Magnatone lap steel wiring issue

[mellotron]

Hi all, I am back for some good ole GN abuse!



I have a 1953 Magnatone lap steel guitar (with matching Varsity amp in "mother of toilet seat" finish). It had not been playable for years, as I got no audio out when plugged in. I recently opened it up and found a broken resistor in the cavity. I replaced that resistor and replaced the old oil-in-paper capacitor.



My problem is that, rather than acting like Volume and Tone pots as expected, both pots now act as Volume pots! I am sure (of course I am!) that I wired it up exactly as it had been before.





Here is a simple graphic of how it is wired up. I am hoping that someone can catch my error and help me get this working properly.

[newey]

Oct 13, 2022 17:52:04 GMT -5 mellotron said:

I recently opened it up and found a broken resistor in the cavity. I replaced that resistor and replaced the old oil-in-paper capacitor.

What are the values for those components? A wrong value for the cap could cause what you're describing. Do you have any way of knowing what the values were originally?

I don't know squat about Magnatone lap steels, but this thing is obviously vintage. You might be well advised to try to replace the components with period-correct stuff. Look at it this way, the value isn't likely to go down over time, so perhaps it's best to restore it to original, if you can find any info or know what the original componenets were.

[thetragichero]

i'd be willing to bet your cap has gone short circuit. replace it with something similar (if you can) and throw the old one in the case for resale purposes (folks like to see original components at least included. they're mostly happy to have a working instrument/amp). i've done this with my `65 guild freshman i learned on (which i'll never sell. i replaced the whole wiring harness because i still like to play it and it has more value to me working top notch. i didn't use a paper-in-oil tone cap like original because a film cap won't ever dry up)

[mellotron]

Thanks, newey. I do have the original components; see pic below. The resistor was a 22K and the capacitor was a .05uF/200V.

The new resistor is 22K and the cap is a Mojo Dijon 47nF/630V. Could that voltage rating be causing this grief?

[newey]

The voltage rating isn't the issue. The rating is the max # of volts you can put through it before it melts itself into a little puddle. With guitar circuitry, you'd be just fine if it were only rated for a volt.

[mellotron]

Why would there be a resistor in the circuit? I have only seen pots, pickups and capacitors in guitar cavities.

[newey]

The resistor is probably there to "tune" the frequency range of the pickup a bit. I'm with thetragichero, try a different cap.

[Yogi B]

Oct 15, 2022 18:04:31 GMT -5 mellotron said:

Why would there be a resistor in the circuit? I have only seen pots, pickups and capacitors in guitar cavities.

While they're not commonplace there's a variety of reasons why a resistor could be in a guitar (too many to list here), the likely reason as to why we don't see more is that every extra component adds manufacturing complexity — another component to keep stock of, another component to solder with possibly no good place to anchor the end(s), and another component that could put in the wrong place.

Looking for Magnatone wiring diagrams led me to this page: www.dennysguitars.com/LinhofBlueMaggieSetElectronics1.html

The first diagram is the most similar to yours, the difference being that the 'hot' lead of the pickup connects to the other side of the resistor, directly to the volume pot (rather than at the junction between the the resistor & cap, as your diagram shows). In this case the resistor would be to prevent the tone control from 'bottoming out' — that is, rather than putting the cap directly to ground at minimum, it's still have to go though the 22k resistor. This would give the effect equivalent to turning the control down to approximately "3", when it was actually turned to "0".

Maybe, maybe, that explains what you're hearing? I don't find that so likely though, because even at zero a regular tone pot should give noticeable output, unlike a volume control.

It's difficult to know — part of the problem is you're not exactly comparing apples to oranges, regarding volume & tone. A more standard (voltage divider style) volume control (as shown in the second diagram of the above link) does a far better job, both in terms of volume reduction and treble retention, versus the shunt only control used in the first diagram. In other words, it may be that the volume control sounds more like a tone control than to what you're accustomed with on other guitars.

Relatedly, in your version of the diagram, my interpretation of the resistor is that it's there to give the shut-style volume control more to work with. (Accounting for DC only) for the output voltage to be halved (−6dB) the resistance of the shunt must be equal to the series resistance. Without the resistor this is only the 3k8 (according to Denny's diagrams) DCR of the pickup coil, (assuming a log pot) that'd mean turning the volume knob to about 13%. With the resistor, the total the series resistance is upped 25k8, thereby raising the −6dB point to around 33%. (By way of comparison, the more usual voltage divider style wiring gives the −6dB point at around 75%.) This is why your wiring seems entirely plausible.

---

There is another possibility that I thought of, and probably the worst case scenario: the pickup could be open, i.e. have a break in the coil wire. This can happen and still leave you with some output due to capacitive coupling between the layers of the coil either side of the break. The output would be very weak & very trebly, but this would cause the extra loading from either a reduced volume or tone control to behave almost identically.

[mellotron]

Great find on that website, Yogi B! I had searched and not been as successful.

My old .05uF cap measures .08uF today, and is not shorted end-to-end. I will put it back into the circuit and see how it all performs.

If it still misbehaves, I will move the one pickup wire to the spot indicated on that first diagram.

Then again, maybe it DID operate differently all along, and all I had ever done years ago was play it both pots fully dimed...(?)

It IS a rarely-played, "yeah, I've got one of those" instruments that I've had for about 45 years. Then, when it stopped working altogether, it became a Challenge!

[mellotron]

OK, I finally got through some other projects so I could experiment again with the lap steel.

The original .05uF cap was reading about .09uF, with rather high ESR and Vloss. I decided not to reintroduce it to the circuit.

I played the guitar at full volume and tone, then turned tone down to zero. While there was some sound coming out of the amp, it was negligible.

Then I moved the wire from between cap and resistor, down to the center tab on the volume pot.

Now the volume drops, but maybe only by -6dB. I wouldn't say that it reduces the highs by much. While I would prefer it to act like a "regular" guitar tone pot, I will accept that Magnatone had something different in mind and will leave it wired like the top diagram at www.dennysguitars.com/LinhofBlueMaggieSetElectronics1.html

Thanks again, Yogi, for pointing me to that valuable site.

[sumgai]

I'm surprised that no one else* noticed this little factoid: the pickup 'hot' wire is not going to the vol pot at all, except as a handy connecting point. It needs to be moved, not to the center terminal of the vol pot (i.e. the wiper), it instead should be attached to the currently-shown "free" terminal on that pot.  Moreover, move both the pickup wire and the cap's lead to the vol pot's "free" terminal.  That leaves the resistor hanging out in mid-air, but you don't care because it needs to find a new home in a parts bin on your workbench - it serves no valid function in what should be a text-book example of the KISS principle.

Now your circuit is up to snuff.   Errr, provided that the parts all measure out at the expected readings.  As to that....

newey is probably the Master Disciple of the ChrisK mantra - Always Measure Every Component Before You Stick It In A Circuit.  And take notes of your readings!  (And I'll just add my own addendum - keep those notes in a place where you can find them later!  CRS creeps up on you like a cancer, and it's no fun going bald as you snatch your hair out, searching for that notebook you could swear you put right there in the back of the drawer.)

HTH





sumgai

* Obviously I was asleep at the switch as well, so I'm not deflecting blame here, I'm equally guilty. But this morning when I brought up the thread to see what's what. I copped an Elmer Fudd - "someting's vewy scwewy here".