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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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RaMaBo:

the one elco in the 55V power supply section reading about 8000µF could be replaced too to get the job very well done, It's nearly out of tolerance this could be due to high temperature and age. If you find a elco with about 10000µF and the correct voltage (or a bit higher) and with the same size (maybe a bit smaller) you could change this one too. Saves you same trouble in a few years maybe.Wink 

I was thinking that too.  That one is close to the limit of the 20% tolerance....but it is in tolerance. I did look for suitable replacements.  I found several brands that make 10000uF at 63V, 80V and 100V in the 40mm (D) and 80mm (L) size.  They vary in price from around $10 (Panasonic T-UP series) to $70 (Cornell Dubilier).  Vishay, Nichicon, Epcos and Kemet have them in between those prices. Some of the components are 35mm diameter instead of 40mm but those should be fine.

Rich
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Rich replied on Wed, Aug 28 2013 8:05 PM

I don't know where you find the perseverance, John.  Good work.


Søren Mexico
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Rich:

I don't know where you find the perseverance, John.  Good work.

Well, its just the "we never give up" drive, and John has it. And John keep them coming we really enjoy it.

RaMaBo: We need your valuable inputs more often.

 

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

sonavor
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Thanks for the nice comments. This is a very enjoyable hobby and I really appreciate the support you guys provide on this forum. I wouldn't be able to get through these projects without it.

-sonavor

sonavor
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Nothing too exciting today. I recapped the three last boards (FM, FM Interface, Filter & Tone Controls). There were a couple of uh-oh moments but I was kind of expecting them. To recap the boards I disconnect the wires and pull the boards out of the Beomaster. After putting the boards back, I had some problems receiving the FM signal and I lost the left channel. The connections are very touchy. I went over all of the connectors again, still no left channel. But the FM tuner started working again. The left channel was out in both the headphone jack and the speaker outputs. I ended up pulling the connector on the left output amplifier board that gets the preamp output  and connected my scope to see if a signal was getting that far. Sure enough it was. So I plugged the connector back in to the output amplifier and checked my headphones. Sound was in both channels. Next, the speakers. Sound in both channels there. One or more of the connectors I touched must not have been making good contact.

Here is the recap on the FM board (just three electrolytic caps on this one). C44 is the only replacement cap on the signal path.


sonavor
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Here is the FM Interface board (just two caps replaced on this one).


sonavor
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Last, here is the Filter & Tone Control board (seven caps replaced here).


sonavor
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I think the FM tuner on this Beomaster 8000 is not up to the level it needs to be.  One check I always try in my workshop is to take my coax antenna connection off my Beomaster 4400 and try it on whatever tuner I am testing out. When connected to the BM4400, I get almost full scale on the strength meter and the sound is really, really good. So using that connection on the BM8000, I get just an okay reception and the signal strength is around 3 or 4 on the scale. Before the FM section recap, I was getting about the same signal level.  Maybe a little less.

RaMaBo
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RaMaBo replied on Fri, Aug 30 2013 11:22 AM

The signal meters can't be compared directly because there are used completely different chips for FM demodulator. As far as i remember the CA3189 (used in the BM8000) has an logarithmic scaled output for the signal strength whereas the TCA chip in the BM4400 has a somewhat different  scale. So don't give to much on the reading of the signal meter.

What's an ok reception on the bm8000? is there a bit hissing in the background? If so try mono reception is it better then?

i never heard both but i once had a self made receiver with the CA3189 and TCA4500 and it gave me a fine reception with an external antenna about 30 km away from munich.  it gave me a really nice sound from FM, even Austrian radiosation from Salzburg (about 100km) was receivable in stereo nearly hiss free. But it depends also on the tuner frontend.

How far away is the station you are listening to, local or distant radio station?

 

Ralph-Marcus

sonavor
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The stations are within 40 miles and I have a large FM antenna in my attic. I see what you are saying about the meter scales so that wouldn't be a valid comparison. My comparison right now is just by ear. But I own several tuners and with the same antenna source, it appears the BM8000 is not receiving quite as well.

I would like to go through the FM adjustment procedure without adjusting anything at first...just look at the measurement results.

FM testing is something I want to learn but I am not familiar with yet. I have a Panasonic VP8193A AM/FM Signal Generator available to me as well as a 200MHz oscilloscope. The procedure as outlined in the BM8000 service manual is not very clear to me. The service manual says: Connect a sweep generator to the aerial input and set the receiver to 87.5MHz.  Connect an oscilloscope to 1TP22 via an RC-probe. Then look for a symmetrical IF curve. It doesn't say what to set the generator to so is that measurement with nothing applied? I am guessing the manual is implying that the generator is set to 87.5MHz. But is that just a sine wave and what should that be a 1mV signal?

