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Beomaster 1900 15v supply

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Saint Beogrowler
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Saint Beogrowler posted on Fri, Mar 31 2017 11:30 PM

I've got a beautiful Beomaster 1900 that has distortion with volume increase and intermittent right channel failure.
What I've done:
replaced the electrolytic and the filter caps. Replaced the idle trimmers and adjusted per service manual. Replaced the R151 trimmer for the 15V supply. I was only getting 13V at TP15 and no changes when adjusting the R151.
All lamps are working.
13V at TP 16 with a good wave form on the scope (I'm still learning to use this though)
13V between the rectifier and R147.
Green rectifier test good.
D50 rectifier test good.
2A quick fuse replaced
still 13V.
C95,96 100nF caps replaced, (one was a tropical fish, one wasn't)
13.5V now and still no changes with trimmer R151.

What am I missing?

 

Peter-

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Saint Beogrowler
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Peter-

Søren Mexico
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What is your AC current

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Dillen
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If D50 is still the original part, I suggest you replace it.
They often  measure fine but they are known to break down under load.

Martin

chartz
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Dillen:

If D50 is still the original part, I suggest you replace it. They often measure fine but they are known to break down under load.

Martin

Yes, Martin is right - isn't he always Wink -, I repaired one 1900-2 with that same failure and symptoms. A replacement of the bridge solved it.

Jacques

Saint Beogrowler
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Thanks for your help guys,

AC to the transformer is 123v at standby and 95v when on.

My D50 bridge rectifier- VS048, 2A 50v

Can I use a 3A 50v bridge rectifier? They seem more common.

I see a KBPC 1005 in my parts unit Beomaster 2400.

Peter-

chartz
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Sure you can. Go ahead! 

Jacques

Søren Mexico
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AC current from transformer to D50 (9) (10) in the drawing ??, 95V when on doesnt sound right, please measure again

Collecting Vintage B&O is not a hobby, its a lifestyle.

Saint Beogrowler
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I swapped the d50 3A Bridge rectifier from my parts unit and success! 14.5v at tp15 and easily adjusted to 15vdc. New rectifiers are in my mouser cart now. Left channel still not working though.

Picked up a working Beomaster 2400 for peanuts locally and popped it open to do comparison testing.

I had tested the AC going into the transformer before. Tested AC at (9) and (10) and at the rectifier on both of them and found 15vac on both via the oscilloscope.

I have a diagnostic learning question, I've mostly just practiced using my new (to me) scope signal tracing on simple non-b&o receivers. Tracing power supplies with the Scope, especially AC, I get different readings than with my DMM set to AC. I can find the expected voltage with the scope with waveforms that seem appropriate but with the DMM I get inconsistent voltage readings. Is this expected?

Peter-

Saint Beogrowler
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Also in this BM2400 I just opened it has a 6a 50v bridge rectifier D50. Seems like higher amp ratings are ok.

Peter-

Søren Mexico
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I ran my BM 2400 with a 4A 80V rectifier, no problems, now go after your left channel, cross checking with the right channel, with your scope

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Saint Beogrowler
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Testing is going well. Music playing now in both channels.

First tracked the signal to balance slider, a usual suspect but since I couldn't get anything from the right channel I thought it would be somewhere else.

And indeed after very thorough cleaning I could get a good consistent signal all the way to the speaker jacks. But when I hooked the speakers up I got no sound from the right channel either set 1 or 2. I repeated the testing, retraced the signal back through. I kept staring at the waveform with the probe in the round pin female speaker jack and wondering why I couldn't turn the waveform into sound.

Dumb, I was grounding my scope probe to the grounding nut and of course the wire connecting the ground flat pin between set 1 and set 2 on the right channel had become dislodged most likely when I replaced the filter caps.

Peter-

Saint Beogrowler
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Broken trace in the radio section took a little while to find, on one of those vertical mounted resistors.

Phono sounded muffled, compressed and not very dynamic as I expected. Swapped C205 and C305 for poly 0.47uf caps and it made quite the difference. Those red drop caps... wood was in decent shape so I just hit it with some Howard's Restore a finish, looks beautiful.

All in all a fantastic receiver, well worth the effort. Just wish it had more inputs for all my digital demands. Yes - thumbs up

Peter-

Saint Beogrowler
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Thanks for all your help guys, I don't know if I would have ever figured out the rectifier that tested good being bad.

Peter-

chartz
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chartz replied on Fri, Apr 7 2017 12:58 PM

Well done then!

Jacques

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