Hi Peter,

yes, it's frustrating to have pages from the service manual missing and no, I don't have those pages either, I'm afraid. But you can download the MK3 manual, which is complete. The boards and component designations have changed a bit, but I don't think that B&O changed the circuit design substantially.

But first look at the power supply block diagram in your mk1 manual (page 2-9). There you find a truth table showing which voltage will be present in which mode.
In standby you should have +5V (one of the many) and some 12V on the +25-30V rail. That 5V (let's call it 5Vstby to avoid confusion) comes from IC5 and TR37 (and yes, you should also be able to measure it at the emitter of TR32).

That's where you should start your troubleshooting. Use the MK3 manual to get an idea of how the 5Vstby regulation circuit works.

rgds,

manfy

Hi Peter,

I do have the BS9000 MK1 type 2521...2530 service manual # 3538847 09-96 but...

The diagrams M and N are here on other page numbers than you mentioned.

I have attached my 2 pages related to the power supply

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and the power switching.

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the diagram named L in my manual is called "motor control and detection"...

HTH.

By the way - where in Germany are you located? North or South?

Greetz,

  hemenex

Hi manfy,

thanks for your help. I attached 5V from an external power supply and found that there is a short circuit on the standby voltage. After disconnecting the servo and other units the shorting stays - must be somewhere on PCB3.

Thanks so far

Peter

Hi hemenex,

thanks a lot for the schmatics - now i was able to locate the defect. I suppose it is diode D22.

Unfortunately it is placed in between the impedances L11 and L9 an difficult to unsolder.

I will post more information after removal success.

Thanks

Peter

p.s.: I am located in south of Germany, nearby Mannheim.

Hmm, what makes you think that D22 is shorted? It's a rather solid 3A, 40V freewheeling diode.
Granted, it may be under quite some stress in such a buck converter, but so is C97, the 10V, 8200uF electrolytic capacitor -- and e-caps are more likely to fail in SMPS circuits!

Thanks, hemenex, for the missing circuit diagram. That must be an updated/corrected version because we do have Diagram N on page 2-26. But ours shows more typos and different transistor models.

--------------------------

PS: I just found a picture of that board on the net. I see that the inductors and the large electrolytic caps are leaded components. That makes it easy: desolder them first and you have easy access to D22.
In any case, what I was trying to point out above was: Be sure what component is damaged before you wildly start desoldering and re-soldering SMT components. There are several other components on that 5Vstby rail that are equally likely to fail.

Another word of caution: This board also holds the microcontroller. Pay attention to ESD safety when handling that board and when doing the surgery. If the uP gets shot, you're dead in the water! 😂

(But ebay to the rescue: I found one guy who offers a used (but probably untested) replacement board for a mere USD650.- 😉  )

Hi manfy,

you are completely right - it also could be a defective capacitor around. I measured the resistance between these components and ground and found lowest value between D22 and groud - this was my reasoning...?

But desoldering of L9 is a good idea. Nevertheless, in MK2 schematic there is a capacitor parallel to the diode which is not there in MK1 - so i first have to check this.

Thanks

Peter

Peter wrote:

But desoldering of L9 is a good idea. Nevertheless, in MK2 schematic there is a capacitor parallel to the diode which is not there in MK1 - so i first have to check this.

Yes, I've noticed that too in the mk3 manual. There's nothing to check. This 470pF/100V cap is just filtering out the switching noise.
Much more interesting, though, is the fact that the Beo guys changed from an SS34 to an SS36, ie.from a 40V schottky to a 60V version !!!
If you find that it's really D22 that has died, we know they had a problem in all MK1 power supplies with voltage spikes caused by the L9/C97 circuit. I seriously doubt that this Schottky died of over-current since it can handle peaks up to 100A.
Additionally, if that diode died of excess voltage peaks, I can almost guarantee that C97 has taken a beating too! ....about 20 thousand beatings every second it was powered on to be exact. (considering that the DC/DC converter runs at approx 45kHz).

You catch my drift, right? Desolder C97 too and test it out of circuit. Capacitance and ESR - if you have the tools.

To round it off, I would guess that the Zener D30 is also a good candidate for failure. Even though it's outside the LC circuit, I'd guess it will participate in the freewheeling job that's done by D22.

D30 is what I wanted to point to as well. It is a Zener so at regular 5V it shouldn't do anything.

I think it's an overvoltage protection if for any reason the 5V line should go up higher. Seen this kind of protection elsewhere.

Switching current for the regulator is only D22's task...

I'm not sure it was neccessary to go up to the 60V Schottky; The higher the inverse voltage of schottkys the higher is their Vf.

I've been there as we had an assembler thinking he'd do us a favour by using a 100V instead of our 40V Schottky ending up getting hot. Vf was almost as high as the one of a standard silicon diode (more than 1V). Was in an switching regulator, too.

hemenex wrote:

D30 is what I wanted to point to as well. It is a Zener so at regular 5V it shouldn't do anything.

I think it's an overvoltage protection if for any reason the 5V line should go up higher. Seen this kind of protection elsewhere.

Right! But they removed this Zener completely in the mk3 design and replaced it with multiple filtering caps.

I guess this mk1 was an early DC/DC converter design for them and they anticipated high inductive spikes because of high load on the 5V rail and a fairly high inductance of L9.
After collecting some actual experience they probably realized that the Schottky was fast and reliable enough and that the Zener brought no benefit. So, they dropped it in their first redesign, the mk2, and added the noise filter C120 and upped the reverse voltage on the Schottky.
The change from SS34 to SS36 was surely not just an "ad-hoc change by the manufacturing guys" because they may have run short on 40V Schottkies. Such things do happen all the time but in that case the change would never find its way into written documentation!