Crawls Backward (When Alarmed)

You may recall the unsoldering of the can capacitor on the Vibro Champ a few posts ago.

I don't do them a lot so I usually rely on the method I wrote about. But no more.

I also needed to replace the AC line cord.  The ground is soldered to the chassis, so I figured it was About Time that I sprung for a big powerful soldering iron.

So here it is - it's a Weller SP175.  The '175' stands for the wattage.  I almost got a 120 watt iron, which probably would have been fine, but I figured I should just get a pretty high powered one.

The only downside, as you can see, is it's big.  If you speak Imperial measurement, it's over 11 inches long.  That's about 28 centimeters by my estimation.  Almost as big as the Vibro Champ chassis!

 Here's the ground I'm going to unsolder.  The typical big blob of solder.

The iron heats up very quickly - about 2 minutes and it was ready to go.  It gets HOT - you can feel the heat if you're using it in a vertical position.  You also need a good holder to set it in when it's hot - the flimsy "stand" that comes with it isn't particularly safe.

It's also so big that you need to be careful not to touch other leads when you use it - it will melt them instantly.  Fortunately, I didn't do that.

As with any soldering iron, the tip surface area and keeping it tinned and clean are the keys.  The tip on this puppy is about half an inch (13mm) wide, so it's idea for this work.

About 15 seconds and the joint was melted!  Where have you been all my life?

Here's the new AC cord in place.  I was able to easily solder the ground to the chassis.

Still waiting for my new speaker from Weber, but I have a couple more things to do in the interim.

In our last installment of work on the RCA VTVM, we rebuilt the power supply.  Probably an overstatement since it's one diode and one capacitor.  But still, doesn't "rebuilt" sound like a major undertaking?

Now we can calibrate it.  This is a straightforward job, and fortunately RCA even gave us instructions in the manual.  I'm not going to show the whole operation, just a couple of highlights so you can get the idea.

Before I get going, I wanted to share a picture of the cool RCA 'meatball' logo that's on the meter face.  This is a variation of the logo that RCA used beginning when the company was founded in 1920.  There were slight modifications to the logo until the 'modern' block letter logo was used beginning the the 1960s.

I like the "lightning bolt" on the tail of the 'A.'  Radio was in its infancy when this logo was created, and the logo reflects the excitement of a new age.

Ok, enough art criticism and back to the calibration.

When the VTVM is turned off, the meter should return to zero.  If it doesn't, you remove the small cover over the meter adjustment lever and gently move the lever until it the meter needle is at zero.

Mine wasn't at zero, so I did this adjustment.

Now we'll proceed with the electronic calibration.  To do this, the VTVM has to warm up "for at least 30 minutes," says the manual.  Turn it on, and go off and do something else, such as watch qualifying for the Canadian Grand Prix.

The calibration procedure essentially consists of using a "known source" at specific voltages, connecting the meter, and adjusting various trimmers to calibrate the meter to the voltage.

Here's how I got my specific voltage readings to use as a source.  I connected my trusty Heathkit IT-18 Capacitor Checker to a 500 volt capacitor as if I were going to check it for leakage.  The checker has a switch for different DC voltages:  3, 6, 25, 75, 150, etc., volts up to 600.   I plugged the checker into my variac - which let me vary the AC voltage (line voltage) the checker is running on.  That way I can choose, say, the 75 volt range, but dial down the voltage until it reads 50 volts.

You can't get any DC voltage right off the outputs of the Heathkit.  However, if you connect a capacitor across it as you would to check it for leakage, you can then measure the voltage the capacitor is seeing.

Two of the leads on the capacitor here (it's a 600 volt cap) are from the IT-18, and two are from...
...my Fluke DMM.  This is a super accurate meter.  I just adjust the Heathkit range setting and adjust the variac it's plugged into until I get the exact voltage I need to calibrate the RCA against.

Then we just follow the RCA calibration instructions.  Basically you connect the RCA's probes to the voltage source at a specific voltage and adjust.

Here's an example.  The first adjustment is to get the RCA's meter to read full scale - exactly 50 volts - on the 50 volt DC+ range.  With 50 volts applied to it, adjust the DC+ trimmer until it reads exactly 50 volts.

Then you reverse the RCA's leads, change it to DC- and calibrate it for DC-.

Straightforward to do.

Here's the reading after the first adjustment is done - full scale (i.e. meter all the way to the right) reading at the "5.0" mark on the scale.

