R-390A Question

[kc2ifr]

My 390A has a problem and before I dive into it, I thought Id see if any of u fine folks have had the same problem.
Some times when I unkey the transmitter the receiver looses sensitivity. If I hit the t/r switch a few times, the sensitivity will return to normal. The problem is getting worse of late and it takes more keying and unkeying to get to return to normal.
Any ideas??
Its not the antenna changeover relay.
Thanks in advance......
Bill

[W3SLK]

Bill,
   I'm in the same boat as you. Sometimes I will unkey and be listening when it sounds as though the band just fades right out. Switching it to stand-by and back sometimes will restore it. Otherwise, I have to resort to cycling power.

[W3NP]

Sounds like the relay that opens and grounds the antenna on the back of the R390A may be hanging up or have dirty contacts.

[The Slab Bacon]

If youse guys are using Dow-Key relays, check your recieve side of the antenna contacts. Mine gets funky once in a while and i have to clean / adjust it. If I use it all the time it seams to clean itself and is not much of a problem.

also, check to see if the antenna relay in the 390-A is working properly.

                                                        the Slab Bacon

[flintstone mop]

Relay contacts sound like a good place to start. Especially if you are using a T/R relay that has a little age. Maybe some TX power is getting into the front end and driving the AGC against the wall....?
Fred

[WA1GFZ]

It could be an AGC problem causing the AGC recovery to be very slow. It may be a leaky cap in the AGC delay circuit. or dirty relay contacts. Try another RX to verify

[W3SLK]

Fred, et al said:

Relay contacts sound like a good place to start. Especially if you are using a T/R relay that has a little age.

Been there done that guys. I've even physically pushed the plunger on the relay. I'm talking about the one that is physically in the receiver. This isn't a 'one minute its there, next its gone' type of problem but is a fade away. Almost if someone is slowly turning the RF gain all the way down. You can still hear the RF oscillator running, but there arent' any strong signals. Even the crystal calibrator signal is either weak or non existent.

[w3jn]

I suspect an intermittent in the filament string somewhere that's getting slightly jarred when the relay clicks over.  Key here is the slow fade away as the tube's emission dies.  IIRC the relay is powered off the filament windings of the exformer thru a selenium rectumfrier so there may be a short there somewhere also.

[WA1GFZ]

I would monitor the AGC line. Say the AGC was slow to come out of full gain then the radio would be saturated and in blocking mode.
A stinkonium rectifier lets you know when there is a short.

[W3SLK]

John said:

I suspect an intermittent in the filament string somewhere that's getting slightly jarred when the relay clicks over.  Key here is the slow fade away as the tube's emission dies.  IIRC the relay is powered off the filament windings of the exformer thru a selenium rectumfrier so there may be a short there somewhere also.

Thanks, John I'll keep that in mind but as Franco said "a 'stinkonium' rectifier lets lets you know there is a short". Also, everything is still working, as mentioned before, the RF osc. is still cooking. But the symptom is very similar to what you had mentioned. BTW, I blew a fuse on the DX-100 last night. I only hope that was the only problem or the 6AR5 rectifier arcing. I've about had it with replacing transformers.

[w3jn]

I figgered something was wrong last night, Mikey, when you disappeared in mid-buzzard 

[W3SLK]

John said:

I figgered something was wrong last night, Mikey, when you disappeared in mid-buzzard

Ha, ha!! I found the culprit, a bad 5V4, only half of it was working. I think this was the one that was arcing over when the drive xformer went tango uniform last year. I should have replace it then.

[WBear2GCR]

Given that it somewhat of a similar radio... my R-388 would suddenly have no siganiganal and then suddenly have normal operation. Turned out it was an oxidized socket/tube pin on one of the two tubes sitting on the PTO. Pull-em in and out or wiggle 'em when it drops out, see if that fixes the bugger!

     _-_-WBear2GCR

[Steve - WB3HUZ]

Two totally different radios. 51J and 75A maybe but not 51J and R-390.

[WBear2GCR]

No PTO??

Still has 455kc IFs... can't be that different, it's a receiver?

        _-_-WBear2GCR

[Steve - WB3HUZ]

A S-38 is a receiver too. Wanna compare that one to a R-390? 

The 51J uses a 500 kc IF, different conversion scheme, completely different mechanical arrangement, less bullet-proof front end, less tubes ... blah, blah, blah .... than a R-390.

[WBear2GCR]

Whatever... 

I said that when one of the tubes on my PTO failed to make a mechanical connection the symptom was the same/similar to the problem Bill had.

The 75A has a 455kc IF, iirc.

Honestly I have never looked at the schematic for a 390, so I don't know what the differences in the conversion scheme might be. Perhaps it is worth a look see. But the front end, the tube line up and the mechanical part are irrelevant to my comment. If the 390 uses a similar PTO arrangment then it is succeptable to a similar problem one would think.

And yes, we can compare receivers, especially those that use similar topologies, similar frequency sub sections and identical or similar parts (like the Collins PTO). At least I think we can do that sort of comparison...?

Dunno what the problem here is...

          _-_-WBear2GCR

[Steve - WB3HUZ]

No problem. I have a S-38 I'm ready to trade for a R-390.