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
Here is the component side of the 03 board
Here is the area I believed to be the problem area - TR104, R158, TR103, C136
The reflowed solder joints
The 03 module reinstalled
When doing a restoration, there are 2 components which I replace, without taking into account whether they are good or not.
1. All the electrolytics - as is commonly done and well-known here.
2. All the trimmer potentiometers. The original open, exposed trimmers are subject to dust and smoke contamination, which causes the oxidation. The original carbon-based units have a power handling capability of 0.1W. New 10mm drop-in replacements (either Piher PTC10 or Bourns 3352) are sealed, have a Ceramic element, and a power capacity of 0.5W. They cost approx $1 each at major distributors, so it is a no-brainer to replace, without even thinking about it. It saves so much hassle afterwards.......
Menahem
Well done John, once more!
I too have had trouble with those trimpots. Especially the RIAA ones which are the feddback loops of the circuit.
Jacques
Excellent thread and excellent work!
//Bo.A long list...
Well, today the left channel dropout problem of my Beomaster 8000 returned. I haven't pulled it back to the bench yet but I am wondering if I ought to suspect 3TR103 (MPS-H54, PNP) and/or 3TR104 (BC-546B, NPN). Could those transistors be failing after the amplifier heats up?
It happened when I was testing out the recap of one of my MC120.2 speakers. I wanted to compare them to my MS150 speakers, just for a reference. After about fifteen minutes I was surprised to not hear any sound from the left speaker again. I tried various input sources but they were all missing the left channel just like the problem had been before. I put everything in standby and let it sit for an hour. The left channel started working again which leads me to think it is a component heating up and failing. I ran the test again - this time I kept the music at a low volume (2.0 on the display). The result was the same, eventually the left channel stops. I am going to check my stock of spare transistors. I may have a BC 546B but I doubt if I have an MPS-H54). I do have two spare BM8000 preamplifier boards (03 module)...that I don't know the condition of. That gives me some options on spares. I am wanting to restore at least one of the spare Beomasters so maybe I will start with the preamplifier module.
I'd say small transistors failure is always possible, but very unlikely. Have you checked the volume adjustment chip? Those sometimes fail.
sonavor:Could those transistors be failing after the amplifier heats up?
Those transistors are so reliable - I would not even consider them as a cause before verifying the following steps:
Step 1 - resolder all solder joints.
Step 2 - check all wired connectors on that PCB to the other PCB's.
Step 3 - replace any potis in the signal path with new Piher or Bourns units
Step 4 - check the ESR's of all those old capacitors on the PCB's (better to replace, but if not, check)
Step 5 - check the functions of the volume/bass/treble/balance pots, and spray with DeOxit or similar.
98% chance that the problem is identified in one of the above steps, and NOT the transistor. You've got too many old, aged passive components on that PCB, to be able to confidently ignore them as possible causes.
If you want a quick ID, then you need an oscilloscope, and probe the signal in and out, bisecting the route, until you find the trouble spot.
Check the speaker switches.
Martin
Thanks for all of the great suggestions. I hope to get the Beomaster back on the bench tomorrow or Monday. Meanwhile I am cleaning off my bench for it and recapping my two spare preamplifier boards. The last time I had this problem (and thought I fixed it), I had isolated the left channel loss coming out of the 03 preamplifier board. Messing with that connector, the sound would come back. So I resoldered all of the components and connector pins involved there. That worked and I used the Beomaster without any problems until this last week. I will re-establish what point in the path the left channel gets lost at, then try to trace back where I last have it.
I have a couple of new test equipment toys I am wanting to try out once I get the Beomaster beast back to the bench.
Here is a picture of my bench test equipment. It is a little congested but you have to do with the space you have. Some of my equipment I am still learning how to use. I still have a lot to learn for sure. But that is an enjoyable part of the work.
My latest test toys are for using a PC to run tests with. I like dedicated audio test equipment but there are so many options now with audio test software.
I finally got my dummy speaker loads for both channels ready. Right now I only built a dummy load for 8 ohms and 200 Watts. It should actually handle more than 200 Watts with the big heat sink. I realize that the dummy load doesn't provide a true speaker load simulation but it is good enough to make my own power amplifier output measurements. I may build a 4 ohm version later but I think the 8 ohms will be fine for now.
I can see an HP Distortion Analyzer, a Tek 465 or 475 scope, a Panasonic RF Signal Generator, a dual Power Supply, your famous Dim-Bulb Tester, and what looks like a frequency counter. How's that?
