The AM Forum

I am a novice tube receiver troubleshooter and thought I'd ask for help from those more skilled than I am. On initially powering up RF sensitivity is great, but it declines over about a 30 minute period. If I switch the power off and on, full sensitivity is restored for a while. What is the most likely cause? I am guessing it's a cap but which circuit function cap is most likely to produce this symptom? I'm betting on something leaky in the RF amp section, perhaps a cap associated with a screen grid. The rcvr is a WW 2 vintage R 5A ARN 7 ADF. I've replaced all bad caps accessible on the chassis bottom but many more reside inside sealed shielded compartments mounted on the upper surface of the chassis. Any advice appreciated.

Not too long ago I had a Hammarlund HQ145x that did that. Turned out to be a tube that worked until it got hot. It was either in the front end RF amp or first IF section. The tubes all tested good so I started swapping them out with known good ones starting at the RF amp. I was lucky that the first or second tube replacement solved the problem.

Rich

You can try making DC voltage measurements at initial turn-on and then again after the trouble is evident. When you see a change, you've found where to start looking for the problem.


Don

If it is not the tubes or the rectifier or capacitor in the Power Supply, consider replacing any El-Menco (rectangular) mica capacitor in the oscillator (Converter) circuit.

That cured the problem in a Hallicrafters S-108.


Phil

I've seen this happen where a grid resistor is open or has drifted to a really high value.  What happens is the grid develops an increasing negative charge and with no place to drain off the voltage, it just keeps increasing thereby cutting off the tube.  Turning the radio on and off dissipates the charge.

Also depending on the design of the AVC system, the DC return to ground (thru a resistor or inductor) could be open or drifted high.

I came upon some NOS carbon comp resistors that I was going to use in a project. I took the time to measure them before installation, only to find that they had drifted ridiculously far out of tolerance. Like, not even close to the correct value. I couldn't trust them even to maintain the value that I measured, so they got tossed.

I also had some NOS wire wounds like the ones used for HV bleeders. Half of them were open-circuit. A HV bleeder is a critical part. I couldn't even trust the ones that measured good, for that application. I had to toss the lot. After that experience, I now design multiple parallel resistors into bleeder applications so that if one fails the bleed is only partially reduced.

So again, I suggest making measurements not only when trouble-shooting, but when building too.


Don

Don makes a good point here ... a bit of component history surfaces here .... reliable electronics came about as a result of the space exploration programs of the 1960's .... impetus was gained from watching missles blow up at launching .... its amazing what can happen when a whole country has its posterior on the line  :o :o :o

I very much appreciate all the helpful suggestions. Thank you.

Mark
AF6IM

Many years ago, I worked at a company that made RF power amplifiers. We had been sub-contracted to build a power amplifier which transmitted video from a certain device to allow its direction to be remotely controlled while in flight.

During our continuing production and shipment of the units, several of them were returned to us from our customer, having failed pre-flight testing. They came upon my bench to be repaired. Upon inspection, I found that each unit exhibited the same problem. A hi-rel electrolytic capacitor had failed, spectacularly. A pattern was developing — which was reinforced when one of those capacitors failed while I was testing a newly assembled unit. After retrieving the body of that capacitor, which had shot across the room, I read the date code on it.

All the failed parts had been sitting on the shelf for a very long time before being used. We were building a new product with old parts. I was able to convince the powers-that-be to toss the old parts and buy new ones. Problem solved.

Reliability matters. Especially when you only get one chance.
https://www.youtube.com/watch?v=n4zEmeWmhh4 (https://www.youtube.com/watch?v=n4zEmeWmhh4)