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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

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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 Fri, Jan 27 2017 6:54 AM

Thank you Christian!!!

I did try to be so careful when I made the heat sink. I even used thermal paste between the copper "cage" and the transistor. But I see, it didn't work. I was pretty sure myself that it would disperse heat enough, then perhaps it was shorted. I was thinking when I made it - "I hope the outer part of the transistor is properly insulated, otherwise I am screwed"..

So the AC128's actually get hot enough to need a proper heat sink. I wasn't sure of that either.

Even with Danish cookies and strong coffee, to remove that heat sink and fix it and put it back must have been lots of difficult work!

Looking forward very much to testing it and comparing it to the almost unrestored white Beomaster 1000 that I also have.

Christian Christensen
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. Sören, As we can buy danish coockies in Sweden, beoing is easier in this country compared to other countries Wink

My re-capped M75 are my precious diamonds.

Jeppe
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Jeppe replied on Fri, Jan 27 2017 10:56 AM

I am investigating the possibility to remake a few pieces of that front rail that holds the radio scale, between the keys, on the Beomaster 1000. It seems they often break. On one of mine it is broken, and I want to replace it. Now checking if they can be 3-D printed or molded. It seems it can be done.

Anyone here who is interested in having this spare part? In the photo is a typical broken rail.

Jeppe
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Answered (Not Verified) Jeppe replied on Tue, Feb 28 2017 2:40 PM

I am still working on my Beomaster 1000's..

Now I have to bring the black Beomaster back to life myself. Christian pointed out that this is actually a germanium version, although it is black. I thought it was silicon, so that I could compare them.. But a nice surprise sort of.

Now however I am left to try to fix this germanium equiped Beomaster myself. I have changed all the capacitors. When I started it up after the recap there was only a sound going "wowowowo.." from both channels and a thin smoke pillar from somewhere. I have checked that the capacitors are all properly put in place. I have checked what I believe is the vital resistors, and the zener. I suspect a transistor in the power stage somewhere. It is probably the same fault as the earlier white BM1000 (which is now under Christians care), which had blown a AC128.

Now the question is - how do you search for faulty germanium transistors?
I know how to measure silicon ones. But these germanium transistors give the oddest readings, they are all over the place and I don't know how to check if they are alright or not!

How do you guys do it on your germanium machines?Confused

tournedos
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Jeppe:
Now the question is - how do you search for faulty germanium transistors?
I know how to measure silicon ones. But these germanium transistors give the oddest readings, they are all over the place and I don't know how to check if they are alright or not!

How do you guys do it on your germanium machines?Confused

Germanium transistors are quite "bad" compared to silicon transistors - for example, they may have a large reverse leakage current that can mess up simple multimeter diode tests. Regular cheap multimeters with a hFE range may fail as well.

The best option would be a real transistor tester, if you do enough work to justify the cost - I haven't looked in a while, but I would expect that you could find a cheap but useable one from China nowadays. Or perhaps even build a small test bench:

http://tagboardeffects.blogspot.fi/2012/08/germanium-transistor-tester.html

(theory for this more extensively here:
http://www.geofex.com/Article_Folders/ffselect.htm
)

--mika

Jeppe
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Jeppe replied on Wed, Mar 1 2017 8:40 AM

Thanks Mika!
It might be a good idea to get me a transistor tester, for example the Peak DCA55 mentioned in the link you sent. It seems it tests germanium as well.

And they can be tested in-circuit?

I am determined to get this BM1000 in working order..

tournedos
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Jeppe:
And they can be tested in-circuit?

Not really. And be careful when (de)soldering Ge transistors, they are very sensitive to heat. That's why the leads are often left quite long, and the old advice was to hold the wires with pliers while soldering so that most of the heat doesn't reach the actual component.

That Peak instrument looks nice, I might get myself one.

--mika

Jeppe
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Jeppe replied on Wed, Mar 1 2017 9:44 AM

Oh dear, they must be desoldered to be checked?Erm..

Addition: I got an idea from youtube, it may be enough to desolder them to test them, you may not actually have to remove them from the board if you can desolder cleanly. This is an advantage as some of them sit in heatsinks and it means quite a lot of work to remove it all.

tournedos
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Well yes, and of course it is enough to disconnect just two leads (but remember that one of them is usually connected to the transistor case if it is metal).

Lot of work, but if you find a failed transistor, you would've had to remove it anyway. And some brain work (and an oscilloscope) will usually help to isolate the problem areas, so you don't have to check every component. You'll learn more as you go and every job will be easier in the future!

--mika

Jeppe
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Jeppe replied on Thu, Mar 2 2017 11:28 AM

Thanks Mika.
I ordered the Peak Instrument. No oscilloscope yet, perhaps later.
Although I am still a novice I have learnt tremendeously since my first post. Thanks for all your help so far!

Jeppe
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Jeppe replied on Sat, Mar 11 2017 12:04 PM

The restoration is moving ahead slowly.. I haven't given up yet.

Is there anyone here with knowledge and time enough, who feels compelled to delve into these difficult topics below: Geeked

1) How do you adjust voltage?