The rest of the calibration is similar - set the voltage, set the range on the meter, and adjust as needed.  It only takes about 15 minutes.  You also adjust for AC volts and resistance (the "Ohms" scale).

This one calibrated perfectly.  I mentioned in the first post about the meter that my other RCA wouldn't calibrate.  I need to troubleshoot that one - I suspect some to the circuit resistors have drifted in value. 
In the meantime, I have a newly serviced VTVM I can use! 

There's a bit of a lull in the action in the Dungeon on the Vibro Champ project.  I ordered a new speaker from Weber and it takes a couple of weeks for them to make it and ship it.  I also got a new speaker for the Champ 600 while I was at it.  So the Vibro Champ is on temporary hiatus.

That doesn't mean there isn't anything to do.  I started this quick project servicing and recalibrating a RCA model WV-98C 'Senior VoltOhmyst' VTVM.  "VTVM," by the way, stands for Vacuum Tube Volt Meter. 

As it turns out, I actually have two of these things.  The one above is a recent acquistion.

I've had this other one for a few years.  It's not-quite-accurate and needs to be gone through.  Which generally means checking or replacing any out-of-tolerance precision resistors.  A fairly big undertaking.  When the opportunity arose to score a second one, I took it.

If you compare the two, you'll see the only differences are cosmetic.  The one I just got is actually older - early 1960s.  It has the old RCA 'meatball' logo and older font face.  The second one has the 'modern' RCA logo, and a different font on the control panel.  They both have different knobs as well. Other than that, they are electronically identical.

RCA probably made thousands of these things.  They're still a staple in the old-radio-and-amplifier repairing world.  They're reasonably accurate, and that large meter (about 5" or almost 13 centimeters) is easy to read. 

A VTVM is great for things like radio alignments, where it's easier to read the meter swing up or down versus a digital meter where the numbers are changing.  Not only are they functional, but they look really cool to boot.

You have to love a piece of gear that comes with a schematic and layout diagram.  Not to mention calibration instructions.

Here's what I'm doing first.  Replacing the power supply filter cap and the selenium diode rectifier.

The filter cap (the cylinder on the right) actually tested ok, but I replace these as a matter of course.  This one is about 50 years old, and it's not worth leaving it in and having it fail.

The rectifier is the square component with the 'fins' to the left of the capacitor.   You'll see these in radios and other gear made all over the world during the 1950s and 60s.  It's a diode that rectifies AC to DC.  They took the place of rectifier tubes in consumer gear - they were easier to manufacture, didn't require a special transformer tap to run them, and they don't produce heat.  However, 50 years later, they are prone to failure and are easily replaced with a three cent modern silicon diode.  Which is what I'm going to do here.

Here's the old selenium rectifier alongside its modern replacement.

In addition to the size difference, the modern diode offers far superior performance.

When replacing a selenium rectifier, you need to check the B+ voltage with the new diode in place.  Generally, there is much less voltage drop across the diode and you'll need a resistor in series with it to drop the voltage to the correct amount.

In this case, there was only a few volts' difference - in fact, the B+ is exactly spot on (88 volts) with the new diode.  What a bargain.

This VTVM was made using a circuit board instead of point-to-point wiring.  PC boards were just coming into use by the early 1960s.  So replacing the two components is just like you'd do on modern gear - unsolder the old, bend the leads on your new ones, and solder them in place.

Now I'm going to see how well the VTVM will calibrate.

You may recall from the first post about the Vibro Champ that two of the control pots have broken shafts.  They're CTS pots with plastic shafts.  One of them, unfortunately, is the intensity pot for the vibrato (tremolo).

Most Fender amps with tremolo used a 50K reverse audio pot.  On the Vibro Champ, however, a 25K pot is used - the circuit is entirely different than other Fenders. 

Mouser does sell Alpha pots with that value, so I could put a new one in.  However, I have a small stash of old pots and I'm fairly confident I can just swap out the broken shaft for a good one from another pot.  Since the pot value and taper is determined by the carbon track in the pot, this should work.  That way I'll be keeping the pot mostly original.

I did a post a while back about opening up pots and 'modding' them to rotate faster to make them better suited to volume swell effects.  

This is the same process.  Here's my donor pot with a good shaft being taken apart.  First, bend back the tabs that hold the pot together.

Then disassemble the pot.  Note what goes where so you can put it back together!

