Problem with shifting volume: Zenith K-731

[Nolan Woodbury]

Over the last few days, one of my favorite sets has developed a problem.

Once warm and playing for 5-10 minutes, the volume will suddenly increase (with, it sounds like, an audible increase in treble too) and if left unattended, will eventually drop to, or perhaps lower than the original level. It isn’t (I don’t think) a reception issue, as the Zenith exhibits the same behavior on weak or strong (FM) stations. It just occurred to me I haven’t tried it on AM, so perhaps I’ll do that before reaching for the toolbox.

Could it be a voltage issue? A failing volume pot?

I'm set to pull the chassis out and have a look, but if this is a common problem perhaps one of you can provide a short cut. I began using this radio out of the box and it plays somewhat regularly. Because of its tone, it's a unit I'd like to keep and listen to more.

Awaiting the collective wisdom of the group to rain down upon me...

[RetroHacker]

Check the bias resistors. I've got a Delmonico that does something similar. After it's been on for half an hour the volume drops, eventually away to nothingness. Typically these sorts of problems are caused by old, drifting carbon composite resistors. Go in with a meter and start checking anything over one meg or so. Replace any that are out of spec. Those old carbon composite resistors like to drift upwards in value with age, and tend to change value when they get hot. Modern carbon film resistors are available cheaply at any good electronics store, and even most of the bad ones (Rat Shack). More than likely you'll need 1/2 watt ones. Rat Shack has a 100-piece assortment pack for a few bucks that works pretty well, remembering of course that you can combine resistors in series to add values together to substitute for unavailable values. Connected in parallel, the sum of the reciprocals of the values of the resistors is equivilant to the reciprocal of the total resistance.

Good luck.

-Ian

[Nick_the_'Nole]

I have a similar problem on my Zenith, C725 or something, no model number tag on it... Anyway, I checked the resistors and everything seems to be reasonably close to rated value. Some of 'em have drifted to about 7% out, but they were tolerant to 10% anyway, no? Of course, I wasn't gonna check 'em hot, since I've had bad experiences doing such things. I *did* however, find some extremely leaky electrolytics while I was poking around. I haven't replaced them yet to know for sure if they're the entire problem, but you didn't mention whether yours had been recapped yet, so could it be that simple?

I really want it to be that simple, 'cause I don't wanna track down bad resistors again...

[Nolan Woodbury]

A faulty bias resistor makes sense. In the back of my mind, I was thinking it could be something worse, like amplifier failure. I have a pretty good selection of bits I got with one of my old sets but I'm unsure of some the values. This is a good excuse to buy the assortment pack.

I haven't touched this radio. I took it out of the box and plugged it in, but kept a close eye on it for the first couple hours. I suppose it would be a good idea to re-cap the set, but I've got a handy test electrolytic (using a good capacitor with two alligator clips) bypass tester that I've used if I don't find any leakage. Good idea? Or re-cap anyway? The radio has no hum, excellent reception and good volume.

I'm babbling. I need to buy some supplies, grab the ohmmeter and pull the chassis. I hope it's a simple fix. I'm pretty new at this, and get the distinct feeling I know just enough to get myself into trouble.

[RetroHacker]

If all the resistors are OK, it could be a cap. Start checking around the output tubes, and work out from there. A leaking cap can cause similar problems - as the set heats up the cap starts leaking and the tube draws more current and then the cap leaks more and then the tube draws even more current... ad infinium until the tube goes into cutoff.

Both possibilites (resistor or capacitor) are pretty likely. I've just seen more resistors cause this problem than caps in later radios, a lot of the smaller table sets (AM/FM) used ceramic disc caps.

-Ian

[Chad Hauris]

Sounds like it's time for the freeze spray! use isolation transformer when checking...power line is connected to chassis.
Spray the freeze spray on resistors and ceramic caps and see if you can get the audio to come back once it's heated up. Do not spray it on the hot tubes. YOu may have to use a heat gun or hair dryer to help heat up the area to coax out the failure mode.
Probably not electrolytics as there is no hum...only other electrolytic in this set is in the FM ratio detector (if it used that system).

[Nolan Woodbury]

I thought it would be a good idea to post the results of this encounter...even if it took me nearly a month to finally get to it.

My copy of "Antique Radio Restoration Guide" (by David Johnson) says two-thirds of all tube radio problems are tube related, so I checked them first. I didn't have to go any farther.

The radio was dead, so I removed the power cord from the back and plugged it in. Using a stick I began nudging the tubes back and forth. The 7MO7 chassis uses two 12BA6 R.F amplifier tubes in parallel, and when I touched the right one the radio sprung to life. I unplugged it and after pulling the tube, I noticed a quite a bit of corrosion on the pins. I cleaned them with a small piece of Scotchbrite then doused the socket with electronics cleaner. Feeling pretty good about that, I yanked the other six and did the same thing, although I didn’t see any of the white stuff on any of the other tubes.

I played the set for over three hours today with no problems. Good volume, reception and tone. Absolutely no hum. The radios sits for three weeks and I fix the problem in five minutes? Sheesh. All that was left to do was put away the (still sealed) box of resistors and caps, my soldiering iron, the new tubes and my un-used can of freon spray. I'm a radio repairman! Well, not really, but this simple fix proves you don't (always) have to be one to solve a problem.

[Paula]

Hi Nolan,

Glad to hear that you got your radio working. I have a K731 that I listen to every day. It looks just like the one pictured as your avatar. I did some work on it to make it reliable for a daily-listener (see this thread: http://audiokarma.org/forums/showthread.php?t=18773). Very nice radio, that K731.

Paula

[Sandy G]

Paula ! Great to see you-have missed you !! Whatcha been up to? Any interesting new sets to tell us about...? Hope everything's been going well for you !! Don't stay away so long...-Sandy G.

[Paula]

Hi Sandy! I bought a house a few months ago, and my radio projects have temporarily fallen off the radar screen. Been really busy painting and fixing up the place (including setting up a place to work on radios!)

Nice to hear from you!

Paula