Try disabling the AFC and see if that helps tuning a bit.
If the radios IF circuits are not 100% trimmed, AFC can easily confuse things.

I'm not sure about the local regulations in your area with regards to grounding.
In Denmark, most radios are just running on live and neutral with no grounding and
I would suggest, if needed, to seek a good grounding directly at the radios metal chassis.

The balance control is the shaft-less potentiometer at the back side of the radio.
It looks just like a normal screw, right next to the speaker output DINs.

Martin

Dillen:

The balance control is the shaft-less potentiometer at the back side of the radio.
It looks just like a normal screw, right next to the speaker output DINs.

It looks something like this:

Thanks Martin and Ville, i wondered what that screw was for, it was not even described in the service manual i have. Will do the grounding later, here in France all the wall outlets are grounded when in a new construction, and with each renovation of old appartments they are obligatory replacements.

I reassembled the whole unit for a tryout last evening and it's just a beauty glowing in the night. However, after a while of listening there was like an overcharge of electricity, and the speakers let out a few short thumping sounds in series, but continued to work. I noticed that the levels were quieter, and that i had to turn up the volume half way up (so i'd guess about a 50% less efficient than before where i was at the first quarter), and while i was trying it out, another series of short thumps appeared and the unit shut down. On opening, i saw that one 250mA fuse was out. When changed and plugged, one of them always burns out now after about a second.

Sigh. It would have been far too easy, now wouldn't it.

Any idea what it could be this time ?

Sebastian

Well, basically I suppose you can start from the beginning of this thread again but
it sounds like it could use a good overall service or maybe a complete restoration overhaul.

Martin

Yeah, i hear you, Martin.

I'll try and tinker some more, with elements already covered, read around, and see if i could maybe learn something. I actually love doing stuff by myself, be it whatever, and it's still frustrating, even though i'm able to admit it's way over my head, to close the box and take it to the shop. But hey, one must be able to admit defeat. But maybe not just yet.

When you mention complete restoration, it reminds me of my question about the cap replacement kit ; do you or anyone propose one (i saw that you do/did for certain models), or does any exist ? If not, of what would it consist if i wished to update the audio path (once the Beomaster is functioning) ? I imagine the caps (all of them ?), but is there more ? I haven't seen any threads about this for the 900K.

Wow, quite an adventure, and it's pretty fun.

Sebastian

It's correct that capacitor kits are readily available for most Beomasters etc.
Beomaster 900 came in several different versions and kits are available
for some but probably far from all. However, if one does not exist, one
can be constructed.
Still, apart from a few candidates, like f.e. the large power supply filter cap
and the cap #514 on the amp board, generally most capacitors in the BM900 are
still performing surprisingly well.

No matter how skilled the technician, there's always a risk of introducing
new faults when servicing and replacing components and it could just end
up adding to the confusion.
I would suggest that you locate and if possible correct the fault(s) before
starting a recap.

Martin

Understood. That's a very tactful way to put it

All in good time.

I'll have a look at it, and keep you posted.

Once more, thanks for all the help you have provided (and probably will have to again).

Sebastian

Hi, it's me again. So, i didn't turn into an amazing technician overnight, but i did learn some stuff by reading and measuring. I've gone through a few components, verifying their correct state, and thus eliminating possible causes, but find myself again at my wit's end. And so was hoping that maybe you could enlighten me.

Here's a quick summary : the 900K blows one of the 250mA fuses each time it's turned on. It can be one or the other.

The parts i've verified and truly believe to be OK ;

- the four AD139 output transistors

- the SP1446 + resistor

- the 1000 uF power cap

- the two 800 uF output caps

- the cap 514 (80 uF)

- the resistors (0,39) 546, 547, 643, 644

- the rectifier

So i desoldered the leads from the power cap positive, and when the rectifier is connected, the fuse blows. But i tested it and believe it to be ok. So i moved to the next in line, and that is the 9.1V Zener diode (912 on schematics). And here i'd need some guidance ; i measured it both ways with diode test and ohm meter, on board and desoldered, and it gave 0 every which way each time. But i saw some discussions elsewhere that stated that Zener diodes need to be tested in circuit with load and what-not. Would you confirm this, or do the tests in my case suffice to conclude that it's dead ? Also, would a dead Zener seem like a plausible cause to my problem ?

