Blaupunkt Parade 94.720 (1964)

In 2023, this radio was offered on the Dutch forum on old radios. It looked like a solid German transistor radio with FM band. The person selling it said it was working fine. I liked its typical 1960-ies design. It was nice that it had an FM band. So I bought it.

I collected the radio. It was quite heavy, a sturdy German radio. I took a look at the inside It was complete but one thing struck me: there was some greenish corrosion on the PCB. I thought I needed to make a proper evaluation of this radio, and maybe cleaning and repairs would be needed. After this, I put it on a shelf and forgot about it for two years. I had a look at it every now and then but I didn't feel the need to get my hands on it. Until now.

This radio has 11 transistors:

• AF106 as RF pre-amplifier for AM and FM.
• AF135 as oscillator and frequency changer for FM,
• AF136 as oscillator and frequency changer for AM, or first IF for FM,
• AF138 as first AM IF or second FM IF,
• AF137 as second AM IF or third FM IF,
• 2xAC122 as AF preamplifier and driver,
• 2xAC117 as output stage,
• AC122 as AGC control amplifier,
• OC45 as AFC stage.

This is a portable radio that can also be used as a car radio. One simply slides the radio in a mounting bracket. On the bottom, there is a connector to connect to the car battery and loudspeaker. On the side, there is an antenna plug. I wasn't planning to use this as a car radio but I found that an interesting detail.

When I tried the radio after 2 years, it was completely silent.

I looked for a manual on the Web, but couldn't find a suitable manual for the “Parade” model. But when I searched for the type number 94.720, I found a model “Blaupunkt Derby 94720” that looked exactly like my radio. There were sources in Japan and Germany citing this Blaupunkt Derby model. Even on the NVHR schematic library there was a Derby 94720 model. Apparently, Blaupunkt had once used a different name for this model.

I found out this radio is not really built for easy servicing. Although its weight suggests sturdyness, several plastic mounting components were broken, and then there was this green stuff on some of the boards.

After detaching the speaker from the frame, I saw that one of the speaker wires had come loose. I resoldered it and got sound for two minutes, and then none. Using a signal tracer, I found there was signal on the output transformer. Measuring continuity, I found that one of the speaker wires wasn't connected to the transformer. How come? The schematic shows a switch, one of the 3 sections of the AFC switch. It switches the speaker from one tap of the secondary to another. I don't get the relationship of output amplifier loading and AFC, but why not?

The switch is soldered to a small circuit board. That board is carrying oscillator coils and trimming capacitors and the AFC and range switches. I had to detach the ferrite antenna to reach the contacts of the switch. Measuring continuity, the switch seemed faulty or disconnected. The manual points out there are soldered vias on the double-sided PCB. Maybe one of them had a dry joint? The terminals of this switch were barely reachable to solder but I could get to them reflow the solder joints. Then the speaker had a constant connection and the radio was working properly. Looking back, this might also have been a case of bad contacts that could have been fixed with contact cleaner on the switch.

After reattaching the ferrite antenna, I made several small repairs to the plastic mounting pieces of the whip antenna. Maybe this radio had had a fall some time ago. Using acetone and heat, and a piece of wire embedded in the plastic, I rejoined the broken pieces.