Help with Ibanez PT-909

[david1991ross]

I have an old Ibanez PT-909 with the square switch in my possession but it is not working. It powers on but it doesn't phase. I can hear the effect when I turn the width knob but it's very subtle. I was told to play with the trim knobs which bias the circuit but that didn't seem to have an effect other than varying the tonality of the phasing but not the intensity. The only way I can get it to phase is by turning the width knob back and forth. I'm not sure what needs to happen for this to sound like it should. Whether it be replacing components or something else.

Here is the schematic: https://www.tonehome.de/app/download/8115651/Ibanez_PT900_schematic.gif

The link is a .PDF file. What component would need replacing? Does anyone know where to start with this or what the problem might be?

Edit: I know this isn't a digital circuit but I didn't know where else to post this.

[Rob Strand]

I was told to play with the trim knobs which bias the circuit but that didn't seem to have an effect other than varying the tonality of the phasing but not the intensity

It's a good start.  (Unfortunately the side-effect is you have to reset the pot.)

The fact it didn't work points to a problem with the Low Frequency Oscillator (LFO).
That's the part of the circuit around IC104, C113, C114 the Width pot, the Speed-pot.
There two common problems:
- If the capacitors are electrolytics, the capacitors C113 and C114 could dried out.
   If it's tantalum it's probably not the issue.
- the other is either the Width pot or Speed-pot has become faulty, or a wire has come off.
For this case: set the two pots to maximum and with a wire short points 12 and 8 on the Width pot,
then points 7 and 9 on the Speed pot.  If it comes to life you know where the problem is.
Then you can try cleaning the pots.

Make sure that schematic matches your board.
If you have a square switch, which is a two-contact push-button it probably is.  I remember there's a few variants of phasers which look quite similar from that era.
[EDIT: I think the issue was the round switch era.]

[Mark Hammer]

This thread should be on the main "Build your own stompbox" forum, where most repair requests show up.

That part aside, Rob's instincts are correct I think.  Yoou indicate you can produce phasing by playing with the bias trimmer.  That implies that:
a) there is nothing wrong with the bias trimmer
b) there is nothing wrong with the FETs the bias trimmer affects
c) there is nothing wrong with switching FET Q105, or indeed any part in the path between the phase shift stages and pin 5 of IC101 - the mixing node.

Which leaves the LFO not doing it's job.  And, given the age of the pedal in question, Rob's guess that maybe one or both of the electrolytic caps used in the LFO have dried out, seems pretty spot on, if you ask me.

Given that it may be burdensome to get replacements of a suitable size, here's what you can do for the time being, to confirm Rob's hypothesis.
- desolder and remove C113 and 114.
- find the largest value non-polarized cap you have sitting around and install it between pins 1 and 2 of that chip
- set the Speed control to the slowest setting.

If you have replaced the pair of 47uf electrolytics (that produce the equivalent of a 23uf non-polarized cap) with, say, a 470nf plastic or ceramic cap, the slowest speed produced would be about 50x faster than with the dual 47uf, so you'd want to keep the speed as low as possible to be able to actually hear a sweep.

Once confirmed that the dual electros are the culprits, you can replace them.  The schematic indicates they have a 10V rating.  GIven how difficult it often is to get caps of that value with anything lower than a 16V rating, you have the freedom to shop around for anything that fits the available space.  Caps of that era tended to be fatter for any given value/rating, so you shouldn't have much difficulty.

[david1991ross]

I got it to work. It was the connection on the speed knob that was the problem. I resoldered the joint and it came alive. Big thank you to the both of you for the assistance! It sounds awesome.

[Ritalin1000mg]

Hi,

I see this is quite an old thread, but I have a very similar problem so I figured it made sense to post my question here.

To recap:

I own an Ibanez PT-909 v6.
It powers on, but no sweep.
When I turn the width knob back and forth, the unit kinda sorta phases. Turning the other knobs doesn't affect the sound at all.

I tried playing with the trim pots. No luck.
I checked the connections to the width and speed pots, they look fine.
I tried shorting points 12 and 8 on the Width pot, then points 7 and 9 on the Speed pot. That didn't do anything.
I replaced C113 and 114 with new 47uF electrolytic caps rated 16V. It still doesn't work.

And now... I don't have any idea what to try next.

This is my first post on this forum and the first time I try my hand at fixing a pedal. I know next to nothing about how any of it works. I didn't even own a soldering iron 3 days ago and I bought it specifically for this.

Also, I don't even like phaser, I just like things to work.

Can anybody help?

[ElectricDruid]

Welcome Ritalin!

Have you got a multimeter? Could you test the resistance of a few things, or give us voltages at certain points in the circuit?

If the answer to all of the above is "No", then you either need to decide whether you care enough about this thing to want to shell out for more tools and learn a new skill, or you need to find someone who's already equipped and experienced. For a one-off fix, maybe it's not worth it. If this is something you want to keep doing, it's great fun once you get into it - there's a whole world of mods and DIY pedals to get into, and a lot of it is stuff that you can't get anywhere else.