You can see the grease on the bottom of the housing.  This gives the pot a nice feel with some resistance.  For the fast-spin mod, you'd want to take this out.  Since this is an amp pot, I'll leave the grease in there.

I then took the intensity pot apart.  Notice that CTS has a color code for the pot - the plastic disc and shaft are purple! 

This is a good opportunity to clean the rotating contacts and the carbon track while the pot is in pieces.

Usually we spray contact cleaner into a pot through a hole to attempt clean it.  Once you've opened up a potentiometer like this, you realize that spraying into one with contact cleaner is a hit-or-miss affair.  It's a lot better to take it apart to clean it if you can.

I put the pot back together using the shaft from the donor pot, and the carbon track and housing from the original pot.

I tested the resistance and it's correct.

Here's the 'rebuilt' pot back in the amp.

The tremolo works!   And it looks original - if that's important to you.  (Oddly enough I believe I'm going to change the volume pot for an upcoming mod...go figure).

I did this for the Bass control as well.  I was just going to replace it with a new CTS 250K audio pot, but the schematic specifies a "J" (30% audio) taper.  So I just put a new shaft in that one also.  Interesting to note that the J taper control has a blue shaft and disc.

Now it's on to the recapping of the Vibro Champ.  This is the Main Event - the aluminum multi-section Mallory 'can' capacitor that contains the three filter caps.  Lots of pictures in this post, and lots of fun.  I'm going to show you what I do to get them off and replace them.

You will see these a lot on radios and amplifiers built in the mid-1950s through the 1970s.  They may be most well-known on Fender Champ, Vibro Champ, Princeton and Princeton Reverb amps, but I have seen them on gear built by Zenith, Hallicrafters, Dynaco, Pilot, Eico, Heathkit and others.  And not just American junk - I have Eddystone, Grundig, Marshall, Diora, Luxor, and other European gear that use this type of capacitor.  This approach can be used with any can capacitor.

It's pretty obvious how these caps came to be popular.  By the 50s, circuits were typically more complex, tubes became smaller, and chassis followed suit.  High-voltage electrolytics were historically large, bulky components.  But can caps allowed them to be made smaller and easier to install.

Most of the time, you can just bend some retaining tab back, or undo a clamp and the can will come out.   But in Fender's case, they soldered two (or more) of the tabs to the chassis.

The chassis, of course, acts like a big heat sink, so it takes a lot of heat to melt the solder.  I've used different approaches - this time I tried two 140-watt soldering guns at once.

But one of the tips is broken, and with not a lot of surface area to get the heat down, I was largely unsuccessful.  What to do?

You need big heat?  How about a propane torch?

This should really be done with a smaller torch, but this is what I had.  I dialed it back to the smallest flame I could get (smaller than the one in the picture), and used foil to cover anything that might melt.  Don't try this at home!

Then I put one of the guns on high heat right on a blob of solder with a piece of solder wick under it, and hit it directly with the torch.

It worked. Ha.  (A really big soldering iron is on my list of tools to buy).

The solder got broken up enough to the point where I could use an old bench chisel to chip it off, and then raise the tabs.

Sounds like a lot of work, but it took about 10 minutes in all to get the can out.

Here's the can removed from the chassis.  In the US, Mallory was the company that made these.  Again, I've seen other brands on English and European stuff, but the concept is the same.

Note how the different sections are indicated on the can.  The 40uF/450 volt section, for example, is indicated on the bottom by a half-circle.  The others are marked with similar symbols. 

Now, you can get new cans from CE Engineering (they use the old Mallory tooling), or JJ and F&T.  They are all high quality parts and will work well.  However, they cost a bunch of money.  The CE can with these same values runs about $35 US.  Rather than spend the money, I have another process I generally use for restorations:  I cut the can open and put modern radial caps in it.

First step is to open the can.

You *may* be able to open the crimp at the bottom and peel back the edge that's crimped down and remove the innards from the bottom.  I've done this in the past, but this particular can was crimped down to the point that I couldn't get a tool into the seam to open it up.

This is actually what I wind up doing 8 out of 10 times: cut the can open along the rim.  You can use a hacksaw, or, as I did here, a Dremel or other rotary tool with a heavy-duty cutting disc.  The can is light aluminum and will cut easily.  Try not to hack it up too much.  You'll soon see why.

Well, goll-lee.

The top of the can slides right off, exposing the innards.

