The original HRO loudspeaker unit had an internal transformer, and presented an impedance of around 7000 Ohms to the receiver. The loudspeaker sockets on the rear of the HRO are wired directly to HT+ and the anode (plate) of the output valve, which means that it would be possible to get a nasty shock from them.
The speaker sockets were supposed to be shorted together if the HRO was ever used without a loudspeaker attached (i.e with headphones only). If this wasn't done it could result in excessive screen grid current and damage to the audio output valve.
My first modification was to fit a loudspeaker output transformer to the inside rear face of the chassis, with the safe low-voltage speaker output via the re-wired terminals. The transformer I used was a 'universal' type with several tappings, but fortunately it came with a data sheet showing the different ratios. One of these gave a primary impedance of 6500 Ohms with a 4 Ohm secondary load which is close enough for this application.
Plugs of the correct size to fit these old sockets are difficult to find today, so I decided to swap the sockets for screw terminals. I used an old screw terminal strip which looks the same as other terminals fitted to the HRO.
Further along the chassis, two more screw terminals were provided for muting the receiver during transmission. As this was done by interrupting the HT supply to the receiver, these terminals form part of ther HT wiring and carry full HT voltage.
The two wires to the terminals were simply disconnected and transferred to a new tagstrip, installed inside the chassis near to the original connections. This method ensures that the wiring can easily be restored in the future if required.
I'm getting close to the point when I can apply power and see if it works. With a set of this age I suspect that there may be a few problems ahead, but perhaps I may be pleasantly surprised.
More to follow... so keep listening!
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