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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi all,
New here, I have quite a few vintage B&O products, mostly working, but a while ago I acquired a BM 4400 as a non-runner and thought I'd have a stab at getting it working.
I've been through and re-capped it with one of Martins kits. Having turned it on, I'm getting some very peculiar behaviour from the fm tuner so started to investigate and got as far as realising there is no 35v feed to the IF board.
Ive traced this back to the power supply and I only have 6v across the top of R2 and R4 in the pic. The same on the 220 cap. The other side of the cap is showing -41v though.
Any ideas what's going on here?
A bit of extra info - I suspect the fm front end has been messed around with (ordered a known good replacement) and the 'on' light is also glowing very dim, despite the bulb being replaced.
Thanks in advance.
Broken ground connection somewhere?Where did you connect your ground lead from the meter, when you measured the voltages?
Note: The 220uF cap does not go to ground.
Martin
Thanks for your reply Martin.
Ground lead was connected to the chassis when measuring voltage. I am seeing 14v from each of the blue wires into that diode (D3), is this correct?
Perhaps the 220uF cap has a bad solder joint... Will take another look at it this evening.
If that's AC, then it sounds about right.
Voltmeter in DC volts.Negative lead connected to the metal housing of the FM tuner frontend.Please take readings with the positive lead at: Both ends of R1 Both ends of D5 The "top point" (interconnect) of R2 and R4
That's 14v on the each of the blue wires, DC. They should be 35v also then I'm guessing?
Readings are as follows:
R1, -41v on the cap side. 0v on the other.
D5, 14.7v on the bottom (from OIC1) and 4.7v on the top.
Top of R2 and R4 are both showing 4.7v also.
Your descriptions of the nodes you are making your measurements at are confusing to me.Can you specify on this Beomaster 4400 diagram where you are making your measurements?-sonavor
Apologies, hopefully this makes a bit more sense
Yes!
Much better.
As a quick test you might try removing the FM boards and remeasuring that 35 VDC supply line where you are currently measuring 4.7 VDC. That would tell you whether the problem is on the FM board or in that 35 VDC regulator circuit.
-sonavor
I suspect there is something " floating" . If there's really 41V across R1, then it should get warm. Check for connectivity between traces and solderpads. You might have damaged a solderpad during replacing. Wouldn't be the first one where traces are cut from the pads.
Removed the FM boards and still seeing only 4.7v. It wouldn't be anything as simple as that diode (D3) failing?
I'll do a bit more digging and check the traces. Thanks everyone.
Did you check TR1 and TR2?
Ok ive just pulled both transistor from the board and although my meter doesn't have a transistor function, ive checked with the diode and both showed O/L when measuring from base to collector/emitter. I also swapped the leads around as aware these are PNP and NPN. So ill replace these as a start...
BC142 I can find on Ebay and a quick google shows I can replace BC256B with BC556B - is this correct?
Thanks again.
BC556B should be fine.
Just an update on this, I’ve replaced both transistors and now have 35v.
Unfortunately the FM section still isn’t working though. Can’t get a signal, no response from the meter at all but both tuning lights are permanently on. I think this is a bit beyond my abilities now...
For info, both tape and phono inputs are working well and the amp sounds great. May just live without the FM tuner for a while...
Check the black leads running from the tuner frontend to the IF board.They are known to break, both internally and from their solder joints.
Martin,
ive just been back and checked all the wires - they are all fine. However, when I switched it on this time, the tuning lights stayed out for approx. one minute, then both turned back on again. Still no FM signal and the radicator didn’t move.
Am I correct in thinking this points to a component failing as it warms up? A transistor somewhere maybe?
Cheers,
Kieran