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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi everyone,
Today I decided to plug my Beomaster 4400 for sound quality assessment purposes.
It still sounds wonderful BUT when I first switched it on there was a loud, and I mean LOUD plonk in the speakers, with some kind of crackling surge in the upper frequencies. The overload beacon lit and went dark again. The protection circuit had cut power and of course no sound. I turned it off, then on again and it worked fine thereafter. It had stood unused for quite some time.
Two days after, same thing exactly, loud crack, protection on, and then everything okay.
Nothing heats up and there is no noise - as expected - when idle. The amp can play for hours and hours without any trouble. Idle current it perfectly fine, and so is offset. I can switch it off and on many times, no problem.
Jacques
Hi Jaques,
the trouble never ends
I guess it's the problem of an old electrolytic capacitor somewhere in the power-amplifier. It just takes to long to load it to its nominal charge and that's why you get some more voltage at the output and the protection circuit switches off. Next time you switch it on the capacitor is already 'precharged' and everyting works fine afterwards.
I'l have a look at the circuit diagramm a bit later. Maybe i can find some obvious candidates for replacement.
Ralph-Marcus
Yes I was there too Ralph-Marcus.
But then the receiver was fully re-capped (Martin's convenient kit).
I'll have to check for an inverted electrolytic or something like that. Nothing is bulging though.
okay had some time to study the circuit diagram.
There's Cx38 (where the x stands for 100 (=L) or 200 (=R) as the most suspicious (for me). Cx47 and Cx48 are possible but not probable.
Cx34 might be the cause for the clipping indicator 'problem' but this is only a 'cosmetic' one i guess.
The two reservoir capacitors (10.000µF C8 and C9) are quite unlikely but possible. If positive and negative voltage rise different the DC balance in the amp might get worse and so triggering the fault switch.
Well you got some things to check now . Good luck
EDIT: I shouldn't compose a quick reply during one hour in parallel to my regular work.
Yes check the correct polarity of the C's. Is in Martins Kit a new elko for every elko in the power amp or 'just' the minimum? There are only 5 elkos in each amp channel including the coupling Cx32.
In Martin's Kit, only the two reservoir caps are not supplied!
So those two 10000 μF are indeed suspicious, what you say does make sense and I had already thought it might come to that. They hardly ever need to be changed though.
My money is on one of the small signal transistors.
Martin
Dillen: My money is on one of the small signal transistors. Martin
Thanks for your input Martin. An explanation perhaps?
Unsoldering the amp input capacitor (C232) makes the problem disappear. So I've got to find a bad component somewhere in the preamp...
Edit: in fact, I realized that it is not the amps that cause the power-on thump, since separating the power stages from the preamp showed absolutely no noise in the speakers.
Okay, found the culprit!
Martin, you were right, TR210 - BC 559B - was leaky. It's the last PNP before the power amp. I replaced with BC 558B I had in the box. For good measure I replaced the BC 549 with 548!
Seems to work fine. Is BC548/558 instead of 549/559 fine by you?
Now, as a bonus, the power-on thump has become very discrete, and powering off is silent.
Thanks!
I am fully convinced Martin could answer our Beoproblems in his sleep :)
Andrew: I am fully convinced Martin could answer our Beoproblems in his sleep :)
I thought he did!
Jeff
Beogram 4000, Beogram 4002, Beogram 4004, Beogram 8000, Beogram 8002, Beogram 1602. Beogram 4500 CD player, B&O CDX player, Beocord 4500, Beocord 5000 T4716, Beocord 5000 T4716, Beocord 5000 T4716, Beocord 8004, Beocord 9000, Beomaster 1000, Beomaster 1600, Beomaster 2400.2, Beomaster 2400.2, Beomaster 4400, Beomaster 4500, Beolab 5000, Beomaster 5000, BeoCenter 9000. BeoSound Century, S-45.2, S-45.2, S-75, S-75, M-75, M-100, MC 120.2 speakers; B&O Illuminated Sign (with crown & red logo). B&O grey & black Illuminated Sign, B&O black Plexiglas dealer sign, B&O ash tray, B&O (Orrefors) dealer award vase, B&O Beotime Clock. Navy blue B&O baseball cap, B&O T-shirt X2, B&O black ball point pen, B&O Retail Management Binder
And be has the teacher's intelligence of not giving too much away, so that we can do our homework and still learn by ourselves at the same time.
A beoproblem, a new word! Nice one Drew!
Hard luck!
After a few days of trouble-free operation, I turned the 4400 on this morning, and click! I heard some humming just before, the overload beacon lit briefly.
I switched it off, then on, click again. After three or four attemps the amp worked normally and still does now.
I hadn't noticed the very brief humming before.
There is absolutely no ripple and noise is in good order, that is very very low, with absolutely no hum even at full volume.
A touchy beast, this.
And the saga continues….. as a wise person recently said to me.
Honestly Jacques, these aging electronics are certainly full of drama, even after a rebuild.
