[R-390] R390 progress 6AK5/6AJ5

[Dan Merz]

Roger,  I like your approach.  But I like to examine circuits within the
radio. It helps me understand some of the whys and hows for the adjustments.
I'm just about at the point to do what you are suggesting.  I replaced four
resistors in the two oscillator circuits,  two were probably not that far
out but since I had it open I replaced them and one was definitely well aged
and about twice its original value.  Pretty tight quarters in those little
compartments.  With a couple more checks on things,  I'll put it all back
together and hook the switch shaft back into its coupler.

The waveforms are definitely cleaner looking now,  probably mostly due to
starting the alignment at 31 and going from there.  I've only done 80
meters, 40 meters and 20 meters as a quick check on what I'm doing and
seeing that it works ok before putting the oscill chassis back in.  I'll
start over once I get it all reassembled and pull out the tube extenders
that I have in now.  I think most of the problem I was originally having
with 40 meter band was due to bad positioning of the t402 output transformer
on oscill #2.  I was fooled because so many of the bands seemed to be
working ok.  The switches in the oscillator chassis are so buried that it's
hard to deox them so I didn't try.  I really haven't seen any indication
that these switches are dirty or  are not making good contact.

The Engineering Report for the 390/389 says 6AJ5's were used in the
oscillators because of their lower power requirements but the biggest
discussion of the tube was the justification for using it in the 1st rf,
based on other criteria for cross mod,  overload and agc characteristics. I
suppose there was some thinking that reducing the number of tube types might
be important but I didn't see that mentioned.  In the 390a,  the design went
to 6ak5w's for the oscillators so either the low power consideration went
out the window with the demise of the 6aj5 1st rf or the 6ak5 was deemed
better for other reasons.  I didn't have the heart to change the resistors
to match the 6ak5 design of the 390a.  I'll probably look for more 6aj5's
down the road.  Because I just re-read this stuff,  I mention it - I imagine
most of you already know it.  That's a pretty interesting archived report
Dan.  

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Flowertime01 at wmconnect.com
Sent: Monday, May 02, 2005 6:36 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] R390 progress 6AK5/6AJ5

Dan Merz,

Sounds like you are making progress.

Start hanging a DC volt meter on the diode load. You are looking for -7
volts.
every place the procedure says tweak and measure some point, just tweak for
maximum signal on the diode load. 

Put the scope away.
Hang a DC meter on the diode load.
Hang a 600 ohm 1 watt resistor on the audio output and an AC volt meter with
a DB scale across the audio output and resistor.

When the BFO is on read the AC volt meter for 1/2 watt of power. The DC volt
meter will peg off the DC scale with over -30 volts.

When the BFO is off read around -7 volts on the DC load.

If you have more than -7 volts (with the BFO off) back the signal drive down
of reduce the DC gain in the IF deck.

Just do all you adjustments for maximum signal on the DC load.
The goal is signal you can hear. The DC load is where you can hear it.

Set the receiver to CAL and the BFO off. Pick a 100 KC dial setting. Start
at
31 so you can adjust the transformer and the cap for maximum signal and a
single peak.

On each band roll the KC knob around for a maximum cal signal and tweak the
OCS trim cap for Max peak. Just monitor the DC load and peak them out.

You need to get a signal generator on the antenna and start looking at your
signal to noise. Until you see how close to the minimum 10:1 you are. How
close you are to the 20:1 you should have. And how close to the 30:1 you can
get, you just have no idea if the problem is a real problem or not.

How old is the receiver?
How exact is your line voltage?
How exact was the TM reference?
How exact is your meter?
How does your meter load your circuit?
How does your scope load your circuit?

Just to many questions to mix in.

Go to the bottom line. DC load voltage and Signal to noise ratio.
Then any thing you try either makes an improvement, makes no difference,
makes the situation worse.

Use the same test for every thing so you can at least compare the results.
You will wonder for ever about voltages on grids and is the TM correct.

At least your down to the 7Mhz band. And it sounds like your narrowing in
the problem. Sounds as if their are some side benefits coming out of the
effort as improved performance on other bands.

Roger KC6TRU
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