Thank you for the many suggestions offered for my problem.
On the most-likely list were spurious oscillations, a bad solder connection
or a cracked chip cap. So I hauled the panel back onto the workbench again
today; the preamp is mounted on an aluminum plate with bypassing relays, and
power and control line protection circuits.
A closer visual inspection of the pcb was done using a monocular- nothing
obviously wrong was noticed. I then hooked up a milliameter in the + supply
lead and could see a difference in current draw of about 1 milliamp whether
it was out of the box, or in it. Interesting! The preamp PCB is very rigidly
mounted to the cover of that die-cast box, and just gently bringing the box
to mate with the cover could produce that change in current. Nothing was
disturbed mechanically on the pcb and cover, so it did not look like a
flexing bad connection problem. A spurious oscillation now seemed most
likely. To facilitate my investigation, I took off the bnc connected cables
from the input and output, and put on a couple of small 50 ohm terminators.
The current still changed by about a mil in or out of the box, but now it
increased when in the box, formerly it had decreased. What did that mean??
With it out of the box, I could now probe all parts of the circuit with my
fingers to see if I could change the parasitic- this had absolutely no
effect- the current draw remained rock steady as I did this. I then took a
0.1 ufd cap and probed every trace on the pcb with one lead while the other
lead was on common, and still no change at all in the current draw. I would
have thought that if there was a parasitic, then at least I ought to be able
to increase or decrease its effect with this, but no.
OK, now time to get out the shotgun. In case there was a bad bypass cap
somewhere in the RF circuit, I soldered in a 100 pf disc on the source trace
and another on the "cold" side of the drain resistors. The only result of
this was that the current was now going down again when put in the box.
Looking at the schematic, it seemed to me that the 9V regulator was a little
light in capacitance on the input side, and a bit heavy on the output side.
Checking an app note, indicated that a bit more capacitance was recommended
on the input side. I added a 10 ufd tantalum on the regulator input, and
another 0.1 ufd on the feedthru cap side of the reverse polarity protection
diode, and voila! The current draw was now the same whether in or out of the
box.
Next, hooked it up in the shack and tuned in a weak beacon to check the
tuning. When doing this I noticed that the variable across the input coil
"peaked" with it at max capacitance. Hmmm, did not notice that before, so
soldered a 2.2 pf disc across it, and retuned. Put it in the box, and it
continues to work, in and out of the box make no discernable difference.
Why it had stopped working in the first place, and which of the above
actions cured it remain a mystery, but at least now it is back in its home
at the base of the tower, and has protection against the bugs or mice that
might get in.
Thanks again for all the suggestions, wish I could have reported the
definite cause and cure
73
Chet, N8RA
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of Chet, N8RA
Sent: Friday, July 04, 2008 2:09 PM
To: 'VHF Contesting Reflector'
Subject: [VHFcontesting] DEMI VHFLNA 2M preamp puzzle
I thought this preamp was working OK for a few months, but it is not now.
When it is out of its die-cast box, it seems to hear and tune up just fine.
But when I put it back into its box, it still outputs a higher level of
background noise, but the input signals are not amplified.
If I just crack the lid a bit, it again hears OK, but when completely shut
it does not.
Input detuning? But there is not way to tune it when it is in the box.
Any ideas??
Chet, N8RA
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