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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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Success...the FM tuner works again. I tried several stations with a small antenna connected and the sound is good.

As before though, it is partial success.  The right channel is still out. Something is not connected or is out.  After I replaced the TR200 PNP transistor on that board the idle and DC offset adjustment all went fine.  The clipping lamp is staying off now. There has to be some problem with the right channel output board though. I'll have to go back to that board and see what is going on.


sonavor
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Both channels are working now. I was pretty sure there was probably a connection problem on the right channel. Tracing the signal path I found the PCB pad for positive side of the C200 capacitor (that I replaced) had come loose.  So no signal was coming into the output amplifier. The pad had completely come off so I put in a new 4.7uF capacitor for C200 and left the positive lead long enough to reach the next node in its path. 

Now the Beomaster 8000 is playing again.  I listened to a little Led Zeppelin I and it sounded great. So far I have tested all of the music source inputs except phono.  All of the right side PCBs have been recapped. The only electrolytic I didn't touch was one mounted on the side of the frame. It is a 10uF, 63V capacitor.  I think it is C35. 

The next step is to recap the left side output amplifier although it is tempting not to right now. The amplifier section appears to run nice and cool and the receiver is sounding good.  I will sleep on it and decide tomorrow.

Søren Mexico
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As always a good job John, As the unit is open do the recap, then its all done, and you can forget it and just enjoy.

Collecting Vintage B&O is not a hobby, its a lifestyle.

Rich
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Rich replied on Fri, Aug 16 2013 5:48 PM

Great news and great work, John.  Except you should test with Led Zeppelin II.  On vinyl.  Yes - thumbs up


sonavor
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Rich:

Great news and great work, John.  Except you should test with Led Zeppelin II.  On vinyl.  Yes - thumbs up

Haha, I'll do that for sure. 

sonavor
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The repair so far was to fix the clipping lamp problem. That was diagnosed to be a transistor in the right channel output amplifier board and the display board. Both of those boards are located on the right side of the BM8000 so I also replaced the electrolytic capacitors on that side (which included doing the microcomputer board and the power supply board).  That leaves the left side of the BM8000 which has the left channel output amplifier board, FM and FM interface boards and the preamplifier board.

Before continuing with the recap I decided to make a few measurements. Earlier this year I built the Classic Audio Tester that Beoworld member Frede designed. It provides a nice way of connecting a audio device (unit being tested) to a computer (through an audio card) for testing. Various test software can be installed on the computer that interacts with the sound card to supply test signals and to read back (and measure) results. The sound card I am using is an external USB 2.0 device by Creative called the E-MU 0204.  It can be set to various sample rates for output signals and input signals.  The software I am using is the Rightmark Audio Analyzer. It has a test routine that performs a series of audio tests and produces the results. That saves a lot of time trying to set up and run each test manually.

I am still very new to using the software and tester so I feel like my use of it right now can only be as a reference point to use in before (recap) and after (recap) of the Beomaster 8000 preamplifier section. I am not set up to run testing of the power amplifier section as the audio card input voltages do not handle high voltage levels. To eventually do that I will have to build an interface that has the correct attenuation to bring down power signals for measuring through the device. The Beomaster 8000 has a two pin connector (signal and ground) for each output amplifier board (right and left). That input to the power amplifier board is the output of the preamplifier stages (including filtering and volume). I made two cables for each channel that route the preamplifier output to the Classic Audio Tester measurement inputs.  I routed the Classic Audio test output signals to the BM8000 Tape 1 inputs.

At the beginning of running the Rightmark tests, the software does some testing of the signal levels. That initialization had me adjust the BM8000 volume controls to the level shown in the picture below.

Here is a picture of the overall set up. 


sonavor
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Here is the test setup connections on the BM8000 output boards and the Classic Audio Tester


sonavor
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I am only able to run the audio tests at a 96K sample rate. When I completed the tester a few months ago I am certain I was able to run higher. The computer I am using in my shop is an older (Shuttle) one that is running Windows XP.  Maybe there is some driver issue. The 96K will do fine for now. Eventually I will upgrade the PC on my workbench.

