Crawls Backward (When Alarmed)

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Recapping the Zenith H845 Radio

After I reconnected the filter cap can, I brought the radio up to full voltage.

This is the setup I use.  On the left is my trusty General Electric variac.  This is a big transformer you can vary the voltage on (as the name implies).  "Variac" is actually a trade name for autotransformers (the correct technical name) made by General Radio, I believe.  Much like "kleenex," it became the generic name.

In the center are two voltmeters - a more modern one and a vintage Weston model.  These are wired to the variac to indicate the AC line voltage.  Why two of 'em?  Why not!

On the right at the top is a vintage Weston ammeter.  This lets us know how much current the load (e.g. the radio or amp) we have plugged in is drawing.  It's important because if something is drawing a lot of current, there is a problem.

And on the bottom right is an RCA isolation transformer.  Not something I use a lot, but I'm using it on this radio because it has no power transformer - the AC voltage in goes right to the power switch.  The iso transformer breaks, or isolates, the radio from the wall circuit - it's a safety measure so I don't get electrocuted if I touch the chassis, which carries AC voltage.

The isolation transformer is plugged into the variac.  The radio is plugged into the isolation transformer.

I like to slowly charge new capacitors if I can - 40 or 50 AC volts overnight and then 10 volts per hour until we're at the line voltage (120).  I'm of the mind that charging the caps is like charging a battery.  It may not be necessary, but I do it.

Once the caps were charged - and the radio was playing fine - I installed a modern silicone diode in place of the original selenium rectifier.  I used a 1N4007 - they're like 3 cents a pop from Mouser.

The diode - the old blue selenium one as well as the small black modern one - is what converts the incoming AC to DC to run the high voltage parts of the radio.

You can see how the diode is wired using one terminal of the old rectifier, and then the positive end goes to the first filter cap.  The old rectifier is now out of the circuit - nothing is connected to the tab on the red (positive) end.

The seleniums can go bad over time - and blow up making smoke and a bad smell.  They look really cool, but they're unreliable.

Now let the recapping begin!

These are the first 2 caps I'm replacing.  Probably the hardest ones to do, since they're a bit buried.  The red arrows indicate where they're at.

On this first one, one lead of the cap wraps around a post that's part of an antenna connector on the back of the chassis.

So I use some solder wick to get the old solder off, so I can see the lead clearly and unwrap it.

Some folks will just snip the lead, especially if it connects to a point like this.  Unless it's really hard to get to, I avoid that.  I just prefer doing a nice clean job.

That method of wrapping a lead around an old connection is usually called a "quig."  Some restorers will wrap a lead around a terminal where there are a number of connections, flow solder on, and call it done.

I think that's a really lazy and sloppy approach.  For one thing, you won't have a good mechanical connection to the joint - usually a tab or terminal.  For another thing, you're soldering on top of the old potentially corroded solder, which doesn't make for a good connection.

The solder wick - as the name implies - wicks up the old solder.  When I get a couple inches that look like this, I just snip it off and throw it in the trash.

The stuff comes in various widths and lengths.  I get it inexpensively from Mouser.  It's gold.  Well, actually, it's copper braid!

Not sure how good this picture is - in the large version you can probably see the old lead unwrapped from the post. 

Repeat the desoldering on the other end of the cap and then withdraw it from the radio.

Here I'm making a loop to go around that antenna post, and clipping off the excess lead length.

Once I slip the loop over the post, I'll squeeze it with my needlenose pliers to make a tight joint, then solder it in place.

Solder isn't glue - I try to make a good mechanical connection always.

Another modern versus vintage component picture.

The old cap was rated at 200 volts.  The new one is rated at 630, yet it's much smaller.

You need to replace caps with the same value and at least the same voltage rating as the old ones.  Since modern caps are so much smaller, I generally keep only 600 or 630 volt caps on hand.

The modern caps also have superior electronic performance compared to the old ones.

One down, one to go.

Here's another trick.

If you have a component attached to another component which is sensitive, use a heat sink.

Here we have the old cap which connects at one end to an RF coil.  These coils are a bit fragile - they're made of very fine gauge wire that's soldered to small tabs.  They're generally covered with wax so they don't absorb moisture.  Heating them up too much might accidentally desolder a coil wire or melt the wax too much.

So here I have some hemostats on the tab (red arrow) where I'm going to desolder.  You could use an alligator clip as well.  The heat sink will absorb the heat from the soldering iron and prevent it from transferring to the coil.

Here's the new cap before I soldered it.  You can see how clean the tab on the coil is - there are two resistor leads and the capacitor lead going through it.  With all of the solder removed, I could easily thread the new lead through the tab.

I soldered it (with the heat sink on it), and then snipped that excess lead you see sticking out.

I hate excess leads with a passion.  I once worked on a radio that a friend had "recapped."  OMG.  He left all of the component leads their original length!  Things were sticking up everywhere.  Nothing was trimmed at all.  What a mess.

He had asked me to look at the set because it wasn't working right.  No wonder.  Sloppy work, lots of leads everywhere.  I cleaned it all up and got it going.

If you work on radios or amps like this, use the original wiring as a guide to workmanship.  Or I should say workwomanship, since the vast majority of assemblers in those days were women.  Smaller hands, an eye for detail, and brains.  I've seen some sloppy original wiring, but most is very well done.

I think that doing good, clean work is not just the way to ensure the gear will work right, but it's also a tribute to the folks who built these things originally.  It only takes a little more time to do quality work.

Now on to the rest of the caps.  The fun part will be replacing all the old paper caps in the signal path with modern ones.


Post a Comment 3 comments:

  • Charlie said...
    August 16, 2014 at 11:05 AM
    I have just replaced all the caps in my H845; If I replace the selenium rectifier with a diode, the B+ goes up considerably. Did you change out the resistors in the PS to compensate for this? My goal is to get the +100 volt supply on target, as it runs the tuner section. Chip Townsend, Tallahassee FL
  • Yr Fthfl Blggr said...
    August 17, 2014 at 9:55 AM
    Hi Chip -

    I didn't have to put another resistor in the power supply, which is a little unusual. As you probably know, modern diodes are much more efficient than seleniums, so the B+ is generally a bit higher.

    I've had a number of sets where I've had to put another resistor there. On this one I expected to, but the voltage with the diode alone was right on..

    It's a bit of trial and error as to the value for the dropping resistor. Try a 100 ohm 5 watt resistor as a start and see where that gets you. Generally somewhere between 50 and 100 ohms will do it.

    I don't think the voltage is *that* critical - if it's within 15% of the schematic value (which I don't recall right now), you should be fine.

    These are great sets. The sound quality is wonderful.
  • August 19, 2014 at 12:39 PM
    Thanks Blogger;
    I changed the 22 ohm off the rectifier to 50 ohms, 17 watts (a surplus resistor) and the 330 ohm to 350 ohms at 5 watts, to keep the heat dissipation down, as the chassis is buttoned up at the bottom. These changes put the B+ on the audio amp at +130 volts and the +100 volt source right on target. The dial calibration was very close upon completion of the repairs, re-capping, and restringing the drive shaft, so I left the calibration as it was.

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