djeans
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Post by djeans on Mar 27, 2012 6:30:56 GMT -5
When I was 16, I bought a guitar from a guitar teacher at a local music store. It had a hand wired custom active pre-amp. Several years later, I removed it because one of the wires had come loose and it stopped working. Now, several years later, I've decided that I want to put it back in, but I can't remember what the wires were connected to. I was hoping that someone here could make an educated guess as to where each wire should connect. It has 3 wires coming from it. A black (that I assume would be the ground), a red wire, and a white wire. I have some pictures of it, if I can figure out how to post them.     When it was originally installed, when switched on it boost the gain of the pickups considerably. It was a little noisy, but it was still cool. Any help or suggestions would be much appreciated. Thanks |
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Post by reTrEaD on Mar 27, 2012 7:42:46 GMT -5
Hello djeans, welcome to guitarnutz2. I was hoping that someone here could make an educated guess as to where each wire should connect. That's a fragile looking construction method. As near as I can tell, you have a switch, op-amp (the round metal can), 4 resistors, a capacitor, and a trim potentiometer. To make a guess where a loose wire might go, we would need to determine what's going on with the parts that are already connected. There are 8 pins on that op-amp. Lots of connections to figure out. TBH, it would be much easier if you could get a drawing from the guy who assembled this, as it's a one of a kind sort of thing. But it might be possible to reverse engineer, with a LOT of work. If you wanted to go that route, I'd suggest starting with the pinout diagram of a 741 opamp, since there are many other devices that share the same pinout.   The add the resistors and capacitors to the drawing and see if the circuit begins to make sense when connected to the switch. Maybe we can guess where the loose wire goes, but no promises. It's going to be a royal pain for you to translate connections that are made in 3 dimensions to a drawing on a single plane, but it can be done. And that would be the first step, if you want to move forward. |
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djeans
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Post by djeans on Mar 27, 2012 16:18:30 GMT -5
Getting any info from the guy that created it isn't likely, since that was over 20 years ago, and he has fallen off the face of the earth. I'm pretty sure I figured out where the loose wire was supposed to go . . . There was a lug on the Switch that had solder and frayed wires on it, but nothing connected to it (visible in last pic, center of the switch) which I assume was where the loose wire went. The loose wire was from the red lead of the 9v battery if that makes a difference. What I am lost on now, is how to wire this to the pickups on the guitar. Any suggestions? Like I said, I think the black wire would be the ground (back of one of the pots) but no idea where to connect the white or red wire to. The pickup is HSS with 1 volume, 1 tone, and a Jackson style 5 way switch that has 8 lugs on the back of it like this www.guitarelectronics.com/product/SWL52/5-Way-Import-Lever-Guitar-Switch-Ibanez-Jackson-Yamaha.htmlThanks again for the help |
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Post by reTrEaD on Mar 27, 2012 17:05:04 GMT -5
Getting any info from the guy that created it isn't likely, since that was over 20 years ago, and he has fallen off the face of the earth. I'm pretty sure I figured out where the loose wire was supposed to go . . . There was a lug on the Switch that had solder and frayed wires on it, but nothing connected to it (visible in last pic, center of the switch) which I assume was where the loose wire went. The loose wire was from the red lead of the 9v battery if that makes a difference. The red lead of the 9v battery eventually has to connect to one of the pins of the opamp. IF this particular opamp has the same pinout as a 741, that would be pin 7. (see diagram in previous post - pin numbering is as viewed from above.) Look for the lug of the switch next to the one you suspect, in the same direction as the toggle goes. Iow, vertical in the last pic. If there is a connection to pin 7 from that adjacent lug, that suggests that the red wire did originally connect to the suspect lug. Dunno what the original connections were. Might have been between the pickup selector switch and the volume control, but more likely between the wiper of the volume control and the output jack. Two sections of the switch were likely used to route the signal either through the preamp or bypass, depending on the position of the switch. Expect a total of 3 wires to connect this to the guitar. 1 from the wiper to the switch, 1 from the switch to the output jack, 1 from the preamp to ground. The ground wire probably could be identified if it goes to pin 4 of the opamp. The other 2, idk. |
