Invader Jim

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Friday, August 12, 2016

Misc. Stuff: Add a degauss coil to a video monitor

This circuit was used to add a degauss coil to a JVC TM-22U portable 5-inch color video monitor. Here's a PCB layout.

Pressing the Trigger button activates the degauss coil for about 5 seconds (obviously this only works if the monitor is not on battery-only power). Power for the circuit is taken from the TM-22U's regulated power supply. There are easily-accessible points (via the terminals of a chassis-mounted power resistor) for 10.6v or 16.8v. I chose 16.8v because 10.6v doesn't let the relay close hard enough for my liking, which can cause eventual reliability problems and failure of the relay. At lower currents I wouldn't be concerned (because it wouldn't matter), but a certain amount of controlled arcing in the relay helps keep the contacts clean and insufficient coil voltage will hinder this process.

D1 provides positive-voltage protection for the 555's input. R1 and C1 lock out the input so that holding down the Trigger button (or repeatedly mashing it) won't affect the timing. C1 may need to be adjusted depending on your chosen time-delay and should provide a pulse width just narrow enough to reliably trigger the 555.

The coil, PTC, and relay were pulled from an old TV and the coil re-wound to fit the monitor's CRT. The PTC is a Murata "Posistor" PTH541A3R0Q11 (3 ohms). Since this is a small CRT, some series resistance will probably be needed (remember to figure wattage) or else the initial pulse might be strong enough to dislodge the CRT's shadow mask (or cause other damage), rendering the CRT useless. I used one of these Dale 100R 50W resistors that I had in the junk box, which lets 1A of current flow in the coil (over 100W would be needed for continuous operation but this circuit only operates for 5 seconds and at most every few minutes until the PTC cools off). The resistor has a heatsink and doesn't get that hot, but it's still a good idea to bolt it to an aluminum sub-chassis with heatsink grease.

The timing is set by R2 and C2. The math is: t=1.1RC where t is seconds, R is Ohms, and C is Farads. As shown, it is about 5 seconds. If you change the timing, the coil's series resistor will also need to be changed so that the PTC will heat up fast enough to "fade" the current in the coil before the 555's time delay is up. If the degauss coil shuts off too soon then the CRT will be magnetized at whatever state the coil was in before it shut off. With this PTC and a 5 second delay, 100R is about the maximum series resistance that still gives the PTC time to heat up properly.
3:00 am - 0 comments - 0 Kudos
Wednesday, January 15, 2014

Tone capacitor myths

See also: p?t=1666193

Many people like to think that the type of capacitor used in their guitar's tone control makes some sort of tonal difference, so they will toss the ceramic or poly film caps that came stock and replace them with expensive audiophile caps like Orange Drops, Black Beauties, or any of several other types.

Obligatory Disclaimer: This is an attempt to debunk the "mojo" surrounding tone capacitor types as used in guitars (that is NOT to say that they don't have such properties in other circuits). If you are someone who fervently believes that your expensive new-old-stock audiophile tone caps sound better than your OEM tone caps, and that anyone who says differently speaks heresy, this is your opportunity to stop reading this and leave. Again, I am not implying that audiophile caps have no "mojo" properties, I am suggesting that these properties can't possibly be perceived in any shunt-to-ground application, such as a guitar's tone control circuit.

First we need to look at how a guitar's tone control works. Here is a schematic of a typical guitar (for simplicity, only one pickup and its associated volume and tone controls is shown). Also keep in mind that a guitar signal is not a single frequency, but is made up of many frequencies all superimposed on top of each other.

Notice that the tone control circuit is parallel to the signal path (it is strapped across the signal and ground). When the tone pot is rolled all the way up (the wiper rotated all the way to the unconnected terminal), there is a 500k resistor in series with a 47nF cap to ground present across the signal. A cap's reactance varies with the applied frequency (reactance is basically the AC equivalent of resistance) with it being the lowest at higher frequencies, causing them to pass through the cap very easily. The pot's resistance does not change with frequency. So the total impedance (resistance+reactance) of the tone circuit will be 500,000+Xc where Xc is the reactance of the cap *at a specified frequency*. Put simply, this puts a very light load on the signal (ignoring the parallel loads of the volume pot and whatever the guitar is plugged into) and only the highest frequencies in the signal are bled off because the load gets heavier as the frequencies get higher.

When the tone pot is rolled all the way down, it is out of the circuit (being unconnected at one end) and only the reactance of the capacitor is present across the signal. This loads the signal quite heavily--but again, the load varies with frequency. All the highs are dumped

from the signal, along with a good chunk of the mids depending on the cap value. The cap's reactance is highest at lower frequencies so the lows stay in the signal. With a big enough cap the tone circuit could let all the frequencies in the signal pass and the tone control would then become a terrible volume control. Conversely, very small tone caps will subtly shave off very high frequencies to smooth out the sound without it sounding like a useless, muddled, treble-dump.

It should now be obvious why the tone cap type is insignificant in this circuit: the output signal doesn't even pass through it. Whatever bits of the signal do pass through the cap are shunted to ground and out of the signal path, never to be heard ("ground" can usually be thought of as "oblivion"). Since this is a passive tone control (it uses no amplifying devices) it can only cut frequencies out of the signal. The circuit is not boosting lows, but is just throwing away highs.

