Phototransistor and black sealant on it/PCB

Thread Starter

gbd1010

Joined Mar 17, 2021
11
Hello,

I have 2 questions if anyone can spare their time. (I'm very much a novice so apologies if these questions are very basic)

I believe I've encountered a broken phototransistor on a stereo system I'm working on and I wish to replace it.
I've never replaced a phototransistor before and the one I wish to replace has what looks like black sealant on its rear and also on the circuit board.

~ I've attached a photo. The broken phototransistor is on the left. The one on the right is working

My 2 questions are...
1. Can anyone tell me what this black material is and do I need to reapply it when I replace the phototransistor?

2. I'm very unfamiliar with phototransistors. The one I wish to replace is model PH101. But I'm having trouble sourcing one. Can anyone suggest a modern equivalent? The Voltage in is approx 9.4v and it must be able to detect light from a standard white LED.

Thanks,
Daniel
 

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Thread Starter

gbd1010

Joined Mar 17, 2021
11
Hello Yaakov,

Thank you very much for responding.
It is indeed a hard resin like substance. I guess I'll try and remove it and hope not to break the PCB.

There is no photo of the phototransistor in the link you provided so I cannot compare it with my existing phototransistor.
It states that it's PH101-4.
Do you know the significance of '4' in this instance as the service manual for my stereo just lists PH101?

Thanks
 

Ian0

Joined Aug 7, 2020
3,739
Is the phototransistor attached to a LED to make some sort of home-made optocoupler? Is it an LDR not a phototransistor? (if you can see a wiggly track under the lens it's an LDR). If so, then you do need to reapply some sort of opaque goo to keep the light out - or put some black tape around it.
Is it really broken? They don't go wrong often.
My guess is that it is some sort of muting or volume limiting circuit. If so, can you live without it? Perhaps you could remove it, and it would still work.
 

Thread Starter

gbd1010

Joined Mar 17, 2021
11
Hello Ian,

Thanks for responding. I reassembled the stereo last night so am unable to look through the lens at the moment. I never looked too closely at the lens part so I can't say if there is a wiggly track there. But I'm pretty sure it's a phototransistor as that's what it's listed as (ph101) in the stereo service manual.

The 2 phototransistors work in tandem with a LED/bulb each and are used to determine if there is a 7" or 12" record on the record platter. If a 7" is present, then light is blocked going through to one phototransistor and the voltage out from it drops, if a 12" record is present then light is blocked going through to both phototransistors and the voltage out from both drops. Effectively they are working as logic gates I guess.

So last night, I disconnected the output from what I believe to be the broken phototransistor ~ effectively keeping its output in a LOW voltage state. This meant that in terms of logic, I was able to achieve the outputs required for the system to determine whether there is a 7" (Low + High) or 12" (Low + Low) record on the platter. The only time the output logic now differs from the original is when there is no record on the platter. It should be High + High, but I have Low + High (i.e. It thinks there is a 7" record on the platter, but there isn't)..

However, I can't see any functional disparity. I have tested the stereo and it is now detecting record sizes correctly and it doesn't matter if it thinks there is a record on the platter when there isn't as this doesn't activate any other sequences/motors as far as I can determine...as far as I can determine!!! Time and testing will tell.

Obviously, I'd still like to fix/replace the phototransistor issue, but failing that, I'll stick with this solution unless there is a fault in my logic.

Thanks,
Daniel
 
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