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Old 23rd May 2024, 6:55 pm   #1
HatOfTheCat
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Default B&O Beomaster - "floating signal" in one channel

I'm refurbishing a Bang & Olufsen Beomaster 1100 receiver. I've first replaced ALL the electrolytics in this unit because several of the plastic cases had already burst open and revealed their innards (several others were on their knees when tested) and also a repaired fault in the 33v supply so am now testing with a 1Khz signal from my pocket oscilloscope into both LEFT and RIGHT channels. The RIGHT looks fine right through the output of the power amp stage, BUT on the LEFT side the signal on my scope is acting like it has a floating ground Waveform is perfectly shaped but its jittering and floating about vertically. I've done dozens of tests across both channels (the scope ground clamped to same bit of the chassis for all) and its clear this "floating" seems not to be present at the negative side of C128, see attached schematic, but is definitely there at base of transistor TR105 (BC253B) and progressing further through the stage its there and by output side of the filter capacitor its pretty wild. I'm powering via "dim bulb" for now, have 8 Ohm power resistors connected to speaker outputs as a load. So far I have tried changing TR105 for a new transistor with no effect and just now also removed and tested the 3 small capacitors (C130, C131 & C133) all tested okay. I have couple of new like for like replacements for TR106 (BC171B) if that is the fault. Suggestions as to what I should be testing or looking for here would be gratefully received. Am just working my way testing the resistors for now.
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Old 23rd May 2024, 7:29 pm   #2
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Default Re: B&O Beomaster - "floating signal" in one channel

Have you looked at both the supply voltage and also the ground points with the scope?

You should see 'nothing' on those 8 ground points on your diagram.
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Old 23rd May 2024, 9:20 pm   #3
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Default Re: B&O Beomaster - "floating signal" in one channel

Mooly - no signal found with scope at any of the GND points. The 48v power rail is common to both channels so I'd expect problem on both if was that but I will check out in next couple of days as have just packed up for the evening. I've also reflowed all the joints on this part of the board in case of a dry joint even though I couldn't find any or a continuity problem in the tracks.

I've put up a vid on Youtube here of what I'm seeing on the scope for RIGHT vs LEFT channel https://youtu.be/sPDvXPb61JY

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Old 24th May 2024, 6:39 am   #4
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Default Re: B&O Beomaster - "floating signal" in one channel

That looks a very low frequency disturbance. Why just one channel and not both I can't really say unless its being pushed over the edge by a higher bias current in one channel vs the other and so draws more current under load on that channel.

I would try it with a much higher load impedance (say 100 ohm) as a test and provided there are no other worries try it on full mains without the DBT. If there are doubts then turn the bias down on that channel first.

In fact you could turn the bias down on both channels first as a test and see if the issue resolves even with the DBT.

So there may actually be nothing wrong here and its just an artefact of the bulb and supply variation getting the better of it.

Also while it is bouncing around like that you could check the supply to that channel on the tops of R146 and R152 and see if the supply is changing... I think it will be.
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Old 24th May 2024, 7:06 am   #5
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Default Re: B&O Beomaster - "floating signal" in one channel

The output of that amplifier should be floating at DC, otherwise C135 and Ca36 aren't doing their jobs. Defining the DC level on the output and also charging C135 at turn-on depends on there being a resistive path to chassis external to the amplifier.

But one channel is doing something odd, and the other channel isn't, so there is something to track down. Because of the AC coupling in the output of these single-supply amplifiers, it is better to probe the junction of R158-R159 this helpfully has a voltage marked on the schematic. Probing here, you'll not lose the DC and very low frequency information which could give useful clues.

Notice C132 in the feedback network. This acts as close to a short at audio frequencies so that R151/R149 sets the closed-loop gain of the whole power amp. But at DC, C132 applies unattenuated feedback. For DC wandering to be slow, there needs to be a slow variation in something making noise, OR a long time-constant filter removing the faster variations.

Pocket oscilloscopes are usually a bit too simplified in their inputs and you may not have a real, switchable AC coupling capacitor. Many of them use software to remove DC when you want to see an AC component sitting on a DC offset. This is limited in its utility by ADC resolution being wasted by having to be scaled to cover the larger DC component. So you have to use these things with a bit of care.

Overall, I think I might suspect TR105 as the most suspicious component. Some plastic-cased transistors suffer internal corrosion over the long term and can go noisy. Otherwise it could be any of the other transistors and various resistors. If you get desperate, these are all cheap components so just working round replacing things is not actually a bad approach. This isn't a blown-up amplifier where you have to forensically trace the sequence of destruction to find the cause. It's working enough so you can look at one thing at a time.

On further thoughts, R146, R147 are highish values and set the mean DC level input to the amplifier which sets all the DC conditions through the amplifier. C129 filters this DC so we have a potential low frequency noise source in the input bias resistors along with a filter to make it slow.

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Old 24th May 2024, 7:01 pm   #6
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Default Re: B&O Beomaster - "floating signal" in one channel

I am struggling to understand what the DC levels should be.

