[TenTec] can't service the omni-VI mixer

[Stuart Rohre]

NTE and other cross reference general replacement semiconductor 
companies should have crosses to the MPS devices.  Don't overlook 
Mouser, distributor.  They have vast stocks and fast service at 
reasonable prices.  There are still surplus electronics places like 
Electronic Goldmine that often have out of production items.

Substituting bipolar transistors really comes down to getting the right 
polarity, NPN or PNP and other major parameters.  A current rating and 
voltage rating close to the original, and frequency cutoff similar to 
the original, or higher than the application you are replacing.  Get the 
Beta as close as you can, but remember, there are wide ranges of beta 
that can do the job.  The mixer may not be a high gain stage, due to its 
intended function.  Most cases of abnormal noise would not be in the 
active devices, (transistors) but in connections, or in faulty passive 
components, resistors, capacitors, and other such components.
A rebuilt mixer board that is close to the original, is better than a 
non working mixer in a radio just sitting on a shelf.

Don't overlook loose plug in connections from the board to the 
motherboard.  Sometimes connectors may wear to cause intermittent noise.

The wiggle test helps here.  If wiggling something causes a noise, look 
closely at what moved.  Don't be too agressive, but the more experience 
you get with troubleshooting, the better your "touch" will be at finding
causes of small problems.

-Stuart
K5KVH