View Single Post
Old 21st May 2019, 10:02 pm   #148
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: More Chinese AM radio kits

Quote:
Originally Posted by Avid_Nerdlinger View Post

"Mostly, it appears, that the stages, specifically the IF stages, are unstable and are oscillating." - isn't it more likely the audio stage?
Possibly, it would take the audio stage to be oscillating at radio frequencies and be hetrodyning with either the received carrier or the IF signal. Its easy to find out, for that just temporarily connect a capacitor such as a 1000pF (probably 100pF would do) across the collector to base of the transistors in the audio stages. That will kill any RF response and tell you the answer there. If that is it just reduce the values if the audio sounds muffled, until it is clear. Of course with a scope it would be dead easy to see if there were oscillations coming out of the audio amp.

Motorboating, if its truly that is often just a common impedance in the power supply feed to the audio output stage and its driver stage, meaning not enough power supply rail decoupling between the two. It can also be due to a high resistance power switch or poor filtering capacitor across the battery.

Whistling can be a sign of IF instability or oscillation. In this case to tame that down you cannot add base to collector capacitance as it will make it even more unstable. One good trick is to add a resistor in series with the transistor's collector, say 47 to 100R. This will suppress any HF & VHF oscillations. A 100R series base resistor can also work too.

An IF can also oscillate if there is a feedback pathway due to poor filtering of the AGC line. If there is just an electrolytic cap only, filtering the agc line, then try connecting a 0.1uF ceramic cap across it.

Last edited by Argus25; 21st May 2019 at 10:17 pm.
Argus25 is offline