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.