Welcome to Beoworld !

What a nice radio !
The first thing would be to check if there is any voltage anywhere.
I suggest measuring AC voltage on the rectifier primary first.
Then DC on the secondary side. Let us know the results.

The dial lamps are good indicators for voltage and transformer function
so I suggest you make sure two good ones of correct wattage are mounted.
The stereo indicator lamp is not important right now but will also need
to be of correct wattage when a new is fitted (different from the dial lamps).

Martin

Welcome from me as well, and congratulations! A 900K was my first B&O piece as well

I'd add that you need to either measure the fuses or replace them with new ones - those glass tube fuses can be blown (or corroded) even if they seem OK. Sometimes the wire breaks when you remove them and the end cap gets twisted a little.

You have a rather early example there as the type designation is still S290. It got the normal four digit type designations in a year or two after introduction I think.

Hope it will be a simple fix!

Hi,

thanks so much for your quick replies, and your warm welcome.

I started by measuring the fuses, and what do you know, i should've certainly done it even before posting here ; the second mains fuse (the 0,25A one with a little "springy" wire), even though perfect visually, is dead. The other two are fine. So i'm off to try and find a new one right away. I'll try and source the correct lamps at the same time (if i get it right, that's two 6,3V for dial, and one of minimum 19V for stereo).

Will report ASAP.

Thanks again.

Sebastian.

Ok, so with a new fuse in place, when plugged to electricity, the SP1446 regulator transistor at the back gets extremely hot within seconds. I disconnected the unit immediately. Now what does this say ?

[EDIT] In the meantime, i looked a little closer, and the red wire leaving from the SP1446 has melting marks (seen how it gets hot, no surprise) [EDIT]

Sebastian.

Most likely a short somewhere.
Let's have an ohm reading (unpowered of course) across capacitor 514.
It's a 64uF electrolytic capacitor sitting at the rear center of the amplifier board, providing
a good and easy place to check amplifier voltage, easy to measure across from the solder side.

Martin

Dillen:

Let's have an ohm reading (unpowered of course) across capacitor 514.

On the scale of 200, i get a reading of 00.3 (which means to me it's not shorted, right ?)

[EDIT] In fact no, it's exactly the contrary, i believe. It should go up progressively if it was working, right ? Told you i was useless in electronics. [EDIT]

I also had the time to mess around in my distress, and desoldered the SP1446 with its resistor to measure them, for what it's worth :

- the resistor = 13 ohm (i couldn't find it's supposed value in the schematics, what should it be ?)

- SP1446 (scale 200k) : C-E = 00.5 / C-B = 00.7 (no idea if i did that right ?)

Sebastian

OK, now desolder one pin of capacitor 514, lift it out of the board and measure again, this time we need two readings;
On the board tracks and directly across the capacitor.

Martin

Ok, i desoldered the minus (-) leg.

Across the capacitor = slowly raises to infinity (so now it means it was ok then).

I'm not sure what you mean by "on the board tracks", but if it means the solder joint of the leg that is still soldered (+) on the underside of the board, and directly the desoldered leg, then that's the same (raises to infinity), or if you mean the solder joint of the soldered (+) leg and the now desoldered empty (-) point on the board, well, it gives zero.

I'll be making a go at the local electronics store today to get the lamps, is there anything i could get that could be suspect and would be better off changed anyway ?

Thanks for your patience, i'm not really making this any easier now am i...

Sebastian

OK, you can solder back the capacitor 514 now. It seems to be OK.

The output stage transistors could be worth checking too but I doubt the fault to be here because the resistance is
so close to zero ohms. I would expect to see the emitter resistors reflected as the minimum ohmic value if an output stage is shorted.

You can desolder one side of the large power supply filter capacitor and measure across it like you did capacitor 514.

Martin

Dillen:

You can desolder one side of the large power supply filter capacitor and measure across it like you did capacitor 514.

That's the enormous blue one (1000uF), right ? Same thing, raises slower but goes on forever and seems fine.

Sebastian

Yes, that's the one.
Does it read a steady zero ohm across the cap when mounted ?
Also check the output stage transistors, all four across collector/emitter and
across the two output caps (800uf) at the rear panel.

