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Technical Tips from Mark W. Persons
Dayton AF315 AM Receiver Modifications

This is a Dayton Industrial AF315 AM Receiver. 

It is a very good choice for EAS monitoring, off-air monitoring, and now for AM/FM repeaters.

However, Dayton Industrial is no longer in business so new ones are not available.

One small problem the receiver has is poor low-frequency audio response and high distortion on audio frequencies below 100 Hz.  The situation gets worse as the received signal strength is increased.  The receiver AGC (Automatic Gain Control) is the culprit.  It was trying to change receiver gain while following low frequency audio.  The fix is to install a 1000 mfd/25 VDC capacitor in the AGC circuit to slow it down.  Here, it is connected to one of the "S" pins, which are sometimes used for an external signal strength meter.  The positive side of the capacitor goes to the "S" pin. 

An additional problem was a peak in audio frequency response of about 6 dB at 180 Hz.  The capacitor solved that problem too. 

But wait, there is more to the story!  You could choose one of two IF (Intermediate Frequency) filters when ordering a receiver, and one of two post-detection de-emphasis audio filters by jumper option in the receiver.  This chart gives measurement data on a typical Dayton AF315 Receiver and compares it to the NRSC (National Radio Systems Committee) recommendations that were later adopted by the FCC.  The plan is for audio to be pre-emphasized (high frequency boosted) when it is broadcast and de-emphasized (high frequency attenuated) at the receiving end to reduce noise when audio is heard.  If the standard is followed exactly, the result is flat frequency response.

Most radios today have the audio high frequency response rolled off sharply after 3 or 4 KHz to reduce noise and listener complaints.  They use narrow band IF components by design.  Wide band IF receivers can only work well when the received signal strength is relatively high and when there is little nearby noise to interfere with the desired signal.   

Testing was done in January of 2011 while using the above mentioned 1000 mfd capacitor in the AGC circuit.     

Audio Frequency Response

Wide Band IF

Narrow Band IF

50 us
75 us
50 us
75 us
Low frequency response
with AGC capacitor added
30 Hz 0 dB -5.8 dB -5.8 dB -5.8 dB -5.6 dB -0.9 dB
50 Hz 0 dB -2.5 dB -2.6 dB -2.6 dB -2.5 dB -0.1 dB
100 Hz 0 dB 0.0 dB 0.0 dB 0.0 dB 0.0 dB 0 dB
1 KHz -0.75 dB +0.9 dB +0.4 dB +1.3 dB +1.2 dB  
2 KHz -2.5 dB -0.1 dB -1.1 dB 0.0 dB -0.7 dB  
3 KHz -4.3 dB -0.1 dB -3.0 dB +2.3 dB -3.8 dB  
4 KHz -5.6 dB -1.4 dB -4.8 dB -4.7 dB -6.6 dB  
5 KHz -6.9 dB -4.1 dB -6.4 dB -17.2 dB -19.3 dB  
6 KHz -7.8 dB -5.6 dB -9.0 dB -29.7 dB -32.0 dB  
7 KHz -8.6 dB -7.1 dB -9.6 dB -40.7 dB -41.8 dB  
8 KHz -9.3 dB -8.6 dB -11.8 dB -45.7 dB -46.8 dB  
9 KHz -9.7 dB -11.2 dB -14.2 dB      
10 KHz -10.0 dB -14.6 dB -17.3 dB      

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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