B&O Beomaster 1900

[Tube Radio]

I have a B&O Beomaster 1900 that I am repairing for someone. It blew the right channel output transistors so I'm replacing all the output transistors along with the driver transistors. I am looking for two BC5488 transistors, two MPSH54 transistors and two MPSA06 transistors. What transistor is that? I tried the number and came up with nothing. Also what is the wattage rating for the .47 ohm resistors in the emitter circuit of the output transistors?

[Tube Radio]

I found the MPSH54 and MPSA06 transistors. So all I need is the BC5458 transistors.

[tri-comp]

You do NOT need any BC5488 transistors, 'cause there aren't any!
Look closer and you'll see they're BC548's. Most likely you have a BC548B version
That's just a common run-o-the-mill NPN general purpose non-specialized european everybody-use-it Philips and/or Fairchind transistor.
Substitute by BC547~BC549 or BC147~BC149 or BC107~BC109 (OLD) or just about ANY NPN you can think of like Toshiba 2SC1815 and a million US-types (mind the pin-out!!!)
Need the schematics? PM me with your e-mail address for a free copy.


Data on BC548 attached.


rgds,

/tri-comp

[Rolf Zetterberg]

I think you have misread the transistor numbers slightly.It's easily done.You should look for BC548B,BC 545B and so on.They are sorted for current amplification and gets an A or B or C after the actual number.A means lowest gain.
Don't you have the schematic?Normally it's available here,but for some reason not the 1900?
http://www.vintage-restauration.com/4.htm
Sen me a PM and I'll help you with a service manual.
PS.I must say that I admire you for working on these-I've stopped doing that many years ago when I saw how increadibly complicated they are!

[Tube Radio]

Yes I did. I went with what the service manual I ordered yesterday listed. The person I'm fixing this for told me today that some of the bulbs are out also which I will try to repair. Yes it is complicated to work on. They have a wierd way of controling the volume which uses a circuit that varies voltage a bulb shining light on four photoresistors.

[tri-comp]

You must replace the bulbs, all of those which are gone. If you don't the receiver will exhibit all kinds of instability. It may cause hum in both channels or refuse to switch between sources. You'd never guess these faults come from defective bulbs but none the less! Probably because the power-supply stabilizing circuit is oscillating due to lack of loading :scratch2: Think about this if you plan to replace the bulbs with LED's with a lot less power-demand.
Apart from fixing this problem I would strongly suggest to replace ALL electrolytics in RED plastic made by ROE. If the housing is ORANGE they're not likely to be defective. If they're RED they show cracks and leaking at the top, mostly.
Actually it's not so complicated a construction. It's easily broken down into manageably parts (like the opto-coupler volume control) and the idea of FM pre-set's/Source-selection with SAS-570 IC's is just something they stole from the first B/W or Color TV sets featuring pre-set channels.

BTW, there's a big difference in the contruction of the electronic volume-control between Beomaster-1900/Type 2903 (Oktober-1976) and Beomaster-1900-II/Type 2903 (Oktober-1979). Be sure to use the right schematic for faultfinding :thmbsp:

rgds,

/tri-comp

[Nakdoc]

The 0.47 ohm resistor is 3 watt. I have some of the plastic sheets for the volume and balance "displays" if yours have craked due to heat. I also have FM rabbit ears and FM external antenna plugs.

[BrassTeacher]

I found the MPSH54 and MPSA06 transistors. So all I need is the BC5458 transistors.

If you get stuck for parts, PM me. I have a few B&O 1900 and 2400 chassis in my "organ donor" room!

[Tube Radio]

BrassTeacher I sent you a PM.

[Tube Radio]

I ordered the power supply caps and bridge rectifier. I used these: Bridge rectifier http://www.mouser.com/Search/ProductDetail.aspx?R=GSIB660-E3%2f45virtualkey61370000virtualkey625-GSIB660-E3 and power supply caps http://www.mouser.com/Search/ProductDetail.aspx?R=380LX562M035H022virtualkey598 50000virtualkey5985-85-35V5600. I see why people don't like to work on them. These are very compact which made it hard to find caps to fit.

[BrassTeacher]

I ordered the power supply caps and bridge rectifier. I used these: Bridge rectifier http://www.mouser.com/Search/ProductDetail.aspx?R=GSIB660-E3%2f45virtualkey61370000virtualkey625-GSIB660-E3 and power supply caps http://www.mouser.com/Search/ProductDetail.aspx?R=380LX562M035H022virtualkey598 50000virtualkey5985-85-35V5600. I see why people don't like to work on them. These are very compact which made it hard to find caps to fit.

Well, if you can figure out the correct disassembly of the unit with little to no assistance (and no hammer), at least you have passed the audition!:D

[Tube Radio]

Yep so far I have. I dread replacing the bulbs that are burned out though. I will do that once I get the unit to stay powered up after I install the new output and driver transistors. I like its features, but it is difficult to work on. If this wasn't for a co-worker I wouldn't be working on it.

[Tube Radio]

Everything is in except for the power supply components. I will wait to replace the transistors until I install the new bridge rectifier and filter caps. Where can I find replacement bulbs? I'll take a look and see if I have any of those red capacitors. If so they will be replaced. Are there any other problem areas I need to look at?

[Tube Radio]

I have a bunch of capacitors that are reddish-brown. Are those the ones that need to be replaced? Should I replace all of them or just the ones that are cracked?

[BrassTeacher]

I'd replace all those reddish-brown caps.

For light bulbs, just about any 12V 70mA bulb that you can stuff in the light box is fine. I get the ones with leads, and snip-n-strip the wires to the length I want, then just push it into the box. Do be careful of not only the voltage rating, but the current draw as well. B&O uses varactors for tuning as opposed to tuning caps, and a voltage drop as much as a few millivolts will cause all kinds of crazy stuff to happen.

[Tube Radio]

I'll do that. I will see if I can get the bulbs the service manual calls for. I may as well replace all the bulbs since they are all the same age. I was afraid I would have to replace all those caps. Looks like there is a ton of them to replace.

[Tube Radio]

Here is the part number for the lamps according to the service manual. 8230046 12V 1W. Any one know where I can find those? Judging by the schematic I have 15 to replace.

[Nakdoc]

That would be 12 volt 80 ma....a bit easier to find. The MCM 25-1370 is 12v 50ma...a bit dimmer . 800-543-4330

[Tube Radio]

I'll check that out.

[Tube Radio]

I found exactly what I'm looking for http://www.rapidonline.com/productinfo.aspx?tier1=Electronic+Components&tier2=Optoelectronics&tier3=Miniature+Lamps&tier4=5mm+Wire+ended+filament+lamp&moduleno=32227.