Crawls Backward (When Alarmed)

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Replacing Signal Caps in Audio Circuit in Zenith H845

I'm starting to recap the Zenith H845 radio.  Perhaps I should say, more accurately, I am continuing to recap the radio.  You may recall in our last installment I restuffed the can filter capacitor.  It's done, reconnected and installed in the radio, which now plays well with no hum.

Since this radio is known for its good tone, I'm going to replace the signal capacitors with good quality ones in an effort to make it sound really super good!  Which brings me to the point of this post.

If you've read about audio (aka hi-fi) or guitar amps on the interwebs, you may have read about capacitors and their affect on tone.  Tone being that mysterious, magical thing that everyone is after.  I'm going to avoid hyperbole, but I did want to use this radio as an illustration of how you might approach selecting, changing, or experimenting with signal capacitors (aka 'caps').

A quick digression.  In the vintage radio and amplifier world, one of the components that most often fail, or are about to fail, are capacitors.  In the old days, the majority of capacitors were made of paper and a dielectric element - usually a metal foil of some sort.  Because paper absorbs moisture from the atmosphere over time, and because moisture would cause a cap to short or otherwise fail, capacitor manufacturers would dip their caps in melted wax to coat them.  After the wax dried, it would provide protection of sorts against moisture.

Some companies also used tar - this was commonplace through the mid-1930s.  And sometimes you'll see a sealed can of some sort - recall the aluminum can I just restuffed.

Of course, 40 or 50 or 80 years later, the vast majority of these capacitors have failed simply due to age.  Wax, as is turns out, is not a great way to protect capacitors.  This is why modern caps are sealed in some sort of plastic.

In radio and amp restoration, we commonly replace capacitors as part of a restoration or renovation.  Most capacitors are in a circuit to block DC voltage.  When they fail to do that, they are referred to as "leaky."   In most instances, you cannot determine if a capacitor is bad by looking at it!  They must be tested at their rated voltage.  While it's true that some electrolytic caps may have leaked their innards, but that's not the meaning of the term "leaky."  Frequently on radio forums a novice will say "the caps look good....".  Well, they may look good, but I bet they're bad!

So many of the caps you'll see in old gear are bad that it's just easier to replace all of them rather than test - although testing is a fun exercise.  This wholesale replacement is called "recapping."  (Or, by the spelling challenged, 'recaping.')

Now back to our Zenith.  I'm going to replace all of the old wax-paper caps.  Some of them are in the signal path - meaning the circuit that the actual audio signal takes.

On the right, I've highlighted the capacitors that are in the audio path (circuit, chain, whatever you want to call it).  If you click on the larger version, you'll see the three most 'important' caps.  They are all "coupling" caps - they "couple" signal from one place to another.

On our Zenith, these are marked on the schematic as C27, C30 and C34.  C34 in particular is the most critical cap - it passes the audio signal to the output tube.  I'm going to replace the old caps with Russian K40-Y paper-in-oil caps.  ("Paper-in-oil" refers to the internal construction of the caps.)

Some other notes on the caps I've highlighted.  C31 and C29 are part of the tone circuit.  Not as critical, but again a place where you'd want to use a quality cap (e.g. Orange Drop, etc).  Another key place is C35.  This is what was called the "quality capacitor."  Its function is to roll off super high frequencies.

Finally, see the network of R25 and C37.  If you trace the wiring from the right side to this network (it's actually a bandpass filter), you'll see it connects to the output transformer, T2.  The signal actually would flow from the transformer, through the filter and back to the volume control.  This circuit is a negative feedback circuit - its function is to smooth out the frequency response.

By using good capacitors (a whole other discussion, one that will start a lot of arguments) at these points, you'll ensure you're getting the most fidelity out of the circuit.  I won't say it will have "killer bass" or "creamy mids" because audio is so subjective.  But you will be getting the most out of the circuit as designed if you use quality parts.

Just a couple more pictures.

One of the great things about these old sets is that they're wired point-to-point.  Meaning you can trace or follow components and determine where they're located.

Here I took a shot of the radio and highlighted the caps we've been looking at above in the schematic.  In this picture, I had already replaced R35, C29 and C37.  But the old ones are still in place elsewhere.

Here's where most of the caps are.  Not surprisingly, they're physically located near the volume and tone controls and the output tube.

You can clearly see the old paper caps.  A couple of the caps are ceramic disks - C22 and C31.  They typically don't fail, but their design makes them less suitable for audio, so I'll replace them as well.

It is possible to restuff the old paper caps.  I've done it on special radios.  Basically you take the paper caps apart, put a new cap inside the paper shell, and melt wax on them.  It's tedious, but it's worth it if you want to keep the original look.  In this case, I'm not going to bother since this is a fairly common set.  I will however, keep the old caps in a plastic bag stashed inside the set for posterity.


Post a Comment 2 comments:

  • Kelso said...
    February 13, 2017 at 8:38 PM
    The schematic I have shows C35,C34, and C28 in alternate locations. Why is this?
  • Yr Fthfl Blggr said...
    February 15, 2017 at 10:13 AM
    The schematic is not a layout diagram. It's drawn to show component connections, not the layout.

    If you trace the connections on the radio they should match the schematic.

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