I have a Marantz 10b, and if you are familiar with them, you know that they use 3 ldrs, two of which are used for stereo/mono switching.
The ldrs are also the 10b's achilles heel, failing quite often.
I have been looking for a DIY replacement for years, and I just happened to come across this:
https://tubedepot.com/products/opto-coupler-ldr-for-fender-amps
Marantz 10b schematic:
http://www.zilla.li/All_Tubes/10B_Downloads/index.html
Anyone have any opinion on its suitability?
The ldrs are also the 10b's achilles heel, failing quite often.
I have been looking for a DIY replacement for years, and I just happened to come across this:
https://tubedepot.com/products/opto-coupler-ldr-for-fender-amps
Marantz 10b schematic:
http://www.zilla.li/All_Tubes/10B_Downloads/index.html
Anyone have any opinion on its suitability?
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looks like the product from Tubedepot uses a neon bulb to illuminate a CdS photo-resistor. Two of the marantz 10b LDR's are dual's, only one single shown in the schematic
You could do this -- get a couple of NE2H neon bulbs and a pair of CdS photo-resistors and mount such that each cell is facing the glass envelope of the neon bulb. Epoxy them together. Seal up with heat-shrink tubing.
The silonex and vactrol LDR's use an LED illuminator. They are sensitive to temperature so i don't know how well they will work in a tube circuit as the temperature changes.
Interesting problem. Not insoluble.
You could do this -- get a couple of NE2H neon bulbs and a pair of CdS photo-resistors and mount such that each cell is facing the glass envelope of the neon bulb. Epoxy them together. Seal up with heat-shrink tubing.
The silonex and vactrol LDR's use an LED illuminator. They are sensitive to temperature so i don't know how well they will work in a tube circuit as the temperature changes.
Interesting problem. Not insoluble.
Thank you both for the comments.
My two concerns have been:
A.) temperature (and since they are used in a tube circuit, I believe that has been addressed.)
B.) frequency response. The frequency response of the LDRs have been questioned in discussions I have participated in in the past. I have no way to know that unless I test the frequency response, and I presently don't have the equipment to test them.
I called Tube Depot after posting this message, and they no-longer have a tech on board, and don't know anything about the application, or the 10b. considering the price, I think I will purchase a couple of them, and try them out. Any more comments?
Thanks.
My two concerns have been:
A.) temperature (and since they are used in a tube circuit, I believe that has been addressed.)
B.) frequency response. The frequency response of the LDRs have been questioned in discussions I have participated in in the past. I have no way to know that unless I test the frequency response, and I presently don't have the equipment to test them.
I called Tube Depot after posting this message, and they no-longer have a tech on board, and don't know anything about the application, or the 10b. considering the price, I think I will purchase a couple of them, and try them out. Any more comments?
Thanks.
Opto-switch failure is a very common issue in the 10B. I fixed many of these by opening (saw-cut) the sealed cans and replacing the two LDR's with new ones (from Philips) which I selected for identical "ON" (conductive) resistance. Then re-seal the can which must be perfectly ligth-tight and you're done.
The problem with the Fender opto's is that they don't make a dual (stereo) version. You may try using a pair but they won't be matched and won't trigger simultaneously due to variations in the neon bulb characteristics. Also, you can't connect the neons in parallel because the first to ignite will drop the voltage too much and prevent the other neon to light. The neons must be series-connected but then you may need to change the value of the dropping resistors. It's feasible but some experimentation will be needed. For the single muting switch (RPL1) the Fender opto should work. Still, the easiest (and most elegant) way is to carefully open the original cans and replace the LDR's inside.
The problem with the Fender opto's is that they don't make a dual (stereo) version. You may try using a pair but they won't be matched and won't trigger simultaneously due to variations in the neon bulb characteristics. Also, you can't connect the neons in parallel because the first to ignite will drop the voltage too much and prevent the other neon to light. The neons must be series-connected but then you may need to change the value of the dropping resistors. It's feasible but some experimentation will be needed. For the single muting switch (RPL1) the Fender opto should work. Still, the easiest (and most elegant) way is to carefully open the original cans and replace the LDR's inside.
