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First attempt at Beomaster 2400 'refurbishing'

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jesperb
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jesperb posted on Thu, Apr 26 2018 12:39 PM

Hello All,

Starting my first real repair project on a Beomaster 2400 - no previous BeoFixing experience, but lots of electronic DIY. This model came from my Dad, which is why i said yes to try to fix it, besides always having wanted to having fixed a Beomaster by myself (counting on a little help from my friends). It will be a rather long project, because summer is creeping up, but if it rains as much as last year, could be a quick fix after all.

Problem is 'no audio - hissing sound in both channels - volume control does not work'.

Took the lid off last night, things of immediate notice :

-End panel (left) missing - anyone got a spare in dark rosewood ?

-Sliders in really good condition, no dust, no coffee spills - nice and clean.

-Bass section, ceramic caps are 'crushed' (see pics) ?

-Output trannies (IC200,201,300,301) have been repaired previously and BADLY resoldered.

Cheers,


Beogram TX, Beovox S45, Beovox MC 120.2, BeoSound 1, CX50, Beovox S75, Beomaster 2400

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Dillen
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Answered (Verified) Dillen replied on Thu, Apr 26 2018 2:50 PM
Verified by jesperb

The polyester caps are not part of the kit. They are usually still fine.
And it's not uncommon to see cracks and flaking off of the outer layer of the "tropical fish" types - though in this case it
seems to be a bit more than usual. You could replace them, but I wouldn't suspect them at this point.

Capacitors and balance control slider much more likely.
But of course, any and all signs of previous repairs should be looked at - and in this case tidied up a bit.

If the lamps are still the original, you will need new ones when you are done servicing the rest.
The filaments in the old lamps become brittle from heat and age and the unavoidable vibrations from servicing will
break some. Replacing those will then break more. You have a fresh full set, might as well replace the lot - but wait until
repairs to the Beomaster is about done.

Martin

Dillen
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Verified by jesperb

The balance potentiometer is special in that it operates from the center outwards, - and with the channels in antiparallel, rather than
from one end to the other in parallel.
In other words, at center position both channels should read zero ohms.
As you move the slider left the resistance in the right channel increases while the left remains at zero - and vice versa.
You lower the signal in one channel in each direction from center. At center no channels are lowered.

The resistive tracks only varies from the center towards each their ends. The other ends (respectively) of the tracks doesn't vary.

Martin

Dillen
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Verified by jesperb

If the Beomaster will switch on briefly - even if only for a fraction of a second - you should be able to tell if one output has
a high DC level, pointing to a problem in that channel.

Martin

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BEOVOX141
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jesperb:
Try do a search, lots of discussions about the lamps.

I know, and that is exactly why I am asking. A  consensus seems to be missing...?

I remember Dillen hinting at the original parts list as being wrong...

jesperb
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...i go my lamps from Dillen (too).

Beogram TX, Beovox S45, Beovox MC 120.2, BeoSound 1, CX50, Beovox S75, Beomaster 2400

jesperb
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Got a little spare time, which immediately went into the Beomaster 2400 on the kitchen dining-room workbench (no room for guests this week).
Looking at the 'volume up mute' issue, where volume would play briefly and then mute for the first 3-4 taps on 'vol up', I measured out 'any and all' voltages around P3 and P5, all voltages matching the values on the diagram. Finally I gave up analyzing the muting diagram, couldn't quite understand it anyways and decided either 3TR3 or 3TR5 should go. Mainly just because they are both involved in muting and the diodes and voltages around them, had all measured out fine. 3TR5 (BC547B) out and in with a new BC547C and now 'volume up' works. Old BC547B tested out fine on the PEAK DCA-75 but the legs were 'rusty' so maybe just a 'tired' tranny Embarrassed

Final issue left to fix : Loud hum in the earphones which disappear when i touch ground, so maybe a grounding wire desoldered itself somewhere? I see no caps left to recap in the signal path from tape-in to the headphone plug. No hum when playing on speakers. Voltage selector is at 220V.

Beogram TX, Beovox S45, Beovox MC 120.2, BeoSound 1, CX50, Beovox S75, Beomaster 2400

jesperb
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I thinks this project is almost done. Did a FM tuner check yesterday, reception is surprisingly good, with only a short length of 0.75mm2 wire pushed into the antenna plug Big Smile

Only headphone hum left to fix. Tried two different jack-to-mini-jack converters and 3 different headphone types so an issue with the electronics.

Cannot find any shorts or loose ground wires, the Tape-In DIN plug is rock steady in place.
Hum increases when i touch the metal end of the electrolytic caps on the sound control board and almost disappears with volume down (=mute).

Not sure if that would be an indicator ? 

Will give it a final troubleshooting try and - even if the remaining hum will hurt - wrap it up.

Beogram TX, Beovox S45, Beovox MC 120.2, BeoSound 1, CX50, Beovox S75, Beomaster 2400

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