Crawls Backward (When Alarmed)

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Zenith H845 Finishing Touches

Now that the Zenith is recapped and aligned, I can put it back in the cabinet and put it to work!

First I need to clean it up a bit.

One of the things I love doing on old radios is cleaning the dials.  Most of them look really neat, and when they're clean they look great.  They have a vibe you just don't get with modern digital dials.

Usually they're glass with silkscreened numbers.  In this case, the dial is plastic, since it's relatively modern (1964).  Either way, you need to be careful cleaning the glass.  Do not use any kind of glass cleaner!  It will remove the numbers from the glass!

I've found that some camera lens cleaner along with lens cleaning paper works very well.  If you consider that camera lenses are something you don't want to scratch, I think you'll agree this is a good idea.

You can use a glass cleaner on the outside where numbers aren't printed, since there's no danger of removing the numbers or lettering.  Even so, I generally clean the outside of the glass the same way I do car windows - hot water and clean lint-free cloths.  Make the water as hot as you can stand.  That way you don't get chemicals on the glass at all.

Speaking of chemicals - I do use Novus plastic polish on the plastic parts.  In this case, that's pretty much the whole front panel.

I've become a convert to Virtuoso cleaner and Virtuoso polish for instrument finishes.  They work wonders.  You can see the grime I got off the top of the cabinet.  Not as bad as some guitars I've worked on, but it's still there.

Putting the chassis back in the cabinet is straightforward.

There's a screw post that supports the dial mechanism frame - top arrow in the picture.  You need to get that aligned.  There's a lockwasher and nut that go on the screw.

Also the two dial lights - the other arrows - have to be seated in their...seats...on the dial glass plastic. 

If these aren't lined up, the chassis won't slide in all the way.

There's also a small square piece of stiff foam that goes under the middle of the chassis to support it.  I didn't know it was there until I heard something sliding around in the cabinet after I took the chassis out.

The support has a dimple on it that lines up with a matching dimple on the bottom of the chassis.  Those clever engineers thought of everything.  It's easy enough to use long tweezers to slide it into place.

Those two pins at the top are part of the AC interlock.  There's a matching socket on the back panel.

Then we put the chassis screws in from the bottom and tighten them up.  There's a pointed lockwasher molded into them - nice touch.  That way you can't lose the washers.

We don't want to forget to solder the antenna leads back on.

You can see the other half of the AC interlock - the box near the top of the picture.  The idea is the consumer can't remove the rear panel and have the radio still plugged into the wall. 

I decided to leave my labels on.  Why the heck not?  It might save someone some time in the future.

I also put a masking tape label on the bottom of the chassis, as well as the can cap, indicating the date they were recapped.


 Screw on the back panel...


...and slide on the knobs.

I may repaint these at some point.  You can see where the chrome plating has worn off over time.

The tuning (outer knob) and the band selector knob slide on the same way.

Here's the finished radio!

It was in nice shape to start with and now it looks great.  Super sensitive and it has tons of bass!

Note the "high fidelity" logo at the top - yes sir!

I bet it would be really amazing with some modern speakers....nah.  I will not mod this radio!   It sounds great as is.

The heritage of this set goes back to the late 1940s.  I have an earlier model - from about 1950 - in the restoration queue.  Zenith made thousands of these sets, and they were extremely popular.  With their great performance, it's easy to understand why.

Here's a real period touch on the dial.

The red arrows indicate the "CD," for "Civil Defense," or "CONELRAD."  These markings at 640 and 1240 kHz on the AM band indicate where you would tune in the event of an emergency.

I have a couple of sets with these markings.  Here's more about CONELRAD.

Here's a shot of the dial in the dark.  Not the prettiest one I have, but it still has that neato 60s feel to it.  I plan to make this one a "daily driver" and listen to my favorite classical music station with it.

 
 

FM Alignment on the Zenith H845 Radio

With the AM alignment done, we move on to the FM alignment.

To accurately align the FM circuit, you will need an oscilloscope along with an FM sweep generator.  It's possible to use an AM generator and VTVM - many old alignment instructions have directions on how to do this.  But the most accurate alignment will be with a scope.

In this case, I'm using my Tektronics 453 scope.  This scope was introduced in 1965 and was state of the art - two channels with a 50 mHz bandwidth.  Even today, it's plenty of scope for old radios and amps.  This one is ex-IBM.

The process for FM alignment is not too different than the process for AM.   The real difference is that we measure and observe voltages differently.

But the process of connecting a signal at specified points, reading the output, and adjusting IF transformers is the same as for AM.

One of the differences is that the IF frequency is much higher - in this case it's 10.7 mHz, aka 10,700 kHz.  And it's frequency modulated rather than amplitude modulated. 