[EDIT] Oh, and should it be dead, i couldn't find its wattage on the schematics, on the web i saw 400MW is that correct ? [/EDIT]

Sebastian

No matter how you measure a zener, it should never read zero ohms on a meter.
The reading one way should be around 0.6-0.7V (measured with a diode tester).
The other can vary a lot depending on the type of zener etc but should never
be lower than 0.6V.
Desolder and lift one end out of the board and measure it again (that is to
make sure that you are measuring ONLY this component and not something else
in the circuit - in this case a nearby 125uF capacitor plus other things).
There are also a couple more transistors in that circuit that would
need testing.

The 1N757A is a halfwatt zener.

Martin

Dillen:

No matter how you measure a zener, it should never read zero ohms on a meter.
The reading one way should be around 0.6-0.7V (measured with a diode tester).
The other can vary a lot depending on the type of zener etc but should never
be lower than 0.6V.
Desolder and lift one end out of the board and measure it again (that is to
make sure that you are measuring ONLY this component and not something else
in the circuit - in this case a nearby 125uF capacitor plus other things).

If you're saying that it shouldn't read zero no matter what, dead or not, well i don't know what to say. I tried it in both ways, on board, one leg free , desoldered, and just tested it again as per your request, and it just always measures zero, ohms or diode test.

... The ohm measure E-C is zero. So this one is ok...
Absolutely NOT.
Full continuity (zero Ohms) is never acceptable for a semiconductor. 

I suggest you replace the zener and both AC... transistors and check the regulator once more.

Martin

Just to clarify how to make sense of transistors with a multimeter, using the diode test range:

• C-E: infinity (i.e non-conducting) BOTH WAYS
• B-E: infinity the other way, conducting the other way
• B-C: infinity the other way, conducting the other way

So it is not enough to test just one direction. Which way will conduct on the B-E and B-C tests will depend on whether the transistor is NPN or PNP type. You can think of a transistor as two diodes with either their anodes (NPN) or cathodes (PNP) connected together at the base.

Any other results will mean that the transistor most probably is shot. However, these tests are not enough to prove that it works.

You will need to use the diode test range, as the resistor measurement ranges may not have enough voltage to make especially silicon semiconductors to conduct even in the pass direction.

One caveat with germanium semiconductors, like all the AC and AD types in this Beomaster: even if they are still OK, they may leak a little to show some conductance even in the reverse direction - however, the reading should be clearly different from the pass direction.

Oh wow, just when i thought i had understood something.

I'll try and locate the parts, and will report for more. My my...

Thanks Martin and Mika.

Sebastian

Dillen:

I suggest you replace the zener and both AC... transistors and check the regulator once more.

Done, done and done. Measuring against new components, it was evident that the zener and the transistors were busted (it's outright silly how i can't seem to get my head around the basic concepts on this). The regulator seems fine to me (but i'm not sure that should convince you). Changed the parts, tested, and everything is working fine again. Yes, sir, thank you very much.

Incidentally, with what could i replace the original SP1446/2N555 if one day i need to (can't source the original, except on eBay but is it worth it) ? I found and picked up four AD162 for spares should the AD139 go bust.

The hum below is still there, but very discreet, not at all my primary concern. However, i changed the first input AC126 transistors on the amp board at the same time to see if i could get rid of the hissing, but no. They weren't dead but measured half the values of the new ones, so it can't be a bad thing in any case. It is a bit of a bore this hiss. The sound is also still somewhat darkly veiled (i tried with external speakers as well to verify). Will have a look at the amp components. Any suggestions as to where to look or what to change that would better the amp signal (the sound/musicality) most dramatically to start ? The cap 514 ?

And so it goes, up and down. But for now it's up.

Cheers Martin & Mika.

Sebastian

Perhaps you are just expecting too much of it. Remember, this is a ~45 years old radio from the very early days of transistorized technology. I have three BM900's and they all hiss. The germanium transistors are noisy by themselves, as are the old mass carbon resistors.

The dark sound may or may not be normal, hard to say without actually hearing it and comparing to other units. I presume it might be improved after replacing some capacitors, Martin will probably know better. I have played mine through all kinds of "proper" speakers, and the sound is IMHO surprisingly good - but you have to keep that in perspective.