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djeans
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Post by djeans on Mar 27, 2012 19:07:57 GMT -5
Here is what I can figure - the opamp has LF356BH marked on it, and according to what I could find on google, has the same pin arrangement as the one you posted. Pins 8, 1, and 5 - viewed from the top of the can - have been clipped off. Pin 7 connects to one of the vertical center lugs on the switch, and to one of the Resistors. If I understood you, then I think I was right about where the red battery lead goes. The black batter lead connects to the metal side of the opamp can. Picture of the battery leads  Pin 4 of the opamp is soldered to the side of the can with the black battery lead and one end of a resistor. The opposite lead of that resistor is connected to the black wire, 2 other resistors, and one lead of the capacitor. The red and white wires are soldered to lugs on the switch. Not sure how to number the lugs. More close up pics of the wires for reference.    I really appreciate your help, and patience with my lack of electronics knowledge. |
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djeans
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Post by djeans on Mar 27, 2012 19:15:56 GMT -5
By wiper, i'm guessing that's the 5-way switch, right? Any idea which of the 8 lugs it would connect to? Currently, the pickups are wired to the first 3 lugs, and lug 4 and 5 are bridged together. The last 3 lugs are not connected to anything.
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Post by sumgai on Mar 27, 2012 21:12:03 GMT -5
dj, Hi, and  to the NutzHouse!
By wiper, i'm guessing that's the 5-way switch, right? Any idea which of the 8 lugs it would connect to? Currently, the pickups are wired to the first 3 lugs, and lug 4 and 5 are bridged together. The last 3 lugs are not connected to anything.What rT was speaking of was the output of your volume/tone section, which comes after the 5-way selector switch. The wiper of the Volume pot should go to your White wire. The Red wire goes to the output jack. The Black wires are grounded, as you've already surmised. You can test this without having to install everything into the guitar and stringing it up. Simply unsolder the wire from the Volume pot's wiper, the one that goes to the jack - you've now "broken" the connection that would ordinarily send your signal out to the amp. Insert the pre-amp as noted above, using either alligator clips (but not everyone has a bucketful of them laying around, I admit), or else just lightly tack-solder the wires into place. Do the following, in order: -> If you've hooked up a battery to the pre-amp, disconnect it again. -> If all of your pickups are currently disconnected, hook one up. Select that pup (or any one of them, if they were never disconnected in the first place); -> Plug your axe into your amp, and turn the amp on, with LOW volume; -> Turn the guitar's Volume up to 10. -> Place a screwdriver on the pickup's magnet(s), and pull it away sharply - you should hear a "pop" or a "click" from the amp's speaker; -> If not, turn up the amp's volume to maybe 3, and try again. -> At this point, you are either blowing out your eardrums, or getting nothing. If it's nothing, then flick the pre-amp switch, and re-test. -> Now you're getting something, right?  -> This is the 'Normal' position, please make a note of it; -> Turn the amp Volume back down to almost nothing; -> Hook up your battery; -> Flick the pre-amp switch, and do the screwdriver thing again.... -> Adjust the guitar's Volume pot accordingly. Now, there is one fly in the ointment - I may be wrong about those Red and White wires. Experience tells me (and reTread, when he thinks about it) that I've got it right, but just in case Mr. Murphy has hidden a turd in the punchbowl, you may have to reverse those two wires - White to the output jack, Red to the Volume pot's wiper. HTH sumgai |
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Post by reTrEaD on Mar 27, 2012 22:00:13 GMT -5
