col
format tables
Posts: 474
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Post by col on Jul 11, 2021 9:08:14 GMT -5
So, for what are the isolation transformers used? Is it to prevent the the two sets of pickups from loading each other and reducing bass frequencies? In one aspect, the exact opposite — which was my original thought before I started talking myself out of it, by thinking about how the phase switching would work. In conjunction with the transformers they increase the cutoff frequency of a highpass (low cut) filter, which is formed from having an inductor (half of each transformer) to ground. Without the resistor, the cutoff frequency ought to be lower than the fundamental frequency of the low E (so as to have little to no effect on the signal), but ought to be significantly higher when the resistors are present. This also leads us to a realisation about the pickups, they must be low impedance; otherwise 510Ω would do very little to shift the cutoff frequency. And so the second purpose of the transformers is revealed: to convert the low impedance signal from the pickups into a higher impedance signal, closer to that of a normal guitar (though still comparatively quite low). Thanks, Yogi. |
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Post by Yogi B on Jul 11, 2021 9:19:47 GMT -5
How can all the mono switch settings be the same if pickups are combined versus selected alone? Ah. I believe this is because Solderburn was measuring the resistance value of the inductors, not the pups themselves. It is not possible to measure the resistance of the pups from the jack socket. Yep, the differences in resistances of the pickup selections are hidden on the other side of the transformer. All we can 'see' measuring from the output jack(s) are the transformer(s)'s output coil(s), hence we'll always get the same reading.
solderburn, if you've still got the pickguard off, could you take a few readings between ground and one of the centre terminals of the blue switch (where both of either the green or orange wires from the transformers, and red wires from neck pickup(s) are connected). In particular, with the blue & yellow switches pressed; with that selection the pickups should be open, thus that measurement should give the resistance of just the other side of the transformer (in order that we can then remove that value from the other settings, with which it is parallel). And to double check my table, a reading for each option where only one switch is pushed. |
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Post by solderburn on Jul 11, 2021 11:33:38 GMT -5
Yogi B, today I will open it back up and get the readings, no problem.
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Post by newey on Jul 11, 2021 11:45:54 GMT -5
Yep, the differences in resistances of the pickup selections are hidden on the other side of the transformer. All we can 'see' measuring from the output jack(s) are the transformer(s)'s output coil(s), hence we'll always get the same reading. Ah Ha, I had thought the transformers were only affecting some, not all, of the pickup settings, like the inductor in a Varitone set up. Your statement about low-impedance pickups being used clarifies the need for the transformers. |
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Post by Yogi B on Jul 11, 2021 13:59:49 GMT -5
Today on "Why Must I Make Things Needlessly Complicated For Myself?", the following gorram table. (Well this is my 666 th post, so something this beastly is quite appropriate!) (Switches are shown from the point of view of the player, for example: the first row has all switches off; the second row has only the red switch pushed; etc.) | Switches | Pickup Selection |
|---|
| B | | B × R × (N + −M) | | B + −M + N | | −M + N | | B | | B × R × N | | B + N | | N | | B + N | | (B × R × −M) + N | | B + −M + N | | N + −M | | B + N | | N | | B + N | | N | | B × M × N | | B × R × N | | N | | N | | none (circuit open) | | B × R × N | | N | | N | | N | | N | | N | | N | | N | | N | | N | | N |
Anywhere where "B × R" appears in the output, the selected bridge pickup is the one that positioned under the other side of the strings, as previously discussed The discrepancies I noted are:Which I reckon should be B + N, the same as when just green is pushed. And, secondly:Which may be a typo? I reckon that should be just B, not just sounding the same but being exactly the same as when just yellow is pushed. |
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col
format tables
Posts: 474
Likes: 25
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Post by col on Jul 11, 2021 14:34:27 GMT -5
I note, Yogi, there is no M-only option. It is a strange beast.
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Post by solderburn on Jul 11, 2021 14:42:46 GMT -5
Wow!! Yogi B, your gorram table is brilliant! It's awesome and so helpful to be able to see a logical representation what's actually going on in this crazy wiring scheme. I really appreciate your help! Legendary!
I think you've created by far the most comprehensive explanation of what's going on with this oddity. I'm printing out the table to pin it to my wall!
And i assume you are correct regarding those discrepancies, i think i was starting to lose my sanity there. Once i get the guard back on i'll do a tap test to confirm your findings.
THANK YOU SO MUCH, Yogi B! Too cool!!
