Hello,
Need help in trying to isolate the problem with a homemade GS-35B amp.
The amp. worked okay 6/7 January in the ARRL RTTY Round-Up. Today,
the symptoms are:
* SWR varies as the load capacitor (C2) is decreased when
operating into an antenna (<1.4 : 1 SWR). Into a dummy load, the
output increases until a value of C2 is reach and then the output is
steady as C2 is, further, decreased. SWR does not change. (SWR
meter is a LP-100 Digital Vector RF Wattmeter)
* The tune capacitor (C1) does not produce a noticeable increase
in the grid current as its capacitance is increased or decreased
after C2 reaches the point of minimum capacitance for maximum output.
* The output is ~900 W for 3500 V at 750 mA. Previously, this
plate voltage & current would produce ~1400 W output.
* Half as much drive needed for twice the plate current. The
input is ~35 W rather than the 80 W previously needed to reach the
750 mA plate current.
* Grid current is only 150 mA for the 750 mA plate
current. Previously, >250 mA of grid current would flow for 750 mA
plate current.
Things tried so far and results:
* Inspection found no burned or arcing marks on any parts in the amp.
* Parts and connections are tight.
* The tank circuit (PiL) was tuned to the previous tune/load
values when the amp was working okay (~1 : 1 SWR, 50 Ohms) using
MFJ259 on the amp. output, 2200 Ohm resistor from the tube plate to
ground, and amp. transfer relay shorted.
Things to try:
* Check filament voltage.
* Take the bandswitch out of the amp. and thoroughly inspect.
- Look for any corrosion that may be keeping the switch
contacts from fully making contact (some corrosion is seen on the
silver plated part of the rotary)
- Look for any between-the-terminals arc tracks
The other homemade GS-35B amp of the same design but using a
different type of bandswitch and output transfer relay in the output
tank circuit is working okay.
Any ideas on where to check will be appreciated.
73,
Mike, K4GMH
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