Capacitance Decade Switch for Guitar - Page 1
Forum section: electronics / projects

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 20, 2023, 02:26:45 am
content: Hi,I have a question I'm hoping I can get some help with. There's a decade switch (capacitance only) that Les Paul used on some of his guitars. It's an 11 position switch, but only used 4 capacitors (10 capacitance values and 1 bypass position). Here's how they show it in the diagram:I can't find a detailed diagram or any more info on the actual switch. I assume it's playing games rotating things in and out of parallel or series to get the appropriate values. Based on the spread of the resistors, I assume it would go something like: 47nF, 56nF, 68nF, 82nF, 100nF, 120nF, 150nF, 180nF, 220nF, 270nF. The numbers wouldn't likely be exact, but I imagine close enough. Or maybe it had some other funky combination of capacitance values (I will eventually be able to measure the values on a different guitar).Any ideas of what switch would be used and how it was wired to accomplish this?Worst case, I can get an 11 position switch and just load it with those values, but it would be cooler to make it more like the original.Thanks,Josh

author: csaaa
author_rank: Newbie
timestamp: September 20, 2023, 02:43:15 am
content: Online measurement may be affected by interference from other components, especially capacitors, and may not yield accurate results

author: themadhippy
author_rank: Super Contributor
timestamp: September 20, 2023, 02:45:09 am
content: The 10 positions with 4 caps sounds like could be a bcd switch,more than likley custom made to get the added bypass

author: ledtester
author_rank: Super Contributor
timestamp: September 20, 2023, 02:47:42 am
content: The cap values are basically in a geometric series with common ratio 2 so the decade switch looks like a "binary thumbwheel switch":https://www.mouser.com/c/electromechanical/switches/thumbwheel-switches-pushwheel-switches/?output%20code=BinaryHere is a project to create a low-cost multi-decade capacitor substitution box out of several of these kinds of switches:https://www.eevblog.com/forum/projects/low-cost-programmable-decade-capacitor-for-prototyping-convenience/

author: themadhippy
author_rank: Super Contributor
timestamp: September 20, 2023, 03:04:32 am
content: QuoteThere also available in standard rotary switch style,for examplehttps://int.rsdelivers.com/product/lorlin/bcks1001rs/lorlin-16-position-bcd-rotary-switch-150-ma-250-v/7392915yea it says 16 way,but shifting the end stop soon makes it 10

author: floobydust
author_rank: Super Contributor
timestamp: September 20, 2023, 03:46:00 am
content: That is rotary switch is from a long lost era. It was common many decades ago. Radio "band switch" I think it was called.Original probably made by Centralab but I've never seen an old catalog.Not an ordinary single pole switch. The wiper is an oddball strip that switches in more (in parallel) at the last group of positions.So I think SPST selection of the caps then add in parallel combos. The wiper is not entirely a single pole.I have some similar parts I could post a pic of the wiper, they totally drive me nuts trying to draw the schematic for them, just weird but great for band switching or tone controls.

author: kripton2035
author_rank: Super Contributor
timestamp: September 20, 2023, 06:12:13 am
content: you also have this diy one that uses the same principlehttp://kripton2035.free.fr/LabTools/lbt-capacitordec.html

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 20, 2023, 11:54:45 am
content: Quote from: csaaa on September 20, 2023, 02:43:15 amOf course, but the switch can be bypassed out of the circuit and tested safely. I'd also test the capacitance of the bypass position and compare to see if it's truly bypassed.Anyway, it sounds like I'm going to have to do it the simple way with a standard 11 position rotary.Thanks for the input everybody.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 20, 2023, 11:59:42 am
content: For anybody curious, this is an example of the actual controls in one of the models with the decade switch. To keep things more obscure, they have the white ring around the rotary switch.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 20, 2023, 12:58:15 pm
content: Quote from: KungFuJosh on September 20, 2023, 02:26:45 amConnecting the capacitors in series is not possible given that all four have one of their ends commoned and grounded.

author: themadhippy
author_rank: Super Contributor
timestamp: September 20, 2023, 01:29:32 pm
content: QuoteThinking about after a caffine refill , it wouldn't need to be custom if it was a 0 to 10 bcd switch as in position 0 the 4 outputs would be open so the capacitors would be out of circuit.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 20, 2023, 04:10:54 pm
content: The MR50 - Rotary Selector Switch is high-rel

author: floobydust
author_rank: Super Contributor
timestamp: September 20, 2023, 06:38:54 pm
content: That looks like it's a "decade switch" by CTS. Again, could not find an old catalogue of theirs.The capacitors form a tank circuit with the pickup's inductance, so the peak frequency is to thesquare rootof C, it's not a linear relationship. That's why I say it's wired as a band-switch arrangement. You can see the rotor is a 1/2 moon.I'm confused why the cap values are basically huge 0.22uF for example. Compare this graph, Response of a "Fender Stratocaster pickup (1972) with 10 MOhms ohmic load and eight different capacitive loads".Taken fromhttp://www.buildyourguitar.com/resources/lemme/covering 47pF to 2.2nF or 1/100th of the values in OP's pic.The capacitors look like dipped tantalum bead, the offset white dot is polarity. They would be part of the instrument's sound.