Menahem Yachad: I can see an HP Distortion Analyzer, a Tek 465 or 475 scope, a Panasonic RF Signal Generator, a dual Power Supply, your famous Dim-Bulb Tester, and what looks like a frequency counter. How's that?
That's really good Menahem. You are pretty close. In the photograph there is an HP 8903A Audio Analyzer, a Tektronix 475 oscilloscope, a BK 1823 frequency counter, a Panasonic VP-8193A FM-AM signal generator, a Nakamichi T-100 Audio Analyzer, an Owon 5022S oscilloscope, and a T-E power supply.
sonavor:a Panasonic VP-8193A FM-AM signal generator
Where did you get that by the way? I would really like to have one or similar, but all fleaBay sources seem to be in Pakistan or Indonesia. Could be legit, but I'm not going to send my money into either one of those countries...
--mika
tournedos: sonavor:a Panasonic VP-8193A FM-AM signal generator Where did you get that by the way? I would really like to have one or similar, but all fleaBay sources seem to be in Pakistan or Indonesia. Could be legit, but I'm not going to send my money into either one of those countries...
I was lucky a couple of years ago. A US equipment company was clearing some stuff out on Ebay. They listed the one I got pretty low but were open to offers. Menahem hooked me up with a technician in the US that knows how to check the units out so I bought the generator on the condition it passed inspection by the technician. It did and it is a great unit. Later that same year I had the opportunity to get a Panasonic VP-7723A audio analyzer but passed on it. I regret not getting it now. I haven't seen a good deal on one since.
I bought my VP-8193A from the same company. Later, I bought a VP-7726A Audio Analyzer from Kangrong Scientific in Taiwan. It was as described, in mint condition, both inside and out, but there was something not right - I just couldn't put my finger on it - so I sent it back. They refunded me, except for the shipping both ways. So I ate about $200, but proportionate to the unit's value, it was acceptable. I would buy again from them, but this time stipulate that any problem, generates a complete refund.
I put the Beomaster 8000 on the bench last night. When I first turned it on, the left channel was not playing so I thought everything was good to locate the exact source. Once I attached scope probes, the left channel came back.
The next step was to connect the probes and leave the Beomaster playing to see if the channel goes out. I know from before that one probe location is the preamplifier board left channel output so I put one probe there. The next place was back a ways between the source input (Tape 2) and the output. I picked the Ext. Filters In/Out jumper because it is easy to get to.Here is the schematic of the preamplifier board and the areas I am monitoring. Keep in mind that some of the points I circled are for the right channel because the schematic only shows the right channel in detail (since the left channel is the same). My test is with the actual left channel components of the board.
Here is the Beomaster with the scope probes on the left channel preamplifier.
Here is the left channel as it shows up on the scope. Channel 1 is blue, Channel 2 is red.
The Beomaster 8000 played without a hitch for a couple of hours.
During this measurement I played with the TP2 pre-amp volume pot (R122, 20K Ohm pot) and observed I could make the signal go away by maxing out the pot one way, then bringing the signal back by turning the pot the opposite way. As mentioned by other Beoworld members, those pots could be a source of the problem. The last time I had the Beomaster on the bench I re-flowed the solder joints of those components and sprayed De-oxit in the openings (and worked the pots about 50 times each).
What bothers me about that theory is that when I observed the left channel out, I switched to the FM tuner and it was missing the left channel as well.
I will leave the Beomaster on my bench for today and keep testing it to see if I can get the problem to come back. I am tempted to try one of my other re-capped pre-amplifier boards but I will wait and see how the current one plays out on the bench testing. If the problem happens again, I will first try working the volume level pots to see if that restores it.
If it comes to replacing the volume level pots, what is a good replacement?
It's been a while since I was inside a BM8000, so don't take everything I say as gospel .
The switching IC's are also a point of failure, so I'd put a scope at the in and out pins.
It's also the point where the FM signal gets switched into the preamp circuit, so that could account for that discrepancy.
However, remember that an IC needs stable power to operate correctly, so make sure that the decoupling caps at the IC power rail are in TOP shape.
Finally...I was able to finally get the failure this morning. The left channel is still showing at the external filters jumper but gets lost between that point and the pre-amplifier board output.
That places the problem between the left channel Ext. Filters and the board output at P16-1
I sprayed some CO2 dust cleaner on the area of those components for a while and the left channel finally came back. I didn't use enough precision on my cooling spray to tell which area is the problem for sure so I will have to wait and try it again. But it appears that it is temperature related.
