Beomaster 2000 Type 2801: First look inside

Continuing on with the Beomaster 2000 Type 2801 I opened up the cabinet to see what sort of restoration tasks lie ahead.

As with other Bang & Olufsen amplifier/receivers from the mid-seventies the cabinet used a lot of M3 flat head screws.

The bottom cabinet plate surprised me a bit in that it is plastic.  Other B&O receiver bottom plates I have worked on have always been metal so that is kind of interesting.  Maybe they were trying to cut the weight down some since this is a pretty large cabinet.

With the bottom plate removed I see that everything inside is clean and rust free.

The top cover of the cabinet surprised me next. 
While it is removable it also has two slotted rails to support just tilting the top of the Beomaster cabinet up for servicing.

The mechanism looks better than it operates though. I found that it is easier to work on the Beomaster with the sliding supports removed.

Note in the photo (at the bottom) a clear plastic piece.  When I opened the top of the cabinet that piece was loose.  It is one of two lens for the tuning scale.  The other one was still in place but this one was loose.

Here it is after I put it back in place.
The other side has a piece of tape plus it is glued.  I don't know if that is the way it came from the B&O factory or if someone has worked on it in the past.

For now I will just glue this end of the loose lens using some Aleene's Tacky Glue (clear type).

Another issue I discovered with the tuning dial assembly is one of the screws that attaches the assembly to the underside of the cabinet cover has a broken plastic tab.

I will glue the plastic pieces together and will fashion a U-channel piece of plastic on the 3D printer that will span the cracks. When epoxied in place the U-channel part will support the broken plastic tab so it doesn't get stressed in the direction that it broke.

The next problem area were the five preset tuning dials.
The round knobs for tuning the presets were loose. They are plastic wheels mounted to a metal shaft of the controls.

I removed all five preset controls so I can clean the controls with fader lube and then re-attach the adjustment wheels with some epoxy.

There is one other place I will need to create a 3D part for but I will cover that when I get to it in a later post.  The end of the sliding cover for the Loudness and Tape select buttons has a plastic filler piece that is missing.  That will be a simple rectangular piece of black plastic to print on the 3D printer and glue in place.

The last things to examine are the circuit boards that have electrolytic capacitors to replace.

Here is a quick look at the Beomaster 2000 circuit boards.

The first picture is of the output amplifier assembly.  It is PCB 7 and has four power transistors at the back that are attached to heatsinks.

There are eight electrolytic capacitors to replace and two 250Ω trimmers for the no-load current biasing of the output amplifier.  I will replace the single-turn trimmers with nice Bourns multi-turn trimmers.

This picture is the IF Section & Decoder board (PCB 2).

There is one 470uF, 40V electrolytic capacitor to replace.  The rest of the polarized capacitors are tantalum type capacitors.  

You will find a lot of discussion for and against tantalum capacitors in restoration projects if you search for the topic on the web.
Some people say to always replace tantalum capacitors no matter what.  I believe that is because the tantalums tend to fail badly when they do fail.  However, they actually have good properties for audio preamplifier, filtering, tone control and radio tuner circuits.  They are not particularly good for power amplifier circuits and notice that the B&O engineers do not have any in the Beomaster 2000 output amplifier.

The tantalum capacitors have good temperature stability, low ESR, low leakage and low noise (when charging and discharging).  They are an expensive capacitor as well so the B&O engineers did not "settle" for the tantalum capacitors in their design. They selected them for a reason.

There are modern electrolytic capacitors and film capacitors that can replace tantalums so that is a valid option.
My thinking here is that I will respect the wishes of the B&O design engineers and retain the tantalum capacitors.  I will check them out to make sure they are good of course but in my other restoration projects I don't think I have come across many tantalums that are out of tolerance.

...and as you will see, the Beomaster 2000 has a bunch of tantalum capacitor components.

It should be interesting to test the results when I have this receiver working again.

While I am showing PCB 2 it was brought to my attention that the little, round bridge rectifier, 2D8 (a B40C800 rectifier device) is known to cause some issues in these Beomaster 2000 receivers.  So that device will be replaced for good measure.

Another radio component in the Beomaster is the Front End (PCB 1).

It has a 10uF tantalum inside so unless the front end is bad I don't intend on touching it.

The Beomaster 2000 IF circuitry continues onto PCB 5 which also has the preamplifier circuits on it.
Sitting on top of the IF Section & Preamp board (PCB 5) is the Tone Control board (PCB 6).  The Tone Control board is shown tilted out of the way in this photo.

Again, the IF Section & Preamp board has mostly tantalum capacitors but there are a couple of electrolytic capacitors that will be replaced.

The Tone Control PCB only has tantalum capacitors but it has four slide controls that I will clean and lubricate with some fader lube.

Similar to other Beomaster components from this time period B&O engineers used a push button switch assembly for the amplifier control selection.  Pressing the button for a given audio source will also turn the Beomaster 2000 on.  Pressing another source select button makes that source the active, playing music source. The Off pushbutton then deselects the selected source button and turns the Beomaster off.  

On a Beomaster 3000, 4000 and 4400 I always kind of dread the pushbutton switch assembly because if there is a problem with it the repair is quite difficult.  There is the mechanical aspect of the assembly then there is the electrical contacts part.  Those contacts are extremely difficult to access for repair on a Beomaster 4400 so I suspect that is the case here as well.

There should be a way to access the electrical contact areas to spray in some good contact cleaner.
I will have to figure that out on this Beomaster.

I am encouraged by the condition of this Beomaster 2000 though. The push button mechanics all operate so I think that all that is required is contact cleaner.

In the next post of this receiver I will work on the repair of the broken pieces of the tuning dial assembly.