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Beomaster 8000 Project

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sonavor
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sonavor posted on Tue, Aug 6 2013 8:28 AM

Here is the Beomaster 8000 that I acquired as part of an 8000 system earlier this year. I have been wanting to get this receiver working again but kept getting side-tracked. I think it is now time to dive into it. I have the service manual so I think I am set.

 

 


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sonavor
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It is a transformer. But it is listed in the B&O service manual as Lx so I called it an inductor. In any case, my plan right now is to try and fix it but I will probably steal the same part from a Beomaster 6000 (which has that same part: 8020341). 

I probably won't be able to do any more work on the Beomaster 8000 until Thursday or Friday.  Tonight I am going to locate where I stored the BM6000.


sonavor
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I located my BM6000 parts unit and stole the L14 8020341 transformer out of it. The BM8000 service manual doesn't show the actual value.  I had listed it as 10uH but the BM6000 service manual lists it as 100mH. Interestingly, the another common Lx device between the BM6000 and BM8000 is the 8020342. The BM8000 service manual lists that device as 10uH while the BM6000 service manual lists it as 10mH.  I would think the B&O part number being the same means they are the same component. I hope that is the case.

 


sonavor
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I checked out the two 8020341 parts with a magnifier. They do look identical. Both have 0.18mm diameter (33 AWG) wire and two turns through the ferrite bead for each color (green and red).  I ordered some 33 AWG, solderable magnet wire so when it arrives I will attempt to repair the broken one.

sonavor
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Here is a picture of the 8020341 parts - BM6000 L14 component on the top (good condition), BM8000 L2 component on the bottom (disassembled).  You can see where the green wire broke off on the left.


Rich
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Rich replied on Tue, Aug 13 2013 5:33 PM

If it's a transformer, and it has the same number of turns, uh, what's the point?

Been quite a while since I had circuits (Fall, 1987), so I'm pleading bad memory if I'm missing something easy.


sonavor
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Good question. Maybe an expert can provide the answer. My guess it is to provide isolation between the signal from the FM board and the prescaler circuit that is receiving the signal. 

tournedos
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That transformer seems to be for isolation, and balanced to unbalanced signal conversion. By all means try repairing it - there's nothing magical in it, the hardest part will be finding the correct gauge enamelled wire (without having to buy a full reel or something). You can often salvage it from scrapped old radios that are full of similar inductors.

--mika

sonavor
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I was able to find small spools of solderable, 33 gage magnet wire on Ebay so I ordered one red and one green yesterday.

sonavor
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I could have just bought one color and painted a different color on the other strand.  But I didn't think of that at the time. :)

sonavor
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I was able to get most of my next round of parts from Mouser today. The first thing I did was verify my semiconductor tester recognized the BC556 transistor as a PNP and that it identifies the MPSA13 as an NPN Darlington.  It identified both correctly. On the two original transistors, the tester couldn't identify them. 

I finished up with my replacing of parts on the right channel output amplifier. That was to replace the suspected bad TR200 (MPSH54 - PNP) with the new BC556.  I also decided to replace the remaining electrolytic I hadn't changed (C200 - 4.7uF, 63V) and the two trim pots. 


sonavor
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I also finished recapping the power supply board (the two large 2200uF and the 4700uF electrolytic caps).  Those may not have been necessary to change but it is such a pain getting to these underlying boards that I decided to replace them just to be sure. I used a little quick drying Aleene's Tacky glue under the big caps to secure them.


sonavor
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Next I replaced the IC4 Darlington NPN on the display board. That is the TPSA13 transistor that controls the clipping lamp. The replacement is an MPSA13.  The result was a (partial) success.  The clipping lamp no longer illuminates when the receiver is turned on. I also re-adjusted the idle and DC offset voltages on the right channel output amplifier.  I say partial success because having to take the display and microcomputer boards apart for this update means I have to stabilize the connections again. The front volume wheel doesn't do anything again and some segments aren't illuminated. Also, I had sound from the left channel for a while but then it went away. I am thinking it is a connection issue plus I don't have the microcomputer board completely put back together properly.
 


sonavor
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I think I am going to wait now until my wire arrives to fix the L2 transformer. That goes back on to the microcomputer board and will hopefully restore the FM functionality. Then I can put the microcomputer board back to its completed state. I think the wire should arrive by Friday.

sonavor
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My magnet wire arrived today so I was able to repair the little 8020341 (L2) transformer. 


sonavor
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I scraped off the enamel insulation and re-soldered the L2 transformer in place.


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