Zenith 5D611 radio

[electra225]

I actually still have four Zenith radios. All have some sort of sentimental attachment but this one. It has a badly-broken bakelite case and my wife thinks it's "cute" for some reason. I made a special point to divest my collection of most everything Zenith when we held our auction. My luck with Zenith has been rather dismal. Three of the four Zenith sets I still own are decent performers. This one is not. It has what is for me the typical Zenith malady, an obscure fault. This chassis uses a 12K7GT IF amp. It has a grid cap on top of the tube. If I just lay the grid lead on the tube, it plays somewhat okay. If I push the grid lead all the way on the tube, the thing squeals like a stuck pig, another Zenith malady I frequently have experienced. The filter caps are 30uf, and two 20uf at 150 volt electrolytics. My plan is to replace these filters with caps of increased value, 47uf and two 33uf at 250 volt electrolytics. My experience has been that one of Zenith's many maladies is filter caps that are undersized. Zenith is also known to use the output filter cap for cathode bypass duties. I got one Zenith chassis, a 5-S-56 to run somewhat acceptably with larger capacity filter caps, so I decided to try that on this chassis. I have somewhat repaired the broken cabinet. My plan is to hotrod the cabinet so the wife can display "her" radio in the house. How it works is really unimportant, but I hope to maybe learn something by experimenting.

[electra225]

And.......

Of course, replacing the filter caps did nothing. I still have a motorboat at low volume, turning into a banshee scream at higher volume. This very issue is why I detest anything Zenith. If I disconnect the grid cap on the 12K7 If amplifier, the oscillation ceases. I would therefore believe that the issue is ahead of the volume control. My experience with motorboating and squealing tells me that it usually originates in the audio or power supply. I'm missing something, but what?

I am getting a small negative voltage on the grid of the 12SA7 pentagrid converter tube. I'm getting about 2 volts negative. I like to see from 5 to 15 volts negative, so I'm not sure what that tells me. The radio basically doesn;t receive stations, never has since I've had it. The pin 1 terminals on all the tube sockets are grounded, so I tried all-metal tubes in the converter and first audio, 12SQ7 positions, with no improvement. The 12SA7 was a Zenith-branded tube, the first audio tube was Coronado branded. I don't have a 12K7 to sub. I have bridged the new filter caps with no improvement. B+ voltages are really close. If I touch the positive probe of my VTVM to the mixer plate, the motorboating ceases. This suggests to me something is floating, but, again, what? And, on top of all that, I put my finger thru the speaker cone.

The output transformer appears to be built in to the speaker, like a field coil setup. So, a different speaker will likely be made of unobtanium. Only Zenith could build a pile of total crap like this......

[TC Chris]

Long ago my brother gave me a little Philco battery/AC portables from the late '40s, the one with the tambour top. Plugged it in and it oscillated. I dropped a grounded tube shield over one of the tubes, RF or IF, and it quieted down. Probably has a bad cap but the quick fix did it. Sometimes luck is just as good as knowledge.

Chris Campbell

[electra225]

I may have made MINOR progress forward. If I touch the plate of the mixer with my signal tracer probe, I can weakly hear a station. As i move the dial, only the strongest local stations can be heard, but that confirms that the local oscillator is running. Touching the plate of the mixer also kills the motorboating. I will check the grid circuits for an open. If I remove the grid lead on the 12K7 the squealing and motorboating will stop. I believe, at this point, that the issue is in the IF stage. I had hum before I changed the filters, so that is improved. The motorboating and squealing were there when I started. This set may be out of alignment. I can't check that until I get the noise stopped.

[electra225]

This radio didn't work and had a huge hum when I started working on it. Naturally, I recapped it. It is still full of dogbone resistors that I haven't checked. I need to start again, this time paying attention to somewhat proper troubleshooting procedures and quit jumping all over the place. I believe the squeal is one issue, the motorboating yet another issue. Both are controlled by the volume control. And, my capacitors are all at least three years old. I put an electrolytic I had on hand in a record player. It ran about five minutes then started humming. It had opened. So it might be that a cap I put in this radio new is no good. This thing may be a Zenith, but it is an AA5 for pity's sake, so I should be able to figure it out.

[electra225]

I reckon I'm talking to myself on this.

The schematic shows glass tubes as being correct in this chassis. The 12SA7GT and the 12K7GT tubes are original Zenith-branded. So that is not an issue. It is uncommon to see grounded Pin 1 on all the tube sockets, yet find glass tubes specified. C20 the output filter cap is suspect, but I'd think I'd get heightened hum if it was open. No hum. I am almost sure the squeal and the motorboating are two different issues. I am not sure if they are connected, which is my next area of investigation. I will check each resistor, since they are dogbones and are subject to being drifted.

