Hi Martin, thanks so much for the advice, much appreciated.

And well noted to keep power off until changes you suggested are complete.

• I still need to pick up some of the technical jargon, assuming the emitter resistors are the ones below numbered 680 & 678?
• Is the whitish band in the middle an indication that they are toast?
• Output stage transistors are the ones underneath on the chassis ie the 2N5034's ?

Thanks again

Anton

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Yes to all three bullets.

And the idle current trimmer (potentiometer) is the one in the center of the photo with yellow and orange pins (the new component will not have this).
Its correct setting is in the servicemanual. You can use the Type 2316/2317 servicemanual for this.

Martin

Dillen wrote:

Too high idle current is my guess - probably caused by a bad idle current trimmer. Don't power up again until it has been fixed; Replace the trimmer, the emitter resistors and check the output stage transistors.

The right channel will need the same treatment more or less.
And new capacitors in general will solve many problems.

So I got busy over the weekend and replaced the output transmitters, trimmers and recapped most of the main board.

Results:

30V Stable voltage has dropped from 32V to 27.3V measured at point J (see pic below) - however adjusting new trimmer makes no difference.

Left channel still does nothing but hum - it is much softer though, barely audible. Hum does not get louder if you adjust volume.

Left idle current has come down a lot but still sitting at 295mV - adjusting new trimmer makes no difference.

Right channel has come back to life and is playing music (YES! 😁) and have managed to adjust idle current to 24mV.

Bottom of unit in area of output transistors and heatsink are running much cooler, at least you can touch it now without risking injury.

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Do you have a DC reading on the output rail (before the output series capacitor)?
Check the driver transistors and all resistors nearby,

Martin

Dillen wrote:

DC reading on the output rail

Copious amounts of googling left me with no idea where/what the output rail is - going to need a bit of help with finding that. For what its worth I am picking up DC readings in the 27-29V range on the output resistors on the left channel.

• With regards to transistors, from my understanding they cannot be checked on the board, they have to be unsoldered and checked with a meter that that has a socket for transistors. Is this correct? No quick check?

Also picked up the following

• There is a 15mV reading between chassis and power capacitor negative - is this leakage normal?
• There is a 28V reading on the copper heatsinks for the left and right channel transistor, is that normal?

Thanks in advance

Anton

The output rail is the center "lead" running through the push-pull stages of the ampilfier.
In your Beomaster it will be the negative pins of the large output series capacitors, left and right respectively.
round -15V DC will be right.
Do a few DC voltage measurements around the output stage and driver transistors and compare the working and the bad channel.
If you find something that differs let us know.

In some cases it's possible to diagnose transistors in circuit, but the best thing is always to take them out.

"There is a 15mV reading between chassis and power capacitor negative - is this leakage normal?"
- Assuming you mean the power (filter) capacitor POSITIVE, because this Beomaster has positive chassis.
It's so little, it could be your test leads or meter.
No worries here.

Martin

Dillen wrote:

DC voltage measurements around the output stage and driver transistors

Apologies Martin, I did muddle up the filter capacitor positive and negative. Positive has the 15mV reading, Negative is 39.6V.

Output rail DC measurements at output series capacitor negative pin.

Left channel : 0.75V 

Right channel : 13.8V

Output stage transistors

Left channel: 0.756V

Right channel : 13.82V

Other measurements:

400uF x 40w (Replaced with 440uF x 65w) capacitor on negative pin 28.18V

Left and Right channel speaker outputs at pins bounces around 3.95-4.12VAC

All readings taken with volume on nil and balance centered, no input selected.

Points to a problem with one of the output stage transistors or, perhaps most likely, one of the driver transistors.
Can you do some measurements around the driver transistors?
Surely something in that area is wrong.

Martin

Dillen wrote:

Can you do some measurements around the driver transistors?

Hi Martin

I've done measurements around the driver transistors and may have found the culprit.

It was a process of trail and error between left (not working) and right (working). All measurements were taken with transistors on board and not desoldered so readings like resistance are probably skewed. I started with continuity testing until I stumbled across one transistor (U2734) on the left channel (that gave perfect continuity) and then moved to resistance and voltage measurements. Below is my measurement comparison left to right.

Clearly this transistor has issues like a dead short or something, there is very little resistance compared to right channel and high negative voltage between pins E-B compared to the right channel.

As for a replacement I'm at a bit of a loss, seems like this U2734 transistor came from some esoteric/exotic/boutique B&O nirvana. Can't even get a spec on it but saw that BC141 is listed as similar. Would this work as replacement? I assume that whatever goes in there has to be changed on the right hand channel as well. Spec at bottom of this post.

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BC141 Silicon NPN Transistor similar to U2734, see note
Uce/Ucb: 60/100V Ic: 1A β (Ic/Ib): 63-250 N: 3.7W F: ≥50MHz Tmax: 175°C

Update

The left channel has sound again ! 🤩

First my thanks to Martin, without his help I'm very doubtful I would have found the problem so quickly.

It was indeed the left channel transistor U2734  (hard to find specs on) that led me to a similar transistor BC141 (see previous post for spec of BC141)

While reading up on that discovered "paired" transistors and figured that the paired match for U2734 would be U2735 and closest match to that is BC161.

Was overjoyed to find stock of both at my local in Cape Town although the sales assistant did smile and said that stock had been lying there for years.

Replaced U2734 with BC141-16 (you have to desolder the heatsink with the capacitor) and fired her up and had sound but very distorted.

Next replaced U2735 with BC161-16 and magic - both channels were working.

This also cured the huge heat buildup on the output transistors and the BM is running a lot cooler now. As in warm to touch, not scalding hot.

So the old girl is now recapped on most of the electrolytic capacitors and firing on both cylinders. Yaay.

Will address the next set of issues in another post.