Having said that, there are most certainly tonal differences among the various capacitor types. Any application where the main signal, or at least part of it, passes through the capacitor will let the distinct tonal properties of that capacitor to become apparent. The best way, by far, to exploit the "mojo" of a capacitor is to use it as a coupling cap in an amp or effect.

So, in closing, any shunt-to-ground circuit, such as most guitar tone controls, will not display tonal differences between capacitor types. Some people will refuse to believe that, and that's just fine, though people should use their ears and experiment and not blindly buy into hype and parroted myths. Question everything! But no matter what your opinions and prejudices, remember that golden rule of guitar gear:

If it sounds good, it is good.
5:00 am - 0 comments - 0 Kudos
Tuesday, November 05, 2013

GCB-95 mods tures/gear/484555/657204

The mods are as follows:
*6n8/10n/22n/47n sweep caps on 4-way rotary switch
*Whipple inductor and stock Dunlop inductor
*Wah/Yoy mode switch
*Q1- BC109 NOS metal can
*Q2- 2N3904
*2k2 mids resistor
*68k Q resistor
*Grounded-input true bypass
*Volume/wah switch
*RGB check LED
*The switch at the heel selects Wah/Yoy and turns the LED Blue or Green, respectively.
*The switch at the toe selects Volume/Wah (or Yoy, depending on the heel switch) and turns the LED Cyan (Blue+Green).
*The rotary switch on the side selects the value of sweep cap, which changes the Wah and Yoy sounds as well as the timbre of the Volume setting.
*The toggle switch on the other side is the bypass switch. It provides grounded-input true bypass and cuts the pedal's power source so I don't have to unplug the input cable.

--Another mod you could try is replace the 220n cap on the wiper of the wah pot with a 330n. This will simulate the famous ICAR pot taper.
--Something else you could do is put a 1k pot in series with the inductor. This is the Q control on the 535Q wah, or so I've read.

The switch wiring is rather complex but it is very easy and intuitive to use.

Positioning of the Wah/Yoy switch is somewhat critical. You want it directly under the rubber pad on the treadle. To install it, you must remove the "column" on the treadle where the rubber pad is attached to. Drill it out then use a Dremel cut-off wheel (or several) to remove it completely. You want it to be trimmed flush like it was never there. When you're done, glue on a rubber or felt pad to cushion the top of the switch. Reassemble the pedal and adjust the switch height so that it activates easily.

Since the shell is so short at the heel, you may want to try the Alpha #107-SF12020-L DPDT stomp switch (available from smallbear, stock #SF12020-L). I used a normal-sized DPDT stomp with the terminals coming out the side and it was a bit too tall. I had to bend the bottom cover plate (and insulate it) to try to clear it.

The stock transistors are MPSA18. I changed the second transistor to a 2N3904, but in hindsight it should probably be left alone. Since it is just an emitter follower, the super-high Hfe of the MPSA18 better approaches ideal.

The BC109 tames some of the harshness of the wah because its lower Hfe dulls the resonant peak of the circuit. The stock MPSA18's Hfe is WAY too high and it causes a sharp resonance which can be harsh to listen to.

How it sounds:
The sweep caps are a great combo. 6n8 is a trebly sweep that's great for fast single-note riffs and tapping, or when you don't want the heel-end of the stock sweep making it sound muddy. I liked the stock 10n cap so I left it. The 22n is very throaty. The 47n is really low. It almost turns the pedal into a bass wah (a stock bass wah uses 68n). The location of the switch knob allows me to switch with my foot on-the-fly.

Wah mode sounds a little beefier than a stock Crybaby because of the second inductor. In Yoy mode it has a "Woy" or "Eeyoy" sound depending on the sweep cap.

The Whipple sounds a lot like the stock Dunlop inductor (which isn't bad at all), but all the reviews made me expect the Whipple was the best wah inductor made. It's a lot cheaper than a Fasel, though.

The Volume switch gives the signal a real nice warm tone and a bit of a volume and mid boost as compared to the bypassed sound. The value of the sweep cap, in volume mode, sets the range of volume (smaller values make the toe setting louder) and the amount of mids in the volume signal (smaller values for more mids). It's a nifty side-effect. Staying in volume mode and hitting the heel switch to Yoy mode makes a "weh"-like sweep that has little effect on volume.
5:00 pm - 0 comments - 0 Kudos
Friday, June 26, 2009

Rules of teh GB&C.

Current mood: blank

1.  If it exists, there is an ULTIMATE thread for it.
2.  If an ULTIMATE thread for it does not exist, it will soon.
3.  Check the stickies.
4.  Google is your friend. No exceptions.
5.  Kit builds are not builds. No exceptions.
6.  Don't name your edits.
8.  Finish the break, apply Titebond Original, clamp, sand flush, refinish.
9.  Yes, it's possible.
10. New amps are not for everyone.
11. LEDs are over-rated.
12. Killswitches are over-rated.
13. It's not necessarily a bad ground.
14. Build planning threads are lame. Period.
15. Parroting is stupid.
16. Drain your caps or your head will implode.
17. Ending a sentence like this is so annoying!?
18. Parroting is indeed stupid.
19. ^I agree, parroting is so stupid.
20. Thanks in advance!
2:30 am - 9 comments - 14 Kudos