The only thing I checked so far is the first step which is to verify 3.0V is at 2TP21. I measured 3.06V so it appears okay.

sonavor
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I started looking at what I can do regarding measurements on the Beomaster 8000 FM section. The FM testing is going to be new to me so I wanted to see a known FM frequency come in on the antenna and see the expected modulation frequency coming out of the tuner section. Looking at the BM8000 service manual I decided to set up the signal strength meter test. 

The service manual says to connect a signal generator to the aerial input, set it to 94MHz and to yield 1mV EMF (allowing +-75kHz tolerance).
Tune the BM8000 receiver to that frequency (94MHz) and measure 2.0VDC at 1TP30.

So I set up my FM generator to 94MHz. The generator has selectable 1kHz and 400Hz internal modulation frequencies (as well as allowing you to use other ones from an external source). I just used the 1kHz.  I wasn't sure of the amplitude and modulation settings on the generator so I set the modulation at 100 percent and looked at the generator output on my 200MHz scope. I didn't get a signal until I increased the amplitude of the output to around 60dBuV EMF (the output units on the generator). Now that I could see the FM signal, I connected it to the BM8000 aerial input. I hooked up a DMM to the TP30 of the FM PCB because the manual says I should measure 2VDC there when the test is all set up correctly. I also pulled the connector from the preamp to the power amp on the left channel so I could measure that preamp output with my digital scope.

In order to get a signal on the BM8000 preamp output, I had to set the BM8000 tuner to 94MHz and increase the volume so I had an output. I didn't have a good signal at first until I increased the FM generator amplitude to above 60dB.  Then I get the expected 1kHz sine wave out of the preamp section.

As for the signal strength, in order to get 2VDC at the 1TP30 test point, I had to increase the FM generator amplitude to 102.5dBuV EMF. The service manual says the input signal should be 1mV EMF. In looking at converting units to see where my test setup is in relation to the service manual configuration, it looks like 60dBuV EMF should match up with 1000uV EMF which is the desired 1mV EMF. This is for a 75 ohm load. This is from EEWeb but I must not be translating the units correctly since I am having to set the generator output to 102.5dBuV EMF.

So I still have to figure out what I am looking at (and doing) before I even think about adjusting something.


sonavor
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With the FM generator input set to get the 2V on the signal strength meter test, the meter reads about 8 on its scale. I don't find anything in the manual that says what the meter should be showing when the signal strength meter test point is at 2V.


sonavor
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I found this formula -
dBuV = 20log(uV)

So if the output of the FM generator is shown in dBuV as 60, that would mean a uV value of 1000uV.  That would be 1mV EMF that the service manual calls out.  As a check, I connected the FM generator output to the 200MHz scope again to measure the signal out with a 60dBuV amplitude. The scope had to go to its minimum voltage scale (2mV/division) and the peak-to-peak was right about 1 division. That should be 1mV RMS so I think the Beomaster FM tuner should be handling the input at 60dBuV out of the generator. 

Right now it doesn't look like I get an output from the tuner for a 60dBuV EMF level signal. If I am right, I need to determine what FM tuner adjustments to make first. I think I want to just look at adjustments that involve measuring a voltage level and adjusting a trim pot. I don't want to mess with adjustments where the inductors are tuned.


sonavor
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sonavor replied on Sat, Aug 31 2013 10:52 PM

I am still researching this. Being new to these measurements, it is taking me a bit to filter through all of the information. I did review the FM generator calibration test results when I sent it off for checking last year. The VP-8193A was pretty much right on the money for frequency and output (measured as dBuV EMF). So I am pretty confident that what I set  on the FM generator is what I get out. What I am not sure of is my connection from the FM generator 50 ohms output to the Beomaster 75 ohms antenna input. Can I just run the coax cable to it or do I need a 50 ohms to 75 ohms conversion pad?  With my present, direct connection I can see that the signal will get there but I am unsure if the amplitude is correct for making any measurements/adjustments per the BM8000 service manual. The same is true for my FM generator signal check on my oscilloscope. I am not sure of the voltage I am getting there when just connecting the generator cable directly to it.  According to the RF Unit Conversion tool at the EEWeb web site, I should see a peak-to-peak voltage of 1.4mV for an FM generator output of 60dBuV EMF. I was seeing about 1.4 to 1.6 divisions on the scope at 2mV/division. So about double what I am expecting (but is my expected value correct?).

sonavor
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It looks like I need a 50 ohm termination on the FM generator cable connection at my oscilloscope. From what I have read, not having the termination will result in reflection and doubling my measurement...which is what I am seeing.

On the Beomaster antenna input though, I believe I do need a 50 ohm to 75 ohm matching pad in line with the generator signal. So I will get those pieces for my test setup before I continue.

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