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:53:31 pm
content: Slightly strange set of four capacitors: 0.1, 0.15, 0.22, 0.47 uF.For a normal decade capacitor, one would expect 0.1, 0.2, 0.4, and either 0.8 or another 0.4 uF to obtain 0.1 through 1.1 or 1.5 uF.For any case, can you fit a small 4 pole (probably 4 deck) 12 position switch, appropriately wired on each deck, with your four capacitors?I used to have an electrical drafting template that had the usual possibilities for rotary switches included (hard to draw neatly free hand for all the interesting cases), but I haven't seen any on KiCad or similar software packages.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 20, 2023, 09:37:45 pm
content: Quote from: floobydust on September 20, 2023, 06:38:54 pmThat chart shows a range from 10k to 10MΩ, but that's not relevant here. This decade switch is for a low impedance guitar circuit, which is not directly compatible with the external amplifier(s) he's referencing for the external load.For example, the average strat guitar pickup DCR is around 6kΩ each pickup (which is a useless measurement outside of some comparisons). Lots of modern pickups go up 2 or 3 times that. An average external load to get the idea of a pickup in a guitar is around 220kΩ, which I think is also something Lemme calculated, but is now the standard for pickup testing.By comparison, the DCR on the pickups in the guitar this decade switch is going into is a whopping4Ωeach. Typical guitars get potentiometers between 250kΩ to 500kΩ, and this guitar will have 1.0kΩ to 2.5kΩ pots.

author: floobydust
author_rank: Super Contributor
timestamp: September 21, 2023, 04:19:15 am
content: I missed that it's low-Z pickups and I was ignoring the transformer. Is it the small black module in the pic? It looks like a mic transformer.For the rotary switch, it seems to be lost knowledge. "capacitance decade switch", "requires only four capacitors for full decade"."The unusual circuit arrangement of these switches permits real economies in the construction of precision resistor and capacitor decade assemblies since they require only 4 standard resistors or capacitors to complete a full decade of test values."Example CentraLab Q, QSD, PSD. Type N for resistors. Mallory 153L for capacitors, 154L for resistors.I found wafer pics of CentraLab versions where you can see the rotor details, and note that it is a two-layer wafer. You can barely see two levels of terminals in the guitar pic.Not easy to figure out the series/parallel permutations that were done. I think the original switch has the extra off position to make it even more obscure.Ultimately it's a bygone specialty switch I think.

author: Circlotron
author_rank: Super Contributor
timestamp: September 21, 2023, 04:39:30 am
content: Somewhat off topic, but I'm always amazed that such a high quality and expensive instrument has such roughly made electronics. 'Outwardly indeed it appears whitewashed, but inside it is full of dead men's bones and every sort of uncleanness.'

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 21, 2023, 01:05:25 pm
content: Quote from: Circlotron on September 21, 2023, 04:39:30 amThat sort of point to point wiring was the norm for electronics back then, before PCBs were in common use.

author: themadhippy
author_rank: Super Contributor
timestamp: September 21, 2023, 01:21:32 pm
content: QuoteAnd not only is it surprisingly resilient to damage .its also a dam sight easier to repair in the dark at the side of the stage.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 21, 2023, 02:15:17 pm
content: Quote from: floobydust on September 21, 2023, 04:19:15 amNice find, thanks! Sucks that it's not available anymore. I wound up ordering a standard 11 position rotary switch and I'll just use 10 caps. Economy isn't an issue since this is a 1-off restoration.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 21, 2023, 02:17:55 pm
content: Quote from: Circlotron on September 21, 2023, 04:39:30 amYou're surprisingly accurate for Les Paul's personal guitars. He was a tinkerer, and had techs clean up his work, but it still looked crazy.Quote from: NiHaoMike on September 21, 2023, 01:05:25 pmIt's still the norm for guitars. PCBs don't make sense in guitars except for some rare cases where they do weird things.Quote from: themadhippy on September 21, 2023, 01:21:32 pm100%.