Behind the White Wheel

Behind the White Wheel

Saturday, April 4, 2015

Becker Mexico Alive

After rebuilding the power supply, time came to rebuild the radio. Again, these units are made of simple electronics components (resistors, capacitors and some semi-conductors) in addition to the tubes. The Becker Mexico contains 7 tubes and complex circuits that are much difficult to follow without the correct schematics. 

The Becker Mexico view from above; 2 tubes and the Wunderbar mechanism are visible
Thanks to Jeff Miller for posting on the schematics of the various Becker Tube radios (Mexico, Brescia, Europa and LeMans), without them I couldn’t perform any reparation accurately. 

Here is a quick history of the Becker Mexico installed in my car:
  1.  I bought it in 1999 from a retired taxi driver who had kept it for more than 35 years untouched, unused to replace the original Becker LeMans (AM, LW) radio which was beyond reparation
  2. I was 19 back then and had only very basic knowledge in electronics, no schematics and no testers. I only had a soldering iron and an ohmmeter. To my luck, visually a tube was broken (replaced it), cleaned the vibrator pins and plugged in the radio listening to beautiful music.
  3. I changed the power supply's vibrator in 2004 after finding a solid state replacement in Canada.
  4. The radio worked well from 1999 up till 2014 when problems started appearing:
              a-      The radio’s sound started becoming unclear (bad quality of sound)  
              b-      Then the Wunderbar (automatic electromechanical search system) stopped detecting any  station              
              c-       Finally, the radio died on the FM band followed by an extremely weak AM reception

I sent the radio to two repairmen (one of them mentioned in a previous article on this blog) who both failed to do the repair after leaving it at their shops for more than a month; I lost hope and decided to give it a try myself.
I started by studying well the schematics; understanding the different currents, how things work and how the radio is designed. I was lucky enough to find a very interesting website on old radios’ restoration ( that Philip Nelson runs. I exchanged few emails with Phil who suggested I start by replacing all the electrolytic and paper capacitors and see if anything changes.
The radio contains the following electrolytic capacitors:
  • One 50uF 350V
  • Two 10uF 25V
  • One 5uF 30V
  • One 100uF 25V
Replacements were easily found and installed; of course the new capacitors are much smaller making their installation easier than expected. 

100uF new electrolytic capacitor installed
New 4.7uF electrolytic capacitor installed
After replacing these capacitors I turned on the radio to only hear a hum from the speaker.
I continued recapping and changed the several 0.022uF and 0.01uF paper capacitors; the replacements are the smaller much more efficient orange and green drop models that are not polarity sensitive (no + or – connections as the electrolytic ones).
Green drop newly installed capacitors along with an electrolytic one
Orange drop newly installed capacitors
On a side note, always make sure to install the new capacitors in a way that makes them look nice in these “board-less” circuits.
Once this job done, I powered the radio without succeeding in getting anything different than the “hum” on the AM and very far FM reception (barely noticeable and accompanied by the hum).
At this stage I was starting to lose hope; the resistors were the only left components that I didn’t test thoroughly; the tubes were out of any suspicion since I tested them all on my working Blaupunkt Santos radio.  
Before doing so, and to get an idea on the status of the old capacitors, I placed each old one on the tester only to discover that the 100uF capacitor had drifted in value to 500uF and the 50uF one was shorted. 

The old paper and electrolytic replaced capacitors; notice the bottom of the Blaupunkt Santos tube radio
I took few days off then started again examining the values of the many resistors. After 3 long hours of examining and noting values, I found a 500K resistor that seemed completely open! This resistor (R104 on the diagram) connects Tubes EF89 to ECC85 indirectly; hence it plays a major role in the FM Band’s operation.   

The famous 500K dead resistor
100K resistors were the closest I found among the ones I had on hand; I connected 5 of them in series, soldered them to the respective points and powered up the radio…Eureka! Music was heard on the FM with very strong reception and excellent quality of sound!

5 x 100K resistors installed temporarily in series. Notice the 6 new orange drop capacitors
and the new 47uF electrolytic capacitor
5 x 100K resistors installed temporarily in series. Notice the 6 new orange drop capacitors
and the new 47uF electrolytic capacitor
I bought few 500K resistors, installed one adequately and started tackling the AM issue. The AM issue wasn’t as difficult as expected; a good hour spent on testing resistors showed that a well hidden (under many wires and other components) resistor R9 (70K - 2W) was completely fried! I wasn’t surprised when I found out that, especially that it is directly connected to the 50uF capacitor which was shorted. I changed the fried resistor and got strong signal on the AM.

70K Ohms (2W)  fried resistor
Finally, I addressed the Wunderbar issue which was extremely easy as it needed a bit of calibration (calibrating potentiometers P2 & P3 and variable capacitor C126) as indicated in the second page of the radio's schematics. 

Rheostat P3 surrounded by 2 resistors. Watch out this area as it contains high voltage 
(Photo courtesy Becker Autosound)
I tested the radio for few hours and was very satisfied with its performance. I lubed the Wunderbar mechanism, assembled it all and installed it in my car ready for additional years of playing music...
To conclude on this Becker Mexico Tube Radio adventure; I can say that most of these radios are easily repairable and can live for so many years. All what one needs is some logic, basic electronic knowledge (resistors, capacitors), specific testers, a soldering iron and a lot of patience. I admit it was a challenging project/ job to complete but on the other hand, I learned a lot and covered an additional aspect of maintaining my car and all of its components in a perfect shape. 

Enjoying the sound of the Becker Mexico Tube Radio

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 
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 …