12-08-2020, 05:56 PM
Merry Christmas; I hope everyone is staying healthy.
I just finished restoring an RCA Model R-28-P. It played well at first power-up except for a very sensitive volume setting.
Question 1: The volume control of this radio controls the bias of the RF amp and 1st and 2nd detectors. No audio until the volume control is turned nearly all the way up and then minor changes result in VERY large audio output swings. Is this normal?
Question 2: Even though the radio played well, I proceeded to adjust the single IF stage trimmers. I injected 175 KHz into the control grid (cap) of the 1st detector while monitoring the 400 Hz output across the speaker voice coil. Adjustment of the two IF trimmers resulted in MASSIVE audio increases; much greater than a 10X output increase. These adjustments resulted in the radio being super sensitive and prone to squeals and oscillation. Detuning the IF calms everything down but that doesn't seem right. What do you suggest? How should the IF be tuned?
Thanks for your help.
Don
I just finished restoring an RCA Model R-28-P. It played well at first power-up except for a very sensitive volume setting.
Question 1: The volume control of this radio controls the bias of the RF amp and 1st and 2nd detectors. No audio until the volume control is turned nearly all the way up and then minor changes result in VERY large audio output swings. Is this normal?
Question 2: Even though the radio played well, I proceeded to adjust the single IF stage trimmers. I injected 175 KHz into the control grid (cap) of the 1st detector while monitoring the 400 Hz output across the speaker voice coil. Adjustment of the two IF trimmers resulted in MASSIVE audio increases; much greater than a 10X output increase. These adjustments resulted in the radio being super sensitive and prone to squeals and oscillation. Detuning the IF calms everything down but that doesn't seem right. What do you suggest? How should the IF be tuned?
Thanks for your help.
Don
