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Author Topic: Problem with my r-392 receive dying  (Read 4555 times)
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NR5P
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« on: March 06, 2012, 08:42:25 PM »

I always seem to have the weirdest problems with this stuff.  I am having this problem with the receive were it will die sometimes.  I've found that if I move the agc control to calibrate and back to agc it starts working.  Now I know that this is having a direct effect because I can wiggle it to agc off and agc on and nothing helps, I tap and bang the receiver and nothing.  I move the agc knob to calibrate then agc and it comes back to life.  Now I CAN receive a very strong signal through the speaker like a local broadcast station but can just hear enough to tell it's in there.  This goes on every 5 minutes for about a half an hour then this method stops working and the receive stays dead.  I checked the AGC switch and it's making good contact and looks clean.

Now here is the really frustrating part

I thought well this is easy it stops working and i start tracing the signal down...but it seams when I take it out of the case and start tracing it starts working again before I get a chance to really narrow it down!!  The smeter stops working as well so I know it's not in the audio section like the previous owner thought.  Any r-390 techs that may know what to check first? 

Could it be a tube?  I am thinking this because it gets worse when it warms up and it seems if I leave it in the case it won't come back to life until I shut it down for a while. 

Whats really funny is when I got it I couldn't hear anything, so I started with the audio section and started working my way back ruling out every section until I got to the rf and everything seemed fine.  I thought it is in the bandpass system somewhere but no it had started working while I was checking it out! 

One more little problem that I am saving for after I get this major problem fixed is that the receive is dead from 8-15 MHz bands.  I imagine this is a bandswitch but I will get to that project next. 
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NR5P
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« Reply #1 on: March 06, 2012, 09:07:40 PM »

one more thing that I made sure to check was the antenna relay because it shorts the antenna on calibrate mode.  This is functioning properly though
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w3jn
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« Reply #2 on: March 06, 2012, 11:58:33 PM »

Take a gander at this recent thread:  http://amfone.net/Amforum/index.php?topic=30478.0

Could be anything though.  I'd suspect the crystal conversion oscillator.  Keep it out of the case and be ready to attack it when it dies.

The 8-16 MHz band being dead is most likely, as you suspected, either the band switch or a bad coil/transformer in the front end.
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NR5P
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« Reply #3 on: March 07, 2012, 06:11:35 AM »

Thank you. Ya I'm going to try to narrow it down while it's dead. I wanted to make sure I wasn't missing anything. The calibrator shouldn't have anything to do with my problem but it seems it does. I have found that it isn't really dead just very very low. I could here some Ssb last night messing with it after it had gone quiet
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NR5P
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« Reply #4 on: March 08, 2012, 08:32:59 PM »

I've narrowed down the problem in between two tubes.  Here is the location where you can find the schematic if you want to see what I'm talking about http://bama.edebris.com/manuals/military/r392/         

If I inject the 455kc if at the grid of the 1st IF amplifier (v501) if receive is dead or alive I get the same signal out so I know everything ahead of this to the speaker is fine.  If I inject the 455kc signal at the plate of the tube before it, which is the VFO mixer (v801) when the receive dies the signal dies so I figure well it's got to be in between(I verified the voltages are getting to the tube even when silent).  The only thing in between is a 15k resistor and a .01uf capacitor which goes to ground and these are on the plate of V801.  Then there is a transformer (T801) which the secondary goes to the grid of v501. 

Now here is something interesting I've noticed.  The way I have been injecting the signal is wrapping a piece of wire around the tube pin I want to test and plugging it into the circuit and using the piece of wire sticking up to inject the signal, using a capacitor to block DC to the sig gen.  Now 50% of the time or more if the receiver is dead and I am hooked up to the plate (pin 5) of the vfo oscillator mixer AS SOON as I touch the capacitor to the signal generator output the receiver will come alive.  So that has me thinking is it the tube acting funny?  I need to buy some spare tubes for this thing anyways but I have none right now to swap in.  And this is the only 26d6 in the receiver so I have no extras. 

Now this v801 is a mixer and an oscillator so it's loosing oscillation and the 455kc I'm injecting is enough of a bump to get it going again.  Is this the sign of a weak tube?  Because it works great when it's working.  Z801 is an LC coil pack that has an inductor tapped to the cathode of v801 to provide feedback.  There is some kind of adjustment in the schematic but I'm not sure what it's for, it's called(additional controlled circuits).  I guess I just need to order a tube.
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w3jn
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« Reply #5 on: March 08, 2012, 11:38:49 PM »

Could be a bad tube (ie grid emission), or the grid circuit has an intermittent return, so the grid bias drifts - and when you attach the sig gen it kicks the bias back to where it oughta be.  The tubes are cheap for this thing, but I'd also check the resistance of the grid circuit to ground and check against the schematic.
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NR5P
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« Reply #6 on: March 13, 2012, 10:32:54 PM »

I got my replacement tubes and having the same problem.  I'm measuring 96K across the grid resistor is 82K.  It is out of tolerance so maybe it wouldn't hurt to take a look it may not be making very good contact.  I couldn't get it to come back on by touching the plate this time. It only started working once for about 5 seconds and quit again.  I confirmed again the issue is in the vfo section so with a little time I should have it solved soon!
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