[electra225]

I don't have the filter caps wired right. That is the ONLY thing it can be. I'm not sure I understand how the factory wired the power supply in this chassis. The filter caps are in a metal can inside a cardboard tube that is insulated from the metal chassis. IT says that the can is negative. So, since I restuffed the can, I dug the old guts out and put three new caps in the metal can I put the negatives of the caps to the original negative place on the can. Typically, you can put the negative terminal of the VTVM on common negative, the positive on the positives of the filter caps, then watch the caps charge from the battery in the VTVM. Reverse the leads of the VTVM, they will discharge, then recharge. These did that when I first put them in. Now, all they do is peg the needle whichever way I connect the VTVM. Something isn't right. I don't have any hum. The motorboating will stop when I touch any kind of test equipment to almost anyplace in the chassis. None of the grids are floating except the one on the 12SQ7. I don't see any resistors that have drifted far enough to cause the trouble. Everything seems wired right, nothing is touching, values of replacement components are right. Oddly, B+ is right on the nose. Can the filters be wired right for B+ and eliminating hum, but not be wired right for bypass duty? The "floating" grid on the 12SQ7 gets its "ground" from the output filter cap.

I know now FOR SURE I have the filter caps wired wrong. I have all the negatives run to common negative. That is not right. The only one that goes to common negative is the output filter cap. The other two connect together, but I'm not sure how. The terminals are not well defined on the schematic.

[electra225]

Two things. First, a clue. Second, I fixed my miswire.

As to the miswire, I have no defense. I looked at the schematic with only one eye, I reckon. Zenith went to great lengths to at least draw their schematics, if not design the power supply, with an eye toward busting the RCA patents. They did creative drawing in this power supply and I didn't catch it. I had to un-stuff my filter caps, dig out the exploded cap, replace them all, wired right this time. B+ is perfect everywhere with the ISO\/Variac set at 117 volts line voltage, so we are good there. The audio is quiet as a church mouse with the volume turned down all the way....

Now the clue. If I remove the grid cap on the IF amp, the motorboating and squealing are gone. The squealing I have determined is due to poor alignment. The motorboating is controlled by the volume control, so it is in the front end. I have further determined that the motorboating originates between the plate of the mixer and the grid of the IF amp. If I use the signal tracer turned up almost full volume, I can faintly hear a strong local station with no motorboating at the plate of the mixer. With the grid cap removed from the IF tube, I get a hum controllable by the volume control. Then, if I dig around with a screwdriver between the tuning capacitor and the chassis, I get sparks, and a hum in the speaker. I don't think that should happen. The most interesting fact, however, is this. I was tapping on things, trying to see if ANYTHING I did was going to make any difference. I tapped on the antenna terminal with a screwdriver. I heard a scratching in the speaker. Then I connected the signal generator at high output to the antenna terminal, and with the volume on the radio all the way up, I heard a 400 cycle note from the generator in the speaker. How can a signal pass when I have the signal chain interrupted at the IF amp? I got to looking at the schematic and found a path from antenna to audio. The AVC rail. Somehow the screwdriver tap is riding down the antenna, down the AVC rail, then, thru the AVC resistor, finding its way coupled to the input of the volume control. This got me to wondering if MAYBE the feedback path causing the oscillation I have been looking for all day has something to do with the AVC circuit. That would place the feedback and oscillation squarely where I have it cornered. My next experiment will be figuring out what there is in the AVC circuit that is potentially causing feedback and motorboating. This thing is messed up, like that when I got it. Now I know why it was so cheap.....

[Dr. Radio]

Didn't this model use a voltage doubler circuit apart of the rectifier?

[electra225]

I don't believe so. B+ on the cathode of the rectifier is 115-ish. B+ on the output tube plate is 110-ish. I use the "-ish" depending on where I have the Variac set.

I'm giving the AVC circuit scrutiny next. That is about all that is between the plate of the mixer and the grid of the IF amp.

[electra225]

I have found R2, a resistor in the AVC line, has drifted from the 15 megohm spec down to 2.795 megohms. This has been confirmed by analog and digital meters and I have lifted one leg of the resistor just to be sure. So, I'll need to order this resistor and I think I'll get another 12K7GT and sub it just for grins. There are a couple of 470 K ohm resistors in the audio section I'm going to replace at the same time, although they aren't part of this problem. I am not sure if this will solve the problem, but I need to start here, then see what I have.

