ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
It appears BC556 (PNP) is a substitute for MPSH54.
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.
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?
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.
That's an inductor?
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.
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.
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.
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.
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.
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.
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
I was able to find small spools of solderable, 33 gage magnet wire on Ebay so I ordered one red and one green yesterday.
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. :)
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.
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.
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.
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.
My magnet wire arrived today so I was able to repair the little 8020341 (L2) transformer.
I scraped off the enamel insulation and re-soldered the L2 transformer in place.
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.
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.
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.
Great news and great work, John. Except you should test with Led Zeppelin II. On vinyl.
Rich: Great news and great work, John. Except you should test with Led Zeppelin II. On vinyl.
Haha, I'll do that for sure.
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.
Here is the test setup connections on the BM8000 output boards and the Classic Audio Tester
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.
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.
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.
Here is the board after the recap.
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.
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?
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.
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?
sonavor:Today began good and ended horribly
Sonavor its a great job so far ! Bummer about your short but you have to keep going.
I will keep going. I'm just a little ticked off at myself right now. But I will regroup tomorrow.
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 ).
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
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.
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.