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 wrote:

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 wrote:

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 🤒