And yet it worked perfectly... For a while...
I'm going to unplug the amps one after another and see what happens. I still suspect something wrong in the preamp but I can't be sure. What are the odds that another small transistor has given up the ghost anyway?
Why is there hum on both channels before the protection circuit - which was faulty when I got it! - cuts the power out?
If you were located in scotland i would say a ghost is playing tricks on you But haven't heard of them in your region.
Quite mysterious this fault and hard to discover if it comes and goes.
Unplugging the amps is a good start. I would interupt the connection between the pre-amp and the amp and put a small capacitor between amp input and ground but leave the amp connected to the supply voltages. This way there's no input signal disturbing the amp while the other amp is completely disconnected ( no supply voltage and no input from the pre-amp). This way it should be possible to find the part in the complete system which gives the trouble.
I keep my fingers crossed!
Okay then.
Both amps disconnected (one after the other), the protection circuit triggers.
It only stops doing so after a few on-off cycles, then it works normally.
The behaviour is really erratic.
Today it worked first time. Offset 8 mV. After a while, I switched it off. Next switch-on, it wouldn't start, the relay clicked immediately. Why?
I unplugged one channel, the overload beacon lit and the relay triggered.
I unplugged the other channel, no click, no light.
I plugged one channel again, click and red light. Second channel, no click, no light, music!
The order in which I plug/unplug the amps makes no difference.
I checked voltages on protection mode - all in good order.
However I now suspect either a faulty rectifier bridge, with an intermittent inner connection, or perhaps a bad solder in that area, which would explain the erratic behaviour, with a possible momentary cut of the positive or negative 35V line.
I really think that everything else is fine.
Hi Jacques,
There are few things in life as frustrating or annoying as an intermittent electrical fault. However whatever the fault, the component or components will eventually hard-fail and you will have little trouble locating it/them.
At least you don’t have my Beomaster 4400 which requires an exorcist as well as a small version of the flame retardant system used on airplanes.
Hi Jeff,
Yes well, I am now pretty sure it's the rectifier bridge that has an intermittent internal joint between two diodes.
Not at home right now, but I will try a simple replacement just to make sure.
Funny thing is, that transistor I replaced WAS faulty!
I am following, you may be right about the rectifier, and check the 2 big capacitors in the power supply,and F2. If no failure there, check the preamps, then D100/200 D101/201. If OK try disconnecting speaker head phone connection at P11/12 pin3, that will eliminate any failure in speaker headphone switches.
If nothing of this helps, you are back at erratic amp failure. (tracks, solder points)
Collecting Vintage B&O is not a hobby, its a lifestyle.
Thanks Søren. Already checked the overload diodes.
chartz:I heard some humming just before, the overload beacon lit briefly.
That would indicate problems in the power supply. Rectifier or caps.
//Bo.A long list...
Hi Guys,
I would love to help with this project, but you have all forgotten more about electronics than I have ever learned, so with nothing of value to add, I keep silent.
However as we have a collection of Beomaster 4400 experts on this forum I wonder if I can ask a BM 4400 question?
I assume that I am using the AFC incorrectly as with my Beomaster 4400 with all the pre-set stations set accurately turning the AFC on “improves” the tuning accuracy on some stations, but knocks the distant weaker stations out of tune.
Yet I find that if I turn the AFC off, select a station first and THEN turn the AFC on, the distant station is properly tuned with the stereo lamp lit.
If I got it right, however belatedly, and this is what I should be doing, great, if not kindly educate me on how to use this feature properly.
I don’t recall ever having this “problem” with either my Beomaster 2400 or Beomaster 4500. Tune the stations for the pre-sets, turn on the AFC and you’re good to go. But not on my Beomaster 4400.
Back to you Jacques, good luck with your interesting issue!
It seems to me that this is normal behaviour. With AFC on, if no aerial is connected, it can't catch any stations at all, tuning lights remaining dark. Muting also does its job here.
With AFC disengaged, it becomes possible to get nearby stations with no aerial connected, just a bit of wire, with tuning lights and stereo beacon.
Back home.
Damn. I haven't been able to check anything, because the fault hasn't shown up today. I need the amp to do what it's been doing!
No troublesome solder joints anywhere. What if the relay itself suffered from a bad contact, thus not allowing + or - voltage to feed the amps, therefore triggering "itself"?
No, that's silly!
To rule out a bad rectifier, I decided to replace it altogether.
Here's how to replace the rectifier: there are access holes but you have to unscrew the fuse/selector holder to get to the left one. No need to take the transformer out!
Then the thing can be taken out quite easily.
New bridge. It actually has better specs! Technology has evolved... Insulating cardboard mandatory, as well as shorter screws, because the new rectifier is much thinner and you don't want the screws to damage the windings...
Voltage is slightly lower, by 0.2 V. Now the relay triggers at each switch-on.
Capacitors? Maybe, but I haven't got any to try out. And anyway, monitoring both + and - voltages at switch-on show nothing odd.
