'30s Epiphone Electar Archtop wiring diagram -- need one

[Mark Wybierala]

I'm working on this guitar because the output is intermittent. The pots are certainly antiques as are the caps. I've drawn a diagram of how the guitar is/was wired but there has been a little butchery and I want to make sure that I rewire it correctly and correct any mistakes that have been made over the years. The guitar has a single pickup and there are three pots. I can get a 250K reading from one but on another I get an intermittent 7Meg and on the third I get nothing. There are also three oil/paper caps (.002, .05, and .1). I'll do my very best to gently desolder these using any sort of heat sink method I can.

I did an internet search and can get nothing for a wiring diagram. I'm not sure what model epiphone this is. I says Electar on the metal plate on the headstock with the metal plate with the patent#s on the back. There doesn't seem to be any marking or lables inside. The guitar has a square access hole on the back that was certainly made at the factory and a cloth covered square cover for the hole that looks authentic but doesn't exactly match the non-faded areas around the access hole. Its a two color sunburst and the front, back, and neck are single bound. Round pearle dot inlays up to the fifteenth fret. Neck joins to the body at #14 with 20 frets total.

Any info is appreciated. I may email Gibson as they have helped in the past.

[David King]

If Gibson doesn't come through you might try http://music-electronics-forum.com/f11/ there are some very knowledgable folks on there.

[Mark Swanson]

Electrical components wear out, and it doesn't make a lot of sense to try and keep them just because they are "vintage". Pots and switches, and caps just don't last, and replacing them is not looked down on, if you use the right replacements. Gibson probably won't be able to help because I think that guitar is from before they owned Epiphone.

[Joshua Levin-Epstein]

Mark (Wybierala),

How about a diagram of what it looks like now?

[Mark Wybierala]

Haven't attached a picture in a while here but I'll try. Here is how it is currently wired. The tailpiece ground seems odd but I suppose amps were different then and I'm guessing the cap is a safety precaution??? R3 is a volume and R1 and R2 are different frequency tone controls??? R1 and R2 are both 250K I can tell because they are indentical pots with the same markings of 250M.Z -- well, I'm guessing again that it was a different way of annotation kilo ohms in the 1930s. R3 is of the same manufacturer of pots but physically smaller in size and I can't read the marking due to a lot of rust.

Thoughts?

Mark, I'm totally with you concerning the replacement of pots and switches. These are a lot of people who don't see things this way though. I do however at least make an effort to preserve caps if I can. I raid any and all vintage vacuum tube devices that I can.

[David King]

So that far end of R1 isn't grounded?

[Daryl Kosinski]

Looks like a good diagram to me (39 years as an electronic technician). I would replace with all new modern components. For the vintage keep it original fanatics. put all the old components in a plastic bag with the diagram, and place it in the case so if at a later someone wanted to use those old dried up capacitors and scratchy pots they could. You may want to experiment with the value of the cap you labeled .002uf.

[Daryl Kosinski]

As I understand the circuit as diagramed R1 / 0.002uf are a variable high pass filter, and R2 / 0.1uf are a variable frequency shunt to ground. Grounding the other end of R1 would turn it into another volume control. It has some features of a early almost a loudness control attempt. R3 should be valued at near the impedance of the pickup at 440 hz not easy to figure out so try what you have and if it works the way you want then it is good.

[Mark Wybierala]

I'll give it try as it is wired. The cap values are certain for what is in there (maybe). The caps are quite clearly marked. I'll see if they read anywhere near what they should. They are marked with a totally readable value but with no designation of the order of magnitude such as uf, pf, Mf -- just the number -- .1, .05 and .002. I have a cap decade box and I could play with that if I need to.

[David King]

Daryl,

That's an interesting concept of a high pass. I wouldn't have thought much signal would get by that .002µF but I've never played with that circuit.

What's the theory behind measuring impedance of a pickup at 440Hz and choosing that as the value for the volume control? Most LCR meters that can do impedance calculations are limited to one or two frequencies, typically around 1kHz and 100-200Hz.

