Beomaster 900k project

[DHowell1973]

Hi All,

First post on here so hoping that someone with more knowledge than me can help.

I inherited the Beomaster 900K from my grandfather and sorted it about 4 years ago with some worK needed to the front mainly. It worked fine.

Recently I have noticed that it is only working on one chanel. The left chanel is working but is very faint. The balance pot on the back adjusts but it makes the right hand chanel very loud and then just both quiet channels. Would try replacing the pot but cannot find any circuit diagram to make sure I swap it with the correct pot.

Would love to get it to 100% so any help would be appreciated. I have good soldering and fixing knowledge but not experienced on the beomaster 900K
On unplugging the din socket on the main board which connects the speakers the left channel is working but no umph or bass. Right channel working on full bass and ump.
Two of the transistors on the back of the unit get warm where as the other two are cold??
Please help as I want to get it working again!!!

Thanks all.....Dave

[paulsherwin]

As always, the first thing to do is to clean and exercise all the switch contacts and potentiometer tracks using a contact cleaner such as Servisol.

https://cpc.farnell.com/servisol/100.../dp/SASERVISOL

[DHowell1973]

Yep that is all done. Have cleaned all the switches and pots and the whole of the inside as it was a but dusty and white.

Have seen another thread saying about changing AC128 transistors and capacitors on the output board. I have the circuit diagram but it doesn't show values?

[Michael Maurice]

Changing the balance control will not help you, you have a fault in the output amplifier.

I suggest you start with checking some voltages, starting with the mid point. Check the voltage on each of the collectors of the output transistors, one should be -24V (remember this has a positive earth) the other at -12V.

Service information is available from www.beoworld.org

[paulsherwin]

I don't know this design, but if it uses AC128s then you should be aware that these transistors are now developing tin whisker problems. The circuit voltages will be out if that's the problem.

[Michael Maurice]

transistor line up for the output stage is AC151 x 2, AC153, AC127, AC132 x 2 and AD139 x 2

I cant remember if the AC127 and AC132's are attached to heatsinks,. but if they are, remove them and check for volts on their cases. There should be zero volts if there is any voltage then the transistor has developed tin whiskers.

[DHowell1973]

Thankyou all for your response.
I am thinking that the transistors are the way to go as voltages are all over the place.
Anyone know of a place to buy them

Also other people whilst doing this have changed the capacitors? Anyone know what capacitance and voltage these should be??

Have been to Beoworld but found it not very helpful. No replys to my posts and not a lot of info on the 900K.

Thanks
Dave

[John Caswell]

Just as an off chance, B&O used to staple schematics usually in an envelope onto the wooden case of their equipment. (Needless to say I will be wrong in this case)
As far as capacitors are concerned, get the thing working first then if necessary change the capacitors one by one, go by what is on the capacitor, checking after each one. I know it's long winded but its safer, also if PCB based do not, whatever you do, bend any component leads hard against the PCB. You will most certainly curse yourself to hell for doing this later. A 5º bend is sufficient to hold them in place and then if necessary reheat the joint and push them against the PCB to make it look tidy.

John

[DHowell1973]

Hi John,

Am aware they used to do this but unfortunately not in mine!!

Dave

[Cobaltblue]

Circuit info is at Radiomuseum

https://www.radiomuseum.org/r/bang_b...r_900k900.html

As a guest you can download a limited amount.

Cheers

Mike T

[DHowell1973]

Thanks for all the input so far.

Just looking for a supplier of the transistors required??

Thanks

[Cobaltblue]

Hi not sure from the posts so far you need transistors.

If you do there's Ebay or someone like Cricklewood

https://www.cricklewoodelectronics.c...AC-to-AUY.html

Cheers

Mike T

[DHowell1973]

Thanks all for your help so far!!!

Cricklewood Electronics were great for the transistors.