Tubologic, thanks for the reply.
You brought up some points that I hadn't thought of. Do you have a part number/source for the LDR's you mentioned? I would guess that I would need to buy at least 3 or 4 per module to match them, since I doubt that the seller would do that for me. (How close do they need to be matched?) Also, it sounds like you are indicating that you need 2 LDR's with two neon bulbs to trigger them. I would think that shouldn't be a problem if only one NE2H is used to trigger both LDR's, which is the way I *thought* the originals were constructed.
Apparently there is some differences between the NE2 and NE2H. The only real difference I see is the addition of a little radioactive (gas?) to the "H" version, to lessen darkening of the bulb and the breakdown voltage.
You brought up some points that I hadn't thought of. Do you have a part number/source for the LDR's you mentioned? I would guess that I would need to buy at least 3 or 4 per module to match them, since I doubt that the seller would do that for me. (How close do they need to be matched?) Also, it sounds like you are indicating that you need 2 LDR's with two neon bulbs to trigger them. I would think that shouldn't be a problem if only one NE2H is used to trigger both LDR's, which is the way I *thought* the originals were constructed.
Apparently there is some differences between the NE2 and NE2H. The only real difference I see is the addition of a little radioactive (gas?) to the "H" version, to lessen darkening of the bulb and the breakdown voltage.
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@Tubologic -- does the LDR fail because the neon lamp failed completely, gone to a small percent of its output, or has the CDS photo-sensor failed?
Datasheets suggest 10,000 to 50,000 hours use out of an NE-2 bulb before its output decreases 50% (depending upon the current)
I mentioned NE2H as those are the bulbs I used as a voltage limiter in of SY's preamplifiers - i still have a bunch in stock!
Datasheets suggest 10,000 to 50,000 hours use out of an NE-2 bulb before its output decreases 50% (depending upon the current)
I mentioned NE2H as those are the bulbs I used as a voltage limiter in of SY's preamplifiers - i still have a bunch in stock!
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In all the 10B optocouplers I fixed the LDR cells were defective and I never had to change the neon bulb , but once you've taken the time to open the can replacing the cheap little NE-2 can't hurt and could help improving long term reliability.
I can't remember the exact type of the cells I used, they were small half-moon shaped probably made by Philips, which I had in stock. They're not critical and there are many modern substitutes available. In the 10B the LDR's are used as ON/OFF switches in a high impedance environment and perfect matching is probably not needed but as I had many of them available I allways selected pairs with close Ron values.
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Yes, that's right: one single neon bulb illuminates both LDR's in the stereo optocoupler.
I think you can make these entirely from scratch if you desire. I made some a long time ago with some matched CdS cells with a short piece of black plastic tubing. Drilled hole in middle for neon, and placed the cells inside from opposite ends against the lamp and filled the ends with epoxy or RTV.
An LDR similar to this might prove suitable:
NSL-4960 Luna Optoelectronics | Sensors, Transducers | DigiKey
1M dark and 0.5K - 17K illuminated. Check surrounding circuit for impedances. The sensitivity of the CdS cell might be an issue - these are rated at 10 lumens.
An LDR similar to this might prove suitable:
NSL-4960 Luna Optoelectronics | Sensors, Transducers | DigiKey
1M dark and 0.5K - 17K illuminated. Check surrounding circuit for impedances. The sensitivity of the CdS cell might be an issue - these are rated at 10 lumens.
Hi Kevin
You hit the nail on the head. I have no idea what the resistance of the cell (s) are dark, and then lit. Sensitivity is also an unknown. I guess my best bet is to take one apart, and see if I can get a p/n off of one. Perhaps another tack would be to disconnect one unit, and do some measuring.
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