Your problem reminded me of an amp I fixed remotely for a ham in this
county. 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. Actually the schematic shows three
resistors in parallel. K7 isn't shorting out R2 so you have that really
lousy voltage regulation and R2 is being burnt up. In Collins rigs the
coil tends to go open on the relay, but the three series resistors could
be nearly open or the electrolytic, C5, shorted, or the relay contacts
damaged. There is a fuse supposed to protect the series resistor, but
you may have put in a larger one. The amp in this county had burned up
the resistor that dissipates a lot more power than its rated but only
for a few seconds so it generally doesn't overheat.
73, Jerry, K0CQ
On 12/2/2010 11:08 PM, Carlin Royal wrote:
> Hello group,
>
> I am having an issue with my Titan 425 that has me baffled. Upfront let
> me say I am only a beginner with electronics and have limited equipment
> to work with. The symptoms are obvious voltage drop being shown on the
> panel meter/s of the RF deck itself. Upon a successful startup/warm up
> everything appears normal. High voltage registers around 2.7kv and the
> low voltage (assuming 28v) panel lights are nice and bright. When I
> attempt to key down and tune the amp with basically any amount of input
> power (say 10-20w) the panel lights nearly go dark and my high voltage
> drops well below 2kv and more.
>
> About 3 years ago I reported a somewhat similar problem with blown fuses
> after an earlier problem as well as having a terrible buzz on my signal.
> I bought a new Peter Dahl transformer, all new filter capacitors, and
> ordered the new "upgraded" rectifier board from TenTec. These were all
> installed by a reputable repairman and the amp worked great for several
> months. Then this issue with voltages started sporadicly. Sometimes when
> you turned the amp on and operated it, it would work absolutely fine for
> hours. The next time you turned it on, it might do as described above
> with the voltage drops. Finally after about a month or two of this, the
> am now just has the voltage issues.
>
> After much thought with what little knowledge I have on these things,
> that the K1 power relay my either be weak and/or had dirty terminals. I
> removed it, disassembled and cleaned the contacts as well as verified
> the functionality with a disparate 115v source through the coil. It all
> checked good. I have also verified my supply voltage at the outlet and
> checked for any loose wiring in the shack. I have also re-seated all the
> connections in the power supply leading out of the new transformer.
> While in the power supply however, I noticed that the large 10ohm 25w R2
> resistor on the rectifier board was burned up,,, so I replaced that as
> well. When everything was back together, I tested the amp again with the
> same voltage drop results as before. As a last attempt and to for sure
> verify K1 was not the trouble, I bypassed it all together and switched
> the amp on simply with the 30amp 220v shack breaker, again with no
> change in results.
>
> I am way above my knowledge level here, and was hoping this reflector
> could give me some ideas and further things to check before having to
> ship this in for repair again. I have the original manual with a
> schematic that I can fumble through, so any advice please reference your
> answers to things I can physically identify and look up in the manual
> and power supply itself.
>
> Thanks in advance
> Carlin
> N5OE
>
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