Martin

Dillen:

Does it read a steady zero ohm across the cap when mounted ?

Well, at the scale of 2000k it's a steady zero, and at 2000 it's 364 steady. Am i right by understanding that the caps should be tested with the highest value available on the meter (mine is 2000k) ?

You will always get the most precise reading at the lowest possible setting of the meter.
Obviously a serious short somewhere and we will have to isolate some circuits to diagnose further.
On the large blue capacitor, there's usually a bunch of leads going to one of its tags and only one or two
going to the other.
Try desoldering the large bunch from the one tag and measure from the other tag to each lead, one
at a time to see which one(s) has the short.

Alternatively, if the exact ohmic reading was lower across cap 514 than across the big blue cap,
desolder the two red leads that go to a solder pad near cap 514 (usually to the left of it if facing the front
as you would normally do) and measure across 514 again. This will isolate the amplifier section.

Martin

Ok, so just a couple of observations in the meantime ; the more i look at the innards of this thing (it's actually amazing how you can have a sense of doing something just by looking at something incomprehensible, i must have something wrong with me), the more i get the feeling that something fishy has been done to it. I just found in the wire-bunch that runs along the back wall, two naked leads that were wrapped in electrical tape together. They come from the right channel external speaker connector socket. They are the gray/white striped & the black wires that respectively go to the inside of the big pin and the center pin.

There has also been some not-so-pretty resoldering done on the socket that takes the integrated speakers plug. The wires look of origin, but the soldering definitely doesn't. I have a feeling that those loose wires should maybe go to the socket, the length is perfect. For now the five pins of the socket are, from left to right in half-circle by the bottom :

1. black/white wire

2. brown wire that connects with pin 4

3. free

4. brown wire that connects with pin 2

5. red/white wire

Now, what's this ? There was a problem and someone eliminated a channel ? I recall reading here something like this.

Also, another thing, what's the value of the top mains fuse ? It must've burned once i changed the bottom 250mA one because it was ok before, but the problem is there's absolutely no markings on mine for value, and i can't locate it on the schematics, for a replacement ?

[EDIT] Oops, Martin, you posted just before, didn't see it. Will try what you're suggesting now. [/EDIT]

Sebastian

Yes, and just added a bit to it as well.

You need to check those leads against the servicemanual and make sure they are properly insulated.

Martin

Note that the 5-pin DIN pins are not sequentially numbered.
Taking that into consideration, the leads seems correctly mounted.

The two mains fuses are 250mA Slow-blow
The dial lamp fuse is 1A Fast-blow

Martin

Perfect, thanks for all the information. In fact, armed with my newly acquired intelligence, i remeasured the caps at the smallest scale (200), and the 514 measures still 00.3, and the big one is higher no matter which way i test it. So i desoldered the two red wires like you suggested (isolating the amplifier).

Sebastian

Dillen:

You need to check those leads against the servicemanual and make sure they are properly insulated.

Yes, sorry about the rambling, i'll try and refer to schematics so as to not invent my own conventions (thank heavens for your patience).

I've been checking and rechecking the wires with the schematics, and there's something that i just don't get ? In the image included i've indicated on red how the wires are right now (and this time i can confirm it). The output cap (800uF) is connected to the right speaker pin 4 (as it should), the pin 3 wire and one of the 2 grounds from pin 2 are the cut & loose ends that i found taped. It's instead the pin 2 (the ground, is it not ?) wire from the mono speaker that goes to the pin 1 of the internal speaker socket, and the pin 1 wire from mono to pin 4 of the left speaker (here also, as it should). Everything else is as it should. I won't even try to understand it, but surely it's not right ?

Sebastian

Dillen:

Obviously a serious short somewhere and we will have to isolate some circuits to diagnose further.

On the large blue capacitor, there's usually a bunch of leads going to one of its tags and only one or two
going to the other.
Try desoldering the large bunch from the one tag and measure from the other tag to each lead, one
at a time to see which one(s) has the short.