[Dr. Radio]

I think you, as well as a lot of others that have worked on these radios, from the time they were relatively new, up until even now, would like to take the radio and do this to the engineer that signed-off "ok" on this particular design assembly......

[electra225]

EVERY time I get deep into a Zenith radio, I get bogged down in an obscure fault. I was beginning to wonder if it was just me. Thanks, Doc.....

[electra225]

In addition to the above with the AVC resistor, I have tinkered with this chassis a couple of times in the past couple of weeks. I am down to three issues: No/weak reception, hum controlled by the volume control, squealing/motorboating whether on a station or not, worse as you advance the volume control. A good ol' Zenith obscure fault....

Before I slid this chassis from the end of the bench to the trash can, I decided to approach this from a different angle. The AVC resistor being drifted may cause the weak reception, so I put this issue on the back burner for now. I wanted to tackle the squeal.motorboat issue. Since I have nothing to lose, I did the first thing I shouldn't have been doing. I started twiddling with screws. Turning all the adjustments, noting any difference. I turned the second IF trimmers with no real difference. I then turned the first IF trimmers, and got oscillator hash in the speaker. I turned the dial and got a strong local station on 620 to come in, but really distorted. I checked plate voltage on the IF tube and found it about 40 volts low. I had eight volts cathode voltage. Remember if I remove the grid cap on the 12K7 GT IF amp, the oscillation goes away. I then noticed that the reading on the KAW was going to nothing, the dial light went out, the radio slowly died, then came back on. Suspicion then was placed on that 12K7 IF amp that I don't have a spare for. I placed that tube on my B&K 707 tube tester and found absolutely no emission. None. All it did was the filament would light. The tube checker did not show a heater to cathode short, although that is my suspicion. My next step is to order a 12K7 then see what that does. I am holding out hope I can get this POS to work. I hate Zenith radios for a reason......

[TC Chris]

My next step is to order a 12K7 then see what that does. I am holding out hope I can get this POS to work. I hate Zenith radios for a reason......

Still trying to figure out how forgetting to check the tubes is a Zenith defect.... Kinda like blaming Ford if I run out of gas.

Chris Campbell

[electra225]

Guilty as charged.....
One reason I disregarded the 12K7 as being the main problem is that I was getting sound thru the speaker, albeit squealing. When I removed the grid cap on the 12K7, that killed the squeal, so my problem was ahead of that grid lead. I had more than one problem. Apparently, the oscillation squeal was riding thru the dead IF amp via the AFC buss. The IF transformers being so out of alignment also contributed to the squeal. The tube has a suspected heater to cathode short, so that helped both the hum and the squeal. Running the radio chassis for an extended period trying to get the magic smoke to leave finished off the 12K7. Now it has an intermittent filament. I believe part of the reason I have so much trouble working with a Zenith chassis is that I forget how much of an enemy of RCA Zenith really was. They went to extravagant lengths to circumvent the RCA patents. Some of their circuit designs, particularly the power supplies are relatively bizarre. Number one on the list would be Zenith's proclivity to use the output filter capacitor for cathode bypass duty in the audio. This design provides a path for oscillation. Zenith also likes to connect the + side of filter caps to common negative, then use the - side of the filter caps for B+. This design tends to promote oscillation. And it makes it easy to miswire the filter caps. When I work with a Zenith chassis, it is important to set and look at the drawing for a time to familiarize myself with where I can go wrong. Sometimes little AA5 radios cause me more grief than digging into a Stereo Theater.....

[electra225]

Chris, I'm right about obscure Zenith faults. And did I learn something working on this radio? Yeah, but I don't understand what I learned. Good old Zenith and their obscure faults. Never ran across this before......

I replaced the AVC resistor with no change. I replaced the 12K7GT with a Philco that I got at AES. Replacing the tube got me some reception, albeit with a lot of squealing and really wonky tuning. I used my signal generator and signal tracer to check each stage for squealing. If I touched a terminal with the signal tracer, both the squealing and the signal disappeared. If I ran my hand behind the antenna, the squealing increased. I finally got a station at 1060 to come in with the least squealing. Twiddling with the IF trimmers got it coming in pretty well, but only that station. If I moved the dial, all I got was that infernal squealing. The Zenith squeal in radios, the Zenith buzz in TV....

