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Restoring a Beomaster 1000

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Jeppe
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Jeppe posted on Thu, Nov 17 2016 12:48 AM

This is my first post and I am almost completely new to BO! Having had some Tandberg and diverse Danish vintage equipment, I recently has become very interested in BO as well.

I recently bought two Beomaster 1000's. One with white keys and one with black. Both need restoration. I really want to make these work. I am specially interested in the earlier version with germanium transistors.

I opened the white one up today, and it seems a transistor was burned (marked AC128,3). There was black dust above it on the wood cabinet inside. I wonder if this is possible to replace..? Is there any hope?
There may be other problems as well, I don't know yet. But this seems serious.

Where do you find more rare spare parts for your vintage machines?

How wonderful that there was a little envelope on the inside with schematics inside it!


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chartz
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Answered (Verified) chartz replied on Thu, Nov 17 2016 6:25 PM
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Welcome to Beoworld!

And congratulations on your introduction to B&O's wonderful vintage products.

These Beomasters are nice items.

I for one always find the components I need on eBay. The aforementioned transistors are indeed listed there. Do have a look!

Those AC 128 germanium transistors are (were) quite common and not very difficult to find. There were tons of them used in old radios and tape-recorders.

Good luck!

Jacques

Søren Mexico
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When I start a new project, I go to Martin (Dillen on this forum) first of all, he can provide most of whats needed, then onto E-bay as Chartz

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

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Jeppe
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Jeppe replied on Thu, Mar 16 2017 8:03 PM

I see, very clever. Thanks!

One may suspect the regulator circuit. It is the problem both my BM1000's under repair have, probably, making them sound "wowowo.." in both channels.
On this one the AD149 IV is fine, so I suspect the two AC128's, the zener, or something in the vicinity..
But then there is also the burning resistor, which is suspect, isn't it? Either that is a result of a failing regulator circuit, or there is a failing regulator circuit AND a problem in the left channel..

Learning more and more every day..

In a case like this, is it safe to connect to 220V and recheck voltages?

Jeppe
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Jeppe replied on Sun, Mar 19 2017 5:56 PM

Next step today..

Removing the double heatsink with the AC128's. They tested ok.
Using the Peak Atlas tester that Mika recommended. Got one!

I was careful to avoid overheating the AC128's, as I have learnt that they don't like heat. I got myself Goot heat clips. Ordered them from Japan on ebay. Highly recommended!
I also put in a new Zener diode while I was at it.

Since these transistors tested ok (Nr 1: hfe=181, leakage 0,21mA. Nr 2: hfe=52, leakage 0,21mA) I went on to test the transistors AC127-2 and AC132-2, mounted on a heatsink in the channel where the smoking resistor is. They tested ok (AC132-2: hfe=85, leakage 0,66mA. AC127-2 hfe=57, leakage 0,77mA). At least that's what I think is ok. I also removed the smoked resistor, which tests ok but looks unhealthy. In the photo the legs of the transistors are not actually in the board so it is correctly measured although the heatsink wasn't removed.

I also checked and cleaned fuses, took away some corrosion. I resoldered an old odd looking potentiometer joint. And I checked what I believe is the NTC's on the smoking channel but they seem ok, although a more thorough check is necessary to know for sure.
I suspect someone has been in there and repaired many years ago, perhaps exchanged a transistor and NTC's (almost certainly) in the smoking channel.

All looked well today. Next step is to check the rectifier.
My suspicion is that there is something odd in the regulator circuit..

Jeppe
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Jeppe replied on Fri, Mar 24 2017 9:15 PM

Today I tried some new steps on the black Beomaster 1000 with germanium transistors.
I replaced the diode bridge. I checked another transistor (AC128 in schematics but AC152 in reality) in the smoking channel, but it was ok. I replaced the smoking resistor in this channel.
I also built a gadget for using when I powered it up, somewhere I read on the net about this - a recommendation to use resistors and a lower rated fuse, on the amplifier fuses, when powering up. I built this thing with a pot so that I could adjust the resistance from 0-400 ohm. If this was a good idea, this gadget, I am not sure, but I thought it made sense. However, when powering up (and removing the two channel fuses and using this gadget in their place) there was still smoke from the resistor (which was a new 2W 22 ohm resistor). I switched off, maxed the pot on the gadget to about 400 ohms, powered up and let the machine run a little longer this time, about 20 seconds, there was no smoke from that channels resistor, but then there was another tiny puff of smoke from somewhere else which I could not locate (seemed to come from around the bridge or so) and total darkness (lamps). I couldn't see any blown fuses.

