[TenTec] 425 Failure

[Jim Brown]

On 8/15/2011 7:31 PM, Art Trampler wrote:
> Third, anything else I should be on the lookout for and checking?

Art,

Your luck is FAR worse than mine. I've goofed many times and driven my 
Titan 425s with 100W. I see the red light flashing, back off the drive, 
and everything is fine.  Part of my good luck is that I didn't RUN them 
that way, but caught my mistake right away.  The worst that's happened 
to me is failures in the power supply and in key logic circuitry, 
described below, and I've had it happen 2-3 times (although not in the 
past five years).

There are pdf files on my website that tie together the multi-page 
schematics so that the are much easier to follow. This may help with 
your troubleshooting.  There are also notes about things that go wrong 
with HV failures and arcing.  Some are in the power supply, where it's 
common for a couple of circuit traces to vaporize. This is easily fixed 
by using wire to bridge the bad section.

It is also common to have failures of transistors, diodes, and resistors 
on the boards under the tubes (access through the bottom door) when 
there has been arcing.  The transistors and diodes are pretty generic, 
so you look them up and replace with something close.  Check them with a 
good VOM (I use a Simpson 260) making sure that you see all the 
junctions.  There's a power resistor (a few watts) on that board that is 
a meter shunt for grid or plate current, don't recall which that will 
blow with arcing.  That shunt is the only resistor I've ever seen go bad.

These boards are mechanically tricky to get out and back in. It helps to 
tape the tops of the screws on the other side of the chassis before 
removing the boards so that you don't have to restart everything. You'll 
need some short, small gauge screw drivers and a lot of patience. it can 
be done by mere mortals -- I've had them in and out a half dozen times 
over the years with the three 425s I own. :)

The schematics are at http://audiosystemsgroup.com/publish.htm

Note that these are PARTIAL schematics, and they are NOT detailed in the 
RF compartment around the tube and resonating components.

  I've also learned that Ten Tec made running changes over the years in 
details of the switching of capacitors in the output circuit. What I've 
discovered are on the 160/80/75 positions.  If you have to work with 
them, I suggest drawing out what you have, comparing it with Ten Tec's 
schematic, and reproduce what you have in your unit if you must change 
anything. On the other hand, you can be a BIT loose with exact values -- 
it will only affect resonance on these bands, and as I learned after a 
failure of one of them, I was still able to achieve resonance with 
significantly less than the intended capacitance, although I was close 
to or at the limit of the variable cap.

73, Jim Brown K9YC