Good work so far in any case - hope you can now start enjoying this true classic

Yeah, i hear you, Mika. I was kind of submerged into getting it work, and while testing in short periods, the charming faults sort of jumped at my face. I've actually spent the day playing my iPod through it, and the hissing is nowhere to be heard on acceptable levels. So i've already written it off as part of my machine and have stopped worrying about it. Your three (wow, nice) ones seem to confirm this. As i've had absolutely no experience with the model i have no idea what is to be expected and what is not, and as i would of course want so much to get the best out of it, it's easy to get carried away. But, so, yeah, the hiss was there at extreme volume and no music, but it's not supposed to be played like that, now is it.

The same goes of course for the "muddy" sound, maybe it's supposed to be like that. But... i don't know, somehow i wouldn't think so. To some extent i expected it, the lovable "vintage warmth". But, it's not the warmth that comes through, it's just... how to put this ? It's kind of... flat, and muddy. The bass, when augmented, is boomy and just breaks into a wall of overall mud (i'm overstating to describe it, it's not as catastrophic as that really). I get pretty good clarity when lowering the bass to zero, but then it's of course just tinny. I've no idea if it could be the bass & treble section that is at cause actually, or the amp section.

I really wouldn't wish to sound like a whiner about it, it's a beautiful, beautiful machine. And i have no desire to overdo it. I wouldn't even want a modern sound out of it. Just... the best of it within my abilities.

Of course i wouldn't even be at this point without you guys, so kudos to you all, your welcome and help for a complete newbie was amazing. Not at once were your comments destined to make me feel stupid (though i did feel stupid and still do), even though this thread took it's length and i was pretty much lost all through it So, yeah, really, thanks.

It's up and playing, and that was the main goal, and for that i'm a truly happy camper.

Sebastian

I just saw this post now, and I hate to see my friends blowing good fuses for no reason!

I have a homemade solution, which is indispensable on my workbench. Basically, it's an in-line AC power-cable extension, but with a 100W lamp in series with the live AC line. Cut into the Live AC conductor (not the Neutral), and connect the lamp to the 2 exposed wires.

If there is no problem in the radio (or whatever), the lamp remains extinguished, and the radio plays fine.

If there is a problem in the radio, causing a short, the lamp illuminates, and the fuse doesn't blow. The bad component will heat up. After about half a minute of leaving the current flowing, I can identify the bad component quickly, by just feeling around on the PCB. 

I always work with this on all devices under repair, and through a 24-hour burn test. That bright 100W lamp gets my attention pretty quickly, when it illuminates. Only after the repaired device has operated for 24 hours without illuminating the lamp, do I connect the device directly to a regular AC mains socket.

There are 2 slight disadvantages:

1. Do not expect to get full AC voltage at the output side of the power transformer in the radio - if the service manual specifies say 40VAC, you'll see about 36VAC. This is normal and with the DC voltage stabilization at the Bridge, you should have normal DC voltage after the regulator.

2. Some devices with high-draw Switch-Mode Power Supplies cannot start, and will simply pulse - this is not relevant to our vintage B&O radios.

It's real easy to build, and I hope it helps you.

Menahem

Thanks for the tip Menahem, this sure could have come handy (and surely still will) as i must've blown 10+ fuses while trying to find the fault. I'm sure to build one in the days to come.

Sebastian

What Menahem suggests will be fine as a safety precaution for yourself and
your house supply installation, mains fuses and some circuits inside the radio.
However, a 230V 100W bulb will easily draw a current higher than 250mA so would most
likely not have saved any fuses in this case of a shorted transformer secondary
(depending on the transformer load factor etc.).
A smaller bulb maybe, 60W or so, but again that would decrease the supplied
current to a working set considerably.

A variac would be the best thing but the acquiring of such a device is
not always easily justified and a package or two of 250mA fuses aren't
that bad an alternative.

You did well.

The BM900 will never be completely "noise-free". There will almost always be
a slight hum or static if you listen for it.
Try connecting a pair of larger speakers and you'll be very surprised when you
hear what this set can do.
It's just a couple of watts each channel but what a sound.

Martin

I have experimented with a 60W, 100W, and a 250W lamp.

If you increase the radio's volume during non-faulty operation, at some point the lamp will start to flicker, and interrupt power to the radio. This is normal. A higher value lamp will delay the onset of this phenomenon.

However, the objective of the lamp is not to test the radio at full power, only to determine and isolate system shorts, which will be identified, irrespective of the radio's volume setting.

