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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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valve1
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valve1 replied on Fri, Aug 23 2013 7:36 AM

sonavor:
Today began good and ended horribly

Sonavor its a great job so far ! Bummer about your short but you have to keep going.

sonavor
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I will keep going.  I'm just a little ticked off at myself right now. But I will regroup tomorrow.

RaMaBo
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RaMaBo replied on Fri, Aug 23 2013 8:43 AM

Hi,

 

the voltages you measure seem to be fine.

Concerning the blown fuse it's a 250V / 2A type.

Those 3-pin fixed voltage regulator chips (78Mxx) are rock stable but have one weak point: If the output voltage is higher than the input voltage they get damaged. and that's what you probably did: Shorting the input while the ouput is still at 5V shot the regulator chip into silicon heaven (and the fuse too Wink ).

Nowadays a well designed powersupply has an additional fast schottky diode from the output of the 78Mxx (Anode) to the input (cathode) so that the high output will be 'shorted' to the lower input voltage, thus preventing the chip.

 

Changing the 78M05 should normally restore previos state of this nice machine.

Godd luck and keep my fingers crossed.

Ralph-Marcus

sonavor
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I picked up some regulators Friday afternoon. Although I did pick up some spare +-15V regulators, I only replaced the suspected +5V regulator.  Once installed I tested just the power supply board plugged into the BM8000 power main.  The 5V regulator is working again. I couldn't check +-15V as they aren't on in Standby.  So I connected the rest of the Beomaster boards up to the power supply board and gave it a test.  I did have my dim bulb tester in the path.  With AC power on, the Standby light works.  I tried Phono from the front panel. The display lit up briefly, then I saw my dim bulb tester go bright and the BM8000 went back into Standby.  So there  is a short somewhere. I started going through the power supply plugs one by one.  Sure enough, there is one plug that is preventing the receiver from operating.  It is P51 on the power supply board.


sonavor
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P51 on the power supply board has 4 pins.  The P51 wires control the delayed relay circuit. I don't see the connection between this power supply circuit and the +5V regulator problem but P51 is what is causing the dim bulb tester to illuminate and is keeping the receiver in standby. I can see a connection between P51 and the +-15V regulators though. I am going to sleep on it and decide whether to change out the +-15V regulators in the morning.


sonavor
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I decided to make one more check.  With P51 disconnected on the power supply board I can take the BM8000 out of standby.  So with that scenario I am able to verify there is +-15V on those two regulators.  Does that mean there is a short in the relays RL1 and RL2 ?  Or can it still be some component on the power supply board?

Here are the regulators.


sonavor
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I forgot to mention I found something odd on the power supply board.  In this picture you can see that one of the new, recapped electrolytic capacitors (C40) is bulging at the top. Maybe this is a result of the short?  C40 goes from the -15V regulator output to ground (in parallel with D9).


DMacri
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DMacri replied on Sat, Aug 24 2013 1:37 PM
Yes, I would consider that cap bad and replace it. The question is if that was the problem or if it was damaged by something wrong elsewhere in the circuit. The canary in the coal mine.

Dom

2x BeoSystem 3, BeoSystem 5000, BeoSystem 6500, 2x BeoMaster 7000, 2 pair of BeoLab Penta mk2, AV 7000, Beolab 4000, BeoSound 4000, Playmaker, BeoLab 2500, S-45, S-45.2, RL-140, CX-50, C-75, 3x CX-100, 3x MCL2 link rooms, 3x Beolab 2000, M3, P2, Earset, A8 earphones, A3, 2x 4001 relay, H3, H3 ANC, H6, and ambio 

sonavor
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I replaced C40 but in doing so and reviewing the before and after pictures of the power supply board I realize that C40 was in backwards. So the bulging was from being installed incorrectly rather than the shorting of the 5V regulator. 