2) Has anyone experimented with altering the loudness via changing the values of that capacitor we talked about earlier? It is supposed to be located on the volume potentiometer, or close by, at the Beomaster 1000. But this is in general an interesting idea. If loudness could be altered, that is a very interesting thing. Many 70-ies amplifiers have loudness, and then perhaps we can even have different loudness settings, like different loudness strength, if we modify it in this way..? There could be a loudness switch - min, medium and max.. Why didn't anyone think of that back in those days?

Update:
Voltage adjustment is complex, I understand it's not easy to give a concise answer to that question. However, I am researching it.
I understand that you measure at the points J, K and I (or L on some models) in the schematics. Values shall be, depending on model:
I 60 mA, K 150mA/30V, L 9 mA (from manual 2316-2317)
I 60 mA, J 150 mA/30V, K 9 mA (from unknown manual, similar to my germanium version)
The potentiometers are easy to find.

Can't believe I'm actually attempting to do these things..
Any final advice before I go ahead..? So I don't completely ruin it..

Jeppe
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Jeppe replied on Thu, Mar 16 2017 5:18 PM

My quest for a functioning Beomaster 1000 goes on. I never seem to give up, in spite of constant setbacks and about as much knowledge in electronics as a horse..

Today I took on the challenge of checking my black germanium BM1000 for faulty components. The problem with it was that after supposedly working (in another persons workshop) I myself changed all capacitors and when connecting it there was a pillar of smoke and the sound "woowoowoo..". I checked all my new caps, ok (not reversed). I now started with checking the AD149's, all ok, including the number 5.

A resistor in one of the channels, numbered 742 (22 ohm) between AC127 and AC132, was the one giving up smoke like an incense burner in a buddhist temple. The other channel's corresponding resistor is ok. Funny thing that when I measure them (on the board) they both give proper values. How about measuring resistors on board by the way?

Another odd thing, there is supposed to be a zener diode here, numbered 908. Where this is supposed to be I found something looking like a resistor, a 780 MOhm resistor. See my photo. Isn't this strange? Or do diodes look this much like resistors?

Does any of you fellow infectees have any clues to the mysteries above, and perhaps a kind hint of where I should turn my multimeter next?



Dillen
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Dillen replied on Thu, Mar 16 2017 5:44 PM

This Beomaster 1000 must be a Type 2313 or 2314 then.

Resistor 742 (22 Ohm) is the emitter resistor for one of the driver transistors in the left channel output stage.
A smoking resistor is not a sign that the resistor was the reason for the smoke. It's a sign that something
else is amiss causing a too high current flow through the resistor. The resistor could still be fine but
I wouldn't trust it. It's a cheap component and I would replace it regardless.

When 742 starts smoking, it's because transistor AC132-2 draws has a too high emitter current. Either because the
transistor itself is bad, something is driving it too hard, something has shortened or gone open circuit nearby,
or the supply voltage is all wrong.

Did you set the idle current right?
Has the NTC broken?
Is the 2A fuse for this channel OK?

Ideally you should never measure any component in circuit, your multimeter doesn't know which of the components
it connects to in the circuit it has to measure - it just measures.

And yes, that's a zenerdiode.

Martin

Jeppe
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Jeppe replied on Thu, Mar 16 2017 6:12 PM

Martin, thanks for a super quick answer!

I did suspect that the resistor itself was not the problem, so it will not be so easy as to replace it I figured. And that made me distressed, where should I look next..Confused I figured that maybe a transistor in that couple was bad or something was driving it too hard. But unsure of how to find that.

I haven't adjusted the voltage, but Christian probably did it when he had a look at it. I am uncomfortable connecting it now and adjusting the voltage. Would you recommend that? Things get fried perhaps.. But if I don't check and replace for example a transistor, it may just be fried again..

Not that I am totally comfortable adjusting voltage either, but I practiced the process yesterday on a working BM1000 so I think I can do it. Measure points is on the emitter on the AD149's II and IV if I understand correctly. The DC potential at point J may also be important?

Not checked NTC's. Actually I don't understand what use they have in the circuit.

Fuses are ok. The resistor-looking zener will be left alone. Or maybe I should desolder and check it.

Hmm

I know you shouldn't measure components in circuit, but sometimes I did it anyway just to see what happens and sometimes it gives sensible values, but quite often not.. Desoldering all resistors and other components is too much work so I guess you guys zoom in the likely problem area and test the most likely suspects and work your way down.

I bought this Beomaster for 190 swedish kronor, or about 20 euro. It was supposed to have problems, distorsion, with one channel.

My immense gratitude to you guys who help and lead me thru this!

Dillen
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Dillen replied on Thu, Mar 16 2017 7:10 PM

The NTC (Negative Temperature Coefficient) resistor works in the idle current control circuit (together with the trimmer).
Transistors HFE (current amplification) increases as they warm up, so if nothing is done to control it, it would
run astray and end in a melt down.
The NTC assumes a smaller and smaller resistance as it warms up, this is used to close the drivers more and
more as temp. rises (and vice versa).

Martin

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