[TenTec] Model 963 questions

[Stuart Rohre]

Richards,
OK, 850 kHz is a more expected harmonic of a switcher.

Since it is two years old, I am thinking maybe you have a previously 
good filtering capacitor starting to change value or go open circuit, 
increasing the inherent harmonics of the switcher.

You may need a scope to look across each capacitor. You would need an 
isolated (for AC) high voltage probe in the oscillator section, but that 
might also load the circuit and stop or change the oscillation 
frequency.  Likely  the main suspect capacitors will be electrolytics in 
the section after the bridge rectifier, ie the DC section.

You can look at those with a regular 10:1 scope probe, since one side of 
the filters will likely be the negative DC terminal.

Look for discoloration or signs of leaking of any capacitor in the 
supply.  Look for any bad solder joints or loose connections leading to 
loss of filtering.

You might consider sending the supply in for repair with a good complete 
description of your observations, but call them first to discuss the 
apparent problems.  Switchers are the hardest supply to troubleshoot and 
repair, if you are not familiar with their circuits and operations.

There is also a slight chance you have a bad diode in a bridge 
rectifier.  One diode open might upset the apple cart.

Also, have you changed the DC power lead length or size of conductors 
just before the problem was noticed?  Sometimes rearranging a shack 
brings undesired problems to notice.

Other things in this category are rearranging and thus getting the rig 
too close to a switcher supply, thus enhancing magnetic coupling.

Once you have a Good 963, (can you borrow another in your area?) you can 
do side by side comparisons with a scope and speed your conclusion of 
what is right or not.

Good Luck!
73,
Stuart Rohre
K5KVH