[idy]

One thing to know: just because a solder joint "looks good" means nothing. So many times a joint looks good but was not done perfectly, a "cold solder" that is loose inside. So carefully reflowing is good. Too much heat and you can cook a pot or other component though. Worth while reviewing solder technique, what to look for.

Reviving a pedal that just had a cold solder joint is one of the pleasant "rites of passage." There are many less pleasant ones....

After that it will be a matter of locating the LFO, finding out what is not working. Will require a meter.

[idy]

There's not a lot to go wrong with that LFO. You need to confirm with a meter that the pot really is connected where it needs to be, and that that IC 104 is good and getting power. You have already replaced the two caps...of course you have to make sure your new soldering was good...

[Ritalin1000mg]

Welcome Ritalin!

Have you got a multimeter? Could you test the resistance of a few things, or give us voltages at certain points in the circuit?

Thank you for your replies, I do feel welcome!

Yes! I have a multimeter and I can take any measurements necessary as long as I know where to put the probes.

And yes! That did come out weird and I apologize.

One thing to know: just because a solder joint "looks good" means nothing. So many times a joint looks good but was not done perfectly, a "cold solder" that is loose inside. So carefully reflowing is good. Too much heat and you can cook a pot or other component though. Worth while reviewing solder technique, what to look for.

Reviving a pedal that just had a cold solder joint is one of the pleasant "rites of passage." There are many less pleasant ones....

I read your reply and decided that, first things first, I should watch a bunch of tutorials on how to fix a cold solder.

So I did. Then I ordered flux. It should come in tomorrow.

Doing this I realized someone else probably had a go at fixing this thing before me. There was flux residue all over the solder joints between the board and the wires that go to the pots. So I cleaned that, because I saw somewhere that it could cause problems. Can it?

Then I started looking for bad joints while waiting for Amazon. I found one that looks really crappy. It's on the negative leg of C125. It looks dull and rough.

However, you guys seem to think the problem's in the LFO. And if I can read a schematic (and I absolutely cannot), C125 is in the power supply part of the thing. Do you think that could be it anyway?

You need to confirm with a meter that the pot really is connected where it needs to be

I don't know how to do that. And my meter doesn't have that feature where it beeps to signal continuity.

However, when I tried shorting points 12 and 8 on the Width pot, then points 7 and 9 on the Speed pot, I also tried doing that on the board end of those wires, on the joints marked "orange" and "purple", then "green" and "brown". It didn't do anything, which makes me think the problem lies somewhere else. Am I correct in thinking that?

Also, when I say I "shorted", I just touched both ends of a paper clip to the connections. Is that good enough for a test?

of course you have to make sure your new soldering was good...

About my soldering on those caps, I think it's ok. The joints look regular and shiny, about 2-3mm in diameter, quite similar to those around them that I didn't do.

You need to confirm with a meter  [...] that that IC 104 is good and getting power.

How do I do that?

If the answer to all of the above is "No", then you either need to decide whether you care enough about this thing to want to shell out for more tools and learn a new skill, or you need to find someone who's already equipped and experienced. For a one-off fix, maybe it's not worth it. If this is something you want to keep doing, it's great fun once you get into it - there's a whole world of mods and DIY pedals to get into, and a lot of it is stuff that you can't get anywhere else.

I definitely want to learn a new skill. As for shelling out for more tools, well... you gotta do what you gotta do. I'll just take it slow at first, see where it takes me.

I do have a lot (a lot) of ideas for mods and pedals and stuff. This project is a test for me to see if I actually enjoy doing everything involved.

Lastly, I really want to thank you guys again for your time, and effort, and patience. This is starting really well.

[idy]

Measuring volts on an IC: Set meter to DC volts, the right range for your power.
attach black lead to a good ground. Use red lead to check the pins around that IC.
Look at a picture of how IC pins go: You put the indentation or circle "up" and count counter clockwise from top left.
1. 8
2. 7
3. 6
4. 5

Dual opamps (like that one) have power to pin 8 (you should see around 9v there) and ground to 4. (you should see around 0v there.) The others...well, well be looking for oscillating, waves, up and down...

And meters to do a few things, like capacitance, AC, diode check (sound and fV) are pretty cheap and plentiful. Worth having more than one...fun...

[Ritalin1000mg]

I did that. Here's what I found:

1  2.06V
2  8.26V
3  3.33V
4  0V
5  3.34V
6  3.03V
7  8.28V
8  8.99V

And no oscillating. All voltages steady.

[ElectricDruid]

I did that. Here's what I found:

1  2.06V
2  8.26V
3  3.33V
4  0V
5  3.34V
6  3.03V
7  8.28V
8  8.99V

And no oscillating. All voltages steady.

Yeah, that doesn't look right. Pins 4 and 8 are where we expect, so the power supply is ok - +9V getting to the chip. But the rest of it isn't right and suggests that the LFO isn't working. Especially since none of it seems to be wobbling about, which is pretty much the essential function of an LFO!

