A Pair of Beomaster 1900 Receivers for Restoration: Recapping the Main Board and Volume Control Board (Unit #1)

The restoration work on the main board and on the volume control board is now complete.

The Beomaster 1900 and 2400 receivers are low profile so they look small. I always start out these restorations thinking that it should be easy...but once I get started on the capacitor replacement I am quickly reminded at how many components there are.

Here is the main board as I received it.

Here is the main board after replacing the electrolytic and tantalum capacitors.

Here are some details.

As I like to do, I replace the electrolytic and tantalum capacitors that have values 4.7uF and smaller with WIMA MKS capacitors. Those are the capacitors that look like red rectangles. They are non-polarized capacitors as well.

For capacitance values higher than 4.7uF I use high quality 105°C rated capacitors.

I always try to match the original capacitor's voltage rating.

Just like I have seen in the other Beomaster 1900/2400 units I have recapped, most of the capacitors are out of tolerance by 30% to over 100%.

I always measure all of the capacitors I remove. I won't post every photo but these two give you an idea of what is typically found.

This is an original 220uF capacitor...a little out of tolerance now.

Here is a 2.2uF capacitor.

The new, replacements will make a big difference with the old capacitors being so far out of tolerance.

Here is the new 220uF capacitor, a Nichicon.

The only capacitor I didn't change out was a 10uF tantalum capacitor inside the FM tuner. I did take it out and measure it...it was right on 10uF so I decided to leave it in place.

This first Beomaster 1900 unit had some past issue with the rectifier bridge for its +15 VDC voltage power supply you might recall.

I decided to replace the pieced together rectifier bridge with a single bridge device.  I also decided to replace the previously replaced capacitors. They still measured good but I want to start with all new electrolytic capacitors.

The two large reservoir capacitors for the output amplifier voltage rails are typically still within tolerance. The two on this Beomaster were as well but I replaced them anyway due to the effort it takes to change them out. I don't want to have to come back and do them later.

Here are the original two reservoir capacitors.

Here are the new ones.

While working on the main board restoration I decided that I didn't like the looks of the way that the +15 VDC regulator device was mounted. The thermal past looked dried out and it would bother me to know that was in there like that.

I reworked the mounting of the regulator by cleaning off the old, dry paste and remounting the regulator with a SIL-Pad.

That looks better. Note that I also added some hot glue to help support the 2200uF capacitor for the +15 VDC supply.

One more change that I made to the main board was to replace the left and right channel 250Ω trimmers (for the output amplifiers' no load current adjustment) with new, sealed, multi-turn type trimmers.

That took care of the main board.
The Volume Control Board was next.

Here is the before photo.

Here is the after recapping photo.

This shows the Volume Control Board reinstalled.

So far the restoration work has meant the removal and replacement of sixty-four capacitors, two trimmers and the four diodes of the bridge rectifier.

This photo shows the removed parts.

I will have to do all of this again for the second Beomaster 1900 receiver.

There are still the tasks of the Tone Control/FM Tuner board recapping as well as the replacement of the Beomaster 1900 indicator lamps.  Once those tasks are complete I can start testing this receiver...the fun part.

Beomaster 2400 Type 2902: Recapping the main board and the volume control board

I completed the capacitor replacement on the Beomaster 2400 main board and volume control board. This included replacing the reservoir capacitors and the idle current adjustment trim pots for both channels.

The Beomaster 1900 and 2400 receivers are not my favorite units to replace capacitors on. They are very compact and low profile which make things a little cramped inside. Unlike the large Beomaster 8000 these smaller Bang & Olufsen receivers do not have easy accessible and removable circuit boards for working on.

Here is another picture for the Beomaster 2400 before the main board recapping.

You can see there are a lot of capacitors to change out. About eighty capacitors out of the ninety-nine total I will be replacing.  I can't easily disconnect the main board from the rest of the Beomaster 2400 to remove it for recapping as it has several hard-wired connections to the transformer and power supply.  On a Beomaster 1900 one time I removed everything from the cabinet chassis but that still made maneuvering the board awkward as I had the weight of the still connected transformer to deal with.