You can certainly use a 60W lamp for everyday purposes, and increase the wattage if you have higher power units under test - I use a 250W when testing Switch Mode Power Supplies to delay the onset of pulsing.

Once you've experimented with a couple of lamp wattages, and you're familiar with the power requirements of your radios or whatever, you can select (and change) the correct lamp value with confidence.

The higher lamp values will certainly prevent fuses blowing, even in the case of a transformer failure - I've had that case occur also in SMPS's.

Of course a variac would be an ideal solution, but at this point, I don't have one, and this is a very useful substitute.

Menahem

Just to report something strange, and maybe you specialists could explain it to me ; it has now been 3+ days that i've listened to the unit daily for lengthy periods (it plays practically all day), and the sound has significantly changed. I'm positive that it's not (only) my imagination. It has gone from dull and dark to something much more balanced and quite pleasing actually. Sure it's not exactly perfect (as to my expectations), but had i heard this when i fired it up the first time, i probably wouldn't have spoken about the "muddy" sound at all. The treble and bass feel so much more "separated" and defined than in the beginning that it's like another unit to my ears.

This is now especially evident on external speakers, even though well heard through the internals as well. I tried external speakers just as well in the beginning but that didn't change anything on the quality, now it's really pretty pleasant and laid back. Ok, so it's still somewhat "veiled" and there's the hiss and hum, but really, it's rather nice.

I think i've read about the caps drying up and "jamming" before taking up the service again, is this that ? Could it be that after a prolonged period of downtime it just needed some time to "heat up" ? Or have i really just lost my mind and autosuggested myself into a true believer in the lack of better action ? Or is it yet something else ?

Whatever it is, it sounds better and better

But please, do enlighten me if you are in the know.

Sebastian

It's well known that caps take time to break-in. I'm pretty sure that that's what's happened to yours, although I would not be too happy about that. On a radio that old, I would not be playing it at all, unless I had done a complete recap.

These old caps cannot be expected to be within specifications, and may (I say "may") be overstressing some particular semiconductors, which may be unobtainable.

Having established that the radio now is apparently "faultless", I would set about doing a recap now, and then relax in the knowledge that it would be good for at least 10-20 years thereafter.

Also the heat of the renewed operation may stress some soldered joints, which may break, causing possibly catastrophic failure. While you do the recap, you will automatically resolder many joints, and you can keep a look out for any other cracked joints.

Menahem

You can regard these as "new" caps - after not having been used for many years, the caps are reacting to the change in their composition with electricity flowing through them again.

Condition/lifetime of old caps is sometimes related to how the radio was used - if in a hot evironment, the caps would dry out, if in a damp environment, the leads would corrode, etc.

Not to be rude, I trust no radio owner with an old radio, and assume the worst - I have had some real basket-cases (radios, not their owners) in for repair, and I have found that the best investment in my time and the owner's long-term enjoyment, is to do a complete replacement of all electrolytics. Many of these old radios still have the original bees-wax capacitors, the wax having dried out decades ago.

Never touch the mica or polyester caps, unless proven faulty - they were specifically put there for circuit stability - and replacement is generally expensive, and realignment time-consuming.

That rectifier voltage is a perfect clue of bad electrolytics - the big coupling capacitor is not doing its job, and probably has excessive ripple - do you have a scope to check it?

Also, you can probably safely double the size (say 470uF --> 1000uF) of that new capacitor - in the old days, those capacitors were very expensive. Larger caps have better ripple capability. I use Nichicon HE and PW for these types of circuits. Yes, there is a performance difference between different models of capacitors.

I'm sure that there aren't a lot of caps to replace - I've done a few tube radios, and their capacitor count is much lower than a transistor radio. It would be really worth it to use the correct caps.

If you put a list with close-up pics of the caps on the forum, I'd be happy to help you with selection.

Menahem

If it runs fine, I wouldn't worry too much about the slightly high voltage unless
caused by ripple as Menahem suggests.
Try putting your multimeter in AC-volts mode and measure across the large
filter capacitor and let us know the result. Not the best way to measure ripple but it will
give us an idea of the ripple level.

There's an adjustment for the 24V supply on the main board, did
you measure/adjust this after repairing the power supply ?

You have a PM, check your inbox at the extreme top right corner of this webpage.

Martin