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Blaupunkt Stockholm 25.335 (1966)

This Blaupunkt stereo radio has a lot to offer but it has been built a bit on the cheap. The things that make it look cheap are the chipboard case, the fact it has only 2 IF stages on FM and that it uses PCB's and cheap switches. On the other hand, it has a stereo decoder with valves and its sound is really good.

This Blaupunkt Stockholm is a low profile radio. It has the left channel speakers in the radio case while the right channel speakers are in a separate speaker case. The Stockholm 25.335 looks to me both expensive and cheap. It has a nice veneered case and follows the "Scandinavian" style of radios and furniture in vogue during the 1960-ies. It must have looked modern at the time. But the material of the case is chipboard and the veneer looks a bit plasticky. The veneer is a very opaque type of wood, which makes it look a bit dull. Some people think that it is a plastic coating with a wood pattern, as I found out reading an advertisement of somebody offering this radio for a ludicrous price of $85,-. But it definitely is veneer. I checked the structure using a strong lense (the 10x objective of my microscope) and saw the pores in the wood and broken wood fibres on places where small pieces of veneer have chipped off. On two places on the top the veneer is slightly cracked on a spot where the glue holding it to the chipwood has detached, in a way only wood veneer would crack. Scratches on the surface reveal wood, not an evenly coloured plastic sheet. So I was confident and polished the case using Teak oil. By the way, never do that to plastic “veneer”: the printed wood pattern would probably be dissolved an you would get ugly spots. Comparing the veneer to a wood reference book, I think it might be ...

The actual radio (excluding the stereo decoder) has 6 valves:

It also has some semiconductors:

The stereo decoder is identical to the Grundig Stereo-Decoder 6 using two valves and a number of semiconductor diodes:

I got this radio for free from a collector. I had expressed interest in some Grundig stereo decoder 6, which he was offering. As he needed to clear some space, he also gave me three German radios that use these decoders. So I decided to take the challenge (once I found some spare time) and fix these radios and align the decoders. As a measure of preparation, I built an FM stereo transmitter, the SUP1 kit by ELV.

I selected this Blaupunkt radio as the first one to fix, because it has this separate speaker box allowing for a wider stereo sound. I started the repair and peeked inside. One of the output valves was missing, so I dug up a spare ECL86 and inserted it. I got good reception on AM but only weak sound on FM on one channel. Using the mono switch resulted in the pilot lights dimming, so something was causing a short on the heater supply. That was odd. There also was quite some hum on the output. I tried unplugging the decoder but then only weak sound without bass remained. Of course: unplugging the decoder will interrupt the signal path to the AF amplifier. I tried FM reception with a “mono decoder”, i.e. a plug that connects both channels to the output of the FM detector. Now FM reception was fine.

I suspected the power electrolytic to be the cause of the hum. I checked its capacitance, which was a bit low and replaced it. This radio has a silicon rectifier, while the parts list lists a metal rectifier. I found the HV a bit too high, so while I was at it, I added a 39 Ω resistor to reduce the HV supply.

Chassis frontal view

Frontal view of chassis taken out of case, dial glass removed.

After this, the hum had gone. I inserted the plug of the stereo decoder and had to straighten a few pins that were bent. When I tried the radio again, I had sound on both channels. Trying the mono switch, I had no short circuit any more. Apparently, the plug of the stereo decoder had been inserted the wrong way, causing a short on mono. I checked the output amplifiers and noticed the left output valve was a bit weak and replaced it.

Now was the moment to align the stereo decoder. I tried the cores of the inductors, but they were all stuck. I checked their resonance frequencies. They were off, but not too much. I decided to leave them because I didn't want to break the cores. But I took a go at aligning channel separation. There are three trimpots for this purpose. One must set these for minimum output on one channel, when a signal is only applied to the other channel. I used a scope to measure the output signal. Unfortunately, the image on the scope was very confusing. I had to turn two of the trimpots to their maximum to get minimum output. I suspect the channel separation of the signal from the SUP1 test transmitter is not good enough for this.

The last job I had to do was to fix the tuning string for AM. The string was worn and almost broken. I took a fresh length of tuning string and copied the old string. After I changed it, it didn't run well. In this radio, the AM tuning string only has half a loop around the shaft of the tuning knob. This doesn't give much friction. There is no room for one more loop. I noticed the old string felt a bit waxy. So I used a little bit of furniture wax (without silicones!) to add a bit of friction. That worked fine.

The radio is playing ok now. The sound is good, nice bass, but some noise on FM. I'm quite happy with this last-generation valve radio with a real valve stero decoder.

Copyright © 2010 by Onno's E-page         published 2010-05-05