telefunken: 55; Bajazzo
I own one of this set. Of course the DEAC NiCd is dead. So I'll manage to make a replacement. So far soo good.
But I need to make an ersazt battery for the 90 V HT. As they are since long gone, I have no information about them.The only thing is the "type Emce Nr. 760 stamp on the cover of the set. I'll be glad to get as many information you may have. (size, capacity, scan of one model cover to put on the ersazt, schematics, tips for construction ...)
Last but not least, the cas need a good cleaning and restauration. So I'm looking for a replacement of the decorative plastic stapped on the box at the cover opening, as it is broken. (it is somewhat yellow)
Thank a lot in advance for your help and advice !
I have been making high voltage batteries from rechargeable NiMH 9V batteries. I got my packs inexpensively from www.goldmine-elec.com.
NiMH 9V packs are usually made from 6 cells of 1.25V each. This only adds up to 7.5V for NiMH. The 9V value is a standard value that came from 1.5V carbon zinc cells.
I have built packs from the standard sized 9V batteries and from smaller sized 70mAh cels from Varta, shown in the photos below. I got these smaller cells from goldmine, but they are sold out from there now.
They still have the standard 9V size, and some very small 2.5V two cell packs. I have been able to make a 50V pack with 20 of these that has the height of two standard 9V cells.
The following Emerson 838 radio has six 7.5V packs in series for a total of 45V. All packs, except for one pack to the left of the speaker were folded to fit in the space. I charge these packs with 1.4V per cell, which adds up to 8.4*6=50.4V. I limit the max charge current to 50mA. This current only flows for the first few minutes. Then I let the cells charge overnight with the 50.4V supply, without any danger of overcharging.
You can also see a custom pack made from three AAA 1.25V cells for the filament voltage.
The following RCA BP10 radio shows the same type of pack without folding.
I wired the packs onto a socket. Each pack is wired to a pair of opposed pins in the socket. So the batteries are completely isolated from each other in the socket. The card edge plug is wired to the radio and has jumpers to wire all the cells in series for a total of 67.5V.
The cells are recharged outside the radio by plugging the blue socket into a different card-edge plug that has jumpers to wire all the cells in parallel as shown here. The battery voltage at the banana jacks is a nominal 7.5V. My charging method is to apply 1.4V per NiMH cell, or 8.4V for these 6 cell packs. This is a safe voltage for infinite charging time. The power supply I use limits the current to a few 100mA. After a few hours the packs are charged up. Often, I just let the pack charge overnight. There is no danger of overcharging if the charging supply is 8.4V.
When the cells are not in use it is best to store them plugged into the parallel charging plug. This keeps the battery voltage matched between the cells.
Hope this helps. Let us know if you come up with alternate solutions.
Hello Joe !
Very good idea ! I did not thought about it !
For now I am going the inverter route. I have actually started making a 50 Hz inverted converting a 6 V lead sealed battery into a 90 V supply.
I let you know how it goes !
BTW the DEAC NiCad is recharging well and begin to hold the charge ! Fantastic for a 1955 piece of gear !
This approach to battery building started out when I found the inexpensive Varta NiMH battery packs. Some of my motivation, also came from trying to avoid interference from switching power supplies.
Some battery powered radios also perform poorly when tied to ground by the AC power supply. This problem usually occurs in radios with a loop antenna, that start picking up E-field interference when grounded. These radios were designed to operation with low noise from the M-field of the loop antenna.
But, I don't want to discourage your approach, as it may work very well, and will not require continued recharging or battery replacement.
I had been meaning to make a more extensive post about my various battery packs, along with the radios I built them for. Your question just gave me the motivation to make this post. I will post separatelly, given the general applcation of the topic.
That NiCd cell is trully amazing, wow! It is usually a challenge to get more than 3 or 4 years of service from modern cells. Keep in mind that the NiCd cell in your Telefunken Bajazzo 55 also serves as voltage regulator and hum filter for the filament voltage during AC operation. If you remove the battery and apply power you will apply too much voltage to the filaments.
The new post about my NiMH battery packs is here. June 29th 2009.
Yes Joe, I am aware of the fact that without the DEAC NiCd the heaters voltages will be in the range of 3.5 V. so will burn out in a matter of seconds.
This morning I found the DEAC at 1.2 V and it felt at 0.2 V when I plugged a dial lamp on it in less than 5 seconds. When I unplugged the lamp, it recovered to 0.7V. So I start charging it again ;-)
Of course, I do not plan to use it at all, as I own a D Cell NiMh of 8.5 Ah to put in the battery holder. I plan to make a fake DEAC cell containing 2 x1N4007 in series between the positive and negative side of the "cell" and a smoothing condenser to regulate the heater voltage in case one start the radio without the NiMh cell ....(It will help reduce the wheight of the set also ;-)
Regarding the perturbations the switching power supply will make, I am aware of them; I plan to make a 50 Hz oscillator which will force me to use a heavier transformer than a more higher frequency, but which will be easier to smooth and shield (as the set is also a mains one, it has some built in protection for 50 Hz hum and I plan to smooth quite well the 90V output).
I keep your idea of the Varta cells. I know a guy having a set using 67.5 V anode battery so I'll show him your article !
Thanks again !