Zenith Model H-615-W

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Zenith Model H-615-W

Post by Wildcat445 on Fri Sep 20, 2013 7:06 pm

It was raining today, so I could not finish staining the deck. Darn. But I did get some bench time in today on another radio in my "To Do" pile.

The victim this time was a Zenith H-615-W. Chassis 6G05. It is your typical "AA6" with a tuned RF stage. It has a 6 inch speaker and is typically a good performer for what it is. I have a J-615, which has chassis 6J05, basically the same chassis. The 6G chassis has a 35L6 output tube and the 6J chassis has a 35C5. The pilot lights are in different places, otherwise they are the same.

This radio had all the classic symptoms of silver mica disease. After being in operation anywhere from 5 minutes to a couple hours, there would be that horrible crashing sound coming from the speaker. This particular chassis has two different kinds of IF transformers. The first IF is slug tuned, the kind that can have SMD and the second IF transformer was trimmer tuned. This type does not get SMD. Apparently, either kind was available at the factory. The chassis is drilled for either type. The trimmer tuned type has a single center mounting bolt, and the slug tuned type has two mounting bolts, one on either side. This chassis had not been recapped, so I took the opportunity to perform this maintenance repair while I had it on the bench. I even had to re-stuff the filter caps, since there was not room to mount replacements under the chassis. All the resistors checked okay, as did all the tubes. The operation of the radio was unchanged by replacing the caps, but I could continue with repair and diagnosis confident that I was working with known good components. The odd thing about this radio was that I found that I could rap my knuckles on the top of the radio cabinet and affect the crashing sound in the speaker. Sometimes, I could get it to start by rapping the cabinet, and sometimes I could get it to quit in like manner. This seemed odd to me.

To remove the chassis from the cabinet, it is first necessary to remove the pilot light from the radio cabinet. There is slot cast into the bakelite to mount the pilot light. The light socket is mounted to an L-shaped bracket. The wires are really short, and do not allow much movement from the chassis without removing the pilot light. On the bench, I hooked up my VTVM to perform the one test that I knew of that would absolutely confirm or deny the presence of SMD. That is to check for postive voltage on the grid of the tube following the IF transformer in question. Positive voltage here would confirm the presence of SMD. No voltage, no SMD. I used test leads to hook the VTVM to so that I would not have to hold the probes while waiting for the trouble to appear. Eventually, the crashing sound appeared in the speaker. There was no positive voltage on the grid of the 12BA6. I did notice, however, that when the crashing was heard, the pilot light would blink. I did not consider that unusual in the beginning. I got a plastic alignment tool and gave the pilot light socket a good rap. The noise appeared in the speaker. I lifted the light socket with the plastic stick, and could make or break the noise. I turned out the lights in the shop and darkened the room sufficiently that I could see arcing in the pilot lamp socket when I moved or struck it with the stick.

The pilot light on this radio does not light the dial. That would make too much sense. Zenith had to be fancy and have this light illuminate a small red jewel on top of the radio cabinet, I guess so you could tell when the radio was on. The sound from the speaker should be a clue, but whatever. This little pilot light is a wonder in Rube Goldberg engineering. The light socket itself is housed in a hard plastic casing. The L-bracket that holds it into the cabinet is kluged onto that plastic housing. It was not designed to be repairable. I tugged around on the wiring and pulled a wire out of the socket. How does this silly thing come apart? The only way to get it apart was to tear it up to get it apart. I took my sharp box knife and cut a slit in the plastic housing so as to allow the socket to slide out. The bracket had to be taken off the plastic housing to allow the wires to slide out. Or back in. The source of the noise was found to be with the neutral wire on this socket. The positive wire was hooked to the terminal for the bulb. The neutral wire was soldered to the spring that holds tension on the bulb. These wires had been pulled on enough that the individual strands of the wire had become frayed and were touching inside the socket. When the strands touched, the noise appeared in the speaker. This is why banging on the cabinet produced the noise. I decided that to prevent more strain on already too short wires, I would make a couple of jumper wires about three inches long, solder them to the socket, then solder this assembly to the original light wires. This will allow more movement of the chassis without ripping out the wiring again. This operation was accomplished and the bracket was secured to the socket housing with epoxy. I fired up the radio, and thus far at least, the problem seems to have been solved. I changed the 35W4 rectifier just for grins, against the chance that it had been damaged by the short in the pilot light. No more crashing in the speaker. I may not be out of the woods with SMD yet, however. Zenith and Admiral radios of this vintage, the early 1950's, seem to have more than their fair share of SMD. Today, right now, this thing is fixed, the cabinet has been thru the dishwasher, and it's on the the next victim.

