[R-390] R-390A IF deck caps in T501 or T502 Dropped posts

[Roger Ruszkowski]

Dave N3DT,
You wrote, Some of my messages to the list are still not going through, I can't figure it out. 
You did have a correct to address in your mail as it went some where and you believe your mail was sent.
The rebroadcast part of the software has incoming mail filters on the subject line.
If the filter is not happy with the R-390 string the mail does not auto post and goes to an inspection queue
If you did not send your mail from the same e mail you are registered under the mail is not rebroadcast. Those mail post go to an un registered user queue (spam).I am registered as flowertime01 at wmconnect.com. And post from my Hughes, Raytheon, Square D, and Tri County Technical College in Pendleton South Carolina e mail accounts do not auto post. That's the design. (I  have gotten around over the years) Larry, did you change the caps in situ? It would be a real pain to take out T502, and maybe that's why you changed both of them. Some times the problems are like spark plugs. you just change the whole set when you get to the tune up.
Pull the IF deck, remove all the tubes and the transformer can cover to get as much space as possible.
You get to change the parts atop the deck on the bench. It was a bear to troubleshoot because it only happens when leaving the RX off for quite a while and then the gain changes within a few minutes of turn on. It's very noticeable though.  Normal circuit current flows and the mechanical parts begin heating do to conductor resistance. Good ground theory. Some very small mechanical stress point changes the conductance value the only way we think the conductance value can go into a more resistive state. The circuit performance shifts away from peak performance. If the circuit performance comes and goes then it drifts. Either performance is undesirable. In side just one component part where copper ends and magic begins smoke is coalescing.  Smoke is a by product of change. Good skills have gotten you this close to the problem part. Feel free to apply some preemptive maintenance to all the solder in sight. The bottom side of T501 and T502 each have decoupling caps. Originals were called black beauties but most offenders are brown in body color. I missed the nugget where Dave disclosed where in time his IF deck is located.     A can of air may help discern the exact problem. If the small part is heating, mechanically changes size, and changes its electrical characteristics then a shot of cold could reverse the symptoms.
We had compressed air in the shop for dusting out equipment and used it for troubleshooting problems related to these thermal issues.
I have vented butane cartridges to evaporate the liquid and chill suspect parts. There are reasons smoking has been banded inside buildings. You never know what Roger may have in progress and seeking ignition.Franklin flying kites in the rain is not the only fool in the field.
Smoking bands are for your safety from people like me. Paint rubbing alcohol on suspect parts with a Q tip to cool a part.
Some air cooling across the top of the IF deck with the covers off  T501 and T502 may help isolate an exact problem.  
If a top side pass fails to isolate the problem, then set the receiver on end, swing the IF deck out on the bench, and repeat the inspection process on the bottom side of the deck.Age is getting to these small cap parts. We will need to develop some new trick test steps to isolate these now old and mechanically stressed out parts.  Your soldering iron works the other way. Warm a suspect up and see if it twitches. Toast a part into failure and you will know which part you need to replace next. Until you start melting the wax and rosin and mold coat you are ok by max temperature. You may want to hold low heat for a minute waiting for the thermal soak through and mechanical change if any to occur. Finesse and skill on the bench was closer to lumber jacking than cabinet making. The methods worked. No R390's were harmed in the educational exercises and repairs.  One would like to think they are doing preventive maintenance and tuning for peak performance. Yesterday every thing was working well. I now detect one change in performance from yesterday. Spock says I have one problem part to locate and re mediate. If you can get your hands on either the power cord or the antenna cable you can haul any R390 or kin out of the mud and return it to service after maintenance. The bars on the front of the receivers are not handles. You can do the job. R390 and R390/A receivers are so repairable. They were designed and built to be maintainable.  Swapping a mica cap in the T501 T502 frames is a far more robust operation than some printed circuit board repairs I have accomplished. As you begin to use compressed air to de solder connections, do tuck some napkins in place to catch the splattered solder. Its a learning curve and bench skill. Bring your chop sticks home from your next Chinese meal. You can carve them with a knife and shape them on sandpaper to make soldering tools.  You can set the R390 receiver on a work surface, operate it forever, and repair it as needed. Two cheap voltmeters and a small set of hand tools. Put a spline pointy end on your chop stick and get on with the maintenance. More is better and not to be ignored. More is not required to get er done. I like the AN/URM-25 line of signal generators also somewhat very repairable as a reliable signal source while doing receiver maintenance. At age 70 my bench frequency standard choice is still a zero beat against the 455 KHz crystal band pass in Z501 of the receiver on the bench Hang with it Dave, summer is coming and some week end projects into one off hobby activities will get added to the list of joys in life. Respectfully, Roger AI4NI
-----Original Message-----
From: Dog T <agfa at hughes.net>
To: r-390 <r-390 at mailman.qth.net>
Sent: Sat, Jan 26, 2019 1:07 pm
Subject: Re: [R-390] R-390A IF deck caps in T501 or T502