{Collins} Ailing 75A4 on 20 Meters Only

Hello Gents:  I have a 75A4 that  has only a low level of white noise on 20 Meters.  The other bands are all working well.  The bandswitch contacts are all clean,  the tubes are good, and the crystal for 20 Meters was swapped out to no avail.   Introducing noise on the grids of the front end tubes showed signal passage.   Are there any suggestions as to where the problem lies and how to go about fixing it?  Thanks.  Brian
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