Since then I like to disconnect the FM tuner box, reservoir capacitors and heatsinks so I can switch from front and back of the main board while recapping.

I remove a couple of solid wires to the transformer to avoid breaking them while working on the main board.

After that it is a careful and tedious ordeal of desoldering and soldering while switching from the component side of the board to the trace side...back and forth.  This is another picture of the main board after the recapping work was completed.

There was one 10uF, 63VDC capacitor under the metal shield box for the remote control circuit.

I usually replace capacitors with values of 4.7uF and smaller with WIMA MKS capacitors.
An exception is if the WIMA won't fit well as with the two small 4.7uF capacitors on this small B&O adapter board.

This picture shows the two Beomaster 2400 idle current trim pots after I replaced the originals.

When installed back in the cabinet frame the emitter resistors are difficult to attach probes to for making the idle current (or no-load current) adjustment. While I have the main board in service position I like to make sure the leads on the emitter resistors are long enough on the trace side of the board to clip measurement probes to.

This picture shows the +15VDC supply reservoir capacitor after the recap. I used a little bit of Aleene's glue to secure it in place.

The two large 5000uF reservoir capacitors are kind of a pain to change out on these types of Beomaster units. The original capacitor terminals are soldered together as well as all of the connected wires.

In my replacement reservoir capacitor assembly I use small terminal connectors to attach the wires to the capacitors and the two capacitors to each other.

That is a more secure connection and makes future desoldering of the reservoir capacitors easier.

The small volume control board only has six small capacitors to replace.

That leaves one capacitor in the FM tuner box to replace and a few capacitors on the tone control and FM tuning control board. Then the recap work will be complete.

Beomaster 1900 Type 2904: New idle current trimmers and output transistor thermal insulators

I started to work on replacing the electrolytic capacitors on the last board, the tone control and FM tuning control board. Unfortunately I ran out of my favorite 2.2uF WIMA capacitors. There is a Mouser distribution center close to me however so the parts will arrive tomorrow.

In the meantime I will go ahead and change out the left and right channel 250Ω trimmer resistors that are used to adjust the output amplifier no-load current. Now is a good time to change them.

Here are pictures of the trimmer replacements.

Along with that change I cleaned off the old, dry thermal insulation paste on the four output transistors that are mounted on heatsinks in the back of the Beomaster. I like replacing that with new Sil-Pad thermal insulators.

Here are the heatsinks reinstalled.

Beomaster 1900 Type 2904: Capacitors replacement Part 1

Most of the electrolytic and tantalum capacitors have been replaced on this Beomaster 1900 now. That replacement was long over due and badly needed. Over eighty percent of the capacitors I replaced measured out of tolerance or completely dead.

I will start with a couple of pictures showing the recapped main board.

Being packaged into such a low profile cabinet the Beomaster 1900 circuitry is mostly on one big board (labeled module 2 in the service manual).  There are a few other small boards with a few electrolytic and tantalum capacitors but the majority of the task is the large module 2 board.

The capacitor replacement was pretty standard....desoldering, measuring and replacing one capacitor at a time. It takes extra time to measure the old and new capacitors but I like to know that information on each restoration.

The Beomaster 1900 did have a couple of capacitor replacement tasks that took some extra work.

The reservoir capacitors 0C7 and 0C8 don't have a bracket to hold them and utilize the original capacitor's large, flat terminals to connect up all of the ground and power lines to them.

To make that task a little easier I mounted a terminal between the two grounded posts of the new reservoir capacitors to secure them together. I also added multi-tab terminals to the two posts for the power wires.

This Beomaster 1900 has a modification not shown in my service manual in the left and right volume related circuitry. This board wasn't in the previous Beomaster 1900 project I did.

This picture shows the small mod board attached to the main board.

There is one lone tantalum capacitor in the FM tuner (metal) box that I replaced.

The volume control board has two small tantalum capacitors that I also replaced.

The last couple of pictures show the replacement of the +15VDC power supply capacitor 2C92.

That just leaves the capacitors on the tone control and FM tuning control boards. I will show that on the next blog post for this project.