BM 4400 Mexico

[mexking]

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

[chartz]

Hi 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

[mexking]

Hi 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

[Valve1]

Ola gringo, good to see you back Soren.

Eddie

[mexking]

Hola 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

[mexking]

Went 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.

[Dillen]

A drifting idle can be caused by bad thermal contact to the heatsink.
Did you use the proper white heatsink paste?

Martin

[mexking]

Thank 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

[Dillen]

The 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

[mexking]

Getting 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

[mexking]

That 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

[Dillen]

Put a scope on an output.
Check for oscillation.

Martin

[mexking]

Thank you Martin, will do

Its 5-6 years ago that I used my oscilloscope and I didn’t use it that much so I will have to get into learning again, not a problem but will take a little time

Found my old thread here

[Author]

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