OldRadios

Trio established their Kenwood corporate brand in December 1980.

The Trio 9R59D was on sale in 1968 for the sum of GBP 35.00 (“thirty five quid”). I see that the same advertiser was selling the Admiralty B40 for GBP 22.50. The Trio uses a fairly conventional circuit with 8 valves. The RF stage is a 6BA6 (EF93) followed by a 6BE6(EK90) then two IF stages using 6BA6(EF93). The detector for AM is a germanium diode and the audio stage is half of a 6AQ8(ECC85) followed by a 6AQ5(EL95). The local oscillator is half of a 6AQ8(ECC85) and the beat frequency oscillator is the other half of the 6AQ8 (ECC85) audio valve. This set was fitted with an 0A2 voltage stabiliser.

The set has main and bandspread tuning, a scheme that was quite popular at the time, because it allowed fine tuning without a lot of mechanical complexity. Other controls are RF gain and a noise limiter. There is an antenna trimmer. Wavebands are 0.55-1.6MHz; 1.6-4.8MHz; 4.8-14.5MHz and 10.5-30MHz.

This set has two mechanical filters. Alas, this does not resolve to two different selectivity settings. The mechanical filters look like transistor IF transformers and follow one after the other in the IF chain to provide a single selectivity setting of about 3.5kHz at 20db. Translating this into English – the selectivity is too sharp for decent audio quality unless you tune to one side of the received station's carrier. Even then, it is quite distorted. The set does work quite well on SSB on 40metres. Although I don't like modifying radios, I would remove the mechanical filter(s) and put them somewhere safe, replacing them with conventional IF transformers - if the set were mine.

In the radio presented to me for repair, one of the mechanical filters had perished through corrosion. I did manage to fix it, but for a while I substituted a standard transistor IF transformer. I have to say, I preferred the IF transformer to the filter, but then, I usually listen to shortwave broadcasts, not SSB or amateur transmissions. I should add that mechanical filters generally can't be repaired – so please don't bring me all your old filters. It was sheer chance that this particular one was repairable.

This particular radio also suffered from “microphony” - audio from the speaker would feed back into the audio amplifier circuits, creating a howl, if the volume was turned too high. I don't think the printed circuit type construction can be blamed for this, rather it could be the choice of valve (ECC85).

The BFO circuit was also troublesome and although a new ECC85 seemed to cure the problem, I am not sure we have got to the root cause.

This was an inexpensive radio in its day and at the time SSB was all the rage. The designers chose to provide economy SSB reception with reasonable sensitivity and operational ease of use.