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~ 1948 Supro Lap Steel Refurbishment ~ Page 5; Troubleshooting intermittent electronic faults: |
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No, my equipment is not dirty; This trusty multimeter has been with me for a lonnnng time, and survived a myriad of environments and their stains. Old Soldiers deserve respect ! |
Reading the circuits resistance from the output cord jack while operating the volume and tone controls. 250K ohms shown on the meter is reading the volume pot, but tells me that a lesser resistance path should read through the pickup but isn't. |
Correct volume pot resistance mid sweep: |
Correct volume pot resistance turned all the way off; Looping output to ground like a balance control: |
But the tone pot was intermittent around the full on and full off position; Quite common in old guitars because the force of the knob shaft hitting the stops is transfered to a verticle force of a pot's wiper against it's resistor race. This is particularly the case with old Steels whose Player(s) used the tone control for boo-wah / wah-wah effects; Otherwise it's uncommon to find such wear on a tone control, but much more common on volume controls. Here is the tone pot with it's case ears bent upward in preparation for going inside and taking a look to see if the discontinuity can be repaired: |
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Here's something you don't see a lot on a guitar; This pot's center lug is linked to the post case ground by a plate on top that ties ground from the center lug's rivet, and the shaft's boss and one of the casing's ears that fold over onto the plate ! Notice that there is no solder at all on two of the lugs; A dead giveaway that an internal ground is in the pot. |
The pot was made by the Stackpole company; Code 304. The pot dates to the seventh week of 1947; THE WEEK I WAS BORN ! Just in case someone can't read the pot numbers, they're: 304707 (304-7-07) The upper 220-2 is the pot's part number. |
Just as I had suspected; Pointed out below is the worn out race caused by wiper bounce at the shaft stops: |
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And the other stop too: |
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Interesting observations / trivia: Since there is no resistance value stamped on the pot casing, I read what remains of the resistor race and see that the pot is a 1 Meg pot. This tells us something very interesting: Valco / Supro had both 500K and 1 Meg pots installed at the factory, and 1 Meg aftermarket changeout was common. This pot is dated the same year as the Steel's serial number, it's part number (-2) indicates it was a Stackpole pot made for Valco / Supro tone control ....and the knob is a chicken head knob seen on a lot of Supros. The pots soldering indicates it is factory original. Chicken head knobs were popular on the tone control because it gave a better pinky finger throw for boo-wah / wah-wah effects. 1 Meg pots gave a smoother / wider boo-wah (wah-wah) effect; While 500K pots have a much tighter and more rapid boo-wah spot. So in conjunction with the worn race, a 1 Meg pot and a chicken head knob, we can be more than reasonably sure that this pot was worn out from boo-wah effects use. Discussions among Steelers about this configuraton have not come to a conclusion whether 1 Meg and a chicken head knob was a factory model or option or was done aftermarket by dealers repairmen, general market reparimen and owners themselves. But since I've seen 1 Meg pots that have all the indications of factory original, I am SWAG'ing that the 1 Meg pot was a factory model or option, while the chicken head knob was likely a dealer or popular aftermarket changeout .....although it too could have been a factory model or option since it's popularity was obviously quite common. |
Camera angle paralax make the needle look like it's on 500K, but it's actually just shy of 1 Meg. |
1000 x 1000 = 1 Meg |