I think I have the problem isolated now. The left channel went out again and this time I applied cooling spray in specific areas. Starting at the bottom of the board with TR104, I worked my way up to TR101. I just did one transistor area at a time, giving each enough time to cool. When I got to the TR101 & TR102 area, it didn't take long for the cooling spray to take effect and the left channel came back. I let it fail again, the re-applied the cooling to TR101 & TR102 again. The left channel immediately came back.
TR101 and TR102 are right next to each other so it is difficult to determine which has the problem. I am pretty sure I re-flowed the solder joints on those components the last time I pulled the pre-amp board. I will check again though but I may just replace those BC 550B transistors.
When replacing a semiconductor of that generation, always ask the question - why?
Those are generally so reliable, that I hardly ever replace them.
Check around for a bad resistor or ceramic cap, etc, which may start the defect cycle all over again. Those old carbon resistors are known to have high failure rates, even though they look fine, and they'll take down a perfectly good transistor with them.
Menahem Yachad: When replacing a semiconductor of that generation, always ask the question - why?
Hardly, but not impossible, right? If I go by the cooling spray area, the components shown in yellow circles would seem the most likely suspects.Note: Again, the designators on the schematic are for the right channel so you have to change the 2xx to 1xx (so TR201 becomes TR101, etc.).The green hexagons indicate where I monitored the signal and the signal always remained on. The red hexagon is where the signal fails to reach.I guess I should include C130 but it was a new capacitor I replaced during the recap. I suppose it could have a problem though.
Here is a picture of the problem board in the suspected area. The cooling spray focused on the two BC550B transistors seems to correct the problem. Of course the cooling spray could have affected the C130, R151 and c131 components. I don't think R150 is a problem because my scope probe at P15-1/R151/C129 node never loses the signal.
It's not uncommon to see the small signal transistors fail for no obvious reason, some types more than others and,presuming that the supply voltage has been confirmed fine, I agree on replacing the two.Try to match their HFE - or just get two from the same production batch, that'll be fine in most cases.
I checked my stock and I do have some Fairchild BC550 and some Fairchild BC550A supply. Eight pieces of each type. I got them a couple of years ago from Mouser. They no longer carry them.
FWIW - modern Fairchild transistors have a much higher failure rate than ON (former Motorola) or ST. So when I buy new, I try to avoid Fairchild wherever possible. Plenty of reports of that on the web, if you're interested.
Using a Peak DCA meter, the preamp board TR101 (BC550B) transistor measures an HFE=286 and Vbe=0.787V at 5mA.The TR102 measures an HFE=290 and Vbe=0.781V at 5mA.
The BC550A transistors I have measure an HFE around 145 and a Vbe around 0.76V at 5mA.
The BC550 transistors I have measure an HFE around 300 and a Vbe around 0.78V at 5mA.
So your unmarked ones are B-grade. Most of B&O's transistors of that era are B-grade (hFe between 200-450). Choose the 2 which have the closest-matching hFe's and install those.
Menahem Yachad: So your unmarked ones are B-grade. Most of B&O's transistors of that era are B-grade (hFe between 200-450). Choose the 2 which have the closest-matching hFe's and install those.
I will do that. However, it looks like it will have to be next weekend before I can get back to this. In the meantime, I am letting the Beomaster run with one of my spare preamp boards. The spare boards have been cleaned, recapped and solder joints re-flowed.
I have been busy with some Beocord 8004 and 9000 cassette units lately but I took some time to change out the two BC550 transistors. The spare preamplifier board has been working great for a week now and I decided to just open the Beomaster 8000 there in the cabinet. Just swap the preamp boards and continue on was the plan. Unfortunately, with the two new transistors, I had no left channel at all. The Beomaster isn't on the bench so I can't trouble-shoot the problem. I don't feel like disturbing the 8000 system any more as I use it all the time. So I put the replacement preamp board back in and I am going to leave the working system alone for now.
I have another 8000 system I am close to starting a restoration on so I will use that Beomaster to trouble-shoot the preamp board. Before that though, I have an SC60 cabinet I have almost completed a refinishing job on. To go on that cabinet I plan to finish my Beomaster 6000 restoration. So those tasks are ahead of the second 8000 system restoration.
Here is a picture of the Beomaster 8000 opened up in the SC80 cabinet. It is so handy the way the 8000 and 6000 systems from the early 80's open up for maintenance.
Over the last few months I have been able to trouble-shoot and finally fix my troublesome channel loss in my Beomaster 8000. In measuring various signal points on the preamp board I could see the problem was with that area of the receiver. It turns out the problem was with loose or bad connectors on the board. I first noticed that I could nudge P21 and P15 on the preamp board and get the dropped channel to come back.