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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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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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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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jesperb
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After a LOT of fiddling with the touch panel, i finally got all the small 'forks' connected to their respective pins. Now power up through the variac, got standby and LIGHTS!
Switch to 'ON', no smoke, no excessive heat on the ribs or anywhere i can measure.
Adjust TP15 to 15V (was 14,68 not too shabby).

Now : who the he.. has an FM antenna lying around these days Big Smile

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

jesperb
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I guess this is where the real work starts Embarrassed

-Left channel dead. Tried swapping speakers and tape channels (DIN<->RCA plug). Only sound in the right channel.
-Sound fades completely out when i push the balance slider past the 'middle' position (see pics).
-Sound in right is very 'muddy' when i turn up the volume.
-PSU looks fine : C92=19,7V, TP14=11,7V,TP15=15,0V,TP16=15,6V,+-31V also OK,
-No-load emitter resistor voltages are : R256=-4mV, R356=-12mV. Didn't dare try to adjust just yet. Besides the emitter voltage doesn't change after about 10 mins of power up, no speakers attached.


First, I'm thinking the balance slider (22kOhms) is broken mechanically, as it shorts out halfways in both sides (slider bridge is already replaced with Dillens kit)? Compared to the treble slider (also 22kOhms) which measures a nice linear 0<->20kOhms in both sides on bottom pins E-S.

Any thoughts on this most welcome, thank you ?

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

jesperb
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I guess this is where the real work starts Embarrassed

-Left channel dead. Tried swapping speakers and tape channels (DIN<->RCA plug). Only sound in the right channel.
-Sound fades completely out when i push the balance slider past the 'middle' position (see pics).
-Sound in right is very 'muddy' when i turn up the volume.
-PSU looks fine : C92=19,7V, TP14=11,7V,TP15=15,0V,TP16=15,6V,+-31V also OK,
-No-load emitter resistor voltages are : R256=-4mV, R356=-12mV. Didn't dare try to adjust just yet. Besides the emitter voltage doesn't change after about 10 mins of power up, no speakers attached.


First, I'm thinking the balance slider (22kOhms) is broken mechanically, as it shorts out halfways in both sides (bridge is already replaced with Dillens kit)? Compared to the treble slider (also 22kOhms) which measures a nice linear 0<->20kOhms in both sides on bottom pins E-S.

Any thoughts on this most welcome, thank you ?

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

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

jesperb
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Thanks Martin. After your suggestions, it was MUCH easier to search for your handfuld of similar answers to this exact same question Embarrassed

I soldered out all sliders (again) and game them an extra cleaning. Measured resistance and is was as per the specs.

However, same issue : very'muddy' right channel, no sound in the left.
This goes for both phono and tape inputs.

Turning down bass+treble doesn't change the 'muddyness', so my conclusion is, it has nothing to do with the sound adjustments (?)

Tried earphones, sound quality is much better, only a little 'muddy' in both L+R. Headphone balance works sort of in reverse though, little lower in the middle, little higher in both ends.

As soon as i get DIN+RCA plugs i will wire up a cable and listen to tape-out through my 'normal' amp, to isolate further.

Stereo switch - does that have anything to say here ?

Jesper

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

jesperb
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Did a read-up on 'all' 1900 and 2400 posts in the forums and sure enough : I'm testing input from TAPE with a 7-pin DIN....

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

jesperb
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jesperb replied on Mon, Dec 10 2018 12:20 PM

The crushed polyester caps on the control board (see first post+pic) were - well crushed.
New ones in and the sound in the right channel is now finally good. Sadly the new ones are same form factor, so they will be crushed too. Anyone who can recommend another makeor smaller size ? EDIT : I see Beo_Jean already gave recommendedations here earlier.

Very low sound in left channel. Sounding somewhat like cross-over ?

This post should change subject to 'Collateral Damage' : 
Ground wire pulled out in one speaker cable plug. Took some time do debug. 
Volume control right lamp stopped working and volume touch control started behaving erratically. Took no time to debug.

 

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

jesperb
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More collateral damage : Dropped the trafo-end of the Beomaster - rather clumsily - onto the working table. Just a small bump from about 10cm's, but all lights are now out, except the standby LED. 
When choosing ex. TAPE the relay clicks and the Beomaster goes back to standby immediately. Must be the error detection circuit doing it's job.

I guess something short-circuited or a lamp broke. Will start checking power and then lamps Crying

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

Dillen
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Dillen replied on Wed, Dec 12 2018 4:09 PM

Check output stage transistors solder joints
and also where the transformer leads are soldered to the fuse board.

Martin

jesperb
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Transformer leads are (and were) soldered fine. Output transistors soldered fine.
I had already resoldered both before the mishap (know this is not a guarantee, so i triple-checked).

PSU measurements in standby are : C92=21.8V (maybe a little high ?), TP14,TP15=0V (unit in permanent standby), TP16=15,4V, TP17=15,3V.

Checked all lamps, even though i see they influence more when the 2400 will not be going into standby.

Time to measure output transistors ? In-circuit measurement OK or pull them out ? 

Sorry for the basic questions, not the most experienced, in electronic trouble shooting.

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

jesperb
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Got a Peak DCA Pro semiconductor tester and did an in-circuit test of the output stage transistors+diodes.  Marked the results on the pic : green=transistor tested OK, orange=Peak found no component. I also checked the transistors 'not detected' with the multimeter in diode test and then ohmed them out for shorts. I found no shorts and voltage drops for BC and BE looks to be 'normal', for a diode.

Any other suggestions, before i start desoldering the 'orange marked' trannies and give them a new test ?

PS More collateral : The manual output switch came apart as was i trying to measure, if it had a short. The previous repairman had done a proper job of destroying it. Spared me the emberrasment though.

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

Dillen
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No in-circuit test will be reliable in circuits as complicated as this.
Doing some DC-level readings would tell much more.

Martin

jesperb
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Which - for reasons unknown - I didn't measure after the 'standby only' issue occurred Crying
Will return with DC-level readings on the output tonight, thank you.

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

sonavor
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Just to add to what Martin said, the FAQ for the Peak Atlas DCA Pro says that it cannot do in-circuit transistor testing.
It is a nice meter, I have one, but also be aware that transistors that are detected and measure good outside the circuit board can still be faulty. I have run into that a few times. It's no fault of the Peak meter but rather a failure scenario of a transistor. Sometimes their fault is under the load of a circuit. That is why measuring and looking at the voltages for various operating conditions is a better test in that case.

-sonavor

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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