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
I had a read on the forum that in 230v countries a service bulletin was issued by Bang Olufsen that the trafo should be set to 240 rather than 220, so i went to set my BM4400 to 240v.
It made a few strange noices, then Trrrrrrrr from the speakers and the tuning lights went out.
Switched back to 220v and everything went back to normal...or so i thought.
One question; should tuning lights stay on for Tape1 but off for tape 2?
Even when tuning lights go off in Tape2 mode the erradicator and stereolamp is still on...
But thats perhaps just me not remembering behavior correct from before.
But the main thing that happend was that the one channel on Tape one was missing. i could isolate it as I had a extra pre-amp board.
I could isolate it to this area:
The problem could be the transistors or the resistors, as the caps are newly replaced...
Is there a way to go about narrowing it down even further to component level without desoldering each component and testing them?
Also reading the service manual its has different types of transistors for each placement. Is this because BO gives you a choice in replacement? for TR106/206 its like 5 different types? are they interchangable or which one is the correct one to use.
Obviously my though is how to find correct replacements.
visual inspection on TR105 gives BC549C and for TR106 BC253B
thanks
/g
BM 2000 *1974, 2x BM 1200, BM 1001, BG 1200, BG CD X, BC9500,BM3300,Bcord 3300, BG CD3300, Beovox Cona, Beovox CX100, Beovox 1200, Beovox RL60.2
Jacques
Pressing Tape1 doesn't cancel the previous source.F.e. if you listen to FM3 and then switch to Tape1, FM3 stays active and the FM3 button stays down.Many owners think this is a fault but it's actually a very clever thing because this function allows you todo after-tape monitoring while recording.The signal from the tuner goes to the tape for recording, the playback of the tape (in decks that supports this), goesto the amplifier, so what you hear is the tape playing the result of the recording that was made 1/10 second ago.
So yes, if you go from tuner to Tape1, the discriminator lights stay on (as does the tuner).Tape2, however, cancels the previous source and turns the lights off.
The radicator (S-meter) illumination works in series with the ON lamp so is always on.
Martin
thanks Martin, gave me the impulse to get a userguide, and there its all explained, inclusive project tango and how the Fm unit was tested in the Schwartzwald :=) bizare!
I have now tested all the resistors and transistors on tape1, nothing... i all checks out.. any creative Ideas?
Left are the small ceramic caps and the tantals,.. not sure how to test them...
Tape loop? Not sure what that is, could you explain? Something I can make?
Wiggled around the plug ..nada...and then i had the preamp plugged in while the amp was on, and with a probe i could ground the pins, and got "humm" on the working channel and absolutely nothing on the other...
as everything elese checkes out, om not sure what the arcing caused, but for sure it left some scars more than in the preamp, as it got the 35Rail probably some spike on that one, caused the damage.. or so i speculate...
Went over it all again, must be something i thought, measure short on Emittor-collector both directions on TR1 (BC237b) im gonna take it out and make absolutely sure, but not tonight... so i started surfing around for a replacement just incase.
It seems not all bc237b are born equal..a multitude of manufacturers....different hFe, noise etc...would anyone have a recommendation or advice on reliable sources/replacements.. this transistor seems obsolete at all the usual component houses.
BC 547B should do
Collecting Vintage B&O is not a hobby, its a lifestyle.
BC549BIt's a preamp. Any noise introduced here would be amplified a thousand times.
Yes, there is a whole slew of choices here, many different qualities.
In a nutshell, for the
1. PreAmp Stage, you want the Low-noise version BC550B NPN or BC560B PNP
2. Power Amp Stage, you want the regular BC547B NPN or BC557B PNP, but you can certainly use the low-noise units above, although there is no tangible benefit.
These are rated at Vcb 50VDC, which is more than sufficient for the vintage B&O amplifiers.
The B rating is the hFe gain, and that is very important as the entire circuit is biased for a certain gain, and if the gain is outside those tolerances (by using an A or C version), oscillation and destruction may result. B&O (and most European units of that vintage) require the B Hfe factor.
Manufacturers:
The best is to buy Philips or Siemens NOS, mostly off ebay - they show up in lots every so often. Those were made before the Non-leaded Rohs nonsense, and are much longer lasting than the newer unleaded components.
Otherwise, new unleaded units from NXP, OnSemi or Fairchild are an acceptable second choice.
Menahem
According to data sheets BC237B and BC549 has the same noise fig. 2 max. 10
Both the BC549B and BC550B will work just fine, HERE in this application.
Both have IDENTICAL low-noise specs.
It is simply a matter of practicalities for me to choose the BC550B, rather than the BC549B.
Why?
Because I repair tens of machines every month, and having in inventory BC550B (rated at Vcb 50VDC), instead of the BC549B (rated at Vcb 30vDC), means that I can hold less stock, and which is suitable for many more applications.
For the same reason, I stock only the BC546B (80VDC), instead of the BC547B (50VDC), even though the BC547 may be the original spec.
Many thanks for the insight,...im needing those tips now...Its definetly shorted alright, but looking at the schema, it seems to be involved in circuit bringing 35v rail down to 33v.
So I assume a over 35v tolarence would be good replacement.
But it couldn't possibly cause just the one channel on just Tape 1 being dead, it must have taken some more stuff with it that caused another Tape 1 related component to fail.....as TR1 is shorted 35v volt would be hitting the whole per-amp ...? fair assumption?
After applying myself to the limits of googling! :=) this cannot be the cause or right channel issues, Yes TR1 is blown but this circuit is some kind of "soft start" when c1 is charged 0,7v will open TR1 and current will flow... further googling will give me 0.1 voltage drop over the open TR1 which will give a potential change of 1.9 over D1 which is a bit steep... in any case, non of this is the cause... so i measured on, and found I have been looking in the wrong place my incircled original post
This is where the party is! Also Blown and would account for the missing right channel
So a bit of ebay and found bc253b no problem.
Weird thing is i decided to change the 50KOhm trimpots, and the parts list in the service manual is just bizzare and wrong, some missing, others named incorrectly.. or is it me?
Just to update you all, got the bc253b in the mail, changed them both on TAPE1 and also replacde TR1 with 549b...
both the 549b and one of the bc253b were actually bad 100% C-E short... got the new trimmers 50Kohm (i just read the partslist wrong.. sorry)
Set the trimmers using a CDX a CD3300 and Bg1200 9Kohm seems to be some kind of sweetspot.
this thing just sings now... Quite like my BM2000 (1974) but this is thumpier!
Well done then!
Did you manage the switch over to 240 V?