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Hello,
I would like present you a way of repairing the detector arm in Beogram 4000. I acquired the turntable for extremely low price here in Poland. I am still working on it, but today I found a way to repair the detector arm insert, used to optically detect the size of the record on the platter.
The original photoelement is a BP100 photodiode. It is used in a non-polarised state - it works exactly as a photocell, producing some milivolts depending on the lighting conditions. In my detector insert, there were some signs of unsuccessful repair trials - the plastic was melted, the bulb was too big and the BP100 had one 'leg' missing.
I decided to try my luck with an alternative - the photodiode OSRAM SFH206K seemed to fit the insert after some modifications, the size of the element is quite similar to the original one (I don't know if it matters) and it is sensitive to the visible light range. Below, a comparison of a BP100 with one leg and a new photodiode SFK206K.
As photodiode is in the TO-92 package we have to use a small, precise file to make it thinner and a bit narrower to fit the housing. Please be careful, as one can easily destroy the semiconductor structure inside. Here - the brand-new SFK206K (left) and adapted part (right).
After using a precise file, it fits well into the detector housing. Please note the bulb (6V, 50mA) is not the right one and I replaced it with the smaller one. Solder markings are an example of the previous owner repair trials :(
After soldering the insert to the BG4000 arm cables it works without any modifications to the circuit. It also works with blue semi-transparent record (the only one I have). Please note that I had to fiddle a bit with the photodiode placement as to get as much reflected light (from the platter/record) as possible.
Below you can see the bag with photodiodes from Conrad (supplier from Germany). Note the TO-92 packaging information - the shape of the photodiode package is almost identical to standard small-signal transistor.
I realise that some purists may say that it is nicer to use an original part - and probably it is :) But for me this tweak works perfectly OK. In fact I was a bit scared of getting the original BP100 photocell from Martin - it seems extremely easy to destroy.
Marcin
nice repair. I would however suggest these diodes rarely if ever fail, which is why there hasn't been mush of a need until now!
Olly
Well, exactly. The main threat to them is an unprofessional user/repairer. Previous service attempts undoubtedly broken mine.
But now the detector arm works again :)
It's a nice solution to this problem and a cheap try in case of problems but I wouldn't expect this fix to work in all Beograms.The original part is a very thin and tiny glass piece with two leads soldered to each their side.Very delicate and it breaks very easy.Being a real voltaic cell, it converts light to electricity.The Photodiode is "merely" a diode controlled by light. It may output a few millivolts when light shines on itbut I wouldn't expect this output to be adequate for all situations.But, as said, easily tried in the given case.
Martin
A reply to a 3 year old post! I love the internet.
I opened up the detector arm to check the bulb and the existing photo detector fell apart – one of the arms completely disconnected from the side. Damn.
I tried, very delicately, to solder the arm back but it doesn't appear to have worked so I'm going to try your method as described above on my Beogram 4004. Wish me luck.
One question;
Am I right in assuming the earth lead on the SFH206K is the shorter lead, and as such is on the wrong side of the arm? Did you have to cross the lead over to reach the earth contact?
So... no response. No matter.
It worked, anyway, for anyone looking at replicating this mod.
It appears the photodiode can be soldered irrespective of earth.
It's certainly delicate work, and the replacement diode needs to be filed down A LOT to fit in its home, but I now have an LED detector arm operating flawlessly with the OSRAM SFH206K.