R-390A Lack of Sensitivity after TX

[W7SOE]

Hi All,
    I am using the break in feature of my R-390A, activated by my ART-13.  It works great except that I notice a drop in sensitivity after keying up.  If the background noise is at a certain level before key-up it is significantly reduced after.  It will slowly rise back to it's previous level after 2-3 minutes.

I have not noticed a problem during my morning 3885 QSOs but I think the sensitivity reduction is real.

Any ideas?

Thanks

Rich

[K1JJ]

Rich,

As an initial test, put a DC voltmeter on the AGC line going to the IF and RF stages. Look to see if it is getting goosed during TX and slowly coming back afterwards.

My SP-600 did that for a few seconds after keyup/keydown - it was traced to this kind of AGC slow decay problem.

T

[flintstone mop]

HOPEFULLY things are switching over properly and you didn't transmit into the R390A!
Depending on what band, you may have killed a tuned transformer on the RF deck.

It's happened here NOW twice!! All of sudden my receiver is deaf!

Do you have access to any RF signal generators to test sensitivity?

Fred

[W7SOE]

I do have an RF generator.  What is a good procedure for testing sensitivity?

Rich

[WV Hoopie]

Rich,

You may have several issues. First do you have a copy of the Y2K manual? If not, why? It is on reversion three. Second, go thru the R390 archives, http://mailman.qth.net/pipermail/r-390/   might have to sign up to view. Somewhere in this long list is a procedure from, "flowertime", on how with the bare tools to check the sensitivity. Only things needed are a cheap calculator, 600 ohm resistor, volt/ohm meter (one with db scale saves time), and a sig gen.

Are you trying to use the R-390/A to monitor your TX? Maybe not a good thing to do.

Have you recapped the RX? This includes the big rectangular oil filled cap on the IF deck, 2MF@ about 600VDC. All the ones I've tested failed on insulation resistance at rated voltage. 

[WA1GFZ]

Checking sensitivity. Put an (RMS if possible) AC volt meter on the audio and set the level so the meter is at quarter sacle or so. Slowly increase the sig gen level until you see a 3 dB increase in AC output. This is MDS. Now increase it until it goes up 10 dB. Sensitivity is usually measured at a 10 dB increase in audio. The level will change when you change bandwidth by (10 log BW in Hz).

[WV Hoopie]

I checked the R-390/A archives. Look in Dec. 2005 and threads pertaining to A Christmas Wish. This is how it was taught in the tech schools for the armed forces.

Craig,

[ke7trp]

Take 390A off Slow.

C

[W2JBL]

take the R390 off slow AGC. then repair the intermittent antenna relay. by the way monitoring your self on the R390/390A works fine. I have 6 of them and never puked anything, using good quality coax relays (new stuff, not Dow Key). I also welded the contacts on a coax relay one time and dumped full the output of a National NCL2000 (that's a healthy sideband linear) into a pristine Collins R390A and had no damage other than to my hearing. a 7.62mm 168 grain boat tail hollow point at 2550fps directly into the tuner module can also do wonders for your signal to noise ratio on a noisy night... a followup round into your transmitter can help things a lot too!

[flintstone mop]

A slightly simpler method is to use the built-in CAL on the various bands. The band or bands affected will not be able to "hear" the CALIBRATOR.

Have you been able to isolate what band is affected or is it all bands?
There's a lot of things that go on in the 390A that can make it deaf on just one band or all bands.

Fred

[Ed/KB1HYS]

Check relay sequencing.  If the transmitter is using a relay to mute the R-390, it's possible that the key up sequence is unmuting the R-390 just a split second before the transmitter is unkeyed.  It was happening here.  With the BFO on, I could hear the "chirp" when I unkeyed the rig.  It didn't take too long to recover. 

I adjusted the relaying sequencing with a little judicous bending of the reeds in the transmitter & Antenna Switch over relay. 

I have accidentally transmitted into the R-390A with my Valiant (operator error - left the Break in switch in the OFF position).  Recovered and no issues, I guess I got lucky.

[W7SOE]

Check relay sequencing.  If the transmitter is using a relay to mute the R-390, it's possible that the key up sequence is unmuting the R-390 just a split second before the transmitter is unkeyed.  It was happening here.  With the BFO on, I could hear the "chirp" when I unkeyed the rig.  It didn't take too long to recover. 

I adjusted the relaying sequencing with a little judicous bending of the reeds in the transmitter & Antenna Switch over relay. 

I have accidentally transmitted into the R-390A with my Valiant (operator error - left the Break in switch in the OFF position).  Recovered and no issues, I guess I got lucky.

I think this is the most likely culprit.  I am using the relay in the ART-13.  This is a separate relay from the main T/R relay (a vacumn relay) that switches the antenna. 

I tried switching the RX to standby, transmitted, and quickly switched back and there was no loss of sensitivity.  So perhaps you are right, the RX is getting a split second of the TX signal and the AGC gets swamped or something.

I could try your trick and dig into the ART-13 relay box and bent the appropriate contact very close to its mate.

Rich

[WV Hoopie]

Rich,

This might seem to be a little overkill but; on all my R-390/A's I use a delay relay to keep the RX muted for about 0.3 seconds after the TX is unkeyed. The delay relay takes power from the TX for operation, a set of contacts is needed for the delay relay as a trigger. A DPDT delay relay is the best, one set of contacts is used to ground the RX contacts in the RX ant relay. The other set of contacts of the DPDT delay relay is used to open the RF gain terminals on the back of the RX (gotta take the jumper off).

The larger transmitters, Viking 500 & Desk KW, both need a little time for the RF energy to drop before the antenna relay makes the switch to RX. If life is good the relays in the "500" provide some delay. The Desk KW needs an outboard means of delay (provided by a KW Matchbox tuna).

I use a short length of aluminum angle to mount a relay socket and bolt this to the back of the R-390/A. Use those rear bottom ears with the holes. No drilling or blasting needed. Then listen to the TX unkey and adjust the delay relay for a smidgen more delay.

Life is good.

Craig