The AM Forum

From Don, N4WY


Problem: TS-440 with popping and crackling noises in audio especially on AM.
 
With carrier adjusted for 20-30 watts you get reports of popping noises in the transmitted audio. Moving wires about within the IF board area appears to cause the noise leading one to believe a bad connector or solder joint is to blame. As the condition gets worse with time, the carrier power will fluctuate (e.g. 20 watts jumping to 50 watts and sometimes down to a watt or less all with a few seconds timespan). CW and FM modes are not affected. SSB appears unaffected unless the carrier is not quite nulled out and the sound can be heard on top of the carrier.
 
The Fix

In my radio all signal tracing pointed to the balanced modulator IC (455 khz). The actual culprit is VR8 – one of two carrier null pots. Adjusting the pot throughout its range revealed a bad area causing the noise and carrier variations. Replacing the 10K pot solved problem. Alternately, adjust the pot for max carrier null – in concert with VR7 of course. Remove bad pot VR8 and ohm it out noting resistance values on both sides of center. Using two resistors of the same values just noted and in the correct position you can “lock down” the adjustment to a fixed value assuming you don’t have the correct 10K pot with proper dimensions on hand. The “course” adjustment seems to come from VR7 anyway. VR7’s null is very critical and only a hairline adjustment brings about maximum null – in my radio anyway.

Hope this helps other owners of the TS-440.