Problems with Zenith K731

[gdunlapsd]

Hi,

My name's Greg and I'm new to this forum.

I recently acquired a Zenith K731 radio via ebay and am trying to get it up and running. It was "working" when I got it, both AM and FM, but had a loud hum and barely any sound from the stations. I just recapped it with 3 electrolytics to replace the multi section can and film caps to replace the paper caps, and the improvement was remarkable.

However, when I installed a 1N4007 diode with a 68-Ohm 2watt dropping resistor, as soon as I turned it on the resistor started to smoke! Of course I immediately switched it off. I wired the selenium rectifier and stock dropping resistor back in and it worked fine again. Triple-checked my work, no shorts, tried the diode and new resistor again, same result. Resistor measures good, tried another one anyway, same result. For now I'm back to using the selenium rectifier, which is, for now working. I'm quite puzzled - where did I go wrong? I've read of others using the same diode/resistor combination successfully. The chassis on mine is the 7K07.

Another issue - FM was working though quieter than the AM side, was listening to a station for a while when it suddenly seemed to cut out - now FM's really quiet, does work but I have to crank up the volume. AM is still nice and loud. Even before it went quiet there was slight distortion (not horrible but I could hear it on the Ss's) and treble wasn't too good (AM actually sounds much better). Some low hum on FM as well - doesn't seem so much on AM, but there's a lot of AM noise around where I live anyway so it's hard to tell for sure.

What should I look for first? Bad tube, resistor, cap? Unfortunately I don't have any tube testers or much else in the way of diagnostic equipment - just a Rat Shack multimeter.
Any tips on what I should look for would be greatly appreciated!

[DaveWM]

backwards diode? band should be the positive. check the diode with a DMM to make sure its good or get a new one.

[gdunlapsd]

Thanks Dave, I thought I had the diode oriented correctly, but maybe it was reversed - it's gotta be something dumb like that. I'll pick up a new diode tonight and try again.

Still wondering about the weak FM though; I swapped the 12DT8 tube with one from another radio I have and no change, though it was temporarily louder when I swapped the original back in. Heard a pop over the speaker and the volume lowered, and it went up and down a few times with a speaker pop each time, so now I figure it's something loose or shorting; or could a bad resistor or cap do that?

[DaveWM]

low voltage will cause the osc to stop, killing all sound (no rushing sound between stations) the pop could be the sudden start up of the osc. Fix the diode then check the B+. Reversed diode could damage new B+ filter caps.

I generally monitor the B+ and then do a slow variac startup whenever I replace diodes. You can quickly see if the B+ is the correct polarity at the very low voltage to make sure you have no wiring errors in how the diode is hooked up (as in reversed you will see neg voltage at the positive term of the filter cap, but if you see it when its low volt no damage will be done).

[Reece]

Got it memorized: the B+ comes out of the banded end (cathode) of the diode. I pasted up a note of that over the bench years ago until it sunk in.

[gdunlapsd]

Thanks - must've had it backwards, it's working now with the diode. Voltages are about right, no hum, so the caps must still be OK. FM was still quiet, but I replaced some out-of-spec resistors and it's much better.

FM reception seems pretty good now and the sound is clear, but the FM volume is still lower than I think it ought to be - cranked all the way up it's at a comfortable listening level, but I know there should be more headroom. No problems with the AM side.

There are still a couple of resistors out of spec, so I'll replace them tomorrow if I can find replacements around here. If that doesn't do it, would a weak tube cause low volume? Something else I should look at?

Thanks again!

[DaveWM]

AM and FM uses the same IF tubes but for the last, the FM limiter, but I doubt that's it. Might be an alignment issue, esp on the last stage of the discriminator, but with out proper tools and know how you should not attempt to adjust. Unless a resistor is WAY out I can't see how this could be it, like 500% or more. Most tube designs work fine with out of spec resistors, unless the resistor is in a tuned circuit or is gold banded.

[gdunlapsd]

Originally Posted by DaveWM

AM and FM uses the same IF tubes but for the last, the FM limiter, but I doubt that's it. Might be an alignment issue, esp on the last stage of the discriminator, but with out proper tools and know how you should not attempt to adjust. Unless a resistor is WAY out I can't see how this could be it, like 500% or more. Most tube designs work fine with out of spec resistors, unless the resistor is in a tuned circuit or is gold banded.

One of the resistors to the limiter (R25 on the 7k07 schematic) is off a bit, supposed to be 4.7M, reads 5.7M; I'll change it out tonight. There's also a large, 3watt 950 Ohm resistor that reads 980 Ohms, maybe changing that out will help, though it feeds A and B which both read fine.

There is a resistor inside the FM section (R5) that's supposed to be 680 Ohms but reads 717 Ohm; maybe that's the culprit? I don't relish the idea of taking off the metal box that's enclosing it but I may have to.

I agree about alignment - don't want to go anywhere near that.

