Hello.

I have just bought myself a nice Beomaster 900K.  I have always loved the design and look of this thing and I have heard only good things about it's sound. 

This is still not quite a Beomaster. In 1960's a finnish radio company Helvar made some corporation with B&O and they made their own versions of B&O products. At least the 900K and Beocord Belcanto were made. The 900K of them is called Helvar Stereomaster 1000. I don't know how much there is change from the B&O version, at least there is a different logo on the front and rear panels and a different speaker cloth as seen in the photos attached. Quite nice actually! Techically I believe this is identical.

It's working nicely, but it doesn't have the FM stereo decorder. Is there ANY chance to get one? (The Stereo lamp works in TAPE and PHONO modes (I haven't tested the sound yet) but there is no Stereo in FM and I had a quick peek inside and there seems to be the empty for the decoder board and the valve-type connector with the dummy plug). I didn't pay much for this and I can still talk the price down because of the missing decoder. Anyway it sounds great even mono, I will try with TAPE input and my iPod tommorow.

Otherwise this BM is working but will need some restoration, there is a lot of dust inside the dial glass and the wood has some scratches around etc. And perhaps some new and fresh electrolytics and....

I would still like to get the B&O version someday...

(Sorry for the poor-quality cell-phone photos )

That is interesting! I had never heard of this collaboration. The cabinet is different as well, everything on the actual chassis seems the same (except for the things you mention).

That empty Noval socket indeed means you don't have the stereo decoder (the plug contains some components to take care of FM de-emphasis, otherwise done in the decoder). I've never seen any being sold separately myself, but you could try your luck by buying a trashed Beomaster for parts. I believe the decoder in Beomaster 1000 is the same, as well - at least in some versions.

This is quite an interesting model indeed, a B&O radio, made in Finland and rarely seen! (In my case it's even more interesting; the only Beomaster 900Ks I have seen have both been Helvar made!)

The decoder itself seems to be quite a small thing with relatively little components (a place for a DIY project for someone?!). I once had a Beomaster 1000 for repair and I remember having the stereo decoder in my hand and as far as I remember it should fit to the 900K.

I believe the 900Ks are so well made that trashed ones are very rare; I hope to be proven wrong!

Interesting set.
I never saw or heard of this one before.

Basically, there are two different stereo decoders used in the Beomaster 900 and 1000 models but they are not
unique to either Beomaster.
One type will work only in some Beomasters, the other will work in (almost) every type.
Get one from a scrapped Beomaster 900, it will most likely be suitable - but may not be the easiest thing to find.

Martin

Thanks.

I have the BM at work and I will take some tools and cleaning stuff with me today. I will take some more photos, too. I'd be intersted to hear if there is any visible difference (inside) to the B&O version. Also I will try some music thrue the Tape input.

I will keep looking for the decoder.

I have now hooked my MackBook to the Stereomaster. It sounds otherwise good, but there is some troubles with the left-hand-side speaker, all deep notes make a scratching noise. I will open the cabinet to have a look what is going on... could it be a broken speaker cone? Or a capacitor problem?

Other than that the sound is very good!

Probably the speaker surround.
If it's the same as the B&O version, it's a paper surround, not foam. The paper dries out, becomes brittle and starts to crack.
If no parts of the surround are missing, you can brush it over with contact glue of a type that doesn't get stone hard.
Some types will dry to a strong, flexible coating, almost like rubber. The black one from Casco I've found particularly good.
If little parts of the surround is missing, you will have to repair the holes. Special cone repair paper can be found for
a prof. solution but would probably be overkill in cases like this. Tiny pieces of tea bag paper (or coffee filter) will do fine here.
If larger portions are missing, I suggest you look for a replacement speaker.

Martin

I opened the speaker cabinet and checked the speaker visually from behind (I don't know how to deattach the speaker itself or the cloth). It looks like paper indeed. I couldn't see any missing bits but this was rather quick glance. See photo attached. 

There seemed to be some little repairs done before (judging from ugly solders).

Doesn't look too bad.

If you losen the four hex nuts, I'm sure the front cloth can be taken off.

Martin

I believe the speaker frets would drop off if you removed the four nuts visible at each corner around the speaker.

Oh yes. I just didn't have any more tools at hand so I didn't try that! I will take the Stereomaster home today to give it another try. 

The speaker cone looks pretty good but I will remove the frets to have a better look.

And sorry for the over-sized photos, I'll try to replace them with smaller ones....

I opened up the Stereomaster and put some contact cleaner to switches etc. and now it's working better, the switches are almost noiseless and the treble and bass adjusts are now almost working as they should. There is a lot of dust and oxide inside, I recon this has been stored somewhere cold. Nothing too bad, so far. The fuse and it's holder was almost covered in oxide and I gave that a quick clean already. 

Martin, I remember you mentioning that you were doing a list of variations of caps in these for a cap kit. I will most likely replace the caps in this so I will send you the list, too. It would also be interesting to see the comparison to the B&O versions to see if this is made from early/late type. Do you happen to remember the ratings of the big caps (power supply and output coupling caps)?

Ville Laustela:

Do you happen to remember the ratings of the big caps (power supply and output coupling caps)?

Just checked the service manual (again)...

Power supply: 1000 uF 50v and output caps: 800 uF 25v.

Only after reading the service manual I realized that the un-marked pot on the rear panel is the balance adjust. It sounded a bit cracking so it might have something to do with the distortion trouble.... (EDIT: No, it didn't help)

Inreresting thread!

I'm not a techie, but I know that it is not good (on a 900K) to have loud volumes while the cabinet is open. I presume that is the same for this Stereomaster.
Also, going from off, to loud in an instant is advised against in the revised (900) manual.

