Entering the amazing Beoworld with a beautiful Beomaster 900k with tuner issues

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T68 Posted: Tue, Aug 24 2021 7:27 AM

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Hi everyone!

This is my first post in these fine forums. And also first time owner of a Bang & Olufsen!

I recently visited a car boot sale and found a Beomaster 900k in good physical shape. Dragged it home, plugged it in and lo and behold: it turned on and worked! 

...well works to let´s say 67 percent. Tape and phono inputs works. Phono after adding an externa phono preamp. But this seems to be expected since these guys do not do riaa. 

Sound quality is impressive considering the age and size of the built in speakers . 

The radio needs some TLC though . the connector and wire for the internal areal was cut and missing. Adding an external areal produced FM reception but only acceptable for one  station high up at around 105mHz. On lower frequencies I get almost nothing. Some really faint stations can be heard. 

If done some reading of other threads and it seems as if problems with some germanium transistors in the radio section is a common issue with these units. Can the symptoms I experience be a sign of this problem?

What else to consider regarding servicing, component replacement etc on these guys?

Thanks in advance for any help and input!

Cheers!

T

Some pics: 

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Dillen replied on Tue, Aug 24 2021 4:16 PM

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Nice Beomaster indeed.
An internal RIAA preamp was in fact done. It has the phono DIN socket incorporated so was made to replace the original socket, held by two screws or rivets.

The IF-transistors are known troublemakers.
Do some DC voltage readings around them and compare to the servicemanual.
You will most likely see, that one or both of their screened cans have shorted
to one or more of the other pins.

Martin

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T68 replied on Wed, Aug 25 2021 7:27 AM

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Thanks for answering, Martin!

Yes, I will try to troubleshoot the transistors. Just to make sure, we´re talking about these guys?

According to the schematic they are labeled as  two different types: AF116/AF126. I cannot see which I got. Regarding replacements, what are the options? Must it be exactly the same or are there others that are easily available that will work?

I´m going on a travel for a week or so and will be a bit slow with this project and responses for a while.

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Dillen replied on Wed, Aug 25 2021 8:52 AM

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The ones in the red squares, yes.
And they are both labelled AF116 (you misplaced one in the schematics).
Any good germanium transistor with reasonable specs will do.
Don't fit AF116 (or AF117) again, They will all be bad or go bad from internal shorts, regardless of use (Google "Tin whiskers").

Martin

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Tim replied on Sun, Aug 29 2021 9:13 PM

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I read this thread with interest as I'm also restoring a 900K, I hooked it up initially and it works, AND sounds good albiet a bit 'scratchy and hissy', so I've replaced about half the electrolytic caps (many well out of tolerance), but sourcing exact values is difficult - anyone know of any good supplier ?

The other point of interest is the germanium transistors, as I understand, these are no longer made (please prove me wrong !), so surely ANY replacement will be of similar age and just as likely to be "troublemakers" ?

I'm not an electronics expert, just handy with a soldering iron, so could anyone give me a specific replacement transistor number for me to look for ?

I'm a bit wary of the NOS advertised on eBay after reading the above, so any help/encouragement would be gratefully recieved.

Tim

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Dillen replied on Sun, Aug 29 2021 9:33 PM

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Tim:

I read this thread with interest as I'm also restoring a 900K, I hooked it up initially and it works, AND sounds good albiet a bit 'scratchy and hissy', so I've replaced about half the electrolytic caps (many well out of tolerance), but sourcing exact values is difficult - anyone know of any good supplier ?

The other point of interest is the germanium transistors, as I understand, these are no longer made (please prove me wrong !), so surely ANY replacement will be of similar age and just as likely to be "troublemakers" ?

I'm not an electronics expert, just handy with a soldering iron, so could anyone give me a specific replacement transistor number for me to look for ?

I'm a bit wary of the NOS advertised on eBay after reading the above, so any help/encouragement would be gratefully recieved.

Tim

For capacitors, using the nearest modern standard value will be fine.
Instead of 6,4uF use 6,3uF.
Instead of 80uF use 100uF
Etc.

The tolerances of the original caps were quite forgiving anyways (think +20/-50% or something like that).

Germanium transistors are still manufactured.
But modern types are often quite expensive and it could prove difficult to find suitable specs.

It's a different story if you stay within the 1960s and 1970s types.
1st IF could be a AF109r
2nd IF could be AF179
That's what I like to use, anyways.

The above mentioned types will be fine if found NOS, and so will almost any other type, like f.e. the AF124-127 series etc.
It's only certain types of transistors that are known to develop tin whiskers. In B&Os models it's more or less limited to AF116 and AF117.
The low frequency germanium transistor types (AC126/AC151 etc.) can go noisy, but that's a different story and has nothing to do with tin whiskers.

Martin

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Tim replied on Sun, Aug 29 2021 9:41 PM

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Thanks Martin - a fountain of knowledge as always, it's given me hope and something to work on !

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T68 replied on Tue, Aug 31 2021 3:48 PM

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I've  found AF109r transistors at a reasonable cost. Will these work for both IF positions?

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T68 replied on Tue, Aug 31 2021 3:48 PM

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-Deleted-

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Tim replied on Fri, Sep 17 2021 9:59 PM

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Hi all,

I'm still continueing with my BM 900K and making steady progress, almost all electrolytic caps replaced (and regularly testing to ensure all is OK), the germanium transistors are next.  The question is; is it only TR3 & TR4 (both AF116) that normally need replacing ? Are any of other tranistors known to be troublesome ?

Also I'm going to be restoring the case and found that the speakers are not the same, both fit perfectly but visually different, perhaps for better sound seperation, is this normal ? And in addition to that, one is badly discoloured, but not damaged, the other looks visually perfect, both read less than 4ohms. The machine was tested before dis-assembly and sounded OK apart from it being hissy and scratchy.

Finally regarding the case, the sides bottom and back are all in near perfect condition, just the top needs attention, I intend to rub it down with fine sand-paper, then oil it and use beeswax (I'm not a lover of polyurethane), are there any cabinet makers/restorers amongst us that would be happy  comment on my cosmetic intentions.

Tim