So that's it, I believe my journey is about to end. I have no idea of what to do next and with my limited knowledge it's beyond me. I now have 4 Beomaster 1000's. If anyone is interested in taking them over, let me know. Two of them have been totally recapped with top class capacitors, to a cost of maybe 50 euro/machine. But none of them work. I give up.

Jeppe
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Jeppe replied on Sun, Mar 26 2017 4:55 PM

Not sure if anyone has patience or interest to follow my struggle with the Beomaster 1000.. This may develop into the most annoying thread here.. But I have myself learnt alot from similar threads to this one, so I am sure it is not solely for my own profit.

Today after some small checks I connected it again. There were lights and there was no smoke! But I had no sound, complete silence. There may possibly have been a short from a broken cable on the bottom side of the board.

Then I tried to check the measurements according to the service manual, points J, K and I. In this I was probably not very successful, I found a few hundred millivolts over AD149 I and II. I found nothing on the potentiometers 675 and 731. If one is supposed to measure directly on them I don't know but I think so. Experimenting a bit I found 36 volts over the AC part of the bridge but nothing on the single pot for DC potential..

But if anyone has the time and patience to give me some hints about how to measure these points, I will be very interested to hear. It says you should measure K with respect to ground, that is chassis I think, that should be rather straightforward, but how come I get zero? And point J, it's not so easy to know where to put the probes. And sometimes you are supposed to measure with respect to ground and sometimes not, how do you know?.

When one measures points like this from a schematics, does anyone have some advice? These are very important things to do correctly, and I think many amateurs dabbling in their own machines would be interested to know.

Søren Mexico
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All voltages specified are negative with respect to chassis, means chassis is +, but if you have a digital multimeter you will see - voltages if negative DMM is connected to chassis, be careful and select AC or DC when measuring, at point K you should get 30 V DC adjustable with trimmer 1K (906). If not search for failures before this point.

Without correct voltage at point K you cant do anything before this is established

All voltages on the drawing is negative in respect to chassis, measured with tape on, volume at "0", dont do anything to the FM tuner or stereo decoder before you have everything else working

Reading your manual and drawing, test points and how to is described there.READ YOUR MANUAL

 

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

Jeppe
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Answered (Not Verified) Jeppe replied on Wed, Apr 12 2017 11:02 PM
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Thanks Sören. No need to shout though.

I am keeping up my attempt with the Beomaster 1000. The learning curve has been steep, I am still a complete amateur but I have learned tremendously.

I found a shorted AC128 in the power circuit pair. Thanks to the Peak Atlas DC55. A new pair was delivered today, although I plan to replace only one. But with the rate they fry up now I will maybe need the other one soon too.

I also plan to replace all the resistors around them in the power regulation circuit. I was unsure what wattage to choose, so I took 5 watt resistors. My amateur calculations showed that perhaps the 2 watt ones were underdimensioned. I am probably completely wrong about this though. They were rather expensive, for being resistors.

Beo_Jean
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Hi Jeppe,

I'm contemplating this BM 1000 as my next restoration project.

How does it sound?  Did you get yours fully working?  Anything special to be cautious about?

Thanks!

Beo_Jean
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Hi Jeppe,

I'm contemplating this BM 1000 as my next restoration project.

How does it sound?  Did you get yours fully working?  Anything special to be cautious about?

Thanks!

Søren Mexico
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Hi Jean

here my thread, white key versions has germanium TRs, some early black key versions also I think, volume, tone control and balance poties are difficult to clean, my volume poti is still acting up, have a used one from Martin and will change it when time allows

I found 4 different schematics for these, so get the right one

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

Jeppe
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Jeppe replied on Tue, Apr 18 2017 12:43 PM

Hi Jean,
It has been a very interesting restoration project, I wish you best of luck. Maybe we can share our problems in this thread?
I am continuing with mine. Actually I have two BM1000 that I want to restore, but so far I am working on one. It goes slow because I only do it when I have spare time since this is a hobby.

I have another BM1000 which is almost unrestored and working (replaced the three largest caps), it sounds very nice with a pair of closed box speakers, warm and smooth.

It is a bit of hassle to disassemble it they say, and it could be, but I think this is not such a big problem. There are lots of cables underneath, I think I happened to melt the sheath on one with my soldering iron when replacing a capacitor, which caused a short, so be careful with that. I think it is quite straightforward otherwise. Yes, the germanium transistors are sensitive and prone to malfunction, but they are a part of the charm of this machine I think. I also bought a silicon version, but I haven't touched it yet..

