Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Removing and Restuffing the Mallory Can Capacitor on a Zenith H845 Radio

I'm going to restuff the Mallory FP 'can' filter capacitor on the Zenith H845 radio. 

This is the view from the bottom - not as pretty as the top, huh?  It's crowded in there - which is the main reason I'm going to take the can off and restuff it.  No way I'd have enough room to put discrete capacitors under the chassis.

The can was probably one of the first things to be mounted to the chassis when the radio was built, and the other components and wiring were added later as it was assembled.   Now we need to carefully undo it with all the other wiring around it in place.  Lucky us.

I always make a diagram showing what leads and components go where before I remove anything.  It might be a little sloppy, but it makes sense to me and that's the important thing.

I have whole legal pads with scribbling like this.  Sometimes I look at them a year later and I'm thinking "what the heck?"
Here's all the wiring removed from the filter cap.  See how clean those tabs are?  Use solder wick!

There are four tabs on the can that go through slots in the chassis.  You can see them just outside the red circle that's the bottom of the can.  

You can also see that big blob of solder on two of the mounting tabs to ensure there's a good ground.  That needs to come off before we can remove the can.

A regular soldering iron or even a soldering gun just doesn't have enough wattage to melt that solder.  So I use my 175-watt Weller soldering iron, "Big Bertha" for this job.

The iron will melt that solder in a minute or two.  The downside is that it's big, and I need to be careful manuvering it in that small space.

If you look closely, you'll see where I have some solder wick already on the chassis to wick up the solder as it melts.

After I get the solder off, I can bend the tabs back and pop the can off the chassis.

Whoo hoo.

Now on to the restuffing.  I've documented this before, so this is a quick overview.

Cut the can open.  I've used hacksaws and Dremels for this, but the last couple I've just rolled the can under a big kitchen knife.  Works pretty well, and doesn't leave a big seam like the other methods.  But I'm thinking a razor saw with a super fine kerf might be good too.

Here's the opened can.

The stuffing is hazardous, use gloves and dispose of it properly.

The neat thing about this is you can see how the capacitor is made - foil layers wrapped with paper.  There's also some kind of dielectric paste in there - that's the hazardous stuff.

I just hack the whole stuffing horizontally  right off the base.

So now I have a can, a base and 4 modern capacitors.

The can actually was wired to be the equivalent of 4 separate capacitors. The values of the old capacitors were 80 uF, 40 uF (2) and 50 uF (I think).  They were rated at 150 volts, except for the 50 microfarad section, which was rated for 25 volts.

The new Panasonic caps I'm putting in are rated for 250 volts, with the 50 at 50 volts - and at 105 degrees C vs. 85 for the old can.  I like to use higher rated caps in these circuits, since the turn-on voltage on the first sections can sometimes exceed 150 volts.  Just a little peace of mind.  I don't want to do this again any time soon!

You can see how much more compact the modern capacitors are, even with the higher ratings.

After a little dry run, I come up with a way to mount the caps so they'll fit in the new can.  The positive leads all go to the appropriate tabs on the bottom of the can via small holes drilled in the base.

You can see how I have shrink insulation on the leads so nothing shorts out.

The grounds are all connected together, and it also passes through a hole near one of the mounting tabs. I'll solder that to the tab and the chassis when I reinstall it.

Now ve meex oop some JB Weld epoxy to glue the top and bottom back together...

...and we put it together and then gently clamp it in a vise until the epoxy dries.




 
 
 
 

Post a Comment 0 comments:

Post a Comment