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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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I checked the circuit again this morning to double check the results.  The measures were pretty much the same but I have added the voltages at the collector for TR200 (on the Output Amplifier Module). 

On the Display Module (8), IC4 should normally be around 50mV at the base.  Currently, the right channel is pushing that to over 1 V so the lamp turns on full.  With the right channel disconnected, the left channel applies right around 1V or a little less so the lamp turns on, but dim. The voltage at the collector of TR200 on the right channel is way off from the left channel.

I think I will have to pull those output amplifier modules next and check the solder points of the components. I will probably go ahead and change the electrolytic capacitors while the board is out.

Meanwhile, during my testing, the display module segments turn on and off differently between tests. I have been able to carefully put the board in and get the display functioning with all segments. So there is an issue with the board connectors. I am going to wait and work on that problem when I have the output module board and power supply boards all checked out and done since they sit underneath the display board.

 


sonavor
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sonavor replied on Sat, Aug 10 2013 11:18 AM

No luck with the clipping lamp problem tonight. I removed the display and microprocessor boards again so I could get to the power and right channel output amplifier board. I checked and replaced a few electrolytic caps on the power and output board (right channel only).  There were two 100uF caps (C205 & C211) that measured a little over 200uF.  There was one capacitor that could not be measured (C206). I replaced those capacitors and a few on the power supply board.

I re-connected all the boards and tested the receiver - the clipping lamp still stays lit. A new problem occurred though. One of the wire assemblies to the pushbutton control pad came loose (broken wires).  I soldered the wires back to the front panel and the pushbuttons work.  However, the volume wheel and tuning wheel no longer work in controlling their respective functions.  The volume control on the remote does work. The FM tuner is also not pulling in any signals.  The display lights now display all segments...so at least one thing improved.  I can still play a music player plugged into Tape 1.  The sound is good and clear.

sonavor
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After a night's rest I checked the sensors for the volume and tuning wheels. I verified they are getting +5 V so I turned the receiver on and they are working now. I did reseat the connectors from the Keyboard Primary (10) to the Microcomputer (9) so I am going to assume there was a bad connection. These connectors are pretty finicky in their old age.  As I said, the display segments are all lighting up properly now. That changes every time I have to move the Display (8) and Microcomputer (9) boards.

The FM tuning is still not working though. It must be some connection too as I haven't touched the FM (1), FM Interface (2) and Preamp (3) boards. I verified that everything is connected but I am not getting any FM stations. The tuning dial works and I can change the frequency on the display.  With autotune engaged the output remains dead quite.  When I switch autotune off, I get that white noise sound you hear when you are not on any station.  So I need to trace though that part later.

For now, the receiver is playing good but the clipping lamp is still illuminated all the time. I haven't pulled any of the transistors involved to check as the boards are so touchy. Every move I make results in some problem somewhere else.

One thing I haven't checked yet is the BM8000 no load current and DC offset. I will check those next. That will mean moving the Display and Microcomputer boards to get to the right channel output amplifier but it must be done. Here is a picture of the current display with the segments all working again. That will probably change after this next test.

 


sonavor
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I forgot to post this picture from last night when I had the keyboard assembly apart. The keyboard was only partially working and I found that one of the ribbon cables and several wires that broke. The glue that holds the cable to the board had come loose with age and moving the cable while working on the BM8000 caused too much stress on the solder points. I desoldered the pins, stripped new leads and resoldered the cable in place. As I mentioned in the previous post, the keyboard is now working again. I also cleaned the keyboard assembly as there was a lot of dust in there.

 


sonavor
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I performed the no load idle (half of it) and DC offset adjustments on the left and right channel.  What I mean about half of the no load idle is that the service manual says to measure and adjust the voltage across test points TP200 (R236) and TP201 (R237) to 18mV while the amplifier is cold. That part is easy to do so I did that part. The second part of the adjustment is to put my meter in series with the collector side of transistor 203.  That means taking out the board, de-soldering the collector and wiring up some test leads. I didn't want to do that yet so I just did the 18mV adjustment.

After that, I let the amplifier warm up, then performed the DC offset check.  I repeated the adjustments for the right channel.

The receiver still plays fine after the adjustment and I don't notice any difference in the sound. The clipping lamp also remains on.  Here is a picture of the left channel no load idle, 18mV adjustment.

