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!