MXR PHASE 90 SCHEMATICS...

[jamiewoody]

i want to totally re-cap a phase 90.

the symptoms, it is barely phasing. iti s very distorted.

my problem, i need to find a parts list and/or schematic for the certain year box i have here.

the one i am doing this for, says he bought it brand new in '72. i doubt this. (it is block logo, as i understand, this line started in '78). the caps themselves do not have values printed on them.

here is the info i have so far:

serial... 055260

it has 10 caps...all ceramic disk types. 4 green ones, that sort of favor some .01 caps i have used before, but i am not sure.

i feel if i start with replacing caps, maybe the problem will be fixed. it is at least a good starting point.

i've searched high and low for a schematic for this. any ideas? thanks in advance.

[jamiewoody]

[nochtanseenspecht]

i guess that if you just change the blue ones, it will be good again,
if not, you can still change the others

[jamiewoody]

my problem is piecing the puzzle together...the missing piece is the proper schematic. there are so many out there, and there were so many phase 90 designs.

also, since the values of the caps are not printed right on the caps, it is hard to tell which is which.

if anybody can help, thanks in advance!

[jamiewoody]

what is the signifigance of the blue caps? are they the ones that deviate the waveform directly?

[Gordonjcp]

The caps haven't failed.  In something like 20 years of fixing electronic equipment, I've found problems caused by faulty capacitors maybe five or six times.  Also, the values *are* printed on the caps - what do you think the numbers are for?

[Circuitbenders]

i'd check diodes and transistors before the caps, especially if they're not even electrolytic. And yes, the values of the caps are printed on them, those aren't just random numbers, http://www.csgnetwork.com/capcodeinfo.html

Actually the first thing i'd probably check if its distorting is for blown op-amps, or even dry solder joints.

[jamiewoody]

i could try changing every component in it, then i could say i can build one from scratch! ;-)

i have some of those transistors already in stock. in looking closer at the ICs,  it looks like they are not lm741, but something similar. i need to look those up.

[Gordonjcp]

None of the circuits will be *that* different from the ones online.  It helps that phaser circuits usually consist of four near-identical phasing networks (opamp forming an allpass filter, with a FET acting like a variable resistor) and an LFO.  Get the LFO working first, that'll be the opamp hooked up to the speed control.  Its output should be a rough sine wave, somewhat triangular.

[jamiewoody]

i opened this p90 up, and compared it to one of my p90s, and it looked totally different! gazing at the pcb traces, they are totally different lines than the one i have! i dunno...i guess maybe i should just buy every component in it, start replacing them and keep doing that until it works. in the end, i will have opamps, caps, etc for other projects.

hmmm....at a closer glance what i have been considering an "op-amp" was not LM741 but actually UA741! is there a major difference?it's google time!

[Circuitbenders]

just a different manufacturer. UA is Texas instruments i think. LM might well be National Semi but i haven't checked.

[jamiewoody]

ah! thanks...so i can change a ua741 for a lm741

[Circuitbenders]

should work fine.

[Gordonjcp]

Don't just start replacing bits at random.  Work out which bit you need to fix.  Like I said, check that the power supply is working properly then get the LFO running.  From there it should be obvious.

[jamiewoody]

i would assume the power supply works, since it has sound....it is less "phasy" and more distorted mow. so,

there is no dc jack on this model, just a 9v clip. so, the power in diode?