Beomaster 4000 (2406): Installation of Custom Designed Toroid Transformer

This is a follow up to my recent posts about a strongly humming transformer in a Beomaster 4000 (2406). This Beomaster is in great condition otherwise, so it made sense to install a new transformer to get this unit ready for the next 40 years of service. Most modern audio designs with analog power supplies use toroid transformers due to their much better EM characteristics. After characterizing the original transformer current loads in the secondaries, I designed a replacement and had it custom made by the Toroid Corporation in Salisbury, MD. It costs a pretty penny to get this done, but the result is pretty, too (at least from a geeky engineery point of view...;-). 

While the installation of a new transformer is fairly straight forward, the details can be pretty daunting. The first order of business was to remove the old 'rectangular' transformer. Here is an impression of the original state of affairs:

This shows the 'cavity' after removing it:

The challenge here was essentially to fit (and hold securely) a donut into a square space. Furthermore, The Laws of Beolove require to not drill additional holes into any B&O equipment, ever! My technical upgrades are always 'reversible', i.e. the units can be returned into their original state if so desired at a later point in time. So I set out to design a 3D printed mounting bracket that would fit exactly to the mounting holes of the original transformer case.

Since this bracket represents a fairly large part from a 3D printing perspective (the final design took about 26 hrs to print), I began the design process with an initial study that 'put the mounting holes into the right positions in space', but that needed minimal printing effort so the design could be iterated in a  reasonable amount of time (it is difficult to do precise measurements within a crowded enclosure). Here is an impression of one of these study parts implanted into the cavity:

Once the holes were close to the proper positions, I completed the design and created a final version of the bracket that was able to hold the toroid and the main rectifier in place:

This shows the final design after four prints (and about 1.5 kg of plastic filament...;-) that did not fit precisely and had other fit-related issues. It turned out that the 'study' prints were slightly flexible, which allowed me to bend the holes a bit to the right positions, while this final design is very rigid and so there was no more room for mismatch. This shows the toroid and the rectifier added:

Before I was able to install the toroid assembly, I needed to replace the reservoir and speaker capacitors to make a bit more room for the toroid. The original reservoir capacitor is a bit thicker than its modern replacement, and that gave me five more millimeters that were required to fit the toroid:

This shows the installed bracket with rectifier in place and the new capacitors::

Add toroid:

The next step was connecting the toroid to the input voltage selector. This shows the arrangement at the input selector:

The it was time to verify that the connections were made correctly for proper voltage transformation at all four input voltage settings. This was the moment where I would get proof that my transformer design was correct. All this can be a bit confusing since the input selector switches the various primary coils of the transformer in series and parallel depending on the tuns ratio that is needed for a particular input voltage. So I hooked up my 110-to-250V variac to the line plug of the Beomaster (turned 'on') and then slowly ramped up the transformer voltage for each of the input villages while monitoring the secondary voltages. This shows the setup:

The secondaries wires are held securely apart from each other by some carpenter clamps in concert with thick cardboard strips. This shows the measurements for the various input selector settings vs the 50V secondary. 110V input:

130V input:

220V input (and 22V secondary):

and 240V:

After this I put the input switch back into its box together with the fuse holder (secured in place with some double sided tape):

After this I connected the secondary windings to the two rectifiers:

And then it was time for the magic moment! I slowly ramped up the variac and the 'ON' light came on. Then I connected speakers and an antenna, and set it to FM. And everything seems to work! And of course no transformer hum at all! This is Beolove! I still need to procure a nice bolt to hold the toroid in place in its bracket, but I think it is pretty safe to say that this Beomaster 4000 is back in business. I will give it a full recap and then give it some play before sending it back to Italy! 

Beomaster 4000 (2406): Installation of Custom Designed Toroid Transformer - Final Impressions

I finished up the toroid transformer implantation in the Beomaster 4000 that I am currently restoring. I added a bolt to fix it in its 3D printed cradle and used the mounting plate and rubber shock absorber that came with the toroid to secure it in place. Here is an impression:

Lovely! A very happy look in my opinion! 