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Post by solderburn on Jul 11, 2021 15:01:11 GMT -5
solderburn, if you've still got the pickguard off, could you take a few readings between ground and one of the centre terminals of the blue switch (where both of either the green or orange wires from the transformers, and red wires from neck pickup(s) are connected). In particular, with the blue & yellow switches pressed;... I assume it's too late now, but i took the measurement that you asked for. I'm not sure if I'm just doing it wrong (probably), or what. I put the multimeter probe on either of the terminals, then put the other probe on the ground at the output jack. I had the blue and \yello in the down position, the Multimeter set to 20k ohms, and i got a reading that was jumping around from about 15 then after a few seconds it would settle down to 0.00. I tried it on both terminals.   Either way, I'm confident that all of your work is correct, and any error is my own. |
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Post by Yogi B on Jul 11, 2021 15:23:12 GMT -5
I put the multimeter probe on either of the terminals, then put the other probe on the ground at the output jack. I had the blue and yellow in the down position, the Multimeter set to 20k ohms, and i got a reading that was jumping around from about 15 then after a few seconds it would settle down to 0.00 I meant to write that I'd expect the value to be no more than a few hundred ohms at most, so I'd try it on the lower ranges, but unless it's less than 50 Ohms I'd expect some reading on the 20k range even if it was only 0.1k (I don't know exactly how low the typical DC resistance of a low impedance pickups is). Those are the terminals I was describing, yes. Though I was thinking of somewhere closer for the ground probe (like the backs of the pickups or the core/chassis of the transformers), but using the jack works too. |
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Post by solderburn on Jul 11, 2021 16:19:41 GMT -5
so I'd try it on the lower ranges, but unless it's less than 50 Ohms I'd expect some reading on the 20k range even if it was only 0.1k (I don't know exactly how low the typical DC resistance of a low impedance pickups is). Those are the terminals I was describing, yes. Though I was thinking of somewhere closer for the ground probe (like the backs of the pickups or the core/chassis of the transformers), but using the jack works too. lol, just when i start to feel little less stupid. Well, thanks for being so patient with me even though i always seem to be in over my head. At 2000 ohms i get 8 on the left terminal and 9 on the right terminal. At 200 ohms i get 9.1 on the left, and 9.3 on the right. |
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Post by Yogi B on Jul 11, 2021 16:55:18 GMT -5
Why, oh why, did I not think to search on Reverb earlier? That instruction sheet I found before, it can also be found on this listing: 1990's Teisco Del Ray Spectrum 5 Version 4 Electric Guitar with Case - Sunburst (and it's a higher quality image, so I'll update it in my previous post soon). Additionally there's a different listing which includes a gutshot. And there's a translucent pink acrylic version through which you can make out enough of the wiring to see that the resistors appear to be positioned similarly to the above gutshot. Those therefore appear to prove my theory that the odd wiring of the resistors in yours (which causes the bridge pickup weirdness) is the result of a wiring or design error. Or at the very least, there were revisions wherein guitars were wired in the way which makes more sense. Ooh! There's even a listing of an original '60s guitar with gut shots: Which again includes the more sensible resistor wiring, However I am slightly perturbed that the description notes that when switch 1 (orange on this, red on the reissues) is pressed there is no output, though that could be due to some unrelated issue. It also shows that the originals had dual gang volume & tone controls. |
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Post by Yogi B on Jul 11, 2021 17:33:54 GMT -5
At 2000 ohms i get 8 on the left terminal and 9 on the right terminal. At 200 ohms i get 9.1 on the left, and 9.3 on the right. Wow that's much lower than I expected, and means we'd likely be lacking the precision required to make sense of any other readings at other settings. |
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Post by solderburn on Jul 11, 2021 17:52:42 GMT -5
Man, that red one is roached. And it looks like someone took off all the switches then forgot ROYGBIV.
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Post by solderburn on Jul 11, 2021 18:06:06 GMT -5
Those therefore appear to prove my theory that the odd wiring of the resistors in yours (which causes the bridge pickup weirdness) is the result of a wiring or design error. Or at the very least, there were revisions wherein guitars were wired in the way which makes more sense. Would it be an easy fix (for a normal person) to change the placement of the resistors to agree with the more logical wiring? Would you say there's domwthing to be gained by changing the placement? If so, than can you describe what should be done to make more sensible. I'm not seeing a glaring difference. No rush, I assume anyone would be all Teisco'd out after so much. |
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Post by thetragichero on Jul 11, 2021 20:36:07 GMT -5
And so the second purpose of the transformers is revealed: to convert the low impedance signal from the pickups into a higher impedance signal, closer to that of a normal guitar (though still comparatively quite low). the epiphone jack cassidy bass uses a low impedance pickup and a transformer with multiple primary taps on a rotary switch for different tonal options. interesting stuff |
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Post by Yogi B on Jul 11, 2021 21:27:51 GMT -5
Would it be an easy fix (for a normal person) to change the placement of the resistors to agree with the more logical wiring? It should be pretty easy, yes. That depends on if you'll make heavy use of the stereo outputs, and always want the treble & bass strings to be fully separated between the two jacks. If you'll be mostly running it in mono, then the swap will make very little difference. The difference is that essentially all the switching each switch is split in half: the right three terminals of each of the 5 switches deal with the halves of the pickups located beneath the bass strings (and the transformer that's between the neck & middle pickups); whereas the left terminals deal with the switching of the treble side (and the other transformer). Therefore for fully separated channels, the wiring between different switches should stick to the same side — i.e. something connected to the left-hand side of one switch should be connected to the the left-hand side of the other switch, and likewise for the right side (the exception being if the connection is to ground, because that's shared between channels). The resistors on the other guitars that I previously posted follow this rule, the resistors in your guitar do not. In the below image I've highlighted the leads of the resistors: the resistor highlighted in purple is connected to the left-hand side of the orange switch, but the the right-hand side of the red switch; vice versa for the other resistor, highlighted in blue. Therefore you'd need to swap the connections made by resistors' leads on one of the switches. This would be easiest to do on the red switch because the resistor leads are the only thing going to those terminals on that switch, unlike on the orange switch where they're sharing. |
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Post by solderburn on Jul 11, 2021 21:59:00 GMT -5
I totally understand now. Thanks for explaining it so clearly. I think i'll fix it in the near future.
Makes me think it was a mistake in the factory when it was being put together, especially since you found all those other examples of it being the other way which is way more logical. Maybe my guitar was assembled on a Monday.
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