I have an RCI 2950 first generation with low receive in AM only, SSB receive is fine, any ideas would be greatly appreciated.
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RCI 2950 1st Gen
- Thread starter Wky Gatekeepr
- Start date
More than once I have seen C25 go open after 15 or 20 years. Easy enough to check, just poke the solder pads under C25 with the leads of a new 10uf cap. If the AM receive comes way up when the leads of the new cap touch the foil pads, C25 is open.
C25 serves to allow negative feedback for the DC voltages only in that section, but not for signal voltages. If C25 stops filtering the 455 kHz signal, it reduces the gain.
73
C25 serves to allow negative feedback for the DC voltages only in that section, but not for signal voltages. If C25 stops filtering the 455 kHz signal, it reduces the gain.
73
Thank you very much, I was looking at schematic tonight and saw that, I will definitely check it in the morning.
Thank a for the advice
73’s
Fixed mine, C25 was ok but I replaced anyways, But C88 was shorted, replaced and all is well.
Radioman56
Motorola bench-tech
Yes, I've also had that capacitor (C25) go "open" (ie: down into the nF range.!) which causes very weak (microvolt-wise) and very quiet (volume-wise) AM receive.!
But it's more because while the paralleled resistor (R32) allows the Emitter of the last 455 Khz amplifier transistor (Q11) to pass DC current to ground and providing some amount of negative feedback (DC wise) to its Base.., the paralleled capacitor (C25) is then providing an RF ground for the Emitter of Q11 in order to amplify the 455 Khz signal to a much greater degree (than without it).
This is the same way that the Cathode of a vacuum-tube amplifier-stage is (sometimes) connected to ground through a paralleled resistor and capacitor.!
Wherein the resistor is providing a DC path to ground (for the Cathode) and providing some negative feed-back (and / or Bias-voltage) to the Grid, while the capacitor is allowing the AC signal to by-pass the resistance to ground, therefore allowing for more "gain" of that stage of amplification.!
73s from one techy to another.!
That is the difference between DC Gain (resistor) and AC Gain (added capacitor) of a circuit. This increases the gain of the circuit for AC amplification.
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