Here is what I can figure - the opamp has LF356BH marked on it, and according to what I could find on google, has the same pin arrangement as the one you posted. Great. Since we know the pinout, we can use the pins as a reference to what the switch and external wiring is probably doing. Pins 8, 1, and 5 - viewed from the top of the can - have been clipped off. Makes perfect sense. Those leads aren't needed and would have gotten in the way. Pin 7 connects to one of the vertical center lugs on the switch, and to one of the Resistors. If I understood you, then I think I was right about where the red battery lead goes. Good. Let's pause here and be very careful about how we name things. This will help to avoid confusion as we go forward. Your switch has 9 lugs. 3 rows, 3 columns. We need to be VERY specific about how the switch is oriented or we'll get confused on our naming agreement. WITH THE SWITCH ORIENTED SO THE TOGGLE MOVES VERTICALLY... You have three lugs in each ROW (across) And 3 lugs in each COLUMN (top to bottom) The POLES of each column are the lugs in the center row. - When the lever is pushed down, each pole connects to the THROW above it. - When the lever is pushed up, each pole connects to the THROW Below it. If we know which row (top or bottom) that (+) battery wire is connected to, we will know which row of throws is used when the switch is in "active" position. The row of throws used for bypass will have nothing connected to it in the battery "column". Make sense? The black batter lead connects to the metal side of the opamp can. Pin 4 of the opamp is soldered to the side of the can with the black battery lead and one end of a resistor. The opposite lead of that resistor is connected to the black wire, 2 other resistors, and one lead of the capacitor. Not exactly as I had expected, but I think I know what might be happening. Do either of those resistors that are connected to the black wire have their other end connected to pin 7? If so, the Black wire is meant to go to the ground of the guitar. The 9v battery has neither end connected to the guitar ground. The red and white wires are soldered to lugs on the switch. Not sure how to number the lugs. It looks like the white is connected to the row of poles on a column at one end. The red is connected to a throw in the same column. This suggests they are connected together when in bypass mode. There appears to be a gray jumper from the red wire going to the pole in the middle column. Is that right? If we see which pin of the opamp is going to the throw on the middle column that will tell us what the red wire is. Likewise for pin of the opamp connected the throw on the same column as the white wire. I think we're getting close. By wiper, i'm guessing that's the 5-way switch, right? Any idea which of the 8 lugs it would connect to? Currently, the pickups are wired to the first 3 lugs, and lug 4 and 5 are bridged together. The last 3 lugs are not connected to anything. No, I meant the wiper of the volume control. Sorry for not being specific. I'm posting a pic from a guitar with a similar switch. It will help in the discussion. Don't worry about the fact that the other side of the switch is connected to tone controls. The left side is what represents what is going on in your guitar.  There are TWO DIFFERENT possibilities for connecting an active preamp like yours. 1 - BREAK the wire going from the 4th lug of the pickup selector to the bottom lug of the volume control. Then connect the input of the preamp to the 4th lug of the pickups selector and connect the output of the preamp to the bottom lug of the volume control. OR 2 - Break the wire going from the wiper (center) lug of the volume control to tip lug of the output jack. Then connect the input of the preamp to the wiper lug of the volume control and connect the output of the preamp to the tip lug of the output jack. |
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Post by reTrEaD on Mar 27, 2012 22:03:50 GMT -5
Now, there is one fly in the ointment - I may be wrong about those Red and White wires. Experience tells me (and reTread, when he thinks about it) that I've got it right, but just in case Mr. Murphy has hidden a turd in the punchbowl, you may have to reverse those two wires - White to the output jack, Red to the Volume pot's wiper. That would work. But I like taking the long way round.  |
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Post by sumgai on Mar 28, 2012 1:05:49 GMT -5
reT,
The experimenter in me says "what can happen, the two wires can go only one of two ways...."
The Engineer in me says "dope out the circuit, and get it right the first time. But yes, there's no harm in getting it wrong the first time".
So, with that modus operandi in mind, I simply looked to see where Pin 6 was going, and the rest kinda fell into place... if you know what I mean. Doesn't mean I can't be wrong, just means that I'm more than 50% likely to be correct.
HTH
sumgai
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djeans
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Post by djeans on Mar 28, 2012 7:23:14 GMT -5
Wow, thanks for all the replies. I've been at work, but I'll try these out after I've had a ouple of hours of shut-eye. I'll let you know how it goes.
