Home › Forums › Product Discussion & Questions › BeoMaster › BM 4400 Mexico
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mexking.
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3 June 2024 at 01:41 #56213
Time to go through my stored items, the first one is my BM 4400, I did a big job on that one around 2012/13,
I took it apart changed all caps, trimmers and lamps, couldnt get it playing and had to order right and left output amps from Martin, that did it and it was playing ok and without problems, had it playing for a couple of years and remember that it had a scratchy bass poti, I changed it for my second BL 5000. At some point I took it out of store and checked it, and now it was not only a scratchy bass, now no sound from left channel, I put it back in store. Now after 4/5 years I opened it again to check what could be wrong. I connected it with antenna and speakers and found a FM station, both channels working, but after a couple of minutes I could smell and feel that something was getting hot I could feel it on the cooling fins more than hand warm. Turned it of and connected my DMM to check the idle current, around 130 mV on both channels, digital thermometer went within 5 min from 25 C to 50 C right channel more than left channel. Will continue tomorrow, I se a long haul coming up. Ideas welcome
3 June 2024 at 07:27 #56218Hi Søren,
Long time no see!
No ideas yet, but full moral support in the meantime ☺️
My own BM 4400 has been in constant, everyday use for about 10 years now, still going strong. Conclusion: they don’t like to be stored…
Jacques
3 June 2024 at 07:57 #56222Hi Jacques good to see you again, a few of us still there
I will have to pack out my oscilloscope and get working again, but the BM 4400 is not the easiest to work on.
What I dont understand is why the overload circuit doesnt cut in
Søren
4 June 2024 at 06:53 #56249Ola gringo, good to see you back Soren.
Eddie
4 June 2024 at 07:13 #56250Hola Eddie
This is going to take some time, I haven’t touched a B&O or an instrument for some 5 year
Good to see you
Søren
7 June 2024 at 02:07 #56338Went in to check the power supplies, but before that I checked the idle current one more time, I connect my DMM and turn on, the idle current starts at 40 mV and increase to 150-160 within some 20 seconds right and left. I disconnected the power supply to the output amps P11 and P12 and started checking all the supplies 15,24 and 35 VDC all spot on give or take 0.1 VDC, but the USTB 35 VDC shows 38.5 VDC right and left channel. I did a big job restoring this unit some 10-12 years ago, had it working for a couple of years without problems, and as the search function in archived forums does not work I dont find my old thread about it. I am a bit lost here, will have to think about what to do.
8 June 2024 at 07:20 #56358A drifting idle can be caused by bad thermal contact to the heatsink.
Did you use the proper white heatsink paste?Martin
8 June 2024 at 08:50 #56360Thank you Martin
I never thought of that, have been wondering the whole day, same failure left and right and no failures in the power feeds. Ordered new paste and will check on Monday. If this is the failure I may have other units with failures coming
8 June 2024 at 13:14 #56367The modern grey stuff often sold for use with computer CPUs is useless. Some of it dries hard over a week or two.
Silicone pads – some are fairly good, others are junk. I don’t like them, and I throw them away on sight.
The original white heatsink compound, that was also used originally, works best. It will last for decades.Martin
12 June 2024 at 19:36 #56453Getting ready to clean, doing one channel to see difference, the thermal grease feel soft and not dry to the touch. I didnt find the thermal grease that Martin recommend but had to buy the one for CPUs that he doesnt like. Just to test and then change it later when I find the right one, if this is the failure
13 June 2024 at 16:17 #56466That didnt work, cleaned all properly and colocated the transistors again with new insulators and Thermal Paste in the left channel, and have the same failure. I have never had the same failure right and left, so I am a little lost where to start looking but as I never give up I will start checking from the preamps and onward
13 June 2024 at 17:35 #56467Put a scope on an output.
Check for oscillation.Martin
13 June 2024 at 18:25 #56468 -
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