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Jackson
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 27, 2004 - 04:51 pm: | 
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One of my GSNs is overexposing by approximately two stops, according to the Fast/Slow indicators. I've compared the CDS iris opening with a known-good GSN, and they are virtually identical. I have also cleaned the contacts for the Bulb-Auto-Flash ring and the aperture ring. I'll add that both areas appeared pristine, and there was no indication before cleaning of intermittant electrical contact. The same is true for the shutter plunger switch that activates the metering lamps, but I cleaned it anyway.
I have downloaded schematics of the metering for a GS, but it appears there are no adjustment pots in the Yashica circuit. Before I waste too much more time, I'm wondering if there might be a common failure item that causes this. Perhaps the exposure capacitor or a resistor that commonly changes value? Maybe the CDS cell itself? Anyone have any experience with failure trends in this model? |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 28, 2004 - 12:03 am: |
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You got it right - the meter circuit of the Electro35G is NOT adjustable. Maybe you found an electrolytic capacitor named Cx or so in the schematics - that's the way they adjust it. In each Electro35G I opened I found a different value for this capacitor, between 2.2 and 4.7uF, with some odd values in between.
I think you have to set the camera to flash mode and check whether the shutter opens 1/30 sec. If not you should use a different value for Cx.
However, a dried out capacitor will rather loose capacitance, so the shutter speeds should be shorter, resulting in UNDERexposure. In more than 20 years in the electronic business I never encountered an electrolytic capacitor with increased capacitance.
I never encountered a faulty CdS cell in an Electro35G (quite contrary to the Electro35FC, there is hardly one of them with a working CdS cell) but you can try to swap the photoresistor. |
Jackson
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 28, 2004 - 10:52 am: |
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Winfried, after spending some time with the schematics, I realize the cap can't cause this problem. The cap is only in the circuit when the shutter release is fully depressed, not when it is partially depressed to power the indicator lamp circuits. Also, the yellow lamp and the red lamp both exhibit the problem. In other words, the red lamp indicates the 1/500 limit about two stops too large. The yellow lamp similarly indicates the slow speed warning about two stops too large. The two lamps activate about three to four stops apart, which is normal.
Eliminating the cap as a potential source of this specific problem is good news. It means I won't have to hand pick a replacement and measure shutter timing. On the other hand, it leaves quite a few items in the metering circuit suspect. The CDS could cause it (although not likely, based on your experience), as could one of the resistors in the aperture ring and several related components. I don't think it's likely to be TR1, TR2 or the thermistor. Failure in those parts would probably create a more severe problem, such as the lamps not activating at all, or activating constantly whenever the release is depressed.
I suspect this will require comparing resistor values and/or circuit voltages with a known-good Electro. I'll post the results.
Thanks for your help! |
Jackson
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 28, 2004 - 06:45 pm: |
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Winfried, I just completed the repair, and it was indeed the CDS cell. I compared it to the cell in a parts unit in order to make the diagnosis. From dark to light, the bad cell read about 450K ohms to 130K ohms with the camera set to ASA 400. The parts unit read about 115K ohms to 20K ohms under the same conditions. Note that these readings are only for cells installed in the camera and shielded behind the film speed iris.
To fix the bad unit, I replaced the entire iris/CDS cell assembly. I did not swap the PC board. The repaired camera has now been checked with a light meter, and appears to be right on the money.
Cheers! |
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