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Author Topic: R390A Cracklin'  (Read 3359 times)

Posts: 960

« on: December 31, 2012, 07:06:09 AM »

I have been hearing more and more of a crackling sound from my R390A. So, before we get into another new year, I decided to figure out its problem.
The carrier meter CAL control is always a problem to get the meter always on Zero.
Troubleshooting and noise / crackling sound coming from RF deck. This is with RX terminated in dummy load and RF gain FULL.
Start gently tapping on tubes and once again 6C4's, 1st osc and 6C4 2nd osc making noise.
I insert a new 6C4WA in both positions and receiver very quiet and happy again. I was afraid of noisy pf caps in one of those osc. circuits AGAIN. With patience and manual, the removal of the RF deck isn't that bad.
There seems to be concerns over using 6C4WA's
Any truth that they are not good or do not work well in the 390A?

Posts: 5210

« Reply #1 on: December 31, 2012, 08:30:37 AM »

I'm going to be firing up my 390A soon. I've had troubles with 5814A's in the AGC strip in the past but never found a really bad 6C4.

Amazing how a typo 56 years ago still lingers on today. Any of the 6C4, 6C4W, 6C4WA, 6100, etc. will work. From the R-390A "Y3K" manual:

Note 10: There is a severe error in the 1956 Army R390-A manual. It says not to substitute type 6C4W for a 6C4. It stated that the differences in characteristics are such that the type 6C4W will not operate properly in the receiver. Navships 1970 manual lists the 6C4W as the tube to use.
The “W” and “WA” postscript designations refer to mechanical ruggedization specifications. Cross-checking of tube reference data bases confirm that the electrical specifications of all types are identical. A “ruggedized” or “Industrial” equivalent are types 6100 and 6135 among others. All versions will work properly. Some may be quieter or last longer. See Chapter 9 supplement for additional tube information.

Posts: 960

« Reply #2 on: December 31, 2012, 10:41:25 AM »

I saw that long long dissertation on the tubes and issues with the R390A. I got bored trying to find the reference to 6C4WA's. I just replaced the two noisy 6C4's in my receiver with WA's and it stills checks good with sensitivity and the crackling noise gone.
My 390A never saw military use/abuse/repair depots
Given as a gift to a Ham friend of mine who worked at Andrew's AF Base wiring up the MARS station. It was new in its original crate......a virgin radio.
I have owned it since 1993 and it has had many problems. RF deck...crappy pf caps 1st osc.......numerous I.F. module problems with AGC. In fact two hard-to-fix problems with AGC!!! And I installed .1mf caps before and after the hard-to-find mechanical filters. Keep that DC voltage far away.
Replaced all the electro caps in the P.S. and the radio acted like a new radio.
This is a 24/7 operating radio. When the WX warms and lightning season starts, it is turned off for long times of no use. Not a good "crowded conditions, SSB nearby AM QSO type radio". Nice for general listening...The Diode Load gives super audio to an external amp and speaker. Love WBCQ on Friday nights 7PM.
I have blown the RF coils for the 160 band in the RF deck by malfunctioning T/R relays or the radio on one antenna whilst I TX on another antenna. Fair Radio had a nice stock of those plug-in coils.

Posts: 4464

« Reply #3 on: December 31, 2012, 12:51:18 PM »

And I installed .1mf caps before and after the hard-to-find mechanical filters.

If it works it works, but that seems like a whole lot of condenser for a circuit operating at RF... I'd have used something closer to a .001 or whatever I had plenty of in that size range.

BTW:  One of my favorite selections from Unca' Sam's depot manuals involves the R-390 alignment instructions: (paraphrased)

1)  Complete the performance check in chapter XX to verify the radio is operating properly.
2)  Repair any deficiencies found and verify proper operation before alignment.
3)  Place the function switch in the operate position and allow the radio to warm up for a minimum of 24 hours prior to beginning the alignment. This alignment procedure may require more than one day. The radio should be left on overnight if a second day is needed to complete the alignment.

Apparently the DoD didn't sweat their electricity bills back in the day or the cost of bench time............   Wink

The end of the world will occur on April 23, 2018 ( the day after Earth Day. Go Figure ).  If you're reading this on April 24th look for updates coming soon.  If you're reading this after June first, fuhgedaboudit.....

Posts: 960

« Reply #4 on: December 31, 2012, 03:29:57 PM »

Hello AC5UP
I used .01 ........basically replacing the dreaded cap that could short and throw DC plate voltage into the Filters. C553...from plate of the first I.F. and used another .01 to the grid of the next stage.
.01 will pass 455 kc.
Went through the usual checks, after warm-up, and sensitivity is ok
The radio seems happier if it's left on and during the winter keeps the shack from that chill.
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