There are many changes previously outlined for the R4C, and it can get confusing. There is a reason for this, for there are a number of alternatives, and what you do depends upon how radical you want to get. This is complicated by the serial number of your receiver. Early model R4C had a lower voltage transformer, and it isn't easy to get the secondary voltage up high enough for a voltage regulator IC. That won't happen unless something is done in the audio section to drop the current consumed by the audio output stage.

R4C Audio Section Summary

If you want to 'improve' the audio ....

• C100 Change
• Bypass the anti-VOX line with a .005 uF
• Route secondary transformer grounds to the filter caps.

The first 2 items may be all you need.

If you want to improve the audio and limit heat

• EP487 and zener diode regulator/pass transistor
• C100 change
• Bypass the anti-VOX line with a .005 uF
• Route secondary transformer grounds to the filter caps.
• LM380/LM383

And last, if you have a later model R4C ....

• Sherwood audio board and reguletor
• Bypass the anti-VOX line with a .005 uF
• Route secondary transformer grounds to the filter caps.