Decca Portrola 52 Vintage Radiogram

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The Decca Portrola was a table top transportable radio gram that was contained within a wooden case with side handle similar to those on suitcases and this enabled it to be handled more easily.

The Portrola was developed by Decca in London, and it was introduced in mid 1952 at a time when life was still getting back to normal after WW2.

This version of the Decca Portrola is sometimes referred to as the Portrola 52 in view of its launch date because there was also a pre-war Portrola radiogram as well.

This vintage radiogram was a real luxury item and provided good reproduction of both records and long and medium wave broadcast transmissions.

It also has a place in my memory as one was given to me in the mid 1960s when someone was sorting out and dumping some old items they did not want. It worked well for me for a number of years when I was young and could not afford to buy anything else!

Decca Portrola radiogram specifications

The Portrola performed well for its day and it was able to provide a "nice" sound from either the radio or the record player sections.

** Frequency coverage given in metres as detailed within the specification and on the dial.

Decca Portrola circuit overview

The circuit for the Portrola consisted of a total of five valves, four for the radio and audio aspects, and one as the full wave rectifier for the incoming power.

  •   RF and antenna

the signal was picked up by a frame aerial that was contained within the lid of the radiogram. This meant that it was normally better to operate the set with the lid up to obtain the best signal.

To gain the best reception, it could be necessary to rotate the set, although for soemthing of this size, that might not have always been viable.

To overcome this issue there was provision for the use of an external aerial and this used a frame coupling winding to link it toth e RF input.

The RF and oscillator stage used a well proven topology of a triode hexode for the RF amplification, mixing and local oscillator. This was based around V1 which was an ECH42.

The RF input and oscillator used a ganged tuning capacitor to ensure good tracking of the RF tuning with that of the local oscillator, and there was a switching arrangement too switch between long and medium wave.

Interestingly, both ends of the coils for the oscillator are switched to provide compete isolation when not in use.

As usual with this circuit arrangement, the anode of the hexode section had an IF transformer tuned to the intermediate frequency, which for this set was 385kc/s.

  •   IF amplifier

The IF amplifier consisted of a single amplifier stage based around V2 which was an EBF80.

The IF amplifier uses the pentode section of this valve providing the gain required for this section.

The incoming signal from the first IF transformer which is tuned to the intermediate frequency is applied to the control grid and there is another tuned IF transformer in the anode circuit of V2.

The combination of the two transformers gives sufficient selectivity for this radio as a broadcast receiver.

  •   Demodulator

The demodulator uses the two diodes contained within V2, the EBF80. One diode provides the signal detection diode presenting this audio to the audio amplifier.

The second diode is used to generated the AVC - automatic volume control. The voltage which is generated has the audio removed using a filter or smoothing arrangement so that only the voltage element indicating the level of te carrier of the incoming signal is retained.

The AVC voltage is then applied to both the RF input hexode and the IF amplifier. This ensures that signal level at te output of the IF amplifier is maintained at a relatively constant level regardless of the incoming signal strength.

  •   Audio amplifier

The audio amplifier is more comprehensive than one that might have been used for just a radio.

The first section based around the first half of V3 (V3a), a 6SL7GT which is a double triode. This acts as the preamplifier for the gramophone pickup.

It provides the equalisation required from the magnetic head to give a flat (well flat-ish) response at the output.

Different pickup heads are used for 33 / 45 rpm and 78 rpm. These have three lines and they are wired such that the right equalisation is given for the head being used.

The second half of the double triode, V3b, is used as the first element of the audio amplification - the input being switched between the radio and gramophone input from V3a as required.

The output from V3 is then applied tot he output pentode based around V4 which is a 6V6GT output pentode.

The output from this is then connected to the internal loudspeaker using the audio output transformer.

The elliptical speaker gave reasonably good reproduction as it was a reasonable size - I can't remember how large exactly.

Tone control was provided in an interesting way. A proportion of the speech coil voltage was developed across a resistor network and fed back to the V3b first audio amplifier. Adjustment using the tone control gave tone correction.

  •   Power supply

Power entered the equipment and was transformed to the required values using the input transformer which also gave isolation from the mains or line input.

The full wave rectifier valve had both anodes strapped together to give half wave rectification and the output smoothed using a pi network of an inductor and two capacitors.

Operation of the record player

The record player section was very easy to use, although a little basic by today's standards, but it has to be remembered when it was introduced.

The speed selector was ver straightforward - it was located on the record deck itself and had three positions. One was fr 33 rpm, another for 45, and the third for 78.

A different head was required for 33 and 45 to that for 78 rpm. However this old one was easily unplugged from the end of the arm on the record deck and a new the one carefully inserted. A clip was conveniently placed for the unused head.

To activate the turntable and start the motor, the arm would need to be lifted and moved towards the side, and it would automatically stop when the arm moved into the centre of the record. This did mean that it came to rest actually on the record itself, so it was wise to removed it reasonably soon after the record finished.

Another word of warning was that even though with the lid down it provided a better sound, this should not be done with a 12 inch LP playing as the clip used to hold the arm in place when not in use would send the head flying across the record leaving a nice scratch.

I learned this the hard way when I had just bought my first LP, and after this, I always had the click, click . . . . on my Beatles Rubber Soul LP thereafter.

The Decca Portrola was an interesting radiogram. It had a number of interesting features and worked well. It certainly provided me with good service for a good number of years.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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