debug bang and olufsen beomaster 3300 amp model: 2953

[miketurbo]

greetings from Westport CT.

I have recently purchased a circa 1986 beosystem 3300.
it worked gloriously for a few hours. then no sound and smell of burnt electronics from the beomaster 3300 amp model: 2953
attached is a bit of the schematic, I have the full pdf schematic if needed.

all, functions aside from sound output seem to still work on the unit. (cd/tape/phono change, display volume display, remote control, radio station, on off etc.

I'm pretty sure the smell was from resistor edit[r122] burning up. (see picture)
I will replace the resistor, but It seems like this is a symptom of something else.

lots of questions:

1) what would cause this, excess current? from where?

should I replace:

2) any of the capacitors?
but how do I know which ones? (none of them are swollen)
how do you test capacitors while they are in a circuit?

3) the amplifier IC chip?
could this be why the resistor burned up?

4) should I replace the adjacent resistors (207, 206) ? they measure fine with
the multimeter but they look a bit singed, are they compromised?





any help is appreciated!

[eccerr0r]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

hmm... actually after second inspection, seems the design differs from the reference design. Hope that's not the reason why the resistor fried...

[miketurbo]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

@eccerr0r
I'm not great at reading/understanding the schematics TBH
here is the full document if that can help.

https://beomanuals.com/manuals/Beoma...ice-Manual.pdf

[eccerr0r]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

Kind of weird it worked for a few hours, no magic smoke for the first few hours at all?
Wonder if it's worth to experiment with just replacing that 100Ω resistor. Past that, seems something wrong with either design or the STK chip...

[petehall347]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

looks like the resistor decided to be a fuse ..there is a short in the output amplifier .
the manual is really badly copied making it just about impossible to follow it .
is it a hybrid amp ?

[eccerr0r]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

I hate STK chip designs...a lot of troubleshooting could end up with "replace STK chip" and not much else you can do about it.

Looks like B&O just tried to put minimal information on the schematic. As far as I can tell there must be another R205-R208 and associated cap and it's not shown. For curiosity, could you take a picture of the whole board?

BTW what wattage speakers are you using, how loud are you using them? Not sure if this is the problem, but still think it might be a design defect somehow...

[petehall347]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

.

[miketurbo]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

@eccerr0r,
it sounds like I should replace the STK IC, as well as the blown resistor.

@petehall347, is it a hybrid (I just looked this up)... it does not have tubes if that is what hybrid amp means...?

thank you!
here are some more pictures

[petehall347]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

yes that stk module i meant as hybrid ..
its probably shorted internally .

now i am confused looking at a datasheet ... maybe the V makes it different ... https://html.alldatasheet.com/html-p...1/STK4151.html

[eccerr0r]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

Yes the V does make it different than X... think the X and the GS are close enough to be equivalent afaik...

[eccerr0r]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

Ah yes I think I see it now, yes there are two sets as I thought. Schematic is confusing.

BTW, if you want to try to swap out the 100Ω resistor you should, and see if it works once more, at least temporarily. If it works then you may or may not want to try a small mod to see if it will make it last longer: make it like the reference design.
Take a picture of the foil side, might be some surgery needed, have to plan this out...

[stj]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

might want to check those red capacitors in the 1950's style plastic cans too

[miketurbo]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

I swapped out the resistor... no dice
I guess the amp chip next?

here is a picture of the back before replacing the resistor

[miketurbo]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

might want to check those red capacitors in the 1950's style plastic cans too

ok thank you I will start with these.
I just ordered a capacitor tester, I guess you need to pull each one to test properly (?, or use an ESR meter... probably beyond my ability at this point TBH)

I've only every known a capacitor was bad by looking at it.

[eccerr0r]

Re: debug bang and olufsen beomaster 3300 amp model: 2953

Just make sure the capacitors are not shorted - do not worry about ESR at this point (or for this issue, actually). A simple ohmmeter is sufficient, though you may need to remove them to ensure you're not getting false paths.

TBH I don't think they're shorted, and kind of doubt any shorted capacitor could explain this behavior. A shorted STK module could...

BTW what is the behavior of the new 100Ω resistor? Does it fry right away or is it DOA?

Actually one test that may show some more information: With the DUT off and the 100Ω R122 removed or destroyed, what's the resistance between pins 4 and 7 of the STK module? Make measurements in both directions. Then with the DUT on, what's the voltage at pin 4 and at pin 7 with respect to GND?