[mstaton]

The 950 ohm and the 680 are within 10% tolerence. They do not need to be changed. It would be a waste of time and it wont change a thing. That 4.7, yes, is out of tolerence.

[gdunlapsd]

Got it working now; replaced the 4.7M resistor and it made little difference; figuring I had nothing to lose at this point I tweaked the adjustment on the discriminator transformer a little (taking note of the starting position), and it made a huge difference - was able to get the volume up to normal on the FM side; however, it would go up and down with a pop, usually coming back if I pushed down on the discriminator tube.

I had cleaned all of the tube pins and sockets, so at first I was worried about the dreaded "Silver Migration Disease" I've heard about, but it wasn't crackling and didn't sound like "thunder" the way the symptom is usually described, so I figured there may be one or more cold solder joints. I reflowed the solder on all the connections under the discriminator tube and transformer, and now the FM volume is up to normal and it's no longer popping or cutting out.

I may ask a friend of mine for help doing an alignment sometime - he used to be a TV repair tech (including tube TVs from the old days) so he may know something about it, but for now it sounds pretty good.

Thanks again for the advice!

[Jeffhs]

The K731 is one of Zenith's best small AM-FM table radios of the late 1950s-'60s. I have one, an eBay score some years ago, that pulls in stations like a magnet, even with just the line cord antenna. This radio is unique in that the limiter tube also serves as a "third IF" stage, which explains its excellent selectivity. I haven't recapped or done anything else to mine since getting it (except, IIRC, to replace one tube about two years after putting it into service here), but it still plays excellently on the indoor antenna in this area, over 30 miles from most Cleveland FM stations. As I always say about the older Zenith radios and TVs, they don't make them like that anymore. If you have one of these old sets, hang on to it, restore it if you need to and enjoy it. I also have a Zenith H511-Y 5-tube AC/DC radio from 1951 that works well for its age, but to get to that point I had to do some work on it, such as rebending a couple of bent plates on the tuning capacitor rotor that were killing reception below 800 kHz, and cleaning the metal escutcheon plate (the oblong shaped one that has the name "ZENITH" stamped in small capital letters across its length) as it was terribly dirty, probably a direct result of having reposed in its former owner's garage, attic, basement, etc., unused, for years or decades. In fact, when I plugged the set in for the first time after UPS delivered it, the #47 pilot light behind the "Z" in the crest emblem at the top center of the cabinet promptly burned out with a bright flash, probably due to the 3-section filter capacitor having become unformed from decades of disuse. I replaced the bulb, however, and the set worked very well, although at some point two of the tuning cap rotor plates became bent (don't know how), killing the local oscillator from 800 kHz to the lower limit of the broadcast band.

I also have a Zenith C845 AM/FM hi-fidelity radio (the one in my avatar), which was given to me about four years ago by a VK member from Arizona. This radio worked very well out of the box and still works surprisingly well to this day, again with no recapping or other repair work needed.

My third vintage Zenith radio is an R-70 11-transistor AM-FM portable from 1980, another eBay score. This one also worked right out of the box, requiring only three new C-size batteries. It is by far my favorite Zenith portable, brings in lots of stations and sounds wonderful (typical Zenith). It is built like a tank, not like today's plastic headphone stereos and FM scanning receivers found in department stores and discount houses. The R-70 is surprisingly well-built, albeit offshore (Korea); I have been told that it is probably the last "good" transistor portable made by Zenith before the company left Chicago. I had a Zenith 4-mode integrated stereo system in the 1980s, also built to Zenith's standards by its Korean subsidiary at the time, but I left it at my former home and replaced it with an Aiwa AM/FM/CD stereo when I moved to an apartment 12 years ago. Don't know where it is now or even if it still works, although I do know it was used quite a bit by the person I gave it to in '99.

[gdunlapsd]

Thanks Jeff.

Turned out that it did have Silver Mica disease after all, in the discriminator transformer. I performed the operation on it, removed the bad cap(s) on the bottom and soldered replacements below the chassis after reinstallation, and that seems to have cured it. Probably why the adjustment had been off to begin with - someone else had toyed with it before I got it.

I tweaked the adjustment as well as I could by ear, but I don't have access to an O-scope or sweep generator, nor does my friend that used to do TV repair long ago. The sound and tuning still seem just a touch "off" - problems narrowing in on some stations - the spectrum is so dense here the stations are practically on top of each other - so I broke down and gave it to a local repair guy to do a proper alignment - cost me more then the things probably worth, but I didn't get it to flip it.

I'm jealous of the stuff you guys back east have available - I'm in San Diego, and around here you can't find squat, not even for an exorbitant price in antique stores (though I've seen them sell knockoffs as "antique"). The thrift stores have pretty much nothing but black plastic '90's crap. The guy who's doing my radio alignment does have some cool antique radios, I think he's the only game in town though. I'd love to have a vintage TV, but have no idea where I'd ever find one here.