no sound from mxr phase 90/diode missing

[frogman]

I'm troubleshooting an mxr phase 90 and read that the 2 diodes in the power supply are a big reason for this pedal malfunctioning. I opened it up only to find that there is only 1 of the 2 diodes present. This pedal was bought in broken condition so it wouldnt surprise me if the previous owner removed it. I've also never worked with smd before so I'm having trouble grasping how it works.





The diodes in the bottom right hand corner of the picture appear to have 3 solder points. I have seen that some zener diodes have 3 solder points. Now im assuming this schematic is right and d1 is 1n914, but I cannot find a 1n914 smd diode with 3 solder points.

The pedal has the script logo on the front and seems to have been made in 2004.

Can anyone give advice on what should go in this spot?

[jimilee]

The sender is removed, which just protects from power surges. Was the wrong voltage plugged in? If so, opamps are next to check with an audio probe.

[mcknib]

SMD components sometimes have different part numbers the one you want is MMBD914 for the 3 pin 1n914

https://www.fairchildsemi.com/datasheets/MM/MMBD914.pdf

You can get them at Mouser and Digikey

http://www.digikey.com/product-detail/en/MMBD914-7-F/MMBD914-FDICT-ND/717888

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/MMBD914/?qs=1KwYHESJF3B2veQrbV1m7g%3D%3D

[duck_arse]

D1 seems to be a reverse protection diode. the pedal ought to work without it. it could be replaced with just about any diode with the right pinout.

D2 is providing a stable bias for the circuit, and should not be/need messing unless it is carked.

probe the pads on the board with your ohm meter, see if the three D1 pads are actually connected to something. then look at some smd diode datasheets, and see the packages. the same part number, with a different suffix letter, can/will indicate 1 or two diodes in the same package, or one diode with only 2 pins connecting, or 1 diode with all three pads connected.

as an example:
http://pdf.datasheetcatalog.com/datasheet2/b/0dddqjdlya9kl26zhxkkqq2u6icy.pdf

[edit :] also, you probably want to ask a moderator to move this to the general section, as more people will see it there. some people don't like digital.

[frogman]

The sender is removed, which just protects from power surges. Was the wrong voltage plugged in? If so, opamps are next to check with an audio probe.

Checked the opamps with an audio probe. opamp's 1-4 are getting sound at the IN- and OFFSET N2 pins. However,1 opamp #5 is not getting sound. neither is either side of C9.

Checked the voltages and opamps 1,2,3,4 and 6 are all roughly:

pin1 - 0V
2 - 2.4V
3 - 2.4V
4 - 0V
5 - 0V
6 - 2.4V
7 - 9.1V
8 - OV

Most of opamp 5's voltages wouldnt give me a steady reading, they were...

pin 1 - 0V
2 - 2.5V-3.1V
3 - 2.5V-3.1V
4 - 0V
5 - 0V
6 - 2.1V-3.8V
7 - 9.1V
8 - 0V

So something with opamp 5 is really off.






Also thanks Mcknib and Duck_Arse, thanks for the info on the diode. I'll probably replace it since i'm fixing it for a friend just so its equipped with that.

[mcknib]

You'll get the op amp expected voltages at GGG here for comparison:

http://www.generalguitargadgets.com/pdf/ggg_p90_instruct.pdf

Schem etc here:

http://www.generalguitargadgets.com/effects-projects/phase-shifters/mxr-phase-90-script-logo/

Not SMD but the same circuit I would think if you've got 6 x single 741 op amps.

As far as I know you wouldn't get any audio out of the LFO op amp which would have alternating voltages that would be the one connected from it's output pin 6 to the speed pot lug 3 your IC 5?.

You'd have the 1st op amp at the input, the input buffer section, 4 in the phase shifting stage and one LFO then your 2N4125 in the output mixing stage you should get audio at the input out of pin 6 through the phase stage op amps and also down and around up to the 150K resistor (R18 here.) both going into the base of the 2N4125 where the signals mixed and goes to the output something like this:



Please note this audio path was done some time ago and never finished or verified I'd imagine you'd also get audio on the feedback loops of the 4 phase stage op amp's but it follows the rule no audio on things going to ground or coming off 9v hence the 'something like this' - blue in this schem is phased and green clean signal

Anyway hope the info helps if you follow it from IN to OUT you should get an indication of your problem area where you lose the signal, you didn't say what type of signal you're getting out of the op amps is it phased on the phase stage one's? If not and the pedal was bought not working maybe the previous owner messed with the trimmer the phase range is very small on it, if you decide to try adjusting the trimmer make sure you mark it's original position first and move it in very small increments.

