ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Another MXR phase 90

9/1/1997 10:17 AM
M.Romasco	Another MXR phase 90 I've got an MXR Phase 90 that passes signal but dosent sweep. It has five(5) transistor type packages that have no numbers or any identification at all. Four of the five read about 30+ ohms between the center and one outer leg. Does this sound like a correct reading of this type of device? Does anybody know what kind of transistors these are? Any feedback appreciated. Matt.

9/1/1997 10:17 AM

M.Romasco

Another MXR phase 90

I've got an MXR Phase 90 that passes signal but dosent sweep. It has five(5) transistor type packages that have no numbers or any identification at all. Four of the five read about 30+ ohms between the center and one outer leg. Does this sound like a correct reading of this type of device? Does anybody know what kind of transistors these are?
Any feedback appreciated.
Matt.

9/1/1997 7:18 PM
Mark Hammer	They are field-effect transistors (FET's). The resistance between two of the pins ("drain" and "source", usually the leftmost and center pin looking at the transistor's flat side from the front) varies as a function of the voltage applied to the third pin. The FET's most commonly found in compressors, noise gates, phase shifters, or any other device where a voltage-controlled resistor (or its equivalent) are MPF102's, 2N3819's, and 2N5457's. If the resistance reading remains steady at roughly 30 ohms then your sweep oscillator ain't sweeping. If it is sweeping, then you should be able to read a slowly changing DC voltage at the GATE pin of the FET's (the rightmost pin looking at them from the front). You might try consulting the phase shifter schematic found at http://hudson.idt.net/~ljg19/" TARGET="_top">http://hudson.idt.net/~ljg19/. It purports to be "very" similar to the Phase 90. P.S.: Assuming that Len Galasso's schematic is "very" similar to the Phase 90, then you should be able to locate a 24K resistor that goes between the output of the last op-amp/phase-shift stage, and the non-inverting input of the 2nd stage. (There should be 4 other 24k resistors in parallel with the FET's). This resistor controls the amount of recirculation. You should be able to replace it with a 10k resistor and a 25k pot in series to get varying degrees of regeneration/resonance. Len's schematic also shows two 150k resistors summing together at the base of a 2N3906 at the output. If you can locate the 150k resistor that goes from the output of the first op-amp to that 2N3906, desolder one end, and hook up a SPST toggle switch between the solder pad and the free end of the resistor. This will allow you to cancel the straight signal and get vibrato effects, using just the phase-shift section alone. As to what's fried on the Phase 90 as it stands, try checking the connection to the rate control; it may be open circuit or something. TIP: Sometimes, the wiper on a pot starts to lift off the resistive element inside the pot. If you are gentle, you can pry up the lugs of the back of the pot, lift it off, and retension the wiper with the tip of your x-acto blade. Be careful, though. You can make it worse. While you're in there, a good cleaning with a q-tip wouldn't be such a bad idea.

9/1/1997 7:18 PM

Mark Hammer

They are field-effect transistors (FET's). The resistance between two of the pins ("drain" and "source", usually the leftmost and center pin looking at the transistor's flat side from the front) varies as a function of the voltage applied to the third pin. The FET's most commonly found in compressors, noise gates, phase shifters, or any other device where a voltage-controlled resistor (or its equivalent) are MPF102's, 2N3819's, and 2N5457's. If the resistance reading remains steady at roughly 30 ohms then your sweep oscillator ain't sweeping. If it *is* sweeping, then you should be able to read a slowly changing DC voltage at the GATE pin of the FET's (the rightmost pin looking at them from the front).

You might try consulting the phase shifter schematic found at http://hudson.idt.net/~ljg19/" TARGET="_top">http://hudson.idt.net/~ljg19/. It purports to be "very" similar to the Phase 90.

P.S.: Assuming that Len Galasso's schematic is "very" similar to the Phase 90, then you should be able to locate a 24K resistor that goes between the output of the last op-amp/phase-shift stage, and the non-inverting input of the 2nd stage. (There should be 4 other 24k resistors in parallel with the FET's). This resistor controls the amount of recirculation. You should be able to replace it with a 10k resistor and a 25k pot in series to get varying degrees of regeneration/resonance. Len's schematic also shows two 150k resistors summing together at the base of a 2N3906 at the output. If you can locate the 150k resistor that goes from the output of the first op-amp to that 2N3906, desolder one end, and hook up a SPST toggle switch between the solder pad and the free end of the resistor. This will allow you to cancel the straight signal and get vibrato effects, using just the phase-shift section alone.

