I recently bought a ratty Heath HR-10B receiver and decided to try some mods on it since there was not much to lose. This is a very basic receiver and has several flaws that generally made it a poor choice for any serious listening. Here are my changes.
1- Removed the BFO from the triode-pentode last IF and built a MPF102 FET oscillator using the original Hartley coil. I used a dropper resistor from B+ and a 9 volt zener as a regulator. Once warmup is complete the BFO is now very stable. This allowed me to add a product detector and not contend with the BFO leaking into the IF signal inside the 6EA8 second IF amplifier.
2- Added a Pullen product detector made from two MPF102 FETs. The circuit is powered by plus and minus 8 volts developed from the filament supply. I used a capacitive voltage divider to reduce the IF signal to the source follower FET. The source resistor is common to both FETs and the output FET has a 10K drain resistor. 470 K gate resistors to ground complete the circuit. The audio out is cap coupled to the cathode of the original detector diode and follows the original signal path. The noise limiter circuit was left unchanged and still works.
3- Added a .38uF cap to the AVC line and dropped the AVC series filter resistor from 1 meg to 180K. I also added AVC to the first IF amplifier. The time constant is a compromise and works well on AM as well as SSB.
4- Tapped off the first P/S filter cap with a 22K resistor and 10 uF cap to provide independent plate voltage for the oscillator. I also replaced the 6X4 with two 1N4007 diodes. The oscillator voltage was varying 10 volts or more as the AVC changed the load on the P/S which caused much instability in the oscillator frequency. With this change the oscillator supply voltage changes less than a volt from no signal to 20 over 9 signals.
AM sound quality is excellent. The FET Pullen product detector works FB on AM, much better than the original diode detector. SSB reception is also good. With the obligatory half hour warmup SSB reception is armchair copy and audio quality is excellent. There is no need for constant fiddling with the tuning any more. AM does require retuning though as seldom are all the transmitters in a round table on the same frequency and a beat note results.
It was a fun project but now comes documentation of the changes and making it neat.
I have converted the orphaned triode section of the 6EA8 to the first audio amplifier. This provided more than enough gain to overcome the lower output from the product detector and also add negative feedback to the audio output.