[MRCA] Watkins Johnson blues

Ray Fantini RAFANTINI at salisbury.edu
Wed Oct 11 10:37:46 EDT 2023


Ok, here is an opportunity for everyone to demonstrate how much smarter than me they are. I have been struggling with this circuit for about two months now. It's part of the time base reference in a Watkins Johnson receiver. They divide down the 1.0 MHz reference and use that to provide a 50, 10 and 1.0 KHZ source for locking up all the different VCO in the receiver including the BFO oscillator. Fairley standard trick in WJ world, its built using all 74LS196 programable dividers. WJ loves that chip being it's all over many of their products. I labeled three points on the drawing. Point A is the 250 KHZ signal that feeds U-19B, point B is the 50 KHZ that feeds U-18B and point C is the 10 KHZ signal that feeds U-20 along with being the 10 KHZ source for the rest of the radio. U-20A and B provide the 1.0 KHZ source for locking the BFO.
What happens is after the receiver is on for about fifteen minutes the signal at point B disappears, point A is good and point C also disappears, but without an input to U-19B that would be expected.
All the devices, U-18 and U-19 have been changed and that makes no difference. Looked at all the traces and don't see anything obvious but have noticed that someone had come in before me and swapped the same components along with installing sockets for the chips. Checked the supply voltage (VCC+) and that's good with no noise. Did find a week connection on the VCC+ for U-18 because when the socket was installed the thru the board trace failed.
Speculation at this point is that it has to be a failure of a trace on the board so am in the process of installing jumpers on the back side of the board but beyond that have no idea what's going on here.

Ray F/KA3EKH
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