It isn't uncommon for high value vintage resistors to go well out of tolerance, but I was surprised to find that several were more than twice their original resistance value. Some of the smaller value ones were also way off, so I decided to change them all. For this I used 2 Watt 5% carbon types; the larger size of these makes them easier to work with, and they don't look as odd as modern 0.25W ones would.
As expected, the two 10uF electrolytic cathode bypass capacitors were both very leaky. I removed these completely by drilling out the rivets securing them, and replacing them with modern axial types.
The HRO has balanced valve heater wiring to reduce hum. A tapped wirewound resistor is wired across the 6.3V AC heater supply, with the tap at the middle of the resistor connected to chassis. Unfortunately the paxolin former of this resistor was broken. According to the manual, the balancing resistor is 64 Ohms with a rating of 3 watts, so I replaced it with a pair of 33 Ohm wirewound resistors mounted on a tagstrip fitted to the side of the chassis.
Whoever had done the previous work on this set obviously didn't share my obsession with tidyness, so the component replacement also provided a useful opportunity to tidy up the wiring. Many of the solder joints were also of questionable quality and were reworked. All new component leads were sleeved, and anchored mechanically by wrapping round the tags before soldering.
This audio amplifier valve in this HRO-MX should have been a type 42 pentode, but it had been replaced by a 6V6GT as used in the later HRO-5. Comparing the circuit diagrams of both models showed that the cathode resistor used with the 6V6 should be 300 Ohms, and not the original 500 Ohms which had been kept when the valve was changed.
In the next update I'll describe the modifications to the rear connector wiring and the addition of the loudspeaker output transformer.
Keep listening!
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