Jeppe
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Answered (Not Verified) Jeppe replied on Thu, Apr 27 2017 4:07 PM

Today I took on the black key germanium BM1000 again.
I got new AC128's from ebay, and a bunch of resistors for the power section.
Before that it made no sound at all, and I found a shorted AC128 thanks to the Peak analyser.

I replaced both AC128's. It was extremely difficult, because there is little space and because the new transistors were smaller than the originals and I tried to use heatsink paste in the heatsink, it was messy.
The 5W resistors I ordered earlier was of course too large so I returned them and today I had some 2W versions instead. I think however it is unlikely that the resistors was the cause of the silence, and the ones I removed measured ok.

Todays operation had a very odd result, does anyone has any idea of what may be going on?
Before today there was only silence. Now there is a noisy hum and no music, however (music through line in) when switching the unit off music is heard for about 2 seconds, at weak volume.

How very odd, anyone has a clue? How can there be sound only when switching off and where should I look for a fault..?

Dillen
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Dillen replied on Thu, Apr 27 2017 7:20 PM

The power switch has a muting switch too.

Martin

Jeppe
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Answered (Not Verified) Jeppe replied on Thu, Apr 27 2017 8:02 PM
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Thanks for still reading this thread Dillen!

What does that mean, the power switch has a muting switch?

Today was a step ahead, from no sound at all to a humming noise and some 2 seconds of music after shutting off. That must mean something, is current too high somewhere and the power supply regulation is doing this.. I am thinking about it but can't find an answer yet.

I think the AC128's may be weird, again. Those transistors seem hopeless. I measured them before installing, hfe about 76, than one hour later it was 10. Then it happened that the Peak analyser couldn't decide which was C and which was E.
But again, it may be me screwing something else up.
However it made me think, can't we replace those unstable AC128's with silicon transistors instead? I for example found these NOS russian made KA837 transistors, which seem to have very similar specs to AC128's but are silicon. I wonder what would happen if I just put in those instead.

And how come in the schematics there is one AC126 and one AC128 in the power regulation circuit. But on the board it says just two AC128's. I put in two AC128's. What is one supposed to believe?

Today I was close to giving up, again. But then I can't stop thinking of it, I just can't give up. The amount of time and money I have spent on this machine is ridiculous.
Any help is very much appreciated!

Dillen
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Dillen replied on Thu, Apr 27 2017 10:11 PM

In late types the on/off knob operates not only the power switch but also a muting switch.
Is it working correctly?
Refer to the schematics and look into the Beomaster.

Germanium transistors are not particularly unstable. Not more than silicon.
And they are not that easily replaced by silicon transistors.
The bias (DC working point) is different - just to name the first obstacle.

Fitting an AC128 with a slim-line housing in a heatsink bracket for a standard housing, just filling the void
with  heatsink compound will not work.
The heatsink compound will not conduct heat on the scale needed. Far from.
Fit a suitable transistor or replace the heatsink bracket.

If the unit differs from the manual but appears untouched - believe what's in the unit.
If you are not sure, compare to a similar working unit.

The more you replace based on guesswork, the higher the risk of introducing more faults.
Don't get me wrong, but with the amount of work you've already done and components you've already
replaced, - some made better, some perhaps made worse - it makes
it next to impossible for us to take a guess at what could possibly be wrong.
The answer is most likely; A couple of things now.

If you have power supply problem - check the load for possible causes.
If/when the load is fine, repair the power supply.

Diagnose and repair.

Martin

Jeppe
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Jeppe replied on Thu, Apr 27 2017 11:31 PM

Thanks Dillen,
I am honored to get your advice.

One problem is that the schematics are a mess, I have only one silicon BM1000 schematics version, it is not always corresponding to my machines. In this schematics I see no muting switch. Perhaps there is one in a silicon version schematics I have, but it seems to be a fully mechanical switch. Isn't it unlikely that a mechanical switch would somehow reverse it's operation suddenly. If there is an electronic switch I have yet to locate it.

What do you mean with 'check the load'?

One possible problem could be that I haven't adjusted voltages. Maybe if the 30V DC potential, according to the service manual, is too high this could cause symptoms like this? Or the bias adjusments, maybe.
I think it is a very odd symptom that sound is heard only when shutting off power. It intrigues me. Come to think of it, I have a Radionette receiver which displayed the same symptom, it was dead but I could hear weak sound for a second when shutting off. Replacing all caps fixed the problem. For this BM1000 all the caps have been replaced already though. Could it really be the big cap in the power supply.. (not replaced by me, by a pro, looks new).

The AC128's heatsink did get very hot, by the way, not fryingly hot but quite hot to touch. Maybe that's normal.

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