Done. Of the 7 wires, 5 3 2 indicate a solid 1 when measured. They are the 2 black ones that connect to the 2 resistors 512 (512a & 512) 547 & 643 on the same board as cap 514 (the amp board), the two black ones that connect to each PIN 2 of left & right external speaker sockets, and the black one that connects to the selenium rectifier (N° 233 in parts list in the manual, didn't find it on the schematics, the flat board-like thingy against the back wall next to transfo and the big cap).

Sebastian

[EDIT] Oh, and Merry Christmas everybody [/EDIT]

[EDIT 2] Correction on the resistor numbers [/EDIT]

[EDIT 3] After some insulation fixing (or so it would have seemed) on the output transistors, left with 3 problems [/EDIT]

[EDIT 4] Don't ask how, but i'm left with the 2 speaker PIN 2 wires shorted. I left them loose and lit up the 900K with a new lamp and nothing exploded nor burned. On the contrary, nothing got hot and the light worked. Now what's with these speaker wires ? [/EDIT 4]

[EDIT 5] Of course there's nothing with the speaker wires. Right ? [/EDIT]

Ok, can you guess i'm spending this Christmas alone ?

Must've been all the desoldering and resoldering, and the fact that one of the output transistors had some faulty insulation (i don't know, could it ?), but gradually i put everything together, and at each step it seemed fine, until just a minute ago i tested it complete... and MUSIC !

Nothing gets hot, lamps work, even the low current that i felt in the beginning at the back when touching it is gone (a grounding issue ?). Ok, so it only plays on the left speaker for the moment, and there's an underlining hum static  (kind of like inbetween stations) present on radio, tape and turntable presets when nothing's on and the volume is turned up, but hey, that's pretty much 200% better than when i got it. Also, i get only like 5 stations with the tuner (with stereo decoder & the internal antenna).

Now to try and see for the rest.

Sebastian

[EDIT] Ok, the hum is almost gone, another grounding issue. The two main concerns left are the missing right channel (?!) and the static "shhh" when volume is high, on all presets. [/EDIT]

[EDIT 2] So my guess is that initially there was a problem with the right channel, and whoever it was that has tinkered with the speaker wiring that was pointed out in a previous post, "solutioned" it by feeding the mono speaker (that is just a second left channel, right ?) to the right internal speaker (which works, proving that the speaker is ok). Now how do i find what's wrong with the right channel ? Is it on the amp board ? [/EDIT]

Wow, good job !

Swap the leads to the speakers to see if it's a speaker fault or listen carefully as
you power on/off the Beomaster or switch channels for a thumb or any other sign of sound
from the otherwise dead speaker.
Check potentiometers, especially the balance potentiometer at the backside.
It's often left unused for ages and will easily cause a dead-channel problem like this.
Check also for wires broken off the main board, especially below the amplifier section, leads
will sometimes break off when the amplifier section has been swung back and forth several times
and may not be obvious.

The persistent static noise will most likely be one of the AC128/AC151 transistors in
the output amplifier, typically the first or second nearest to the input.
Again, if in doubt or lacking parts, swap a transistor with the matching one from the other channel.

Martin

Dillen:

Wow, good job !

Cheers, Martin, but this is entirely thanks to you. I wouldn't have even tried anything on it was it not for your patient instructions when it was clear (and still is) that i don't know anything on the subject.

Different types of transistors can be used, AC126 is possible.
I seem to remember that the first transistor is closest to the nearest outer
corners of the amp. board. The service manual will tell you which one(s).
It's possible that the same fault is present in both channels but of course, it
could also be caused by something else.
Is the static present also with the volume control set at zero ?

The rockwool is supposed to be in there and you shouldn't judge the sound
with the chassis out of the cabinet. It's a pressure chamber system so depends
greatly on a closed cabinet.

An internal FM antenna (and also an AM antenna) is provided.
The FM antenna is in the form of two (usually black or red) leads in grooves at the
inside of the wooden cabinet. They should have a plug to fit into the red socket
at the back.
If not, you will have to make something up to fit - or connect an external antenna.
In most cases, the internal antenna will be more than adequate.

Martin

Dillen:

Different types of transistors can be used, AC126 is possible.

 

I seem to remember that the first transistor is closest to the nearest outer
corners of the amp. board. The service manual will tell you which one(s).

Yep, that's AC126 for me, will have a look at them.