PAM -Unknown Battery Radio Video
Written by Bryce Ringwood   

Before the introduction of transistor radios in 1954,  post-war portable radios used  miniature B7G based "battery valves". This PAM radio is a little unusual because it is intended to be a domestic table-top radio.

I was absolutely unable to find any information concerning this set, so had to rely on some native intuition to get it to work.

The first problem with servicing these sets, is that the batteries are no longer made. You can power them from a home-made battery pack built up from a 1.5 volt "D"cell and 10 "PP3" 9-volt batteries wired in series (Or make a replica battery containing these), but the cost nowadays could be in the region of R400-00 ($40-00) and the life expectancy would be about 20 Hours (assuming the PP3 has a 250mA-Hr capacity). 

While I was making sure the set was repairable, I used a battery for the filaments and a bench power supply (actually my infamous home made valve tester) to provide the HT.

The first problem encountered was removal of the knobs so that I could access the chassis. Quite a few radio owners believe that the best way to prevent radio knobs from coming loose is to glue them on to the spindle using a spacecraft structural adhesive only known to themselves and NASA. The only safe way to remove the knob is to split it in two using a Dremel with a diamond cutter and then prise the two halves apart with a screwdriver. This leaves you with a knob that has to be replaced and a split spindle, which can still be used with a new knob.  

A previous owner had deemed it neccesary to shuffle the valves around, so that the detector was in the IF stage socket and vice versa. This is quite a common problem. Once this was sorted out, the radio worked quite badly, because there was no AGC to speak of. Replacing the IF valve with a new one cured the problem. Finally, the set needed a new speaker. I used a modern Japanese 4"speaker. 

The only remaining problem with this set, is that we don't know for sure what the valve set should be. I think it needs a DK96 (mixer/osc), DF96 (IF), DAF96 (Detector/st Audio) and DL96 (Output). I replaced the 1T4 that was in the set with a new DF91. (1T4 equivalent). The AGC returned.

Finally, I made a battery eliminator (see projects - includes a circuit diagram for a similar set ) and finally tested the radio.  All in all, it performed quite well, bringing in stations like "Radio Japan" and of course the BBC with ease. It also worked well on Medium Wave.

These small table radios seem to be popular in bar areas and make an interesting talking point. Its probably best to check whether it is a battery or mains set before embarking on a purchase.



Click to hear Mario Lanza on the PAM. The cameraman's hands were a bit unsteady.


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