Zenith K731 - New Project - New Questions

Hi.

First off, I would like to say nice forums. Lots of useful info for ALL kinds of audio equipment.

I recently purchased a Zenith K731. It works, but I am unsure of the FM sensitivity. When I first purchased it the reception was horrible. It would only pick up 3 stations FM in Memphis and they were close to the house. I read a little, and removed the tubes one by one to check for buildup on the pins. Everything was good except there is one tube that sits to the left front by itself that was loose. When I re-seated it the sensitivity improved dramaticaly.

Still, it is dead on AM. Nothing Nada. Just a slight hum when I press my ear up to the speaker. The hum is also present in FM, but with just a tad volume (barely) it disappears. Also, I read these radios had a light on the frequency indicator. That does not work either.

It's a cool radio and it has a permanent home on my living room mantel. I just want to get it 100%. The finish is almost perfect. This particular radio doesn't have the legs that make it stand up, this one sits kinda like an old television did - arches and corner feet. I have read a good bit of praise from these radios on this forum, and it has a real good rich sound. Also, this radio has the red CD logos on both sides of the dial.

I have tinkered a bit with electronics such as CB radios just NO tubes. Can someone nudge me in the right direction as to restoring this radio. I'm thinking a new set of tubes as well as a new set of caps resistors etc.

Thanks in Advance.
Mike

[sydsfloyd67]

I'd say that that vintage Zenith tube radio should be very sensitive on the FM band. In my case, more so than several subsequent quality 70s receivers in my location. I've compared a handful here with a '64 8 tube Zenith using no external antenna at all on the Zenith.-sf

[Chad Hauris]

If it has a selenium rectifier...replace with silicon diode such as 1N4007 with a 100 ohm 10 watt resistor between the anode of the diode and the power line to pad down the voltage to the correct level and help absorb the turn on surge. The problem could be low voltage due to bad selenium rectifier. The old selenium must be electrically disconnected from the circuit. Also use a 1/2 amp fuse right where the power line comes in.

If there is some hum the electrolytics are starting to go bad...replace them and any wax paper caps or black caps with color bands which are plastic encased paper. Measure your power supply voltages and make sure they are OK once you repair the power supply.

Resistors are usually OK and most tubes are OK too...the FM mixer/oscillator tube such as the 12AT7 or 12DT8 is the most suspect to be bad. Don't go looking for tube problems though till you make sure your power supply is ok.

Some of these Zeniths run the dial lamp off a dropping resistor directly from the AC line...in which case it is a 28 volt bulb I believe. The older ones use a neon bulb indicator.

Also please observe pertinent safety precautions...the chassis is connected directly to the power line and there can be a shock hazard from charged caps, etc...you may want to do a little more research on these hazards before you begin as it can be dangerous if you don't take the proper precautions.

Thank you BOTH on your replys. I have let it run for quite a while today about 4 or so hours and it's still chugging along. The volume shifts from time to time, only once getting way louder than it was when I first turned it on. Still no audio from AM though. Does this unit have to an external antenna or is internal one suffecient? I figure it needs an antenna to get maximum reception but I figured I could at least hear a local close oldie station without one.

Also, I remember a little about a friend messing with those old sweep tube amplifiers on the cb band. Every time he would mess with it, he bled down those huge capacitors eliminating a rather nasty shock. Is this needed with this particular radio?

Thanks again,
Mike

I hate to sound stupid, but I can't figure out how to make the control knobs let loose. I don't want to pull them off for fear of damaging something on the chassis. Anyone have a inside secret?

[Chad Hauris]

Sometimes the control knobs have "keepers" on them to keep them from being pulled off as the control shafts are connected to the hot chassis and there would be a shock hazard if the knobs are removed... You need to loosen the knobs by pulling gently and then remove the chassis mounting bolts and pull the chassis back out of the cabinet, you will then pull the control shafts off the knobs.