D.
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djeans
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Post by djeans on Mar 28, 2012 18:08:05 GMT -5
Ok,
Tried Sumgai's suggestion:
Yes - still strung, with pickups, and I got normal sound.
With battery connected, still nothing on the other side of the switch.
Flipped the wires, same outcome. Audio on one side of the switch, nothing on the other, with or without the battery.
Going to work my way through ReTreads suggestion next.
D.
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djeans
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Post by djeans on Mar 28, 2012 19:28:58 GMT -5
Ok, Still no luck, but I'm optimistic that we're getting much closer. Yes. Yes. I decided to draw out all of the connections, maybe this will make it easier to visualize. (maybe not, I'm not a very good artist.)  Still no luck at the moment. D. |
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djeans
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Post by djeans on Mar 28, 2012 20:20:28 GMT -5
Just used my multimeter to check to see if i'm getting any juice to the pre-amp from the battery, and I'm showing 9.23 volts when measured from pin 7 and the outside of the can, so I think that at least the battery connection is good.
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Post by ashcatlt on Mar 28, 2012 20:45:13 GMT -5
Check voltage between each of the other opamp pins and ground..
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Post by reTrEaD on Mar 28, 2012 21:33:20 GMT -5
I decided to draw out all of the connections, maybe this will make it easier to visualize. (maybe not, I'm not a very good artist.) A couple of things here seem odd. The capacitor connected between pin 3 and ground is unusual. But more importantly, the lack of a connection to pin 2 is a sure sign of trouble. Without negative feedback, the gain of this circuit would be off the charts. I would expect a connection from pin 2 to maybe the junction of the right lead of the trimpot and the resistor just above the label "Offset Null". Maybe nothing connected to pin 2 was just an omission in the drawing, but if it's that way IRL, check to see if pin 2 might have been going there but the connection has failed. |
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djeans
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Post by djeans on Mar 28, 2012 21:46:10 GMT -5
I did accidentally omit pin 2. it connects to one side of the trim pot and the resistor that connects to the switch. Sorry - drawing has been updated to correct this error.
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djeans
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Post by djeans on Mar 29, 2012 0:27:04 GMT -5
With the battery connected and the switch in the on position
Pin 7 and ground - 9.24v
Pin 2 and ground - No Reading
Pin 3 and ground - No Reading
Pin 4 and ground - .029 kΩ
Pin 6 and ground - No Reading
Unless I'm doing something wrong - those are the readings from each pin.
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Post by reTrEaD on Mar 29, 2012 7:24:15 GMT -5
The voltage at pin 7 shouldn't be so high with reference to ground. The purpose of the resistors connected to pin 7 and pin 4 is to create a reference point at about half the supply voltage, that will be our ground.
Maybe the lead for pin 4 has been bent and is touching the case of the opamp?
EDIT:
btw, the white wire is definitely the input and red is definitely the output.
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djeans
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Post by djeans on Mar 29, 2012 8:33:41 GMT -5
Yes, it definitely is. It is soldered to the side of the opamp along with the black battery lead and one of the resistors.
So that means red wire to the jack and white wire to the volume pot, correct?
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Post by ashcatlt on Mar 29, 2012 8:43:34 GMT -5
I can't actually see your picture on my phone for some reason, but...
You posted one voltage reading and one resistance reading. What were you measuring for the others?
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djeans
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Post by djeans on Mar 29, 2012 9:03:00 GMT -5
My meter is an automatic digital type. It automatically changes the display based on what is applied to the probes. The other leads showed no reading, so I assume there was no connection made.
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Post by ashcatlt on Mar 29, 2012 9:42:31 GMT -5
So the mter decides whether it wants to show you resistance or voltage? What the he'll good is that?!?
Edit - Actually, I can almost see where it might work, but it's pretty whacked. I still say...