My advice would be to audio probe it first though!

[duck_arse]

just for fun - measure the voltages on the pins of D2. and the pins of the trimpot. then, for extra points, go around the board, find all the test points, marked "TP1" "TP2" "TPn", and measure the voltages there. they are test points for a reason, we just need to work out what the reason is.

IC5 volts look ok for an oscillator.

what is the part # on the row of fets, Q1-4, please?

mcknib - where does "VA" on R27 connect in your blue/green circuit, any idea?

[mcknib]

Here's the full schem with the power supply R31 = 100R, C12 = 100u, C13 = 10n DA, I posted it just to give an idea of the audio path.



IC1 = TL071, IC2 = TL074 and IC3 = TL061

[frogman]

I found that the signal goes the entire way through the pedal. With the pedal switched ON it is only slightly different than the dry signal (pedal switched OFF). No phasing effect happens anywhere in the "phased signal".

While probing the opamps I found that there is no signal coming from any of them on pin 6 but rather pins 1,2 and 5... as well as a faint signal on pin 3. They are indeed 741 opamps.

Is it normal that, when probed, all of the resistors and most of the capacitors in the "phased signal" only draw the signal on one side?

just for fun - measure the voltages on the pins of D2. and the pins of the trimpot. then, for extra points, go around the board, find all the test points, marked "TP1" "TP2" "TPn", and measure the voltages there. they are test points for a reason, we just need to work out what the reason is.

IC5 volts look ok for an oscillator.

what is the part # on the row of fets, Q1-4, please?

mcknib - where does "VA" on R27 connect in your blue/green circuit, any idea?

The fets are 2N5952

2.4V on the side of D2 with one pin, 0V on both pins on the other side.

TP1 - 2.4V
TP2 - 2.3V
TP3 - 2.3V
TP4 - 2.3V
TP5 - 2.2V-3.9V
TP6 - 2.3V

[mcknib]

You've got a distinct lack of 5v where it should be if you compare yours to GGG's D2 - 5v1 zener should give you 5v ish

I'm no expert but it does appear the lack of 5v is a major part of the problem and it appears to go back to pin 3 of IC1 which should be around 4.5v with incorrect voltages your FET's will be mis- biased giving incorrect resistances and no phasing you should have around 2v on the gate and 5v on the D and S from your readings your's will be 2.4 on the D and S


and just to double check are you reading your IC pins like this:

[duck_arse]

mcknib - thanks, I was just a little unsure of the completeness of that dia. carry on.

also, I was about to say a 5V1 zener should not be reading 2V4, so something is pulling that down. but now mcknib has typed some, I don't need to. also, carry on.

[mcknib]

No please do DA you're definitely a man of greater knowledge than me + I'm off to work sadly

[frogman]

Mcknib, you were right about the transistors voltages being off. I'm posting them for the sake of documentation.

Q1-Q4 =
D - 2.3V
G - 2.3V
S - 1.5V

Q5 =
D - .6V
G - 1.7V
S - 2.3V

I did notice that the trim pot on my pedal does not change impedance or voltage when adjusted. Tried every combination of the 3 pins. Is there a certain spot that the trim pot should be for the pedal to function right?

You've got a distinct lack of 5v where it should be if you compare yours to GGG's D2 - 5v1 zener should give you 5v ish

I'm no expert but it does appear the lack of 5v is a major part of the problem and it appears to go back to pin 3 of IC1 which should be around 4.5v with incorrect voltages your FET's will be mis- biased giving incorrect resistances and no phasing you should have around 2v on the gate and 5v on the D and S from your readings your's will be 2.4 on the D and S

The way you worded that makes me think the problem is somewhere in the power supply. Is this an accurate assumption? To diagnose the problem do I need to remove individual components and test them or is there an easier way?