Snip off the aluminum strips that connect the tabs on the bottom to the caps.  The long strip you see is the ground.  Unfortunately, it's hard to solder to, otherwise I'd reuse it.  I just snip them all off.

I have these super duper, high performance Panasonic caps I'm using as replacements. If you think these are too expensive, you can save $5 and get some more generic caps. 

With a little trial-and-error, I come up with a configuration that will leave me with all of the grounds tied to one point, and the three positive sides laid out so they'll contact the appropriate tab on the can for its value.  Depending on your can, you may have 2, 3, or 4 caps inside it.  Radial lead caps tend to be easier to work with for restuffing than axials.

In this case, the grounds are tied together in the middle.  The two caps on the bottom are 22uF - the second and third sections of the can, and the cap on top is the first section.  The older Champs used a 20uF cap; the later ones like this used a 40uF.  I decided to split the difference and use a 33uF.

Drill holes through the bottom of the can near each of the tabs, and also a hole for the ground.

You can see what I mean by trial-and-error.  I had to arrange the caps so that each positive lead would wind up near the appropriate tab.

Just run the leads through the holes you drilled, and solder them to their terminal and trim the leads.

You probably see where this is going now.

Another angle of the tabs on the bottom of the can.  You can see how I ran the ground lead through a mounting tab.  I'll bend that tab down and solder the ground one it's back in the amp.

Next, we mix up a batch of J-B Weld epoxy.  Spread some around the rim of the can and cut edge on the bottom of the can.

Stick it together and...
...voila! 

We have a newly restuffed can cap ready to go back in our Vibro Champ.

I like doing this because it more or less keeps the original appearance.  You will see a lot of these amps where new filter caps have been cobbled together above the cap inside the amp.  I just don't like that look at all.  Sure, you can tell the can has been redone, but I think it's a far better thing than using discrete caps.

Continuing my taking-of-a-break from guitar projects, I have this 1978 Fender Vibro Champ on the bench.  Isn't it cute?  I've wanted one of these for a long time, but I refuse to pay the crazy asking prices.  I managed to get this one at a reasonable price - it needs a small bit of work but it will be easy for Amplifier Boy to get it back to snuff.

The Vibro-Champ was the sister to the classic Champ during the 1960s and 70s.  The 'silverface' models, introduced in 1968, have only 2 circuit changes from the earlier 'blackface' models.  Namely, more filtering (40uF vs. 20) on the first input cap, and the addition of a 330pf cap across the output tube.  This is the usual 'CBS' change seen on virtually all post 1967 models; it was done to suppress oscillation caused by bad lead dress.  More on that later.

Here's what got the price down.  The plastic shafts for the intensity and bass controls are snapped off.  I'd guess the amp was dropped or knocked over and the shafts were broken off.

The intensity pot is a rare bird - it's a 25K reverse linear taper.  This is the only Fender amp that used that value and taper pot.  (Most other Fenders use a 50K reverse linear here).   I'm pretty sure Mouser sells a modern replacement, but I have a plan to resurrect the old pot.

Wish me luck.

The amp does work, but it's down on volume and punch.  I'm going to do the usual service - replace the electrolytics, and fix the pots.  I also may play around with a mod or two.

Taking the chassis out is simple.  The design is brilliant.  Leo Fender, with his early experience repairing radios, knew that these things would need periodic service.  So he made it easy to take apart.

First, take off the upper back panel.  Then unscrew the four machine screws that hold the chassis in place from the top.  They also attach to a chrome chassis strap.

I didn't show this part, but you'll want to put your free hand under the chassis where the screw is - there is a nut there that will drop off as the screw is undone.

Pull the screws and the strap up from the top.

The amp chassis will remain seated in the cabinet - there are blocks that hold it in place.

If your straps are corroded or rusty, you can clean them up - or get exact replacements from Fender.

The chassis will then slide out like a drawer.

How brilliant is that?  Did I say Mr. Fender was a genius?  We take this for granted today, but this was a new idea in the early 60s.

This process is the same for any brown, black, or silver Fender amp made from about 1963 to the early 1980s.  And, of course, modern reissues.

Be aware that with the big amps (Super, Twin, etc.) the chassis are quite wide, the transformers are huge and the chassis weights a lot.  As the furniture movers say, "lift with your legs!"

Oh yes.  Totally stock and unmolested.  Just what I like to see.