Here's the HP 5314A reading the output from the Hickok generator.  One of the shortcomings of old tube test gear is that it takes a while to stabilize.  I give the Hickok 30 minutes to warm up.  At that point, it's reasonably stable, but you still need to keep an eye on it.  After an hour it's pretty good.

Here's a typical readout.  The scope is reading the shape of the wave passing through an IF transformer.  By turning the slugs, we try and get the most symmetrical wave with the highest peak.

We do this for each IF transformer in the set.  Just a small turn of each slug will affect the waveform.

Another downside of old gear is that the signal isn't always 'clean.'  You can see small ridges in the wave - that's DC in the output.  But it's good enough for our work.

After the alignment(s), I run the set for a while on the bench.  My defacto test for AM reception is a local sports station.  It's not the strongest station in the area, and some sets have difficulty picking it up with a strong signal.  Not so the mighty Zenith!  It picks it up easily.

Most sets have a hard time picking up much on FM down in the dungeon.  The Zenith did quite well again - it picks up a lot on just the tiny factory antenna.

And it sounds great, with plenty of volume.

 
 

AM Alignment on the Zenith H845 Radio

I finished the recapping on the Zenith last week.

Here's a shot of the recapped chassis.  You can see the yellow poly caps and also the silver Russian K-40Y-9 paper-in-oil caps I used in the audio chain.

After you recap a radio, you wind up with a pile of old leaky caps.  I'm going to save these in a small plastic bag and staple it inside the set.

I don't know why, the caps are bad.  It's really just for historic value.

Now I'm going to do a full alignment on the set.  I'm starting with the AM alignment - it's the most straightforward.  The FM alignment is more involved, but I'll go through that too.

For this, I'll use my trusty Hickok 288X signal generator.  This generator outputs both the AM and FM signals we'll need for the alignment.

Before I go much farther, I should mention what an "alignment" is.  You may have heard this term and thought "what the heck is that?"

I'm skip most of the technical detail  in this post; I really just want to give an overview of the process.

The best plain-language explanation I've read is in the Bryant and Cones Zenith Transoceanic book, which I'll paraphrase.

An alignment ensures the radio is playing up to its peak performance.  That means that its sensitivity, selectivity and dial tracking are all at their optimum.  If you think of the tuning circuits in the radio as a number of windows in sequential order, an alignment adjusts those windows so they all line up and the signal can pass through.

In brief, what we do is inject radio signals at specified frequencies into the circuit at various points, and adjust tuning coils or slugs in the set until the signal is at its strongest.  That's it in short.

The tuning adjusters vary from radio to radio.  They will be a screw or a hex slug of some sort.  I have a couple sets of plastic adjustment tools so I can pretty much work on anything I'll come across.

The tools are plastic so they don't throw off the readings that we'll take.  If they were metal, their capacitance would make the alignment inaccurate.

Here are a couple of the coils we'll adjust.  Technically, these are IF, or intermediate frequency transformers.  They consist of two coils and two threaded slugs that adjust up and down inside the rectangular 'box' you see.

I'm trying to avoid a lot of radio theory here (you can find it elsewhere on the interwebs...), but this is a critical adjustment with regards to the radio's sensitivity (how it picks up weak signals), as well as its selectivity (how well it rejects adjacent signals close to the one you want to pick up).

You can see the alignment tool has a hex-shaped end - this will fit into the slugs.

We set our intermediate frequency (IF) to 455 kHz.  Most American sets use this frequency.  You can find the IF on the schematic or alignment instructions.

I'm feeding the signal from the generator to a HP 5314A frequency counter.  You may recall from my rebuild of the Hickok posts that I tapped a line from the inside of the generator to run to a counter, in addition to the RF output on the front panel.   Comes in very handy since we can read the frequency at any time and keep the main output connected to the radio.

And with the generator connected per the alignment instructions, we adjust the slugs until they are peaked.  How do we know when they're peaked?

We have a big VTVM (vacuum tube voltmeter) connected to the radio also.  In the old days, the method was to connect the VTVM across the speaker terminals.  That works, but you have to have the volume on the radio up all the way, and it screams a high pitched scream!

The modern method is to connect the meter to the AVC buss.  You can have the volume turned all the way down and still get readings. 

This is where a big meter is great.  You can see the needle swing as you adjust the slugs, and see when you've peaked them up.  You could use a modern DMM, but it's a lot harder to discern readings.  The VTVM is the best tool for this work.  Plus, it looks super cool!

In the old days, there were service manuals printed for all of these sets.  In a number of cases, you'd even find the schematics and alignment instructions in the owner's manual.  That material was important for the serviceman - since there were actually repair people, they would use this for troubleshooting.  It's a real blessing for us modern restorers too.  I have a set of CDs scanned from original manuals that have the repair information for hundreds of sets! 

Next we'll do the FM alignment. 

 
 

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.


 
 

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.

 
 

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.