Your photo's are great, no need to apologise!

Andy T.

I remember hearing that, too. I had the BM off while I checked the speaker. (This reminds me of the warning in the Beomaster 1000: "WARNING: 2x 20 WATTS MAY DESTROY SMALL SPEAKERS") )

The photos were just a bit too big, I resized them just few minutes ago. The combination of the wood and the light-coloured fret is very nice, especially from the side. The top of the wood is actually quite scratched but I will get that fixed later. 

It's a general rule for all pressure-chamber speakers not to run at high volume unless under
"normal operating conditions" (cabinet closed tight).
It wouldn't sound right either.
You will also notice foam strips along the chassis edges to keep it all tight.

I agree, the photos are very good.

The first version of what later became the B&O Beomaster 900K, was also called "Stereo Master" followed
by "610K" to indicate model year 1964.

Martin

Ville Laustela:

There seemed to be some little repairs done before (judging from ugly solders).

The 40-odd year old original solderings can look quite ugly as well! Although the two black caps on top of the amp board don't seem original to me...

The distortion might be a damaged output transistor as well. On my first 900K, one of them had shorted and burned the 0.68(?) ohm emitter resistor (or vice versa). Somebody had just unsoldered the wires to the transistor from the amp board and left them dangling free over there, completly unisolated! Surprisingly, it didn't sound that bad - I looked at the output on an oscilloscope and noticed one half-wave missing

Check those resistors as well - in that 900K, they were I believe 1/2W and seemingly a bit underrated for the purpose. On my other 900s, all of them were much bigger 1W versions. The smaller carbon mass resistors sometimes create a short when they heat too much.

tournedos:

The 40-odd year old original solderings can look quite ugly as well! Although the two black caps on top of the amp board don't seem original to me...

The two black caps are exatcly those that look suspicious. The solderings are ugly and the plastics of the wires around are having melted spots by unexperienced solderer. 

I will check the resistors, too.

very handsome piece!

i really like your first pic and the cabinet on this! i like the slight variance in angle of the panel with the cabinet!

would look great on my desk!!!

don't suppose i'll ever see one available over here (though mika managed a caddy over there)!

keep us posted!

Dillen:

Probably the speaker surround.

I have now removed the fret of the distorting speaker. The speaker seams to be intact, no visible holes or missing bits. It doesn't feel even that brittle.

I am now listening to the Stereomaster with headphones (attached to the speaker output with a resistors) and it sounds perfect. With both FM and Tape input. I will try to clean the volume, balance, treble and bass pots again. Also the external speaker connectors as there is a switch (external speakers can be connected to be the only speakers connected or to use both internal and external at the same time, depending which way round you connect the plug).

EDIT: I just tried with the internal speakers again and still the left one sounds distorted. So there is still something wrong... 

In dark I can see that the dial lights pump a bit when volume is higher; the caps "might" have seen their best days...

And some more photos attached.

Try to play the problematic speaker with a known good external source (be careful with the volume, of course) or even swap the speakers in the internal connector. That should tell where the problem is.

I have now tested with an external speaker and the left channel is distorting with it, too. So there must be an electronical trouble somewhere. 

Some update:

I had a better look inside and find a cutted, white-blue wire starting from the area of the stereo decorder connector and ending somewhere near the rear panel (there are several similary coloured wires). The smaller circuit board (that folds out after removing two screws) with the two black caps has had some repairs someday in history; it seems that two transistors (and the black caps) have been replaced and wired a bit oddly leaving the transistors with very long wires, see attached photos. I think I will re-solder all the parts in there and cut the transistors wires shorter.

I am still not sure of the distortion problem. I am listening with headphones again and I can't hear anything wrong. With speakers there is disctinct distortion on the left channel with the internal and external speaker. I connected my wireless headphones to the Stereomaster and noted that the signal gets very heavily distorted if I raise the volume over the half-scale; could this simply show that the signal gets distorted only at higher volume levels because of some power / capacitor problem? Just a thought. (Can't really test this with normal headphones!) I have cleaned all pots and the speaker connectors.

Be careful with those transistors - those germanium types are very easily destroyed by excess heat during soldering, which is why you often see them with long wires (with insulating sleeves in proper repairs, though...). The ancient trick was to hold the wire with pliers between the component and solder joint while soldering, to conduct heat away from the part. Then again, in those days people didn't have thermostatic soldering irons.

This distortion would probably call for a signal generator and an oscilloscope. How is your headphone adapter constructed? If it loads the power amp less than the speaker, it might not reveal the problem. It could be a partly failed power transistor - another nice property of germanium transistors. They can be difficult to measure since many of them leak a little already when new, and the characteristics can vary a lot from part to part.

But I'd start by fixing that awful soldering mess around those definitely not original caps... and the service manual has a number of nice test point voltages which you should go through to isolate a possible problem area.

I am now testing with the internal speakers now. There is a little segment of low volume that sounds good (not distorted) but once I raise the volume higher it starts to distort. Of course it's difficult to say as the volume is low and so are the deep notes.

The headphone adapter (for the wired ones) has two about 120 ohms resistors (for using low-impedance phones with some Beomasters that want to have over 100 ohm headphones).

In case I can't get any more out of this; Mika, would you like to have a look on this someday?

Then it looks like the amp basically works, but for some reason can't output much power. You could try replacing the output coupling caps, or just swapping left & right for testing. I dunno how probable it is that they would have a problem, though - I've never done anything to them...

Trying to figure out things that have been modified in the unknown past isn't exactly my favourite hobby , but if you get stuck, I'll certainly help when time allows. I have the test equipment and working units to compare to