What I don't like to see, however, is the sloppy wiring Fenders of this period are notorious for.  This isn't too bad, since the amp is so simple.  I've worked on other 70's amps that were awful.  The sloppy wiring affects tone and can cause oscillation, hum and other issues.  Some folks will rewire these amps to clean them up.

Turns out three of the four tubes - the preamp tube, the tremolo tube (aka 'vibrato') and the rectifier are all original!  Very unusual.  The preamp and the trem are Fender-labelled 7025s.  Pretty rare to see them.

Next up, we'll change the filter and cathode bypass caps.

Today I delve into the world of the famous Boss DS-1 distortion pedal and the seemingly endless modifications for it.

Except I'm not doing the mods on a DS-1.

What you see on the left is a First Act MX-510 Distortion pedal.  If you're not familiar with First Act, they make very inexpensive guitars, amps and other instruments aimed at children.  They also make a higher-end line of guitars that have a decent reputation.

The best First Act story involves Paul Westerberg.  He was on tour, stopped into a big-box store to get some shaving cream, and saw a First Act guitar.  He bought it on a whim, and it soon was being used on the tour.  Subsequently, First Act did a run of Paul Westerberg model guitars, which sold for under $150.

Back to the MX-510.  This little pedal is a clone of the Boss DS-1.  It went out of production a few months ago, and Toys-R-Us was selling them for (get this) $5 US!  I bought two of them.  And today I'm going to do some DS-1 mods to it.

You can easily find the DS-1 schematic and layout online, but the MX-510 has a different circuit board.  However, thanks to one of the great folks at freestompboxes.org, I found a diagram of the board tracing.  Armed with that, and a DS-1 schematic, I found all the components I wanted to mess with.

There are 2 boards - the main board, and a second one for the controls.  They're connected by two ribbon cables.

All of the mods I did are DS-1 mods.  They go after the usual DS-1 issues: too much distortion, tone control ineffective, too much treble with distortion, too much compression, etc.

The first thing I did, and certainly the easiest and arguably most effective is to lift the two clipping diodes, D4 & D5.  Don't short them - the pedal won't work if you do.  I just unsoldered one end of each and left them in place in case I wanted to do more with them in the future.

The picture shows their location on the board.  This mod takes care of the compression in one shot.  It also moves the pedal out of the super fuzzy territory back into distortion mode.  I really liked the pedal after this.  If you do only one mod, this may be it.

Next I did the well-known mod developed by the justly famous Jack Orman.  This is the "fat mod."

This mod reduces the gain of the input transistor, Q2, and noticeably fattens up the tone.  Two resistors are changed - R6 is changed to 150K, and R9 is changed to 1K.

I wanted to include a picture of how I desolder components.  Use solder wick.  You can get it in different widths - I have thin for PC boards and thicker for amps and radios.  I've tried solder suckers and they don't work for me.

This will leave you with a perfectly clean pad or tab to work with.  This stuff is cheap - get it online at Mouser.

At this point, the pedal was much improved for my taste.  It now had a lot less gain, and it sounded much bigger, open and transparent.  I'm personally not a high-gain player, and I found a number of usable tones and settings at this point.  But even cranked, there was plenty of distortion for an AC/DC-type of tone.

What still bugged me was the 'fizz' that the DS-1 (and MX-510...) are known for.  This is especially noticeable on pick attack and during note or chord decay.

So I did a couple of things to reduce the treble and cut down the fizz.  One was to change C7, on the control board up to 470pf.  This cut the fizz and treble a fair amount.  Interesting to note that a stock DS-1 has a 10pf cap here, while the MX-510 has a 100pf - which tells you that the folks at First Act are aware the fizz is an issue.

The last few changes went after the tone stack in order to flatten out the midrange dip that's prevalent in the stock pedal.

I changed R16 to 39K and R17 to 33K.  Both are 6K8 stock.  I also changed the tone caps C11 and C12.  C11 is now a .068 and C12 is now a .047.  I also changed out C10, the cap across the diodes, to an .022 in an effort to ground some treble out.

This set of mods evened out the tone and cut down the fizz a bit more.  The fizz is still there, but it's not quite as annoying.

I made up a picture with all of the board components identified, which may be helpful if you decide to mod an MX-510.

This little pedal is a lot of fun to use now - it's much more dynamic and gets a nice overdrive tone.  You can get these pedals very cheaply, and with a couple bucks worth of parts, you can have a 'boutique' version!