works. The PCB is now normal. Now I notice that the CdS cell might be aged. At total darkness the meter shows a small reading. Any ideas? Does the cell needs renewal? Any tips on choosing a replacement CdS cell? |
Aaron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, November 17, 2004 - 09:20 pm: |
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Just to wrap up the issue. The CdS cell in the MR4 is in fact not faulty. There was actually some conductive deposits on the surface between the 2 terminals of the meter cell which has caused the small meter deflection in total darkness. After cleaning that surface with alcohol the light meter is back to normal. As a matter of fact even if I could find a new CdS cell (which is quite difficult, if not impossible for this 40-year old meter), I was unable to remove the old one from the meter which I think is moulded into the plastic meter base.
Aaron |