R146, R147 and R148 form a potential divider. If the supply is 33 V (and neglecting the base current of TR105) the base would be about 12.4 V from ground, putting the emitter at about 13 V. Base current would make it slightly lower.

The 100% -ve feedback for DC from the +ve side of C135 to the emitter of TR105 should bring that point to the same 13 V, whereas it surely ought to be about half the supply, 16.5 V. And it's marked on the circuit diagram as 22 V.
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Old 24th May 2024, 7:21 pm   #7
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Default Re: B&O Beomaster - "floating signal" in one channel

Is the scope signal (left channel) still noisy with no input?
Is C129 doing its job of decoupling the base bias?
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Old 24th May 2024, 7:32 pm   #8
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Default Re: B&O Beomaster - "floating signal" in one channel

Yes, THat 22v marking implies a roughly 44v supply.

There could well be an asymmetry in how far positive versus how far negative the output can swing, well , yoou know what I mean by negative. The VAS gets an added boost in the positive direction from the bootstrap, but the VAS can't do anything in the -ve going direction. But 12.4v on the base implies 13.1 at the emitter of the input transistor. So 16v at the output looks odd.

BUT the base and collector current of TR105 all has to run through R151, and that's 8.2k

A 3v positive direction offset would be caused by 3/8200 = 0.4mA

R150 1.5k has 0.7v across it so 0.7/1500 = 0.5mA so things do fit EXCEPT for the marked voltage numbers.

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Old 24th May 2024, 8:33 pm   #9
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Default Re: B&O Beomaster - "floating signal" in one channel

Thanks for the various replies. Just to answer a few of the queries and let you know where I am now.

- the supply for this section is 48V (the supply to preamp & tone section is 33V, that supply had a cooked 1k 1W resistor, board was very blackened but resistor was still hanging on though measured at 14K and the transistor right by it was begging to be put out of its misery, that all works OK now and is actually powered off the 48v rail right by the RIGHT channel which is the one working ok)
- as I said ALL the electrolytics were replaced before first powering up and finding this fault EXCEPT for the 3000uF filter caps and replacing them also made no difference.
- my first suspect was TR105 (BC253B) and replacing with like for like made no difference
- small caps C130, C131 and C133 replaced these and again no difference, none of the old ones measured out of spec out of circuit
- TR106 (BC171B) replaced with like for like, again no difference
- resistors R146 to R154, R156 & R157 all tested in circuit and/or out and none found out of spec - I have plenty spares for all of these so can just replace anyway
- diodes D102 to D105 all tested ok in circuit
- I've reflowed all the joints in this section (except output transistors) and made no difference

Obvious next steps:

- check C129 is doing its job (is new but defo worth checking this and around it)
- resistor R160 to be tested/replaced
- capacitor C136 to be tested/replaced
- bias transistor IC101(SPS5418), I have some MPSA13 which looks like a good alternative so can replace with that, its a bit of a pain replacing this one as its smothered in thermal paste to thermally couple it to one of the output transistors

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Old 25th May 2024, 8:49 pm   #10
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Default Re: B&O Beomaster - "floating signal" in one channel

FIXED

C129 was the problem I probed the junction between R146 and R147 and was getting about 8v which was mad as it was 29v at same point on the other channel. Removed C129 from board, tested it and it was fine so simply refitted it to the board and no more jumping signal.

Really annoying/confusing/strange as I had already reflowed its joints (together with the rest of this section of the board) with no change. I'd also even measured continuity from protruding pins under the board to next components/ground and couldn't find an open I'll just have to chalk that one up to "I'm and idiot who can't solder properly"

Now powered up direct without the current limiting dim bulb and all okay and sounds good. Next need to check out the FM radio section to see if all working properly (are 4 presets that could be iffy), really hope it is as last one was a total due to a gummed up bearing on the shaft of the tuning capacitor... total 'mare that one was.

Thanks for the steer to look at this part of the circuit from both Radio Wrangler and buggies

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Old 26th May 2024, 6:20 am   #11
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Default Re: B&O Beomaster - "floating signal" in one channel

Hmmm Well done of course.

Any chance there could be something gooey/conductive around there and the heat of the iron cleared it up?
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Old 26th May 2024, 8:20 am   #12
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Default Re: B&O Beomaster - "floating signal" in one channel

I'm not sure it wouldn't be a good idea just to replace C129 outright - it's cheap enough and in your shoes I'd regard it as suspect.
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Old 26th May 2024, 10:35 am   #13
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Default Re: B&O Beomaster - "floating signal" in one channel

Mooly - "something gooey/conductive around there" you could well be right, this pic was taken before I started working on the board and the original capacitors of which several had their plastic cases completely split open. The offending C126, 470uF in pic, was like all the others of this value (except two broken open already) had a crack right around the edge of the top and a line running down length of its case. I did clean up the board with IPA but there's still a varnish like patch around the new capacitor so that could have been issue and I just knocked off the bit that was touching in process of refitting the cap