[Daryl Kosinski]

The theory of impedance matching is getting maximum power transfer. You need to take into account your pickup. volume control, and input impedance of the amplifier. For this an LCR meter can be too much information. The last time I measured a pickup for eg. resistance was 30 years ago but here's how I did it. Using a signal generator (440) feed a signal of less than 1/2 volt into the pickup through a series resistor 10k or more, value not that important, measure the AC voltage across the pickup. Now place a potentiometer around 500k in parallel to the pickup, adjust the pot until the voltage is exactly 1/2 of what you read without the pot. Remove the pot and measure the DC resistance of the pot. That's the DC resistance equivalent of the pickup. You can drive yourself crazy and do this for many different frequencies plot the results and find the sweet spot. The end result will be use whatever parts you can get a hold of and it will sound just as good. Don't overthink.

[Bill Raymond]

I don't think maximum power transfer is an issue in this situation. What you need is voltage transfer and the usual rule of thumb is to load a transducer with at least 10x the impedance. But, I could be wrong--that's just my limited understanding of these creatures.

[Joshua Levin-Epstein]

Apparently Epiphone guitars sometimes had "fancier" tone controls. According the Fisch/Fred book "Epiphone, the House of Stathopoulo" a short lived innovation was a third pot called the "Rhythm Control". I quote "It was said to "enable the player to execute the fastest rhythms without blurring or running together of chords" ". There is no illustration of a guitar with 3 pots. There is also mention of a 2 pot stacked tone control (sometimes 3 pots to include volume) for bass and treble. So your guitar might be a 3 pot version of this.

[Mark Swanson]

I'd say it is definitely pre-Gibson then.

[Mark Wybierala]

I measured the caps.
The .002 measures .005uf
The 05 measures .08uf
The .1 measures .15uf

I'm posting while I'm working and I made what are to me two cool discoveries.

Kinda cool is that these pots easily come apart and I've repaired the two 250K pots.

Here is a picture of one. It may fail when the lubricant I used begins to dry or it may be scratchy when in circuit. It reads nice and smooth with my meter now but time will tell.

[Mark Wybierala]

Here is the second discovery which is way cool. This pot is the same as the first but it has been modified to be a three position switch. You can see that the carbon strip has been removed from the middle of the rotation of the pot. Lugs 1 and 3 of the pot are not connected via what was a 250K strip of carbon and the is only continuity between 1 and 2 at the max rotation CW, and 2 and 3 at the max rotation CCW. There is no connection what so ever in the mid position. I can't show it in the photo clearly enough but the missing carbon was clearly done on purpose. The part# and everything visible on the exterior of the pot is exactly the same between R1 and R2. This pot was in the R1 position of the diagram I drew earlier

[Daryl Kosinski]

I can't see any reason to remove the carbon from any part of a pot that would in this case make it an on off switch. More likely a earlier clueless cleaning. replace it with a new one. Replace the caps to. I had a 0.01 ceramic in my 64 Jazz bass that checked ok but turned over time onto a piezo. You could sing into the bass and it would act like a microphone, feedback was awful.

[Mark Wybierala]

What is ubber convienient with this guitar is the back access cover. I can take the chance of installing all of the existing parts and easily remove them unlike your average archtop. I am a fan of using modern parts but my client is of the other persuasion to an extreme almost fanatical measure. The budget is generous with this one so I do the best that I can to make the client happy. If I can somehow use the existing parts, I will get a bonus next Christmas -- maybe custom mail order steaks like last year.

The pots did not look tampered with but then again, all reassembled, they don't look tampered with now.

[Mark Wybierala]

Daryl, You were right on the money. I opened up the third pot and what I found was an example of what you describe as being a botched attempt at cleaning the pot that is about half way as tidy as what I thought was something way cool. I've ordered all new pots. I found Antique Electronics had pot from PEC that sort of looks similar on the exterior at not silly money like Alessandro. The client will appreciate the look. I'd like to check out the taper of these pots anyway.

[David King]

PEC makes the Alessandro pots. You can usually find the panel mount version for around $10 ea at Mouser or Digikey.
PEC (Precision Electronic Components) are the only pots still made in North America. They should last about forever.