I went back to the station that I could hear and noticed that the audio was distorted. Grid emissions in the 50L6? Here is the weird part, the obscure fault. Replacing the 50L6 with another one not only eliminated the audio distortion, but it also got rid of the squeal. How in the devil does an output tube cause squealing?

Bottom line is that I finally got it working pretty well below 1200 khz. The set is totally dead above there, but I can hear the local oscillator in another radio at 1465. I cleaned the tuner, which didn't help. It does not check shorted with an ohmmeter. I am suspecting the oscillator is not running above 1200, but I can hear it in another radio. The IF in this set is 465 khz.

Another issue is that the chassis runs about 20 minutes after it cools down then just fades until only the strongest local stations are heard. Shut it down, let it cool off for 10 minutes or so, fire it back up and it sounds pretty good. Then the audio starts sounding muddy, followed by very weak reception. Admittedly, the only alignment I have done is just twiddling to get it working. Since it is dead above 1200 khz, one has to suspect that the oscillator is not running at the right speed, or not running at all occasionally. I thought I had a 12SA7 in my test stash, but I must have used it. I want to replace it to see what happens. It could also be the 12K7 IF amp, but it tests good as is the new one I got. I need to sub all the tubes to check if that makes a difference. I may get this thing working yet. I am THIS close. Suggestions would certainly be appreciated.

Another thing, probably self-inflicted. None of my normal test procedures worked on this radio. I still haven't been able to get a signal generator test note thru the radio. Just individual stages. My signal tracer didn't want to ground right to this chassis. I got a lot of noise out of it when I was using it on this chassis. If you peck on the chassis with a screwdriver handle, I get noise like a bad connection. To save my soul, I don't see anything.

[Dr. Radio]

You're probably tired of my random post responses to your troubles...



But, I'm embarrassed. For some reason, I thought the 5D611 was the "upside down bakelight chassis" radio. Otherwise, I wouldn't have been so negative about it. I googled the 5D611 and it's a nice, run of the mill table set.

I guess I'm losing it.



Now, Greg, with obscure problems...you need to look for obscure things.

Yes, try another oscillator tube.

If that fails you...

If this radio relies on ANY rivets to provide a connection to the chassis, suspect those. Just like bad grounds in an old car, mounting points on chassis radios can oxidize even if they look solid.

Although we constant hear, don't change out original capacitors related to the oscillator circuit due to their special characteristics, if you have an "domino dot" mica capacitor, with the age of the radio, maybe, just maybe it has finally inhaled moisture as the lead seals have given up the ghost.

This is the weird stuff that will make you pull out your hair.

[TC Chris]

I was kinda thinking an exorcism might be in order.

Chris Campbell

[electra225]

I appreciate your input, Doc. I'm making progress, but I may be overlooking something. I never turn down good help, so, please, your input is most appreciated in all the messes I get myself into.

The only reason I'm messing with this thing at all is because the cabinet is busted and the wife thinks it's cute and wants me to fix it and hotrod it. The rule is no radios in the house that doesn't work. "Happy hands at home" have been working with this radio. We bought it at an auction for a known "tinkerer" a jack of all trades type guy who had recently passed away. I bought several items at his auction, none of which worked, all of which had been tinkered with by someone who had no idea what they were doing. I found a wire earlier, that the rubber insulation had fallen off of, that was jammed between the speaker and the chassis. Every screw on this radio had been turned. I hope the IF trimmers haven't been turned down so hard something got broken.

I have checked with a magnifying glass all the grounds I could see. They are all soldered, so I reheated those to be sure. I don't see anymore wires that are touching. Several have really crispy insulation, but are intact. The only ground I'm not sure about is how the tuner is grounded. I ran a jumper wire from the tuner frame to the chassis with no improvement. I don't know what else could make a set get weak gradually, then get really weak to the point only strong stations are heard other than a tube. There is no filament opening because the set does not quit completely, all the tubes remain lit. I haven't checked B+ when the trouble starts, and maybe I should get set up to measure that. I checked B+ when I was troubleshooting the chassis and it was about 10 volts high, but line voltage would account for that. I'm running it on my isolation transformer, but not the Variac.

I think my first thing will be to find out why it's fading. That out of the way, then I'll tackle the alignment. And, yes, Doc, it does have a domino cap in the oscillator circuit. I'll look up its value and see if I have one for replacement. Good idea, thank you!!

Chris, if I did an exorcism on this thing, I might not need to fix the cabinet...... I've gone too far now to stop. The thing tried to kick my butt, but it ain't gonna get that job done. I'm not out of the woods yet.....