Regards

WC

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Re: Zenith Model H-615-W

Post by Dr. Radio on Fri Sep 20, 2013 8:24 pm

Nice work Wildcat. Couple-a-thoughts...

You mentioned that the trimmer style IF cans don't have the silver mica disease. Unfortunately there are some out there that actually have the 'crap design' built into them. I have a Crosley "Rondo" that has the trimmer caps on the top, but lo and behold, one of the cans has a trimmer that has a fixed cap built out of the same bad design sandwiched inside the stack of mica sheets UNDER the screw assembly. Mad Mad Mad 

I know there are others out there. I know another enthusiast on another board that battled this same thing. I hope it doesn't give me trouble when I go to restore it, I don't have a plan if it does.

The positive voltage trick--I agree, but one thing I've encountered is the silver mica sheet's contacts oxidizing or loosing connection with the connection tabs. No silver migrating so I don't think I see any positive on the grid. Just throwin' some thoughts out for others.

Good work on tracking down that evil dial lamp socket. I once ran into a weird situation on a 35W4 and #47 bulb in a printed circuit radio. Everything worked fine, but the dial lamp was dim. All voltages checked good, but the dial bulb was not receiving full voltage. Turns out something was flaky with the tap in the original 35W4. It was good, but was preventing a nice bright dial lamp. Swapped rectifier tubes, problem solved. Never ran across that before.

Just food for thought.
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Re: Zenith Model H-615-W

Post by Wildcat445 on Fri Sep 20, 2013 10:39 pm

It took me forever to even believe that silver migration was a reality.  I finally found it to be a reality, so then my next problem was how to diagnose it.  I try to keep it simple.  I got bit with the bad rectifier/bad lamp bug once and that was enough.  It took me a half dozen lamps and two or three rectifiers to learn that hard lesson.  Never again.  When I replace the lamp, I replace the rectifier and vice versa.  I like the Zenith and Admiral sets from the early '50's.  The worst thing about them is SMD.  Nearly every one I have has had it.  I hate messing with SMD almost as much as restringing a tuner.  It will never come easy to me and I will never be good at it.

The crashing sound is caused when arcing occurs, thus introducing positive voltage onto the grid of a tube.  Looks to me like if there was a loss of connection with the connection tabs, there would be no voltage spike, no connection to a grid, thus no crashing sound from the speaker.  Just for my own sanity, I will consider the IF transformers with trimmer tuning to be immune from SMD.  I have not seen enough evidence of that condition not simply being a faulty diagnosis to believe it yet. There are enough other conditions that mimic SMD that over-diagnosis will be a problem until people get more experience with it.  Faulty rectifiers, faulty dial lamps, faulty resistors, dirty/defective volume control, defective mounting bushings on a tuner, and dirty band switches are some of the conditions I have run into that could at least be suspected of being SMD.  Checking for positive voltage on the grid of the tube following the IF can in question has always led me down the right path.  I see no reason to make it more complicated than it already is. Thanks for your input, Doc.

Regards

WC

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Zenith model H-615-W odd symptom

Post by dan88king on Sat Oct 19, 2013 10:43 pm

I've got a nice looking 1938 Silvertone three-bander that plays great. I recapped it and checked all the resistor values. I used to listen to music on it while I worked in my home office, and every so often the volume would drop off a lot. I would reach out and turn my Alladin gooseneck lamp off, and then back on, and the Silvertone would jump back up to normal volume, until the next time. A few justseem to develop a personality over the decades. I never did find out what was causing the problem, and after a few months, got to sort of like it that way.

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Re: Zenith Model H-615-W

Post by Wildcat445 on Sun Oct 20, 2013 5:39 pm

You have a Silvertone radio that "plays great" until it doesn't. Is that not what is known as a broken radio? Turning a lamp off and on should have no effect on a radio in good condition. A radio with a personality is one thing, but a radio with a known defect is quite another.

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WC

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Re: Zenith Model H-615-W

Post by Bill Cahill on Sun Oct 20, 2013 7:53 pm

It's never good to leave a radio that way. While it could be minor, like dirty switches, dirtry volume control, etd., it could have something more serious wrong with it like a shorting tube. This can eventually cause some pretty major dammage to the set. I'd highly reccommend that you find, and, repair the problem.

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