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Re: [TenTec] Titan 425 Major H/L Voltage drop

To: tentec@contesting.com
Subject: Re: [TenTec] Titan 425 Major H/L Voltage drop
From: Jim Brown <k9yc@audiosystemsgroup.com>
Reply-to: k9yc@arrl.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 03 Dec 2010 10:46:03 -0800
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
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 
both supplies.

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
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