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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
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
A scope to see if the hum comes from the amplifiers supply or inputs.
Martin
Before I am using a scope I want to let you know that when the sensitive controls are touched the relay is clicking. Does this eliminate part of the search area?
So far I have done nothing yet, because I would like to have some more detailed advice: the hum scares me off, I am not experienced enough . I am very sorry to admit that.
I want to add pointing out that the problem still occurs when PCB 4 is disconnected from PCB 2 does this narrow the search to what you mention as ''amplifier"s supply.
You say that this is another BM1900. So, if you want meaningful help, you have to give us the status of this specific new system. What have you done to it so far? Is it recapped? Light bulbs checked/changed? Any mods from its original state (such as incandescent to LED mod that you did on the other system, etc etc)?
This BM1900 was standing a couple of months on a shelf before I acquired it, no visible traces of bad caps.
The only thing I did is what you see on the picture, including original double checked light bulbs. I did not (yet) change any capacitors. If it turns out that that is necessary I will first sleep a night on it.
As I mentioned earlier all the lamps light up, relay clicks and hum appears when I touch any of the senitive pins,even with these pcb's detached from the main body, but only as long as I keep my finger there, no selected source is heard. This situation also occurst when all pcb's are in place and connected..I get the impression that the culprit is in the output stage, do I have a chance to find it when I only use my multimeter?.It seems a tough job to use a scope if there is no source to be selected/heard. (In addition I do not own a scope)
In the first sentence I should have written: "somebody's shelf".
The BM1900 was made from 1976 to 1980 (says the internet). So that's good 40 years .... and the best e-caps you can buy today are specified with a lifetime of 10,000hrs. Well, that's formal specs and it doesn't mean that e-caps are guaranteed to die after 10 to 20 years or the specified amount of hours ...and yet, if this were my BM1900 I'd recap it before trying to get into any troubleshooting.
But that's just me. I don't have much hands-on experience on B&O, but all I read on this and other BeoForums goes in the direction of recapping - and that does make logical sense.
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.
Aad Jansse: ...I brought back to a pleasant listening a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .
...I brought back to a pleasant listening a BM2400 and a BM1900-2, both without recapping, see my posts on other threads .
Of course, if some units are still fine then there's no need to break them open and start messing around. You'd normally do that when the unit starts failing and once they're opened up and dismantled, you normally fix all the problems that will happen sooner or later in one sitting -- and the e-caps on old B&O's are one such known problem. But well, it's your time and your money...
Your photo above is not very clear but I believe to see some of those red vertical e-caps that Martin always recommends to replace whenever he sees them in a thread.
If you want to find and fix the source of your hum, your best course of action is to break out the oscilloscope and start tracing the signal paths. You don't absolutely need a signal generator for that. The hum is a signal and therefore it can be measured and tracked with a scope.You said you can borrow that old Russian scope from a friend. Do that! It's a simple single channel analogue scope -- it doesn't get simpler than that! If you know how to handle a multimeter, you can get proficient in its use in one or two hours of playing around with it.
Happy troubleshooting!
The photo was meant to show what elements were detached from the main body while the hum was still there, which indicates that these parts are not the cause of the hum.
Meanwhile I am enjoying the sunshine, which is one of the few positive things during the present hard times.
Tomorow I will be struggling with the scope and multimeter
Thanks for your comments.
Found this in the manual, could this be a trail to a solution?
I did not obtain the 30 V(+ and -) when touching this FM5 or an other pin: the relay sounds like a pinball machine, not allowing the 30 V being steady present .
I do have the 15V supply.
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. 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.
Check for cracked solder joints at the output darlington transistors.Check for shorts at the output stage and for DC on the outputs.
Someway or another a second thread with the same topic has started, is a bit confusing as to which post follows its predecessor
I will follow up on Martin’s advice 2 posts ago.
please check the other thread
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 Jansse:I would rather have seen your post in the other thread ( my fault that there is some confusion).
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!
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.
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.
OK
I will further investigate it, taking your remarks into consideration and keeping you posted.
I tested this one, see 2nd picture
How can I "free" these capsin order to replace them?
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!!
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
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.
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
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.
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.
I corrected my sentence in the post above. I typed and posted it too fast and didn't realize it didn't say what I wanted it to say :-)
What I was trying to say is that anyone changing those two reservoir capacitors must be careful to get them connected back up correctly. The polarity of the capacitors is very important and someone not familiar with those original capacitors may not notice which terminals are negative and which are positive. -sonavor
Taking the 2 caps out and soldering them back I watched carefully the polarity: the familiar hum or bangconfirmed that the polarity was as usual as before, despite the state of the bridge rectifier that I removed from the pcb, tested, found it 100% and soldered back.
i also found 2 lamps burnt on the source selection indicator, replaced them and continued searching.
Before I power on the scope I can also tell that the problem differs depending on which speaker outlet is selected: speakers 1 with a bang, speakers 2 without, however both same reaction of the relay: continuous clicking and nowhere a 31V to be found; according to the manual I should have 15V on TP15, but my patient shows some 115mV, maybe because there is no source engaged.
Pardon me my poor english: the right description of the unwanted sound when the relay clicks is not a hum ( +/- a constant level); therefore I used the word “bang”, sort of popping sound.
TP15 will only show 15V once you're able to power it on - but now the fault switch is putting the unit immediately back into standby. So, close to 0V is fine on TP15.
Where does that popping sound come from? From the speakers? If so, I hope you're not using any valuable speakers you plan on keeping. It wouldn't be wise to have any valuable speakers connected while we don't know what's wrong. If for some reason the fault switch fails you could easily fry any speakers connected to the system.
Did you replace the 2 caps on the 31V supply with new ones or did the originals measure OK? If so, what capacitance and ESR did those caps show?
"The Never ending story":
these are my speakers.
The caps, the original ones, measures are
07C 5052 uF ESR .15 Ω
0C8 4891 uF " .12 Ω
C92 2294 uF " .11 Ω
I can use a headphone instead of speakers
Well, I myself would not connect anything to the speaker jacks at this point.One of the Darlington transistors could be fried, in which case there could be 31VDC on the speaker output. And 31V/4ohm = almost 8Ampere, and that would certainly damage the speakers permanently!So, I would first measure and make sure that the output amp is ok before connecting high-value speakers.
A popping sound in the speakers is a sign of some signal on the output line and that could very well be a DC voltage. That would also explain why the fault switch is switching back to standby immediately.What I would do now is disconnecting the speaker, connect the multimeter between right speaker output and ground, set the meter to MAX HOLD mode and power on. The meter should show the max. DC value at startup. Then I'd do the same for the left channel. If any of them shows a significant DC value (e.g. 0.6V or bigger), I'd know that the fault switch is doing its job and that I have to investigate components in that output line to see where this DC voltage comes from.
Aad Jansse: The caps, the original ones, measures are 07C 5052 uF ESR .15 Ω 0C8 4891 uF " .12 Ω C92 2294 uF " .11 Ω
The values and ESR are still quite good considering their age. But they're clearly aging.
It has been quiet here on the BM 1900 front: I had left my home for quite some time to avoid the stress of being “lockeddown” at home, My wife and I moved temporarely to a cottage (B&B) somewhere in my country wihere there is a less dense population, minor risk to pickup a covib-19 contamination, nor conditions to take my workbench with me.
I’ll be back later