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Another BM1900

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Aad Jansse
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Aad Jansse posted on Mon, Mar 30 2020 4:14 PM

After having brought back to life of my BM1900-2 and my BM2400 with the very much appreciated help of Beoworld members, I dare to start another thread featuring another BM1900.

The problem: when powered up the standby ( red LED ) is activated, but there I stop:

Selecting one of the sources, FM’s, TAPE or PHONO, the 1900 produces a loud hum iinto the speakers. I do not dare to enjoy this longer than a split second, so my question is:

where to start the analysis of the problem.

Aad

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Aad Jansse
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I appreciate your concern as to recapping, but a bit stubborn that I am, i would like to point out that  very recently I brought back to a pleasant listening  a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .

Nevertheless I will as a stated before keep your and Dillon's advice in mind

Aad.

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manfy
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manfy replied on Sun, Apr 5 2020 3:18 PM

You're making progress, I see!
Before, you had a hum when the unit was powered on, and now, the hum is gone - because it doesn't even power on anymore. Unsure
Obviously the fault switch is deactivating the main power. A few pages further in your manual you will find an explanation of the fault switch.
Something must have happened from then to now. What's different from the previous setup?

First thing to check now is the 15V standby voltage and the peak to peak ripple on that rail. Check that ripple in idle mode and while keeping a selector button pressed. It might only happen for a short moment (because the fault switch will activate quickly), but if you set the scope trigger in the right place, you should get a nice image.

Oh, and check the main filter caps on the +/-31V supply. 0C7 and 0C8 are most likely dried up or shortly before dying, and that's why the output stage will come up with all sorts of odd symptoms.

Dillen
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Check for cracked solder joints at the output darlington transistors.
Check for shorts at the output stage and for DC on the outputs.

Martin

Aad Jansse
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Someway or another a second thread with the same topic has started, is a bit confusing as to which post follows its predecessor 

Aad Jansse
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I will follow up on Martin’s advice 2 posts ago.

please check the other thread

Aad Jansse
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Hi manfy,

I would rather have seen your post in the other thread ( my fault that there is some confusion).

Anyway, you gave me enough to sleep on and to use as a possibility to escape from corona depression.

Thanks you all

Aad

manfy
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manfy replied on Mon, Apr 6 2020 8:43 AM

Aad Jansse:
I would rather have seen your post in the other thread ( my fault that there is some confusion).

Huh? There's no other thread! This is page two of your original thread. Bottom left side of each page shows the page counter and allows you to go to different pages.

Aad Jansse:

I will follow up on Martin’s advice 2 posts ago.

Our recommendations are not "exclusive" in the sense that you must only select one and disregard the other. It is just a list of things to check because they are likely to help you solve the problem. Of course, you start with the easiest one and work your way down until the problem is found and solved. Once the problem with the fault switch is solved, you'll probably find that the hum problem is back and you'll continue with troubleshooting.

The power supply is a very fundamental part of every electronics. If the power is no good, then all the attached circuits won't be able to do their job correctly. So, making sure that the power supply is fine must be the first step in every repair process and it never is a waste of time paying attention to this!

Aad Jansse
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first of all: sorry for the "page two"confusion.

Secondly, I understand now that I did not "start with the easiest one" when trying to locate the problem: I found out that the 2D50 bridge rectifier
supplies +/- 24 V to the fault switch. If this causes the trouble I would like to ask Martin for an email for supplying me a B40C1500 replacement.

Aad

 

manfy
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manfy replied on Tue, Apr 7 2020 9:07 AM

Something doesn't sound right here. Please check that again!

What I see from the circuit diagram is that D50 supplies +15V (that's the standby supply).

0D1 should read +/-31V and that's the main power for the output amp and it provides -24V for some other circuitry. The fault switch runs on the -31V rail.
If you read a DC value of +/-24V at D1 this would suggest that the smoothing capacitors 0C7 and 0C8 are practically dead. In this case the DC voltmeter would give you the average value of a rectified sine wave.

Do you have simultaneous AC/DC measurement on your multimeter? If so, you can check the AC RMS value of the rectifier output.
Using a scope would make things even clearer...but well, you keep avoiding that.

Aad Jansse
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OK

I will further investigate it, taking your remarks into consideration and keeping you posted.

Aad

Aad Jansse
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I tested this one, see 2nd picture


Aad Jansse
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How can I "free" these capsin order to replace them?

Aad


manfy
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manfy replied on Wed, Apr 8 2020 8:01 AM

Aad Jansse:

How can I "free" these capsin order to replace them?

Aad

Donno! From here it looks like the caps are held in place by that plastic bracket. You'll have to investigate how to get them out.
Once they are loose, you'll have to desolder the wires that go to PCB 2. Then you should be able to test the e-caps (you won't need to disconnect the wires to GND and the bridge rectiifer as they shouldn't affect the measurement -- unless there is a shorted diode in the rectifier. But that's easy enough to check with the multimeter in diode test mode).

I'm not a big friend of those no-name component guessers, particularly because they don't provide any specs of what they can do. A good multimeter in capacitance test mode is usually more trustworthy. With such large caps you really have to make sure that they're completely discharged before doing the actual measurement.
I hear they are good value for money. You can get those for 5-10 bucks on the internet, shipping included. Just don't look at them as accurate measuring instruments!

Oh and PS: Don't use that component guesser for in-circuit tests! They won't be able to show useful data and since they auto-cycle through all sorts of test modes they might actually do harm to some components in the circuit!!

 

sonavor
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Those are the reservoir capacitors 0C7 and 0C8.  They are 5000uF electrolytic capacitors. They have to be connected properly so they work for ±31V rails.  The BM1900 service manual shows them.

-sonavor

Søren Mexico
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Easiest way to get at all the caps and other components is like this

 


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

manfy
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manfy replied on Wed, Apr 8 2020 6:46 PM

sonavor:

Those are the reservoir capacitors 0C7 and 0C8.  They are 5000uF electrolytic capacitors. They have to connected so they work for ±31V rails.  The BM1900 service manual shows them.

-sonavor

Yes, of course! For testing the whole unit, the caps must be connected to PCB 2. When I said that the wires to PCB2 must be disconnected, I meant that this is for measuring the capacitance only! If PCB2 stays connected, you'll also measure all other filter caps on that rail and the component tester will get confused by all the circuitry. Actually it's probably best to disconnect the wires to the rectifier too, because the low resistance of the transformer winding will confuse this component tester. That's the problem with those "all-in-one" blackbox testers. If they see anything unexpected, they immediately throw up a "component unknown" error.
But for out-of-circuit tests they are quite ok.

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