As to what's fried on the Phase 90 as it stands, try checking the connection to the rate control; it may be open circuit or something.

TIP: Sometimes, the wiper on a pot starts to lift off the resistive element inside the pot. If you are gentle, you can pry up the lugs of the back of the pot, lift it off, and retension the wiper with the tip of your x-acto blade. Be careful, though. You can make it worse. While you're in there, a good cleaning with a q-tip wouldn't be such a bad idea.

9/2/1997 5:33 AM
M.Romasco	Thanks Mark, I checked all connections, re-soldered everything. Strange thing - when I touch the dual op amp's connections w/ my finger, the effect starts to work (maybe my finger is acting like a cap or resistor?) but I can adjust my rate when this happens so that rules out the dirty wiper. I'll check out the schematic you mentioned. Thanks, Matt

9/2/1997 8:55 AM
CJ Landry	Try replacing the op-amp. It should be a TL062 and is connected directly to the Rate pot through a 4.7k ohm resistor. The op-amps are cheap and readily available from Mouser. Christian CJLectronics

9/2/1997 8:22 PM
Mark Hammer	Funny thing about re-checking all solder joints. It assumes that everything in BETWEEN the solder joints is AOK. Could there be a crack or cut in your board somewhere? Keep notes on what pins you need to touch to get everything working, then check the traces that lead between those points. You may find out that you need to solder a jumper to substitute for a cracked pc-trace. TRUE AND TANGENTIAL ANECDOTE: My very first computer that I owned (5 years after I cut my teeth on a fridge-sized PDP-8) was an Acorn Atom; a terrific little 6502-based machine made in Britain. It was a nifty little single-board job with a better keyboard and operating system than anything made in that price range on this side of the Atlantic (for 1982), but it had one little problem. Seems the voltage regulator had been attached to the motherboard without enough of the heat-sink goop between the regulator and the heat-sink. The board got overheated and a number of key pc-traces developed hairline fractures, which would expand and close up after turning it on for 15 minutes. What I ended up with was a computer that needed to warm up before it would work! Since I sunk all my spare cash into it and needed it for my M.Sc. thesis, I wasn't about to trash it. Solution? I left the chassis unscrewed and kept a hair dryer on my office desk to speed up the "warm up" time whenever I booted up. Now, if only I could use a hair-dryer to get WIN95 to boot up faster!!

9/2/1997 8:22 PM

Mark Hammer

Funny thing about re-checking all solder joints. It assumes that everything in BETWEEN the solder joints is AOK. Could there be a crack or cut in your board somewhere? Keep notes on what pins you need to touch to get everything working, then check the traces that lead between those points. You may find out that you need to solder a jumper to substitute for a cracked pc-trace.

TRUE AND TANGENTIAL ANECDOTE: My very first computer that I owned (5 years after I cut my teeth on a fridge-sized PDP-8) was an Acorn Atom; a terrific little 6502-based machine made in Britain. It was a nifty little single-board job with a better keyboard and operating system than anything made in that price range on this side of the Atlantic (for 1982), but it had one little problem. Seems the voltage regulator had been attached to the motherboard without enough of the heat-sink goop between the regulator and the heat-sink. The board got overheated and a number of key pc-traces developed hairline fractures, which would expand and close up after turning it on for 15 minutes. What I ended up with was a computer that needed to warm up before it would work! Since I sunk all my spare cash into it and needed it for my M.Sc. thesis, I wasn't about to trash it. Solution? I left the chassis unscrewed and kept a hair dryer on my office desk to speed up the "warm up" time whenever I booted up. Now, if only I could use a hair-dryer to get WIN95 to boot up faster!!

9/3/1997 5:42 AM
M.Romasco	Thanks For the tips Mark, One thing I need to know is if all the FET's that you have mentioned are the same component. Matt.

9/3/1997 9:58 AM
CJ Landry	What a great idea. That made my day. Thanks Mark. I have a suggestion for booting WIN95 faster. Connect your hair-dryer to the system clock and when you need an extra boost in speed jam the fan speed into high.( Ha Ha ) Christian

Page 1 of 1