At this point I think it would be easiest to tear it apart and put it back together (maybe on perfboard) so that it will work.
Just for fun you could try connecting that loose black wire to your guitar's ground. This should put half the battery through your pickups and bias the circuit up to make it work. Not sure that you won't see smoke when that bias voltage hits the ground connection on whatever you're plugging into, but there isn't really a whole circuit created with the battery there, so you might be okay.
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Post by reTrEaD on Mar 29, 2012 10:04:18 GMT -5
Yes, it definitely is. It is soldered to the side of the opamp along with the black battery lead and one of the resistors. My bad. I got confused on which black wire was connected to the case of the opamp. The case of the opamp is not connected to ground (the "loose" black wire), it's connected to the battery (-) So that means red wire to the jack and white wire to the volume pot, correct? Yes. Just for fun you could try connecting that loose black wire to your guitar's ground. I think this is the way it was meant to be. The "loose" black wire and the junction of the two resistors that span across the battery are meant to be ground. The black wire from the battery clip is definitely NOT ground. I'm still troubled by the capacitor between ground (loose black wire) and the input of the opamp. Seems like that does nothing desirable and shunts the signal. |
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Post by ashcatlt on Mar 29, 2012 10:22:17 GMT -5
I'm still troubled by the capacitor between ground (loose black wire) and the input of the opamp. Seems like that does nothing desirable and shunts the signal. Yup, it's a low pass filter. We don't know the value of that cap. It could be an attempt to reduce hiss, or maybe knocking down a bit of the treble to make up for more harmonics which would presumably be produced when the boosted output hits the amp. I personally like to roll off treble on my booster pedal for exactly that reason. The black battery lead is not ground (kinda goes back to that discussion we had recently), but it what I meant when I asked for voltage readings. We need to know the voltage difference between each pin and the bottom of the battery. |
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djeans
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Post by djeans on Mar 29, 2012 18:44:34 GMT -5
OK, I figured how to put the meter into manual mode - retesting each pin for voltage, we get: Pin 7: 6.53v Pin 2: -2.54v Pin 3: 0.00v Pin 4: -2.077v Pin 6: -1.463v The "loose" black wire was used as the ground point for each test. I don't know how to decipher what the markings on the cap mean, but it says Cm 222 J5A  Sorry about the late reply - I live in Hawaii, so I'm going to bed when everybody else is getting up. |
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djeans
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Post by djeans on Mar 30, 2012 4:12:22 GMT -5
Diagram updated with color codes of resistors - some were hard to make out, but I think I figured them out. |
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Post by yakkmeister on Mar 30, 2012 9:46:11 GMT -5
I don't know how to decipher what the markings on the cap mean, but it says Cm 222 J5A it's a 2.2 nF (2200 pico farads) cap. First 2 digits are the significant figures with the third being a base 10 multiplier. 22 x 10^2. Not 100% on the next bit ... But the J should mean it's a 5% tolerance... 5A - no idea. I would interpret the code to be 222J - 2.2 nF @ 5% +/- en.wikipedia.org/wiki/Ceramic_capacitor |
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Post by ashcatlt on Mar 30, 2012 10:27:08 GMT -5
The pickups were connected to the circuit when this was measured? Both input and the "loose black wire"?
Pins 2, 3, and 6 should all sit at 0. Pins 4 and 7 should be equidistant from 0, with opposite polarity - pin 7 positive. Pin 4 + Pin 7 should equal the battery voltage. That part is close enough, and I suspect that maybe the DC resistance of the pickups may be causing the misbias (the fact that 7 is more positive that 4 is negative).
It freaks me out the pin 2 is more negative than the bottom of the battery (!) and pin 6.
Seems like maybe pin 2 (or something connected to it) is shorting to the bottom of the battery (or something connected to it).
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djeans
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Post by djeans on Mar 30, 2012 15:33:41 GMT -5
Actually, no. Forgive my ignorance . . . I guess I should have known that it should have been connected. Give me a few minutes and I'll remeasure all of those with it connected.
D
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