[mcknib]

Your power supply seems to be fine at 9.1v going in

Just for clarification use the GGG schematic and post voltages using their IC and Q numbers i.e your IC5 seems to be the LFO which is IC6 on their schematic Q1 to 4 would be the 2N5952's pins D, G, S and Q5 the 2N4125 pnp which would have C,B,E pins.

Do a continuity check on the zener to make sure it is the zener one side connects to ground and the other side should connect to pin 3 of the trimmer R22 in the GGG schem and to the + side of C6

[duck_arse]

apropo of nothing mutch: there was a recent discussion of why people hate tantalum caps hereabouts. one reason was they go short across supply lines, a bad thing. looking at yr first photo of the board shows a tantalum cap, right where a bypass for the Vref might be fitted. and, as you're seeming to be having some something dragging down your Vref voltage, like a short or low resistance where it shouldn't be ................

as they say, just sayin'.

[frogman]

Your power supply seems to be fine at 9.1v going in

Just for clarification use the GGG schematic and post voltages using their IC and Q numbers i.e your IC5 seems to be the LFO which is IC6 on their schematic Q1 to 4 would be the 2N5952's pins D, G, S and Q5 the 2N4125 pnp which would have C,B,E pins.

Do a continuity check on the zener to make sure it is the zener one side connects to ground and the other side should connect to pin 3 of the trimmer R22 in the GGG schem and to the + side of C6

The D2 didnt pass the continuity test. I am also getting 2v across the trimpot instead of 3v, so I am going to try swapping out one of those and a few capacitors/resistors. I want to get everything at once, but cant find a suitable D2.

From what i've found it needs to be a Zener diode that is 5.1V/500mW/sot-23(signifies the 3 pins)

I found some forums saying that the 500mW is important for it phasing correctly.

http://www.diystompboxes.com/smfforum/index.php?topic=22174.0

I could only find them with 300/350mW. I looked over mouser, digikey and the internet in general and found nothing. Could anyone provide me with a link or part #?

[mcknib]

300 / 350 mW would be no problem in a 9v circuit, power dissipation or rating is a rough guide to how much power a component can take before it may fail or heat up dissapiting power it's not exact as far as I know.

When you say D2 didn't pass the continuity test do you mean there is no electrical connection from D2 to the 10uF tantalum capacitor + side (C6 in the GGG schematic) and pin 3 of the trimmer?

I would advise checking that cap first as duck_arse said rather than start pulling components out.

It may be an idea to post a complete picture of both sides of the pcb.

[frogman]

300 / 350 mW would be no problem in a 9v circuit, power dissipation or rating is a rough guide to how much power a component can take before it may fail or heat up dissapiting power it's not exact as far as I know.

When you say D2 didn't pass the continuity test do you mean there is no electrical connection from D2 to the 10uF tantalum capacitor + side (C6 in the GGG schematic) and pin 3 of the trimmer?

I would advise checking that cap first as duck_arse said rather than start pulling components out.

It may be an idea to post a complete picture of both sides of the pcb.

Forget the continuity remark in my last post. The grounded pin of the diode is continuous with the + side of the 10uf tantalum capacitor and grounded pin of the trimpot. The - side of the 10uf is continuous with the other (non-grounded) side of the trimpot and other (non-grounded) pin of the diode.

Upon closer inspection, the 10uf appears to have been resoldered back in place. Not to jump the gun, but would turning it around or using another aluminum electrolytic capacitor in the right polarity configuration be the next step?

[mcknib]

Yes I'd try that any 10uF electro as you say in the correct polarity will do fortunately it'll be through hole so a quick one to try especially if it looks like it's been messed with.

Hopefully that'll be what's pulling your voltage down

[duck_arse]

DON'T turn it around. it is polarised for a reason, and it only goes in one way. it doesn't work better the other way round, it will melt/burn/go hoverboard on yo ass.

try pulling that cap out, and you can test for correct volts without it if you don't have a replacement at hand. or use a 4u7, or a 22uF, or a 33, 47, 100uF if you have one near. the value is not critical, same as the voltage of the zener is not critical, it just needs to be enough for your fets. you can go one or two voltage rating higher, again if that's all you can find. so, m a y b e 4V7 would do, or 5V1 or 5V6 or 6V2.