Zenith Chairside: On to the cabinet repairs

 Since it's nice outside today, rather than hole-up in the basement, I dragged the chairside cabinet out to the deck to start on the cabinet repairs.  The cabinet overall is in fair condition; structurally sound but with some minor damage.  The greatest damage is to the front of the speaker grille, where, somewhere in it's life, this radio got whacked hard enough to crack the horizontal bars and chip-out a chunk of veneer.  To start, I'll repair the structural damage by injecting glue into the cracks.

I inject the glue into the crack until it oozes out from everywhere along the crack where it can ooze.  Then, I'll pull it back into shape by clamping the damage between two flat planks of scrap wood.  This will force more glue out of the cracks, but I'm not going to worry about that now...

 I try to get as many clamps around the damage as will fit. Since I don't have many with jaws narrow enough to fit through the grille slats, I augmented my bar-clamps with a carriage bolt/nut threaded through the slats and clamping blocks.  Everything is synched-up tight and will be allowed to dry overnight. With any luck, by this time tomorrow, I'll have s straight grille panel needing only a small repair to replace the missing chunk of veneer.

While waiting for the glue to dry, I put the finishing touches on a Detrola 101 that I've been refurbishing at the same time as the Zenith.  This one also had minor veneer damage but without missing pieces, so it went through a similar repair: using a razor, I enlarged a pore in the veneer above the blister, injected it with glue and clamped between flat blocks.  After sealing/grain filling and spraying with toned lacquer, the repair is completely invisible.

Before: Presentable, but tired.

After.  Came out nice, no?

Zenith Chairside - Capacitor Madness!

Look at that flywheel!  That's what gives these old Zeniths their smooth as silk tuning. 

Normally, I follow a logical troubleshooting process rather than simply replacing parts wholesale, but not when it comes to the paper caps in old radios.  Why?  Experience has taught me that, if they're not bad now, one is bound to fail shortly after putting the set back into service.  It really stinks when you have to reopen a freshly completed radio to replace a $0.25 part, so I prefer to just change them all and be done with it.

As you can see in the picture above, this chassis has quite a few of those little turds.  Fortunately, they're readily accessible for the most part, and things aren't as cramped on this chassis as they are on many others.  Should be an easy job.

Even though this is more a matter of just replacing parts than it is troubleshooting, I still have an order in which I tackle these things - one circuit at a time, beginning with the audio power amp, just as I do in my homebrew projects.  The shot above shows the new capacitors installed in the push-pull output amp and phase inverter circuits.  Before moving onward, I tested the radio after replacing the caps to make sure I didn't accidentally introduce any new problems. 

Something to be aware of about radios produced between about 1939 and the start of WWII is that the rubber insulation on the wiring often will crumble when you move it the slightest little bit.  This usually means carefully rewiring the entire set, not something I particularly enjoy doing.  Fortunately, this set doesn't have much of that sort of wire, so I'll just have to rewire the harness for the eye-tube (shown below the volume control in the picture above) and a few other circuits, not that big of a deal.  Last year, I worked on a '41 Zenith and had almost 40 hours into rewiring the thing.  That's why I usually avoid radios built during that time frame, but this one is so damn cool that I had to have it!

Here's a tip:  I use welding clamps to secure the chassis to the bench while working on it.  The heavy power transformer is at the top of the chassis in this position, and if you don't secure it, the chassis is going to want to roll over and break some tubes.  A chassis stand might be a better idea, there's a fellow in Alaska that makes some nice ones, but I'm cheap and the welding clamps work well enough for me.

You can probably tell that I'm struggling to find something interesting to say about this stage in the process.  Changing capacitors isn't the most exciting thing in the world, but to me, it's cathartic:  I put on some music, pour a cold beverage and relax while plugging away at it.  

Here are some common questions that I'm asked:

"I can't find .05 uF capacitors anywhere.  What do I do?" 

• Use the nearest modern standard value:  For a .05, a .047 will be fine.  Likewise, .022 and .033 can be used to replace .02 and .03 uF.   

"This radio uses capacitors rated at 200 Volts, can I replace them with 630 volt caps?"

• Yes, you can always go to a higher voltage rating, but never go lower.  In most cases, modern capacitors will be smaller than the originals, even when the working voltage is much higher

 "Where do you get your capacitors?"

• The film caps that I use to replace papers are from Sal Brisindi at Sal's Capacitor Corner (www.tuberadios.com/capacitors.)  What I like about Sal's caps is that the leads are longer than others, so you don't need to do a bunch of splicing to get them to fit.  Don't ask me where they're made - the guy's name is Sal and he's from New Jersey. I don't ask questions, Capisce?
• I use Nichicon electrolytics that I order through Mouser.  

That's about all that I can muster to say about this topic.  See you next time!

New Project: Zenith 10-S-442 Chair-Side Console

I imagine that I'm like most people in that my radio production drops off during the summer.  In the winter, it's easier to hole-up in the basement building and repairing things, but I can't help but feel guilty doing so when it's nice outside.  So, summertime tends to be more of a time for acquiring things than it is working on them.  It also seems to be when the best shows and auctions take place, so I'm OK with that.

