[R-390] Need value of R235 from a RF deck

[Craig C. Heaton]

                                                        Today's Adventures

Hello All,

Might want to break out your Y2K manual to follow along. 

So far, in checking out passive components around V205 & V206; C311, C312,
C313, C314, C315, C316, & C320 have been replaced with new silver micas. A
few resistors too boot, R235, R220, & R227. R235 was a 47K ohm resistor, Y2K
calls for a 47 ohm. The other two had drifted up almost 50% and I was nearby
with a hot iron. 

Page 5-15 of the Y2K manual gives some voltages of tubes. The only thing far
off is V-206, pin 7. The value given is -16.6V, this beast is -6.6V. Hasn't
changed with any of the component changes.

New clues for those who are interested, to chew on for a while: Most of the
time the oscillator starts at 151Khz, not 200Khz. To get it fired-up, gotta
tweak the adjustment on the air variable cap. Near max capacitance, the
oscillator jumps to a value near 200Khz then I have to adjust the air
variable to min capacitance for a zero beat to WWV. 

My thought is the air variable should be somewhere in the middle of it's
range when zero beat with WWV????Something is pulling this little bugger.

Ok, last clues for today, beer locker is just about empty and I might have
to declare an emergency or quit working on this beast for a while. 

Page 5-28 of the Y2K manual. Step 20, After getting the oscillator running
at 200Khz, indication is 30DB, should be 40DB. near 0DB running at 151Khz.
Step 21, about 40V P-P, should be 54V P-P, with my Tek 2336. (Good
calibration voltage on the scope, .2V P-P.) Step 22, 9V P-P, should be 13.5V
P-P. 

On step 22 there is a good sine wave, but have to have the oscillator
running at 200Khz before removing V206 or the oscillator will not start. On
step 21, a half decent square wave, sort of at 100Khz. Tops are square,
bottoms are pointed like the sine wave.

Leaning toward something pulling the air variable.

Now returning the list to condition NORMAL, awaiting comments.

Thanks,
Craig 

PS: After dinner, will pull the H202 out of the Motorola and try it again in
the Amelco. Haven't tried it since replace the caps. Might be more than one
thing wrong.



 

-----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: Tuesday, July 29, 2008 4:12 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Need value of R235 from a RF deck

Craig,

I understand that you believe the problem is in fact the crystal cal 
oscillator shifting to some new frequency every time you switch the function
switch to 
CAl.

Ed W2EMN put in some good thoughts on the crystal contacts. 

Remember the crystal oven is also a plug in device. You may want to check
the 
octal socket contacts under the oven can and the crystal contacts in the
oven 
can also.  And as long as you are checking contacts do the tube socket pins 
also.

I think the variable cap is good as you describe it.  As you get two null 
points, you are not out beyond the limit of the cap to yield a proper
adjustment.

Once you get past the easy mechanical ground tie points and socket points
and 
do not cure the critter, you may have to go looking for bad solder joints in

the circuit.

Something is changing resistance and or capacitance in the circuit as it get

switched on each time.

Eye ball the resistors real close. Finding one that flakes under load is a 
real challenge for test equipment. You almost have to get luck with an eye
ball.

I would hate to think you have to live with this problem until it goes to 
failure so you can find it.

If the deck is still open and you have not done it yet, eye ball the RF band

switch contact mesh and make sure it is in its best alignment through all
the 
band stops and on each wafer switch section.

Are you getting any direct mail with additional ideas?

Let us know what you are finding.

Thanks for sharing with us.

Roger AI4NI

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