Behind the White Wheel

Behind the White Wheel

Wednesday, April 1, 2015

Rebuilding the Becker Power Supply


The Becker tube radios that fit the Mercedes-Benz pontons and other models are composed of two pieces: The radio itself and a heavy power supply that is mounted under the dashboard. The two are connected via a thick cable. 


A diagram showing each component of the 1950's Becker Sound System 
(Ref: mbzponton.org) 
The power supply as it appears
As a start, let’s talk about the role and the components of the power supply:
The power supply contains 2 transformers (1 big and 1 small), a buzzer or vibrator (a metallic can that has 6 pins, a vacuum tube (EL84), a 50uF capacitor (in a metallic can too) and a selenium semi conductor unit in addition to many paper capacitors and resistors. The power supply transfers the 12V DC current to 220V AC allowing the several tubes to operate. The radio then sends back the current to the power supply which amplifies it and transfers it to the speaker for the final output (music, news...etc.) 
The power supply is a critical component for the correct operation of these radios; hence rebuilding it is always a necessity.

Big transformer (from 12V to 220V) - small amplifier transformer - vibrator removed in this pic
As a rule of thumb, all paper/ can capacitors should be changed, regardless of what they measure on the capacitors’ tester. If they still haven’t failed, these 50+ years old capacitors will soon do.
Luckily, they are all still available, much smaller in size and much more efficient.
As a start, the first capacitor to replace is the big 50uF can capacitor. The available replacement measures 47uF, which is OK. 

Here is the new (much smaller) 47uF capacitor installed (brown). I kept the old capacitor in its place (far left) for the purpose of originality of appearance
Then comes the time to replace the 100uF ones; always make sure that the voltage on the new capacitors is equal to or more than that on the replaced ones. 

100uF electrolyte and 0.022uF paper capacitors before replacement (in red boxes)
...and after replacement
Finally, I replaced two 0.022uF paper capacitors that both played a role in determining the quality of the sound.
I took the opportunity to test the capacitors I took off and they all tested badly; some of them were even shorted! 
After replacing the capacitors and installing them in what I see as an artistic and safe way, time came to tackle the vibrator issue. For this problem, two choices are available; a permanent one – which is to buy a new solid state vibrator from several online suppliers (much recommended) and a temporary one – which is to open the can (break it) and clean the vibrator points. Prior to finding the solid state vibrators, I used to clean the points of the old vibrator every other year; fortunately, this is not needed anymore after finding the solid state replacement. 

Solid State Vibrator installed
I had bought earlier this year 3 solid state vibrators, so here it goes; I picked one and installed it in the place of the old Kaco vibrator.
With the capacitors and vibrator replaced, it was time to try to operate the power supply. I didn’t touch the EL84 tube as it looked good and tubes rarely fail in these units.
I connected the working Becker Europa corresponding tube radio, a speaker and a 12V source to the power supply and turned the radio on. Few long seconds passed and the sound started radiating from the speaker. A look at the Ampere-meter showed that the power supply was drawing current much more than it should. A quick test showed that the Selenium Semi Conductor was overheating. 

Replacing the Selenium Semi Conductor
As I still haven’t dealt with such a situation before, I tried swapping it with one from a donor power supply; the result was amazing and the Ampere-meter read something between 1.5 and 2 Amps; the magical current that these radios should draw when in use.
I let the radio play for a moment and verified the new capacitors, the resistors, the new vibrator, the transformers…etc. for any sign of overheating or melting but all seemed fine.

Capacitors, Vibrator, Selenium Semi Conductor that have been replaced
I re-installed the power supply’s cover, tightened the 2 screws and felt proud of having a power supply ready to be used for hopefully another 50+ years …


2 comments:

  1. Nice work. Thanks for posting.

    ReplyDelete
  2. Hi which is positive and negative power suply?

    ReplyDelete