The way the LFO works is that the op-amp on pins 5,6, with output on pin 7 jumps from 0V (or close) to 9V (or close) and then the op-amp on pins 1,2,3 charges either up or down to the voltage coming out of pin 7, so we get a triangle wave sloping first up then down then up. And that's what we should see on pin 1. The repeated switching from 0V to 9V on pin 7 should be pretty obvious too!

Turn the power off, turn the speed to fast, then check the resistance between pins 2 and 7. That's the speed pot. You should get close to 2.4K, the value of R127, since at the "fast" end the pot should be close to zero.

With the power still turned off, test the resistance between pins 1 and pin 3. That's the Width pot. You should get 100K.

Big differences from those results might suggest a bad pot (or I might have missed something obvious, happens too often ).

Good luck!

[Ritalin1000mg]

Turn the power off, turn the speed to fast, then check the resistance between pins 2 and 7. That's the speed pot. You should get close to 2.4K, the value of R127, since at the "fast" end the pot should be close to zero.

With the power still turned off, test the resistance between pins 1 and pin 3. That's the Width pot. You should get 100K.

The values I get are pretty spot on:

pin 2-7  2.38k
pin 1-3  95.4k

[duck_arse]

well sir, the 1N5231B at D104 should be pumping out about 5 V, as it is a 5V1 zener. pin 3 of the opamp is connecteced hard-to, and is only showing 3V something. if you can remove the IC from the board, and then test the CP-2 voltage, if it reads 5V1, replace the opamp. if it reads not 5V1, replace the zener.

[ElectricDruid]

The values I get are pretty spot on:

pin 2-7  2.38k
pin 1-3  95.4k

That's a good sign. That means that those connections to the pots are ok, and that the pots themselves seem to be behaving.

I agree with Duck's logic about the zener and the op-amp. Removing the dual op-amp IC104 is looking increasingly necessary, unfortunately.

[Ritalin1000mg]

Okay, I think I kinda got it.

Either the zener diode D104 or the IC is faulty. We know that because of the voltage on pin 3 that's too low.

But now that got me thinking.
As I said in a previous post, the solder joint on the ground side of C125 looked terrible. And as it happens, it's right next to D104 on the board. Like in the same "blob" of solder.

I just had to try reflowing. I mean... I bought the flux.

So I did. A half decent job even.

But it still doesn't work (I know it had to be expected, either there's a connection or there isn't).

Now my question is: are zeners prone to cooking ?
Maybe that crappy solder job on D104-C125 is the culprit after all? And someone cooked that diode while screwing up the soldering?

What do you guys think?

[Ritalin1000mg]

If my theory above is ludicrous, which it might very well be, how should I proceed?

I remove the IC. That I can do.

Then what?

How do I test the CP-2 voltage? Between where and where?

And then, what do I look for in a replacement part? The exact same reference?

If it's the zener, will any 5V1 do?

How about the op-amp? Does it have to be a MC1458?

And lastly, who sells these things? Do you know a reputable seller in Europe? I'm in France. Will I have to buy a billion of them or does that stuff sell in reasonable quantities?

(as I said, I'm new to all this)

[ElectricDruid]

How do I test the CP-2 voltage? Between where and where?

There are various options, but across the zener diode itself would be pretty simple.

And then, what do I look for in a replacement part? The exact same reference?

Mostly it doesn't have to be the *exact* reference. There are usually plenty of "works the same" equivalent parts.

If it's the zener, will any 5V1 do?

Yes, I'd have thought so. It's doing a fairly basic job!

How about the op-amp? Does it have to be a MC1458?

No. Any dual op-amp with the standard pin-out will work. There are some that are frequently used for LFOs because of their low current draw, like the TL022 or TL062. But any common thing will work.

And lastly, who sells these things? Do you know a reputable seller in Europe? I'm in France. Will I have to buy a billion of them or does that stuff sell in reasonable quantities?

I don't know of shops in France, but Germany is well-equipped:

https://www.banzaimusic.com
https://www.musikding.de/Musikding_1

HTH,
Tom

[duck_arse]

might be that your easiest test would be to remove the zener, and tack-solder in a 1k resistor in its place. this will form a divider with the R151 1k, and will/should produce V/2 at CP-2. if not V/2 at the opamp pin 3, then opamp bad [or something else we haven't thought of].

[Ritalin1000mg]

Alright! So I removed the op-amp.

I definitely would have gone duck_arse's easiest route but I just don't have a 1k resistor lying around. As I said, I'm new to this. I don't have anything lying around.

Then I measured the voltage across D104 and got 3.42V. The exact same as what I got on pin 3 of the op-amp before.

So I guess it's the zener.

I removed it. Soldered the IC back on. It was fun.

Now I'm on banzaimusic.com, and I found this:

https://www.banzaimusic.com/5-1V-Zener-Diode-0-5W.html

That would work, wouldn't it?

Well it's 14 cents apiece, and I need one. That's €8.95 for shipping 

I added a TL062CP, just in case.

62cts total...

I guess my question is, what would you guys get as a first-aid kit? So I can make those shipping fees worthwhile.

I see boards by "Electric Druid". Anything to do with you, ElectricDruid?