Grundig UKW-Concert-Boy 58

My Dad owned in the late 50ties a high end portable tube radio, a Grundig UKW-Concert-Boy. I can remember this radio when I was a child: it had a green cover and was placed in the kitchen of our home. Like a lot of old things this radio one day disappeared in the early 70ies.

When I started to work on old tube radios my Dad also remembered the old Grundig. So I hunted on eBay to get one again as a gift for him. What I got was a Grundig Concert-Boy model 57/58 in a brown case. It looks like the green ones were really rare.

The UKW-Concert Boy 58 was playing with a lot of hum, so I had to take it on my workbench.

Most important: I got the schematic for it, thanks to Jos Lauth in Luxembourg. He also sent me an article from the German 'Funkschau'-mag covering the Grundig radio: Funkschau Grundig UKW-Concert-Boy 58

The Grundig radio is able to run on AC 110/125V, 160V or 220V and also on battery power.

It is a 9 tube design:

Rectification on AC operation is done by a Selen-rectifier. For the detector the radio is using Germanium diodes.

I replaced the Selen rectifier bridge for the plate voltage with a 250B800 Si bridge. I also changed the PS caps and used a 1k resistor due to the lower voltage drop of the Si bridge. That was an easy one. More difficult was the work on the heater rectification. Grundig used a single-wave Selen rectifier, followed by a 500µF cap, a load resistor and by an early version of a Nicad rechargeable battery, called DEAC, specified with 1.25V and 3.500mA. For sure the Nicad was dead. Without this battery the heater power supply is a lousy one. I replaced the old Nicad with 2 new C-size 1.2V 1.800mA Nicads, but the radio had problems to run on FM. I assume the internal resistance of 2 new Nicads in parallel was to low, that dropped the heater voltage to much. I did some tests and finished with 1 C-size  Nicad, paralled with 4 * 4.700 µF electrolytic caps. It would be much better to have a 1.5V rechargeable battery stabilizing the heater voltage at 1.5V, but Grundig lacked those batteries also when they made this radio in the 50ies. The Concert-Boy has now about 1.35V at the heaters, which is not perfect, but it is working.

This is the schematic (.gif) for my heater supply solution: Grundig_CB58_heaters

 

The chassis from the front: on the left side the output transformer, on the right side the power transformer.	The new power supply section: 2 * 47µF 350VDC caps with a 1k dropping resistor
The chassis from the back side: on the upper 'floor' is the complete radio section, in the lower left corner the rectification.	The output tube section (EL95 above and DL96 below) and the new heater supply filter section: 4 * 4.700µF caps and a C-size Nicad behind them.