Received an assortment of  "trimmers' today and after doing a simple word search for Trimmers on this forum I've discovered that setting them to a 'minimum' is a good way to adjust the 'no-load' current.

Is that minimum resistance towards R211 or R213 ?

Apologies if this seems obvious but I haven't a clue😠

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Well well well it looks like B&O printed incorrect capacitor polarity markings on this PCB. (c108/208 '5000uf')

I took the originals off (4700uf) to replace them and foolishly trusted the markings on the PCB !  I've only just discovered that I replaced those two biggies on the board the wrong way round! - The evidence is in the old leads on the 4700uf caps which i removed and in the circuit diagram, maybe thats why everything got a little hot ?

I'll find out later when I turn this thing on... probably for the last time before I ceremonially smash it to pieces with a large rock 😮

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Pete wrote:

Well well well it looks like B&O printed incorrect polarity markings on this PCB.

Yes, good catch. The screenprint is wrong.
Negative must be connected to the coils L100/L200. The layout drawing in the service manual does show the correct polarity.

Pete wrote:

Received an assortment of  "trimmers' today and after doing a simple word search for Trimmers on this forum I've discovered that setting them to a 'minimum' is a good way to adjust the 'no-load' current.

😠 I'm afraid you may be drawing the wrong conclusion. It's true that if you turn the trimmer to a minimum (=max counterclockwise position, i.e. turning it towards R113) you will get the smallest voltage on the base of IC100 but that alone doesn't quite guarantee the smallest bias current on the output transistors! But to be honest, it's not immediately clear to me what IC 100 is doing to the output transistors.

If I were you, I'd measure the setting of the old trimmers and use that as the base setting for the new trimmers at first power-up.

manfy

Thank you!

Of the two 1k VR's removed; one is fried and the other is reading 1.1k with 0.6K towards R113 0.5k towards R111 so I'll set the values and see what happens, one channel powered up at a time.

Below is what the service manual says regarding components near IC100

Thanks again

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manfy wrote:

😠 I'm afraid you may be drawing the wrong conclusion. It's true that if you turn the trimmer to a minimum (=max counterclockwise position, i.e. turning it towards R113) you will get the smallest voltage on the base of IC100 but that alone doesn't quite guarantee the smallest bias current on the output transistors! But to be honest, it's not immediately clear to me what IC 100 is doing to the output transistors.

Turning the trimmer towards R113 would take the idle to max as it would drive IC100 off.
To start with minimum idle current IC100 must be on = trimmer slider moved towards R111.
IC100 is clamping the two bases of the output transistors, and it's an NPN type.

Martin   

Thanks for this, but naah, I still can't see intuitively how IC100 sets the idle current of the output transistors. It balances the base voltage of the output transistors and I guess by doing so it indirectly sets their working points and idle currents.
Anyway, never mind. The circuitry does work in principle or else it wouldn't have been realized that way.

The function of TR102 is very clear with this description and thinking of your initial description of the problem, I'd carefully look at that transistor. If this one is shot, i.e. shorted from C to E it certainly would explain why your output resistors R121/122 go up in smoke immediately after power-on.
Please note that the description in the service manual is just a conceptual explanation! If you look at the circuit diagram you see that 21R1 is fed by +50V and not +34V!! And that in turn would nicely explain why IC100 got shot and why all those resistors got fried.

In your first post you asked about TE1088 vs BDX33C. Don't worry about it. The service manual shows that B&O used 3 alternatives for that darlington: BDX33A, TEO1088 and FJ3001.
So, BDX33C is perfectly fine. The only difference between A, B, C, D type is max Vce of 60, 80, 100, 120V respectively.

Do you have a DC power supply that goes up to 50V?? If so, the output amp can be tested nicely on its own: Leave input and output disconnected and slowly increase the supply voltage while watching the current drawn. This way you can measure various points on your circuit at a lower voltage that is non-lethal to the components. This allows you to assess what's wrong without sending additional components into nirvana.

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Cross-posted with Dillen. He's a professional with hands-on experience in B&O equipment, so his word can be trusted! 🙂

Do you have a DC power supply that goes up to 50V?? ...

No I do not but I have a Manson 3-15v supply and connected it up briefly. It drew 4amps at 5Volts ! Needless to say the R221/2 went hot! I suspect I previously killed the BDX's (by putting C2 the wrong way round) there is no resistance between collector and emitter on both BDX's with the board un powered. I'm getting 3k and 6k resistance between the C and E on the TR102/202's - haven't taken anything off the board yet.. 

Not sure what to try next😳

Pete wrote:

No I do not but I have a Manson 3-15v supply and connected it up briefly. It drew 4amps at 5Volts ! Needless to say the R221/2 went hot! I suspect I previously killed the BDX's (by putting C2 the wrong way round) there is no resistance between collector and emitter on both BDX's with the board un powered. I'm getting 3k and 6k resistance between the C and E on the TR102/202's - haven't taken anything off the board yet.. 

Believe it or not, that's actually a good thing to see such a high current at such a low voltage: It shows that you have a major problem with one amp and that is usually quite easy to find - as opposed to intermittend problems for instance.
Your 3-15V supply is sufficient for that.

First to your caps C108/208: You said the speakers were disconnected during first powerup. That's good. The caps were "almost free floating" except for R106 and R107 that connected them to GND. That makes good 5 kiloohm, so with a bit of luck you didn't seriously damage those caps. (I trust you measured their capacitance ok when you reassembled them the right way, right?!).
Since the speakers were not connected, those caps mounted wrongly were certainly not the reason for the excess current.

So, it appears that one channel is working fine and the other has this overcurrent problem. That makes things simple because you can compare measurements from one channel to the other and if any value is substantially off, you can focus on the components in that path.
If the current is sufficiently low at 3V then you can measure and compare voltages. Usually you'd start around the transistors because they are most likely to fail.
Alternatively you could just compare resistance values at various test points without any need for a power supply. Such a high current suggests that something is shorted somewhere in the faulty amp.

thanks for your reply manfy

I'm somewhat confused at the high current at such a low voltage given that the B&O power supply throws out 50v and I also think both channels are now affected after previous tinkering. Originally one channel worked but then I replaced everything in the hope of a simple balanced result - with the capacitors the wrong way round and perhaps the board shorting somewhere on the case as I didn't nip off the component leads enough below the board - so stupid me!

I'll be removing one leg of most of the components to test their values over the next few days (when it's /raining outside!) and I have spent a further £12 for all Transistors/IC's on eBay (arriving Friday) - not a bad price compared to my first attempt via 'mouser' in the USA (-: So will write a result of resistances/uf's before attempting to replace anything out of kilter.

'Lockdown' here in the UK is good for these sort of things 👍😕