On 12/2/2010 9:45 PM, Dr. Gerald N. Johnson wrote:
> In the 425, K7 and the fried resistor are a soft start circuit
> that slows the turn on capacitor charging current surge and on the
> TenTec schematic it's called the surge relay. K7 is energized from 28
> volts DC through a series resistor and a big shunt electrolytic to make
> it close after a few seconds.
The Titan 425 has two power supplies -- one that supplies 28VDC to run
the logic and control circuits, and the 2.7kV supply for the output
stage. The dial lights run from the 28VDC supply, so if they are
dimming, it tells you there is a problem with the 28V supply. As Ron
and Jerry have suggested, that soft start circuit (which is on the 240V
side of the power transformer) is the main component that is common to
On the other hand, I wouldn't rule out one or more bad filter capacitors
-- one in the 28V supply and one or more in the 2.8kV supply. They are
easy to test and easy to replace, but you do need to find exact
replacements so that they fit. Luckily, they are available from good
industrial electronics houses like Allied (where I last bought them).
To test them, find a good analog VOM (Simpson 260 or equivalent,
something that has a meter with a pointer and good Ohms scales). Let the
amp sit with the power off for 30 minutes or so, then connect the meter
probes to each capacitor, one at a time, watching how strongly the meter
moves as you reverse the probes. A lot of movement means a good cap. Run
through the string of eight caps, looking for one or more with weak
movement. That will be the bad cap. It is NOT necessary to replace all
the caps unless you feel rich. It is common for one or two caps to be
bad after they have been sitting for a few years without voltage applied
to them. The symptoms will be HV that drops more than it should when
you transmit, and hum on the transmitted signal.
There is a set of Titan 425 schematics on my website that are a bit
easier to follow than the ones printed in the manual.
73, Jim K9YC
TenTec mailing list