Ted Kendall - I definitely would have ! ..except the only other new 470uF ones I have in hand are literally about 1mm too tall to fit once I put the bottom of the case back on ...yes, I ordered pack of 10 only to find they wouldn't fit so what's in there are second lot I ordered. These B&O receivers are pretty slimline so my own fault for not measuring just guessing. This like ALL the other electrolytics is a new one and I did test each one before installing but they are cheap enough as you say to just pop another one in and I'll be giving the unit a soak test over a few weeks anyway. If it goes out again I'll know... I plugged some throw away headphones in and it sounded like "thwacking" every 2/3rds of a second or so
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Old 26th May 2024, 10:58 am   #14
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Default Re: B&O Beomaster - "floating signal" in one channel

Thanks for the pictures... its possible isn't it. Those ROE caps are a replace on sight item at that age. Interesting. See how the soak test goes.
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Old 26th May 2024, 11:19 am   #15
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Default Re: B&O Beomaster - "floating signal" in one channel

The liquid inside electrolytic capacitors is designed to be highly conductive so only a little ln the loose can easily make the surface of your board conductive and affect higher impedance circuits like that input.

Why, you'll be wondering, doesn't it short out the capacitor it was once inside of? The answer is that electrolytic apacitors don't operate chemically like batteries, they have a solid dielectric in the form of aluminium oxide formed on the surface of one of the aluminium foils rolled up inside the capacitor. The oxidised aluminium foil is one of the electrodes. The other foil has had its oxide stripped-off by electrochemical 'forming' after the capacitor was assembled and sealed. The oxidised foil had been treated to give it a matte surface, greatly increasing its surface area viewed on the sort of scale set by its oxide thickness. This greatly magnifies the capacitance that can be made by that piece of foil. The oxide intimately contacts the surface of the matted foil, but leaves the problem of making the other electrode conform to the rough surface of the very thin oxide. This problem is solved by using a conductive jelly as the other electrode. Conductive liquids are nowhere near as conductive as metals, so the second foil is wound over a thin absorbent layer to act as a large area current collector. This foil is also matted to give it more working area for better connection to the liquid.

So, the conductive liquid DOES short to one of the foils, effectively becoming a conformant part of that electrode. but it does not short to the other foil, because that one is entirely covered in insulating oxide.

The conductive liquid has to be compatible with aluminium and not corrode it.

Aluminium in air reacts rapidly forming oxide on its surface. Aluminium is very reactive, but the oxide is hard and relatively inert. The capacitor electrolyte has one more trick up its sleeve. When a capacitor is made both foils are identical, both are oxidised a bit. The foils are tapes and they get wound up like a swiss roll but the jam in between them is done by two tapes of the seperator material, saturated with the electrolyte. What's built is no use as a capacitor - yet.

So the assembled capacitor is driven with a DC polarising voltage. The oxide on one foil is stripped off, while that on the other is built up. The conductive liquid is working as an alactrolyte in an electrochemical reaction acting to form the capacitor in the final act of its manufacture. Thereafter, it just works as a shape-following conductive electrode. The naked aluminium foil connects to the liquid and the liquid had better not be reactive with naked aluminium.

There's a lot of trade secret recipe stuff in designing that liquid, and some years ago there was an industrial espionage business where most of a secret recipe was stolen from some reputable manufacturers, but it seems they got an unfinished version minus some anti-corrosion additives.

Not rotting aluminium is important but it doesn't have to not attack any other metals, because all the metals inside are aluminium. Once the electrolyte bursts out of a capacitor it does tend to attack copper and iron thins like PCBs component legs and whathaveyou.

So you need to clean the affected board very thoroughly. The electrolyte is water based so IPA is effective, finishing with de-ionised water.

Aluminium electrolytic capacitors need a reliable DC bias on them or else the electrochemical forming process can run backwards, leaving a useless capacitor. This is called de-forming. Sometimes you can bring them back by applying voltage to re-form them, but it doesn't always work.

Good practice is to use aluminium electrolytic capacitors with about 10% of their rated voltage as a fixed bias. Experience and reliability tables say don't go over about 80%

Where a capacitor has let go, and someone has replaced it leaving a board still wet and powered it up, I've seen blue arcing in the liquid all over the board.... rather like Hollywood's idea of an electrical special effect weilded by some improbable super hero.

David
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Old 26th May 2024, 12:36 pm   #16
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Default Re: B&O Beomaster - "floating signal" in one channel

Radio Wrangler - I love the "secret recipe" By way of illustration here are a couple of the failed capacitors in state as found in this receiver, literally in bits. Apart from being a salient example of why NOT to just bang equipment of unknown condition straight on the mains they show the internal construction with the two slightly different foil strips... minus the long escaped important electrolyte with all its secrets
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Old 26th May 2024, 2:51 pm   #17
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Default Re: B&O Beomaster - "floating signal" in one channel

Quote:
Originally Posted by HatOfTheCat View Post
...failed capacitors in state as found in this receiver, literally in bits.
By the 1980's many earlier B&O products were often found to have ROE capacitors cracking or leaking electrolyte. I do recall Beomaster 3000, 4000, 6000 (the quadraphonic one), Beovision 6000 or any set with the 39xx chassis being adversely affected. Not an unusual problem by any means.

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