migrating fm stations on my Zenith K731 - Videokarma.org TV - Video

burnsyaz · #1 03-26-2008, 12:23 PM

Hi there,
I have a problem with my k731. When I turn it on it works fine for like 5 minutes and then the radio stations seem to migrate and eventually within minutes all you hear is static. For example, if I am listening to 93 on the fm dial, the station will start to move down the dial to the right. Eventually I will chase 93 down the dial until it is being received on like 95 or 96 on the dial. Then it just fades to static.
I have tested all of the tubes and they are all in good condition.
The unit is completely stock with the exception of a couple of tubes.
Anyone have any ideas. As always, thanks in advance.

Fisherdude · #2 03-26-2008, 01:16 PM

My first thought is that you probably have a component, such as a resistor or a capacitor, that is failing. It's in good enough shape to allow the radio to power up and operate, but as the bad part gets hotter and hotter it begins to drift farther out of spec, as evidenced by the shifting frequency, until it gets so far off that the radio no longer operates.

The first thing to try is to take the chassis out of the cabinet and do a visual inspection. Look for any parts that show evidence of heat, leakage, bulging, etc.. If you find something that makes you go: "Oh-oh, that can't be good!", then that's probably it.

While you're in there, I would also put contact cleaner on the tube pins and sockets. Sometimes a poor connection can cause a tube to go out of its intended operating parameters.

If nothing is noticeable, then you need a schematic and a DMM to start checking voltages throughout the circuit until you find where something isn't right.

Old1625 · #3 03-27-2008, 01:00 PM

If there are any original tubular capacitors in there they should be replaced--blocking and supply-bypass alike. If a leaky one is loading down the B+ to the FM front end it can sometimes manifest itself that way. A global replacement of those caps may solve your problem.

If the problem persists then look for the simple and obvious things first.

A schematic is a good idea if it is evident that someone might have replaced any of the disc capacitors associated with the local oscillator. Many of those have temperature coefficients. Oscillators are critical in nature. If the wrong part is in there you can get severe drift. Get the Sams and look at the component list and make sure everything is exactly as it should be.

burnsyaz · #4 03-27-2008, 08:45 PM

I'll take a look, but I'm pretty sure the caps are all original. The radio worked great for months when I first got it a couple of years back.

jpdylon · #5 03-27-2008, 11:40 PM

1st, check the 12DT8 that sitting near the tuner assembly. That is the FM oscillator and detector. If its faulty it will cause major drifting problem. This isn't the most likely cause of the problem but its a good place to start

Also consider replacing the selenium rectifier with a modern silicon diode. A 1N4007 would work well for this purpose. You will need to add some dropping resistance to bring the voltages into spec. The silicon diode is much more efficient than the selenium rectifier. When the selenium starts to fail. te B+ will drop, which may explain your symptom.

Definitely replace caps. Given their age the are probably more like resistors than capacitors. If you want to continue to use this reliably its a good thing to do. Most of the zenith chassis of that era are easy to work on and are fairly spacious.

If you plan to run this out of the cabinet or with test instruments, you'll need an isolation transformer to prevent your electrocution or damage to your test instruments.

burnsyaz · #6 03-28-2008, 12:24 AM

Anyone know where to get the schematic? I googled it but I haven't come up with anything yet.

jpdylon · #7 03-28-2008, 10:17 AM

register at diyaudio.com and its yours

http://www.audioasylum.com/forums/tu...ages/5263.html

or maybe a member there can snag it for you?

burnsyaz · #8 03-28-2008, 12:28 PM

Everything looks normal as far as I can tell so time to bust out the meter. Thanks for everyone's input. Maybe I'll get a chance to mess with it some more this weekend.

skippy_ps · #9 03-29-2008, 05:45 PM

I had the same problem with my K731. A new 12DT8 fixed it, as Jordan pointed out. Mine has all new caps though.

Murray

burnsyaz · #10 04-01-2008, 12:59 PM

Ok I ordered a NOS 12DT8. It should be here in a few days. I'll try that before I move on to anything else. The 12DT8 that I have in there now tests as good but we all know how tubes can be deceiving sometimes. Anyhow it will be nice to have a nice new backup 12DT8 just in case that tube is not the problem.

burnsyaz · #11 04-06-2008, 01:00 PM

Well I pulled the chassis, cleaned out the dust bunnies and sprayed the tube sockets. I put it all back together and it works fine now. I didn't have to replace any tubes after all. Could this problem have been caused by the dust? I have heard about large amounts of dust causing circuits to fail before. Thanks again for everyone's input.

Fisherdude · #12 04-06-2008, 03:18 PM

Quote:

Originally Posted by Fisherdude

...While you're in there, I would also put contact cleaner on the tube pins and sockets. Sometimes a poor connection can cause a tube to go out of its intended operating parameters...

Quote:

Originally Posted by burnsyaz

Well I pulled the chassis, cleaned out the dust bunnies and sprayed the tube sockets. I put it all back together and it works fine now...

It's really amazing how often it's the simplest things first. When a radio sits, sometimes for decades, crud accumulates on all contact surfaces. I've had a dead radio come to life simply by clicking all the switches back a forth a few times.

Congrats on getting it working! That's a nice radio.