 


sonavor
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Here is the right channel output amplifier no load idle adjustment. I have replaced the electrolytic capacitors on this board except for the 4.7uF, 63V in the lower right corner of the board. I didn't have a replacement for that one. It is in the signal path though. I measure it and it measure about 6.5uF. I didn't write down the measured ESR value but I remember it was okay. 

 


sonavor
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Here is the left channel DC offset adjustment. This is with the volume at zero, speaker load is applied and the receiver has been on for at least 30 seconds. In this case the BM8000 had been on for a couple of minutes. The right channel adjust was made in the same way.

 

 


sonavor
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The other board I partially re-capped is the Power Supply and Remote Receiver board (6).

I didn't replace the large axial caps. They measured within tolerance on the high side. Their ESR was good. I haven't decided if I should replace them. I was reluctant to replace the smaller axial, Phillips caps. The 10uF caps all measured close to 10uF and their ESR was around 1.2 ohms. The replacement Vishays that I installed were all right on 10uF but their ESR was 2.2 ohms. The new caps have a good ESR value but the measured value of the old ones looked better.  The 1uF, 100V cap I installed measured better than the old cap. The same is try for the three radial caps in the upper right of the board.

 


sonavor
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I revisited the clipping lamp again tonight. In measuring voltages around the circuit with the left and right channel connected and disconnected I have eliminated the left channel as being part of the problem. The clipping lamp glows dim, with the same driver voltage at the base of the Darlington [IC4] on the display board, regardless if the left channel is connected or not. The right channel will drive up the intensity on the lamp when it is connected so it has a problem on its clipping circuit. However, I don't thing the right channel output board has any problem musically. It plays clean and I was able to adjust the idle and DC offset earlier. So it is just something with the clipping detection.

My plan is to replace a couple of the display board clipping circuit components and see if I can get that part resolved. I'll do the same on the right channel clipping circuit but I have to order the parts I need. 

I am also going to look at upgrading the board interconnects. Maybe look for some newer, more reliable connectors as the existing ones are old and have been giving me fits.  I plan on keeping the Beomaster 8000 for a long time (along with the other pieces: Beogram 8002, Beocord 9000, Beovox MS150s and the system cabinet).  So I want to make repairs that will last.

sonavor
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sonavor replied on Sun, Aug 11 2013 10:53 PM

I removed and tested two transistors in the clipping lamp circuit that look the most likely (based on the voltage measurements I took).
One is the NPN Darlington on the Display board (IC4 - TPSA13). The base is driven by the clipping detection signal from the left and right channel output amplifier boards.
Using a semiconductor tester (Peak Atlas DCA55), that NPN device is not recognized as a Darlington. The tester thinks it is low power thyristor. I tried another NPN Darlington that I have in stock and the tester identifies it correctly. So I suspect IC4 as a possible problem and found a replacement at Mouser that I will order.

The other transistor is from the right channel output amplifier board. It is a PNP transistor (TR200 - MPSH54). The semiconductor tester only recognizes it as a common cathode diode network. So I suspect that device as well. I can't find the MPSH54 at Mouser. I prefer using Mouser because there is one nearby. However, if I have to order elsewhere I will. Does anyone know of a good substitute for the MPSH54 PNP?

sonavor
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It appears BC556 (PNP) is a substitute for MPSH54.

sonavor
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Before removing the transistors for the clipping circuit, I traced through and measured voltages in the FM boards. I didn't see anything wrong there. Now with the display and microcomputer boards disconnected (so I could get to right channel output amplifier), I studied the FM signal more closely. I found this break in the wiring on the microcomputer board.  The wire is so small that it is hard to see.  Especially with the cables in place.  Martin had alerted me to check for the small FM signal wires and I am hoping this is the source of my FM problem.

 


sonavor
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The break in the wiring on the FM signal to the microcomputer board (frequency counter circuit) is worse than I thought. When I examined the broken reddish wire, the green wire fell out. So it was actually broken as well. Plus the green wire broke from the inside of the little inductor. I don't believe I will be able to fix that.  I have never replaced that type of component before. Is it readily available?

 


sonavor
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Here is the 8020341 L2 part removed.  The wire measures 0.18mm in diameter.  I am investigating the possibility of repairing it.  My fall back plan will be to rob the part from a "parts unit" BM6000 that I have.  I got the parts unit last year and I am planning to use some other parts from that to fix my first BM6000.  However, it would be nice to not have to rob that unit and eventually produce another working BM6000. But the Beomaster 8000 has a higher priority to me.

 


Rich
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Rich replied on Mon, Aug 12 2013 7:17 PM

That's an inductor?


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