Hi Evan,
I got the amp fixed. One of the metallic spacers that was part of
the
power switch had gotten lodged somewhere. Another ham came over
to try to help find out what was wrong and during a visual
inspection
the amp was turned over and out fell the spacer! I was surprised and
embarrassed since I had looked vigorously for that spacer thinking
it
could be the problem. But it didn't decide to fall out until someone
else
was looking :-)
Anyway the amp is back 100%, now. Thanks for taking the time to
think
about the problem. Its appreciated.
73,
Kris N5KM
On 12/23/2010 11:57 AM, K9sqg wrote:
Kris,
OK on test results to date. If it were me I would then
look into...
All contacts on the bandswitch, both input circuit and
output circuits.
Using a receiver or some RF sensing device, at very low
power to save the tubes etc., see what frequencies are coming
out of the amplifier. This is a check for parasitic
oscillations, the linear acting as an oscillator rather than
an amplifier, and/or the amp acting as a frequency doubler,
tripler, etc. A related check is to connect everything up, go
into transmit on SSB, but reduce mic gain to zero; then raise
the mic gain to normal, speak into the mic, and then
immediately reduce the mic gain to zero. Power output, plate
current, grid current, etc. should all be observed for normal
operation.
The amount of bias voltage applied to the tubes in standby,
and transmit.
The condition of the plate blocking capacitor(s).
Trying one tube at a time as was mentioned might be useful.
And while you are at it, during "one tube operation", try
each tube in each socket.
Not sure if the input bypass capacitors (re: tube side of
input balun) could cause a problem like this, seems unlikely.
Sounds like you have a puzzle on your hands. Please keep
us informed so we all can learn from this.
73,
Evan
-----Original Message-----
From: Kris Mraz <[email protected]>
To: K9sqg <[email protected]>
Sent: Thu, Dec 23, 2010 11:43 am
Subject: Re: [Drakelist] L7 Power Switch
Hi Evan,
Thanks for your help.
Does the problem
happen on all bands?
Yes. All bands.
What happens to
the power output for various tune conditions?
As I dip the plate, grid
current increases and output increases.
I can then increase drive a bit from the xcvr and
grid current and output
power correspondingly increase but plate current
stays the same.
Again, adjusting the Load capacitor has no affect.
Are the grid RF
chokes intact?
Checked OK with ohmeter.
Are both of the
plate circuit RF chokes ok or does either show signs
of heating?
Checked OK with ohmeter. No
signs of heating.
I checked the tubes for shorts and were OK but a friend
suggested pulling
one tube at a time to see if it makes any difference. I'll
try that later
today after I get some Christmas presents wrapped.
73,
Kris
On 12/20/2010 8:02 PM, K9sqg wrote:
Kris,
Glad the switch situation
is cured. This is why I always use an external
switch for on/off.
The thermistors should have
no effect given that they are in the primary of
the filament transformer. Not sure where to start
on this one but that grid current is excessive,
anything above about 225 ma of grid current has
the possibility of damaging the tubes. What makes
the L7 difficult to fault isolate, similar to the
L-75 although the L-75 is much worse, is that you
can't leave the cover off in transmit very long
without over heating the tubes. When you have the
cover off, and defeat both interlocks, the tubes
get very, very little cooling of the base pins,
not a good situation. Hence, power on testing
must be limited to very short intervals. I would
check the grid and plate current meter circuits
to ensure they are calibrated.
Does the problem happen on
all bands?
What happens to the power
output for various tune conditions?
Are the grid RF chokes
intact?
Are both of the plate
circuit RF chokes ok or does either show signs of
heating?
Please keep the group
informed.
73,
Evan
-----Original Message-----
From: Kris Mraz <[email protected]>
To: [email protected]
Sent: Mon, Dec 20, 2010 8:34 pm
Subject: Re: [Drakelist] L7 Power Switch
I found some nylon spacers in the parts bin at
the local Ace Hardware store
that fit the bill exactly. No modifications
required. (Sorry, no part number available).
I reinstalled the switch and while I was in
there I put a thermister in
each leg of the filament transformer primary
just to avoid any future pitting.
I fired it up and... nothing. No power out
except the drive power (about 60w).
So now I've got another question for the group.
I can dip the plate current to about 180 ma. and
the corresponding grid current
rises to about 400 ma. The LOAD control has no
affect, whatsoever. I even
removed the thermisters just in case since
that's really the only thing I changed.
HV is good, T/R relay is OK, filaments light up,
idle current is OK (about 180 ma).
The 3-500Z's aren't shorted.
Can someone suggest where else I might look?
Thanks,
Kris N5KM
On 12/7/2010 8:00 PM, Kris Mraz wrote:
Hi
All,
Last night I removed the power switch on my
L7 amp to
repair pitted contacts. All went well until
I went to reinstall
the switch. The mounting holes on the ears
are larger than
the standoffs they mount to so that the
switch just flops around.
Can someone look at their L7 to see how the
switch is
mounted to the standoffs? I'm guessing
washers are used but
I checked the workbench surface and I shook
the L7 to
dislodge anything loose but found nothing.
Thanks for checking.
Kris N5KM
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