Oh yes, that was the "African" version of Beovox 1200 mkII. A smaller version of Beovox 3000 Type 6220, though actually
quite identical to Beovox 2700 with the Peerless woofer and Seas 87H tweeter.
Not a lot of photos or info available anywhere, so would be interesting to see more.

Martin

BC214, huh? You like to live dangerously, don't you? 😎

BC214 has a max. Vce of -30V and you're getting dangerously close to that limit by turning the trimmer to one of the extreme positions! BC212 with Vce= -50V would have been better. (especially BC212B would be good because of similar gain (hfe) as BC154)
BC214 should be fine as replacement for phono pre-amp transistors, though, because there you have a 24V supply.

But anyway, BC214 should be fine for testing in the power supply and as long as you keep the trimmer at middle position during first power-up....and provided nothing else is wrong in the circuit, of course.

------------------------

One more thing, which should normally go without saying:
When you start the soldering work, visually inspect the solder joints in that part of the board first. If you find any cold solder joints (= damaged solder joints that cause intermittend contact), you should re-solder them. You'll know what a cold solder joint is once you see one.

rgds, manfy

manfy wrote:

BC214, huh? You like to live dangerously, don't you?

 😮 Erm ...... yes but not with audio equipment - thanks for the warning!

So as you first suggested 🙂 rather go with the BC161 as that can handle -60V?

Regards

Anton

Yes, I think BC161 is a good choice here. It's the complementary type to BC141 and the voltage and current ratings are in a good range.

There's only 2 things that can kill a transistor: over-voltage and over-current. DC-gain and cut-off frequency are not overly important in this power supply circuit.
In the amplifier circuit it's a bit different. There you do not want to stray too far from the parameters of the original transistor. A substantially different hfe or transistor characteristic might result in a noticable difference in sound quality or sound image - and that's the last thing you want in a good amplifier. It's especially annoying if you get an audible difference between stereo channels!
So just keep the BC214's for the phono pre-amp that's causing you trouble. The transistors are the most likely culprits in that circuitry.

manfy wrote:

Yes, I think BC161 is a good choice here

Well Manfy you rock, thanks to your help and suggestions the power circuit is stable at 30V 🤩

It was the "BC154" that was the culprit, the moment the BC161 went in everything started working.

Reason why "BC154" is inverted commas is because it is not a BC154 transistor at all.  It was in fact a SGS 87040 (but this had been covered up with green paint, had to scrape it off with a blade).  Very little info about SGS 87040 but google says close to 2N708. Then I remembered that the pre-amp section has about 10 BC154's as shown on the schematic and circuit diagram.  Well on my model 2318 they were all substituted with SGS 87040 except for the 4 in the phone preamp where they were subbed with BC159B.

Pic of my camouflaged 87040 transistor below. Would love to know why they did this thing with the paint? Inferior part they were disguising, attempt at hiding proprietary information ...

Now I'm onto the next mystery - where has my 9V supply gone for the radio circuit?

I'm getting 30V on the collector of BC141. Still gettting 30V when it gets to the input side of resistor #903. Output side 4mV. Desoldered one leg and tested, it is 1.2K ohm. While that was loose also tested #909 - thats 40.7K ohm.  Can't figure out why the voltage drops so much.

Please let me know if you have any thoughts.

Thanks in advance.

Anton.

Login to view image

Glad to hear, that my suggestion regarding the two zeners and BC154 was right.
Now perhaps a shorted 902 (100uF) or 904 (Zener)?

Martin

AntonvZ wrote:

Now I'm onto the next mystery - where has my 9V supply gone for the radio circuit?

Well the 13 hour post approval lag gave me time to figure that out and replace the other zener diode #904.

When I desoldered the original diode and tested it it gave OL readings in both directions .

At least the radio is working again, getting decent reception although maybe not perfect clarity.

Next up will be the dead right phono preamp stage but first want to test a theory I have about Beogram din plug wiring before I start changing things on the amp.

Anton

Dillen wrote:

Glad to hear, that my suggestion regarding the two zeners and BC154 was right.
Now perhaps a shorted 902 (100uF) or 904 (Zener)

Snap - bit of a cross post there (-:

Indeed you were right as always and thanks for all the help.

And as above right again, it was zener #904.

Regards

Anton

Congrats! Good job!

AntonvZ wrote:

Pic of my camouflaged 87040 transistor below. Would love to know why they did this thing with the paint? Inferior part they were disguising, attempt at hiding proprietary information ...

If those 87040 substitutes were mounted originally, it proves that even B&O picks any sort of substitute whenever there's a shortage of correct components 🙁 But on the upside it means that you don't have to worry too much about using any cheap alternative transistors as a replacement. 😐

Now that you have a stabilized supply voltage of 30V, you just need to adjust the idle current for the amplifier and you're in a base state for further works on the unit.

AntonvZ wrote:

Next up will be the dead right phono preamp stage but first want to test a theory I have about Beogram din plug wiring before I start changing things on the amp.

Ok, but don't waste too much time with it. It's very unlikely that there is something wrong with the input wiring, particularly since the left channel is working.

It's much more likely that you have a contact problem at the 'needle-talk filter' switch. 50-year-old switches do have a tendency to show oxidation buildup on their contacts. The circuit diagram shows that the preamp output is switched via that filter button. Just press it off and on a few times while an input signal is connected and if you hear intermittend sound then the switch is the problem.

If not, I'd start out with measuring the voltages at the 3 preamp transistors. Your service manual should show the nominal DC voltages and the one transistor where the voltages are off too far should be the source of the problem.

Just out of curiosity: what kind of device are you connecting to the phono 2 input for testing? Any standard turntable with MM pickup or a Beogram unit with Moving Iron cartridge?

rgds, manfy