[AMPS] Problem with AL-811H
Wt8r@aol.com
Wt8r@aol.com
Wed, 3 Oct 2001 08:47:58 EDT
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I am attempting to repair an AL-811H for a friend. The amp has been abused
due to mishandling as aircraft baggage making several trips to the Caribbean
and back. The outer case shows several dings and one of the threaded holes
in the chassis metal which holds the cover in place has been broken open.
The reported symptoms are: When attempting to tune the amp upon return to
home QTH, there was a pop and a flash inside the amp.
On my bench, I am driving the amp using an Icom 751-A. The only visible
signs of damage evident to any of the electronic components was one of the
two 1000pF DC blocking capacitors from the anode/plate RFC to the tank
circuit was cracked open slightly. I replaced these with two 500pF (6KV)
capacitors for test purposes with no apparent difference in result. Using an
ohm meter, it appears that the DC input blocking capacitors at the top end of
the filament RF choke are good and that no short circuit to ground exists on
the input. The tubes show no internal shorts using an ohm meter. The tubes
light properly, the HV is 1700V and the anode idling current when keyed
(without drive) is 150 mA (which is about right).
I disassembled the rear pc board from the rear panel and inspected the traces
for burns and shorts, but nothing was obvious.
When attempting to operate the amp, the grid current and plate current swing
up and down in synchronism with the drive, but NO RF output is observed. I
am using a power meter and a Bird 2500 watt dummy load. Adjustment of the
TUNE and LOAD capacitor produce no apparent dip nor power output. No unusual
smell or noise is observed.
Any suggestions would be greatly appreciated.
Dave, WT8R
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<HTML><FONT FACE=arial,helvetica><FONT SIZE=2>I am attempting to repair an AL-811H for a friend. The amp has been abused due to mishandling as aircraft baggage making several trips to the Caribbean and back. The outer case shows several dings and one of the threaded holes in the chassis metal which holds the cover in place has been broken open.
<BR>
<BR>The reported symptoms are: When attempting to tune the amp upon return to home QTH, there was a pop and a flash inside the amp.
<BR>
<BR>On my bench, I am driving the amp using an Icom 751-A. The only visible signs of damage evident to any of the electronic components was one of the two 1000pF DC blocking capacitors from the anode/plate RFC to the tank circuit was cracked open slightly. I replaced these with two 500pF (6KV) capacitors for test purposes with no apparent difference in result. Using an ohm meter, it appears that the DC input blocking capacitors at the top end of the filament RF choke are good and that no short circuit to ground exists on the input. The tubes show no internal shorts using an ohm meter. The tubes light properly, the HV is 1700V and the anode idling current when keyed (without drive) is 150 mA (which is about right).
<BR>
<BR>I disassembled the rear pc board from the rear panel and inspected the traces for burns and shorts, but nothing was obvious.
<BR>
<BR>When attempting to operate the amp, the grid current and plate current swing up and down in synchronism with the drive, but NO RF output is observed. I am using a power meter and a Bird 2500 watt dummy load. Adjustment of the TUNE and LOAD capacitor produce no apparent dip nor power output. No unusual smell or noise is observed.
<BR>
<BR>Any suggestions would be greatly appreciated.
<BR>
<BR>Dave, WT8R
<BR>
<BR></FONT></HTML>
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