This is a follow up to my recent post about the redesigned Beogram Commander remote control board, which now works in both (DC-motor) Beogr...
Wednesday, March 23, 2016
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::
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:
220V input (and 22V secondary):
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!