Last weekend, my wife and I attended the Michigan Antique Radio Club's Vintage Electronics Extravaganza, where I was able to add a couple more projects to the queue, including this 1940 Zenith Chairside.  I've been looking for one for about 15 years, and this is the first one that I've found that wasn't trashed or out of my price range, so I jumped on it.

This set appears to be basement fresh, still with all of its original tubes except for the notorious 6X5 rectifiers, which have been replaced with the later GT style.  The cabinet is solid, but in need of some veneer repairs that I'll cover in a future post.   For today, I'm going to go into how and where to start on something like this.

First, I'll start by removing the chassis and speaker from the cabinet so that I can get this thing from the garage to the basement without wrecking my back.  The next two pictures show the chassis, as found, from the front and rear.  It's surprisingly clean for something that probably hasn't been dusted in almost 80 years!

So, where do I start?  Well, in the past, I've found that some Zeniths from this era tend to eat power transformers, so before I get too far along, I ought to check this one to see if I need to source a replacement.  We'll start by replacing the power cord, since this one's has been cut-off - something I love to see because, in my experience, most of the cut-cord sets that I've come across didn't have any serious problems and, better yet, haven't been boogered-up by someone who tried to repair it without the requisite knowledge and skills.

The above picture shows the new cord and power-line filter capacitor.  The domino-shaped capacitor on the bench in front of the chassis is NOT a reliable mica, but what's called a Micamold - a paper cap disguised as a mica.  The give away is the value: .005 uF; generally, real mica caps will be .001 (1000 pF) or less.  The replacement is a .01 uF disk rated to be used safely across the power line, or, as in this set, from line to ground.  In this application, there is no benefit to sourcing a .005 (.0047 in today's parlance), a .01 will work fine and I've got them on-hand.

Next step is to pull the rectifier tubes.  This disconnects the transformer's high-voltage secondaries from the rest of the chassis for the next test - apply power to see if the transformer is transforming.

So, with the transformer effectively isolated, we can fire up the chassis using a current-limited, isolated supply.  This is a low-tech bit of test equipment; an isolation transformer with a 40 watt incandescent bulb in series with the chassis.  If there were a short in the chassis, the bulb would glow, dropping the bulk of the supply voltage and hopefully limiting further damage.

When measuring an unknown, potentially high voltage, I use my disposable Harbor Freight digital meter.  I've smoked two expensive meters doing this in the past, so now I use one of these until I'm certain that the voltage being measured can be handled by my Fluke or Simpson.  I also connect the meter, using insulated jumper leads, before applying power.  Why?  That's 607 Volts, kids.  Respect it, it can kill you.

Great news!  I've got the expected voltage on the secondaries - the transformer is good, the bullet's been dodged.

So, moving on, it's a virtual guarantee that the power supply filter capacitors have failed, so we'll disconnect power and change those.  Now, on models where the filters are mounted above the chassis, I'll remove the filter "can", remove it's guts, replace them with modern caps, reseal the can and reinstall it for an invisible repair.  In this case, the filters are in a cardboard tube below the chassis, so I'll remove this and replace it with modern capacitors mounted to a terminal strip.

The filter "can" is riveted to the chassis, here I'm carefully drilling-out the rivet.

I drill the rivet to the point where I can drift it through the bracket/chassis with a small punch.

Here, I've attached the new capacitors to the terminal strip before installation - it's easier to do it now than after it's been installed in the crowded chassis.  Teflon tubing insulates the flying HV lead.

Here they are in their happy new home.  I used the hole from the original "can" to mount the terminal strip, using #6 hardware - including lockwashers - you don't want this thing to move!

So, at this point, the smart thing to do would be to remove all of the remaining tubes, reinstall the 6X5s, apply power and test to see if you've got the expected DC voltages.  But, I'm a gambler, so I plugged the 6X5s in and fired the thing up, watching for smoke.  I must be living right, because the damned thing came to life, sounding great!  But, I didn't leave it powered on for long because it still has all of its original waxed/paper capacitors, some of which are likely to be bad or on the verge of failing.  So, the test was a brief two or three minutes of glory.

After the successful test, I started looking for potential signs of problems, and found two:  The RadioOrgan board was cracked and the insulation on the wiring at the base of the eye tube was starting to disintegrate.

Cracked RadioOrgan board.

Disintegrated wiring at eye tube base.

I'm not in the mood to deal with the wiring today, so I'll finish up for the day with the easy fix:  The RadioOrgan board.  This gets cleaned and reattached using "super glue".  The key is clamping it and leaving it alone for at least an hour or two so that it can fully cure.  Don't rush this and it'll turn out fine!

That's all for today.   Next time, we'll tackle the eye tube, including the pesky 1 megohm resistor, located in the socket, that's always bad. I haven't had much luck with eye-tubes themselves, they always seem to need replacement.  Fortunately, this set uses the fairly common 6U5 and not the rare and insanely expensive 6T5!