But the burned up surge limiting resistor points the poor voltage
regulation of both supplies to the soft start relay not taking it out of
the circuit. If the soft start relay was doing its job, the resistor
would not be burned up and poor regulation of the high voltage supply
might not show on the 28 volt supply unless the keying relays draw all
that much current.
This Titan is the third amp that has come to my attention this year with
a soft start problem. In the Collins 30S-1 at the Collins Radio
Association, it proved to be the relay coil that was open. In another
brand near here, it was the current limiting resistor that was open. I
can't tell from a distance whether this Titan has a group of three bad
resistors, a shorted time delay electrolytic, an open relay coil, or bad
relay contacts. The last can be tested with power off in several ways.
Simplest is probably to measure the resistance of R2 and then if the
relay K7 is not encased, push the armature to close the contacts.
The open relay coil can be detected by an ohmmeter after unhooking it or
by looking at the voltage on the coil contacts when power is applied to
the amplifier taking proper precautions to prevent HV contact. If the
relay coil is good the voltage will rise exponentially in a few seconds
to about half the 28, e.g, probably 12 or 13 volts if the capacitor
isn't shorted and the resistors are all good. If the voltage rises to 28
volts the coil is open. If the voltage doesn't rise as far as 12 volts
(presuming its a 12 volt rated coil, I don't have a parts list to know
that but the relay is probably marked for voltage) it could be that one
or more of the resistors has gone open or the capacitor has excessive
leakage. The way to check those best requires opening the circuit and
checking them individually. Its not beyond reason to find bad soldering
at on or more of the parts in this delay circuit contributing to the
problem of not enough voltage on the relay coil.
If the relay is enclosed, it might be best to unhook the coil wires and
apply an external 12 volts directly to the coil to see if the contacts
short out the surge limiting resistor. Unsoldering from the coil
terminals has to be done with care to not break internal connections,
its better to break that circuit away from the relay, like on the PC
board presuming the relay is not mounted ON the pc board.
73, Jerry, K0CQ
On 12/3/2010 12:46 PM, Jim Brown wrote:
> 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.
> http://audiosystemsgroup.com/publish.htm
>
> 73, Jim K9YC
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