Need to source the following capacitors which I am having difficulty with:

2.5uf +64v
125uf -16v
80uf -25v
100uf -6.4v

Any help gladly received!!!

Dave

[AC/HL]

Only the last is currently a standard value, although it'll be 10v working.
You'll need to get the next higher value for the others, probably 5, 150 and 100. Again the next higher working voltage.

[Michael Maurice]

2.5uF + 64 use 2.2uF 63V
125uF-16 use 150uF 16V
80uF use 100uF 25V
100uF 6.4V you might be able to use 6.3V but 10 or even 16V is perfectly acceptable.

[DHowell1973]

Well I have replaced all of the Transistors and capacitors but still nothing??

One of the transistors on the back is running hot and I just get a hum out of one speaker??

I am now at a loss??

Help.

Dave

[ekjdm14]

This is why blanket recapping is not generally advised, sadly, let alone replacing all transistors.

So to clarify, originally we had right channel working well/left channel low and tinny but now we have neither channel working and just hum on one speaker? (is this the right channel that's humming? That would suggest to my mind that the original fault still exists on the left channel and there's an introduced fault on the right.

First thing I'd do is recheck your capacitors, making sure they were fitted with the correct polarity (ideally checking against the schematic since there's now no original ones to compare to & bearing in mind some of the +ve will appear to go to -ve due to the nature of Germanium amps, so it can be counter-intuitive). If these are all correct values and in the right way, I'd concentrate first on getting the right channel back to where you started. Personally I'd be tempted to remove the "new" transistors and put the old back in as they were known working, but to be sensible I'd suggest checking voltages on them again and verifying they also are in correctly.

As far as Germanium transistors go, it's not uncommon for "new" old stock items to be in worse shape than originals so it's not beyond the realms of possibility that you've replaced good with bad there. Do you have an LCR meter or even just a multimeter with diode check function? You should be able to check the transistors on the LCR, or with a multimeter see 2 diodes between the leads (also check for any resistance (leakage/short) between B C E and the can that would indicate whiskering).

By the way, I knew a David Howell from school, you didn't attend Cruckton Hall per chance?

[DHowell1973]

I think that it is getting beyond me to be honest.

Is there anyone out there that can repair or know of anyone that can repair the radio

Happy to courier it anywhere in the uk for the right person??

Thanks

[Michael Maurice]

I can probably repair it for you, I'll PM you.

[DHowell1973]

Originally Posted by ekjdm14

This is why blanket recapping is not generally advised, sadly, let alone replacing all transistors.

So to clarify, originally we had right channel working well/left channel low and tinny but now we have neither channel working and just hum on one speaker? (is this the right channel that's humming? That would suggest to my mind that the original fault still exists on the left channel and there's an introduced fault on the right.

First thing I'd do is recheck your capacitors, making sure they were fitted with the correct polarity (ideally checking against the schematic since there's now no original ones to compare to & bearing in mind some of the +ve will appear to go to -ve due to the nature of Germanium amps, so it can be counter-intuitive). If these are all correct values and in the right way, I'd concentrate first on getting the right channel back to where you started. Personally I'd be tempted to remove the "new" transistors and put the old back in as they were known working, but to be sensible I'd suggest checking voltages on them again and verifying they also are in correctly.

As far as Germanium transistors go, it's not uncommon for "new" old stock items to be in worse shape than originals so it's not beyond the realms of possibility that you've replaced good with bad there. Do you have an LCR meter or even just a multimeter with diode check function? You should be able to check the transistors on the LCR, or with a multimeter see 2 diodes between the leads (also check for any resistance (leakage/short) between B C E and the can that would indicate whiskering).

By the way, I knew a David Howell from school, you didn't attend Cruckton Hall per chance?

Sorry no not that David Howell. I am origionally from Cornwall so not me.
I have now put back all of the original components and made slight progress but still major issues.
One chanel is playing but very quietly. There is a bad loud hum. One channel is operating very hot??
Any help appreciated
Dave