With a new C40 installed correctly, the problem has now changed.  Attempting to take the Beomaster out of Standby (with everything connected) still illuminates the Dim Bulb Tester but dimly now. I can go to Phono mode.  When I try an FM station though, the unit shuts off.  I checked the outputs of the voltage regulators again. I have a good +5V and +15V but no -15V. 

Dillen
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Dillen replied on Sat, Aug 24 2013 7:19 PM

Check the rectifier.
It saw an overload too when you shorted the cap.

Martin

sonavor
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I replaced the -15V regulator. That only temporarily worked. When I tested the power supply with the new -15V regulator, power came on, the Dim Bulb Tester stayed very dim (so that is good). Putting the Beomaster into Phono mode displayed correctly on the display and the -15V regulator measured okay.  I selected an FM preselect and the channel displayed correctly.  The Beomaster remained on.  So all looked good.  I removed power and my test leads.  I turned on the speakers and powered back on.  When I tried to come out of Standby the Dim Bulb Tester flashed brightly and put the Beomaster back to Standby.  The -15V regulator is now shot again.  So there is something in its path that is knocking it out.

I removed all connections from the board and re-measured the input voltage through the rectifiers.  They appear good.  The D5 output is +11.51V for the +5V regulator.  The D1/D2 rectifiers are producing +26.05V.  The D3/D4 rectifiers are producing -26.05V.  I checked the D9 diode that is in parallel with C40 (the electrolytic that was backwards).  D9 measures okay out of circuit.

The input to the -15V regulator comes from IC9.  I haven't checked that yet. I only have one -15V regulator left so I won't put another one in until I find what is likely causing it to fail. It is strange that it worked right at first. I don't know if the regulator would have failed if I let power stay on without engaging the speaker but I'm not going to sacrifice another regulator to find out.

The picture here shows what I have measured. The red bar shows the original short that caused this whole problem.


hamacbleu
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Just a tought.., for i'm just an amateur in electronic... but it's strange: i've blew the exactly the same cap a few months ago by doing the exact same mistake... (Maybe because it's strange to see two same polarity face to face i dont know...) it just exploded! However I just changed it and everything was back to normal... So perhaps you should continue searching in the area of the regulator?

by the way, thanks for sharing all this, i'm learning a lot...!

Guillaume

DMacri
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DMacri replied on Sat, Aug 24 2013 10:59 PM
The cap would have sacrificed itself and taken the regulator with it, but it should not have caused any further damage from what I can see. The problem is probably later in the circuit - off this page. I think it would be unrelated to the reversed cap issue.

Dom

2x BeoSystem 3, BeoSystem 5000, BeoSystem 6500, 2x BeoMaster 7000, 2 pair of BeoLab Penta mk2, AV 7000, Beolab 4000, BeoSound 4000, Playmaker, BeoLab 2500, S-45, S-45.2, RL-140, CX-50, C-75, 3x CX-100, 3x MCL2 link rooms, 3x Beolab 2000, M3, P2, Earset, A8 earphones, A3, 2x 4001 relay, H3, H3 ANC, H6, and ambio 

hamacbleu
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... What about the two small 180 ohms resistors linking the power supply to the rectifier, just at the right of the startup board? Mine was burned.. (the one linking the red wire). Don't know if the cap caused this however...

sonavor
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sonavor replied on Sat, Aug 24 2013 11:10 PM

Yes, I was fortunate the misplaced cap didn't explode but it isn't related to the problem that occurred from the short on the 5V regulator. The incorrect installation of C40 was back at the beginning and had been operating that way for over a week. So lucky that cap was built for punishment.  The shorting of the 5V regulator and subsequent attempt to power the Beomaster on with a new fuse and no Dim Bulb Tester in the circuit must have allowed a power surge in the receiver I guess. That probably damaged some other parts (including the -15V regulator).  Whatever the problem is now, it doesn't seem to affect the 5V and +15V regulators. I'll probably have to start taking out and testing the transistors and diodes associated with the -15V path.

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