Fixing a broken Phase90

[mjaskula]

After building the beginner project a year ago, I've decided to get back into this pedal building hobby by trying to fix a vintage Block logo Phase90 that I have lying around. I originally bought it broken, which turned out to be broken connections between the board, switch, power, jacks, etc. Once I got everything wired up correctly it worked fine. While putting the pedal back together the circuit board made contact with the power jack and some components must have blown. Because of school I never had the chance to look at it till now, a year and a half later.

I've never done more than the beginner project and don't really know anything about how this circuit or its components actually work to create the effect.

When bypassed, the guitar signal is passed through but there is also a lot of loud noise/static. When the effect is enabled, the noise remains, and the guitar signal is replaced by an incredibly loud, high pitched, feedback-like noise.

I don't have a schematic for this circuit, as i did not build it. (If anyone knows where to get one that would be great.) I have, however, found an image of the board, posted in another thread.



The lower right hand corner of the image is where the board made contact with the power lead.

I've measured the voltages on all the pins of the three ICs as a starting point. I can measure other voltages if someone thinks that it will help. In the image from left to right we have IC A, IC B, and IC C.

Pin	IC A	IC B	IC C
1: 1OUT	0.74	4.78	3.59
2: 1IN -	0.74	3.42	3.56
3: 1IN +	0.18	0.99	0.49
4: Vcc -	0	0	0
5: 2IN +	0.21	0.03	0.03
6: 2IN -	0.62	4.45	6.2
7: 2OUT	0.64	8.13	4.83
8: Vcc +	9.54	9.54	9.54

It appears to me that there's something wrong with ICA, as its numbers are so different from the other two.

I'm not sure where to go from here to determine which part(s) are busted.

Any insight would be helpful.

[gak]

hi
my knowledge is very limited so i can't help too much...
but here you can find the schematics:
http://www.diystompboxes.com/smfforum/index.php?topic=37893.0
http://www.tonepad.com
and also you can take a look at my voltage readings on this post:
http://www.diystompboxes.com/smfforum/index.php?topic=37831.0
i think there're also another few threads about phase 90 issues and some volages posted... try using de "search" button on the top...

gak.

[petemoore]

Any number of approaches could be taken, pulling the OA and installing a socket there being one of them, someone changed a bad OA today also.
  Because all the voltages of that chip look wierd, [except pin 8], it'd be wierd tho that both amps [this is a dual OA] got hit', but is certainly a possibility.
  You can measure the resistances and continuities around that area as a first resort.
  Coded resistance values, or values measured that don't correspond with the schematic can be caused by current flowing in an alternate path through the circuit than the resistor itself...I'd guess it's not a burnt resistor, because they're 'resistant.

[mjaskula]

Excellent, thanks for the quick responses guys.

gak, i took a look at the links you provided and could not find a schematic that I was sure was the version of the 90 that I have. I am pretty sure that the version I have is neither the Script version nor the Dunlop reissue.

I looked at your voltages and this tells me two things. First, the voltages for my 'IC A' are way off. Second, I don't have any voltages that are constantly changing. I don't really know what these two symptoms indicate. I also realize that I'm comparing my voltages to some that you took when yours wasn't working, and it's probably a slightly different circuit. Though i imagine that the IC's functions are the same.

I did a little searching on other peoples phase 90 problems, but I don't know how to apply their problems to mine, I'm sure that once i get a good lead, other threads will start to mean more to me.

Your thread talks about a component called a Zener (diode?). Which component is this and what is its function? I'm going to guess that it is one (or both) of the diodes located on the left side of the image.

petemoore, I assume you mean that I should pull the OA with the weird voltages, or do you think I should swap all three? Because of a combination of my skill, and this being a vintage pedal, i don't want to change more than I have to. Though the pedal is pretty useless now, so I shouldn't be afraid of messing it up.

You talk about measuring values and comparing them with known values (the schematic?). I don't really know where I can get expected values. I only have a source for possible problem values in other poster's problem reports.

I can check continuity though, and will begin doing that around the problem IC and the point where I think the problem occurred.

Thanks for the help guys. What do you think the probability of me getting this pedal working is? Realistically.

[Hal]

i think he ment to pull just the one op-amp, and solder in a socket instead.  The one op-amp is actually a dual-opamp, it has 2 amplafiers in one chip. 

[R.G.]

Pin              IC A        IC B        IC C     
1: 1OUT   0.74   4.78   3.59
2: 1IN -   0.74   3.42   3.56
3: 1IN +   0.18   0.99   0.49
4: Vcc -   0   0   0
5: 2IN +   0.21   0.03   0.03
6: 2IN -   0.62   4.45   6.2
7: 2OUT   0.64   8.13   4.83
8: Vcc +   9.54   9.54   9.54

It seems like the + inputs are held at a very low voltage, about 0.2- 0.3V. If it were mine, I'd find out why that is before investing much more in it. For instance, does +3.x volts appear across the reference zener?

[mjaskula]

I don't know what a reference zener is. This is a purchased pedal, so I don't have the intimate knowledge of the circuit that I would have if i had built it.

I will search for information about this zener thing and see if I can figure out what its purpose is, it seems to be an integral part of this circuit.

Also, what should I expect the voltages of the positive inputs to be?

[R.G.]

In the photo, below the orange blob of a capacitor in the upper left hand side of the board, there are two diodes. One is baby blue with a black band, the next below it is smaller, orangish glass, with a black band. Both black bands are to the right hand side of the diode. Measure the voltages at both ends of those diodes and report them back. One of them is the reference diode, one is the power supply protection diode.

[mjaskula]

In the photo, below the orange blob of a capacitor in the upper left hand side of the board, there are two diodes. One is baby blue with a black band, the next below it is smaller, orangish glass, with a black band. Both black bands are to the right hand side of the diode. Measure the voltages at both ends of those diodes and report them back. One of them is the reference diode, one is the power supply protection diode.

Upon inspection I see that the non-black ends (I assume that both ends have more technical names than I'm going to use.) of the diodes are connected to ground, and both measure 0v. The black end of the orange diode connects to power and measures 9.52 accordingly. This must be the protection diode. So, finally, the reference diode is measuring 0.025v.

Yesterday R.G. wanted to know if +3.Xv appears across the reference diode. Since I'm seeing much lower than this I believe that we've found a good lead towards the problem. (Or maybe the problem itself?)

I'm off to read about these diodes and their use in the Phase90, as I promised I would. Thanks everyone for helping the new guy out. Keep it coming.