Any DC on the amplifier center rails (the output before the relay) ?
Dillen: Any DC on the amplifier center rails (the output before the relay) ?
There is 1.47 V at both amp speaker outputs in protection mode.
The reservoir caps are good to me, unplugged from the amps, they do keep their charge and - and + keep their correct values in time.
Thanks for coming aboard Martin.
The relay is fine, I can see voltages slowly and symmetrically decrease after I switch off the amp. The new rectifier has made this fault permanent it seems. Or has it?
I have now tested the reservoir caps. They're okay really. Low ESR and about 9500 µF.
I think I've found a bad MPSH54 (TR115) which I can swear tested okay . Is a BC556 okay here - inverted pinout aside?
Nope it doesn't work so well. I've got 30 mV offset, even if the amp now works.
2N 3906 : 25 mV, still not acceptable.
chartz: Hi Jeff, It seems to me that this is normal behaviour. With AFC on, if no aerial is connected, it can't catch any stations at all, tuning lights remaining dark. Muting also does its job here. With AFC disengaged, it becomes possible to get nearby stations with no aerial connected, just a bit of wire, with tuning lights and stereo beacon.
Oh how I admire both your knowledge and courage to just jump in, pull things apart, then put it all back together….. and with a good result.
I have no background for this and this late in life “catch-up” is not especially effective.
I need to do a complete lubrication on my Tandberg 9000X reel-to-reel which has not be lubed since 1971 and I am all stressed about doing this today. However my tape forum has convinced me that my deck is in desperate need of lubrication and that it is nothing short of a miracle that it works at all….. so gotta be done.
Well that and the astounding difference the 50+ point lubrication made on my Seeburg M100B jukebox. Those old jukebox mechanisms were designed to push on no matter what, BUT with proper lubrication everything just zips along silently.
Anyway, I am way off subject (about normal for me) but back to the Beomaster 4400 FM section question, I am using what was billed as a über Hi-Tech FM antenna and the Beomaster 4400 FM section is powerful enough to bring in most local stations sans antenna.
My question was concerning the proper use of the AFC switch. If I just leave it on the AFC improves the reception on the local stations BUT ruins the reception on distant stations… and as you guys know I like listening to KING-FM Seattle, which is 120KM away.
With the AFC on, KING-FM is knocked out to only static, turn the AFC off and then back on again and it is perfect.
So I am thinking that I should turn the AFC off before going from one station to another and once I have decided on one, THEN turn the AFC on. Does that make sense?
Finding an intermittent fault is much more exciting than tuning a radio, but since you guys really know, so I figured why not ask while you are bearvering away at Jacques’ problem.
Piaf: Well that and the astounding difference the 50+ point lubrication made on my Seeburg M100B jukebox. Those old jukebox mechanisms were designed to push on no matter what, BUT with proper lubrication everything just zips along silently.
Mmm... Are you trying to get an answer from Martin, by any chance?
I've located MPSH54 transistors in Begium. Very hard to come by it seems.
Here's the little rascal, TR115. Here 2N3906, waiting for the real McCoy.
Temporary conclusion: two faulty transistors - one being intermittent, the other leaky , and the protection circuit works!
Jacques,
While I am pretty sure Martin can fix nearly anything electronic on the plant, I would not be so presumptuous as to assume he was familiar with 62 year old American tube powered jukeboxes.
Of course, I maybe wrong.
New v. old. The NOS ones appear to be genuine Motorolas!
Why those semiconductors fail is one of the greatest mysteries of the universe... After women of course...
chartz: New v. old. The NOS ones appear to be genuine Motorolas!
the old one seems also to be an original Motorola one . It's got the 'M' sign just before the MPS.
chartz: Why those semiconductors fail is one of the greatest mysteries of the universe... After women of course...
And i think noone will ever get behind this mystery of the women
Those failing semiconductors can surely be explained by bad moon rising, factory failure while sealing the chip or just '*** happens'. At least the transistor is nearly 40 years old. There may slowly come signs of wear
Nevertheless good luck with those hopefully last repair on your Beomaster 4400 for the next years!
chartz:Why those semiconductors fail is one of the greatest mysteries of the universe... After women of course...
A great deal of research has been put into both - and for the former, most of it has at least been scientific research!
I took some courses on electronics reliability at the Uni and have forgotten most of it, but the main reasons for semiconductor failures (apart from misuse and mishandling) are moisture creeping in through the leg openings or the sealing of the case and metal migration - the tendency of metal to actually shift along with electric current over the years. This can attack the tiny bonding wires from the chip to the case legs, or in severe cases even bigger structures. Simplified, even the semiconductors can actually wear in use.
--mika
Good!
With proper MPSH54 in place, offset is now a normal 10 mV.
Why does the BC556, or the 2N3906 for that matter, produce a higher offset - 30 mV and 25 mV respectively - I can't reckon, but nothing replaces the original it seems.
I can supply the address if anyone's interested in those transistors.
And yes, I do hope this is the final repair!