Then I put on the bottom plate:

and with plate installed:

This shows a the detail around the 'gills' of the Beomaster:

I like how the red shines through a bit. Since the toroid represents a performance upgrade of the Beomaster and brings its power supply into the current millennium, I like that it can be seen a bit if closely scrutinized. Putting in a toroid into a Beomaster 4000 is a bit like installing Brembo calipers on a vintage BMW 5-series, if you catch my drift...;-):

Toroids have a much improved EMI performance than conventional "EI" style transformers due to their geometry. That is the main reason why most modern low noise analog power supplies employ toroid designs.

While I am writing this post I ran the unit together with the Norwegian Beogram 4000 that I am testing right now, and I can report that it sounds absolutely awesome! No humming on any input and everything stays absolutely cool including the toroid and I am cranking it up quite a bit right now. Appropriate for Jethro Tull's Aqualung!

This pretty much concludes the restoration of this Beomaster (if nothing comes during the testing period). Here is a picture of the units with exchanged parts:

Beomaster 4000 (2406): Exchanging the Electrolytic Capacitors and Adjustment of the Quiescent Current

After testing the new toroid transformer for a while (and coming to the conclusion that it works very nicely - no hum in the amplification chain whatsoever) it was time to complete the restoration of this Beomaster 4000 and replace all electrolytic capacitors on the various PCBs with new 105C grade major Japanese manufacturer units. While time consuming, this is not too difficult to do, but a few of the capacitors are challenging to access since the 4000 is from a time when board-to-wire connectors were not commonly used in consumer electronics. In other words, it is difficult to take out boards since most connections are soldered. So it is best to just leave everything in and work your way around a few obstacles like the power switch that obscures a solder point or the vertically installed loudness board. But with a bit of patience, a few tricks and a steady hand it is a straight forward process. Here are a few impressions. The first step is usually to take out the preamplifier board that is directly bolted to the frame with four screws next to the DIN jacks on the back. This picture shows the board in its original state:

And after exchanging the capacitors:

Then it was time to work on the two main boards. Unfortunately the preamp board does not disconnect like in the Beomaster 4400 where a rare wire-to-board header allows this convenience. In the case of the 4000 the board needs to be left connected and dangling from the frame while one works on the main PCBs. But this is not such an issue if one props up the Beomaster vertically using a couple of carpenter clamps affixed to the heatsinks (use cardboard shields to prevent scratches).

This shows the output amplifier board in its original condition:

The four red resistors at the bottom are the emitter resistors that can be used to adjust the quiescent current. In between them are the associated trimmers. I replaced these with modern encapsulated units along with the capacitors. This shows the board after restoration:

Next up was the FM tuner and 15V power supply board. this shows the original condition:

And after rebuilding it:

This board has two EMI cans. On the right is the FM 'front end' that selects the right carrier frequency and on the left is the 'detector' that converts the frequency modulation audio information into amplitude change that can be amplified and analyzed into an audible stereo signal. Unfortunately there is one electrolytic capacitor in each can and so one needs to get in there. The easy one is the front end, where the top and bottom covers come off easily. Note that the bottom one has a soldered ground connection which needs to be unsoldered before it can be taken off:

This shows the solder side of the PCB after taking the bottom cover off:

And here is a peek into the top:

The capacitor is in the top part, a brown/black tantalum type. This shows it replaced:

Care needs to be taken to not disturb the inductors. If they get accidentally 'adjusted' one may need a FM signal generator to recover. After putting the covers back on I turned my attention to the detector. Here the board is piggybacked on top of the main FM board and so one has to unsolder it first before the bottom of the PCB can be reached for soldering. The top cover can simply be pulled off. This is how it looks after it is off:

Then I took the board out:

The capacitor is up front center (also brown/black; 10uF). Before one can unsolder it the bottom shield needs to be taken off. One of the pins is soldered into the PCB, making the ground connection, i.e. it needs to be unsoldered first. Then the shield comes off:

And then I was able to put the new capacitor in:

After putting everything back together in reverse I took this picture:

On to the loudness board that is right behind the loudness switch on the front panel. There are two capacitors (red):

I should have replaced them while the transformer was out, but it did not occur to me. So I did some 'artistic soldering'. Copper brain is very useful for such situations when the desolder gun is too big to fit. After a bit of effort I had them out and replaced with new units:

Alright! Almost done. On to the FM preset capacitors that are soldered directly to the preset switches and a ground wire:

I also replaced them with new units:

The next step was to adjust the quiescent current with the new trimmers:

I elected to ignore the prescription of the service manual to measure the current in the output transistors directly with an ampere meter connected into the circuit. The setup allows for that via two pin headers that are not soldered. However, I did to like the idea to connect a multimeter with an internal resistance of about 5 Ohm in mA mode into the circuit and thereby altering the setup. I instead calculated the current that corresponds to the prescribed 80mA quiet current in the 0.15 Ohm emitter resistors in each of the B-style push-pull output branches (12mV), and hooked up my multimeter across these resistors and adjusted the trimmers to get the 12mV. Now this Beomaster is running cool. When it arrived one of the channels got pretty warm indicating trimmer issues, as they are frequently found in this vintage of B&O.

Beolink 5000: Glueing the LCD Screen Back In

The Beomaster 4000 that I recently outfitted with a toroid transformer came with another Beolove challenge piggybacked into the box: A broken Beolink 5000 remote control, which had a detached LCD screen. Unfortunately this Beolink 5000 was beyond repair since the LCD screen had mechanical damage at the contacts end, probably due to a fall of the display on a hard floor or similar. 

We decided to not give up but rather try to procure a potentially fixable unit from ebay. Luckily, a few weeks ago a 'lot' of three Beolinks became available with the promise that one of them was actually still in fully working condition. My customer won the auction and had the units shipped directly to my bench. Unfortunately, it turned out that the LCD screen of the working unit also was already loose,

but it had not detached yet from its ribbon cable. Luckily, the seller had the presence to tightly wrap this unit so the screen stayed where it was during shipping and it did come alive when I put batteries into the unit:

I decided to develop a fix for glueing the display back into the remote body without breaking the very flimsy plastic foil ribbon connector that connects the LCD display to the circuit board. In fact this particular design feature seems to do in 99% of the Beolinks since it simply rips off when the screen detaches and falls out due to ageing glue. The glue seems to be the same foamy stuff they used for attaching the aluminum cover to the Beogram 8000/8002 turntables, i.e. it is no surprise that the LCD screens also pop out at this point in time.

Anyway, the task was to not kill the ribbon connector, while handling everything and cleaning the old glue out etc...It was beneficial that I had the two other units with completely detached screens to practice on. I used them to develop a 'tool set' that would allow me to do the repair with minimal mechanical stress on the ribbon connector. 

Here we go:

I opened the unit up:

Then I carefully pressed the LCD screen out of its frame and moved the PCB and screen a bit to the left to release the board from its tabs on the right. Once the two parts were liberated I supported them with a custom designed 'brace' that would stabilize them in their relative alignment:

I applied the matching top part:

And flipped the entire 'assembly' over to the right:

Now I removed the board-to-board ribbon cable from the display control board:

The two tabs left and right of the ribbon can be pulled out and then the cable comes out:

This liberated the display part allowing me to go to work on removing the glue from the display frame:

The next step was to remove the glue from the LCD. The orange part of the brace now served as support during this procedure. I had designed a screw hole into left side of the orange part that allowed me to bolt down the circuit board to make sure there was no movement between LCD and PCB to protect the ribbon:

After cleaning off the glue it was time to install some industrial strength double sided tape in the frame. I had developed two more 3D printed parts that would allow me to apply the tape and then cut it to size with precision to not impair the translucency of the screen:

and with tape applied:

Then I cut the 'window' out of the tape layers using a razor blade and a matching 3D printed part that fit exactly into the display bay from the top:

This was the end result:

after removing the tape backing

Then I reconnected the display board and flipped it over with the braces in place:

After carefully pulling out the red brace part, I pressed the display into its bay, making sure that it was aligned properly:

And then came the moment of truth! Did the ribbon cable survive this stressful operation? After putting the keypad back on and inserting the three batteries it turned out that the operation was successful. The unit came alive again!

The next step is to figure out how to repair completely detached screens...another day, more Beolove!