Here is the result of the Rightmark tests.  I am going to run them a few more times to verify the numbers are consistent.
Also, keep in mind, you can't compare these measurements with the Beomaster 8000 specs as my measurements are only for the preamplifier section and my test setup is not the same as the tests used for the published BM8000 specs. 


sonavor
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I ran the Rightmark audio tests five more times on the Beomaster 8000 preamplifier (using Tape 1 as the input source).  The measured values stayed pretty much the same but the last four tests all measured closer to each other than to the first one.  Here is a picture of the test results showing the very first test run on the left and the latest test run on the right.  I think the measured values are still very good and I should be able to use these numbers as a reference after I recap the preamplifier board.


sonavor
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Tonight I replaced the electrolytic capacitors on the BM8000 preamplifier board. The two 100uF, 10V caps measured around 130uF. They aren't in the signal path and I replaced them with some Panasonic 100uF, 35V caps.  The rest of the electrolytic caps are 10uF, 63V and are all in the signal path.  Most of them measured in spec (around 11uF and 12uF). Their ESR ranged from 1.0 to 1.6 ohms.  One measured 16uF. I replaced them with audio grade Nichicon caps.  They are all around 9.8uF and have an ESR of less than 0.8 ohms.

Here is a picture of the preamplifier board before the recap.


sonavor
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Here is the board after the recap.


sonavor
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Here are the Rightmark audio analyzer test results before and after the BM8000 preamplifier board recap.  I went ahead and ran a few test runs again before I pulled the preamplifier for recapping. That is the picture on top. It shows the last four test results before the recap.  The bottom picture are the last four test results after the recap.

Of course I also connected the preamp back to the power amplifier boards and did a listen test.  The receiver sounds really good (it sounded good before too). I am glad I went ahead and did the recap as a few of the old electrolytic caps were off.  They could only get worse. Now, with a new set, the board should be good for a long time. The audio test results show me that the board measures slightly better than before the recap. The difference in the before and after tests are too small for my ears to tell. But the testing is nice because it proves to me that the new parts are in place and working as good as before the recap.


sonavor
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Today began good and ended horribly. I successfully recapped the left power amplifier board and re-adjusted the idle and DC offset. Then I checked the right channel idle and DC offset again. All was good up to that point. I put the Beomaster into standby.  Then, I forgot it was in standby and probed the C35, 10uF capacitor that is across the 5V regulator output and common. ZAP. I blew the F1 fuse. It appears I shorted the 5V regulator common with the input. Now I have a non-functioning Beomaster.  I tried another fuse with my dim bulb tester in circuit and I get a nice bright illumination on the dim bulb so I have a short somewhere. I checked the three regulators (5V, 15V and -15V) and they are not shorted or open. So something else is what got zapped by the short. I had been very good about removing power completely between each step until today...and electricity always makes you pay.  So I am dead in the water on this until I can figure what could have been damaged by the short.  Here is a picture of the circuit and where the short occurred.  Any suggestions on what to check first?


Søren Mexico
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I shortet out my BM 2400 0ne time and my BM 901 two times, its not only what you find easily, but also damages that shows up after a while, at my 901 I´m about to change everything in the 2 output stages, just as I did in the 4400, the 901 never recovered completely after the 2nd short, it played good for a while after changing several diodes and transistors, but then started acting up again, so i decided to do it all.

Good luck John, i feel the pain.

Collecting Vintage B&O is not a hobby, its a lifestyle.

sonavor
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Thanks Søren. It is really a pain when it is something as idiotic as forgetting to remove power. I am hoping it is the 5V regulator that is bad.  With everything else disconnected and the fuse out, I connect the P52 power connector to the power supply board and applied power. The output of the D5 full-wave diode bridge is 13.5V.  I checked the signal with an oscilloscope and it is a nice, flat DC voltage. So I think the diode bridge and C31 (4700uF) are okay.  The voltage at C33 (2200uF) and D1, D2 node is 27V.  The voltage at C1 (2200uF) and D3, D4 nodes is -27V. Those go to the +-15V regulators which I don't think would have been affected.

I would think that shorting the 6IC4 5V regulator input and ground would have just taken out the fuse (which it did) and maybe the regulator.  The regulator pins don't measure a short but that doesn't mean it is good.  The regulator is my prime suspect so I will try and pick up a replacement tomorrow. One fear I have is that the 5V regulator failed in a way where it didn't limit its output to 5V. The F1 fuse says T-2A. So is that a slow blow, 2A, 250V fuse?

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