Ibanez PT-9 Phaser LFO Failure

[clarisso11]

Hi everyone,

I've been working on repairing an old Ibanez PT9 phaser (service manual attached). The unit had leaking capacitors and damaged tracks, which have been repaired and continuity of the repaired tracks confirmed.
I think I have isolated the issue to the LFO circuit, but can't figure out what is causing the problem.

Symptoms:

• Dry signal passes. When effect engaged, pitch shifts slightly, but no flanging
• Controls don't seem to have an effect
• Trim pots don't have an effect
• No audible clock noise in the circuit, using an audio probe

Tried so far:

• Went through checkpoints on schematic:
• CP1: 9V
• CP2: 4V (should be 4.5V)
• CP3: 1.58V (should be 4.3V)
• CP4,5: Expected sine wave signal
• CP6: No signal
• CP7, 8, 10: Fixed sine wave only
• CP9: 7.46V when on, 0.69V when off
• Replaced IC101, no change
• Replaced IC101, IC102, IC103 all together, no change
• Pulled and tested Q101-104, all good
• Tested electrolytic capacitors with ESR meter, all good

I'm quite puzzled by the 1.58V at CP3, I suspect that's why the LFO is failing; just can't figure out what's causing this.
I measure 4V and then 1.58V across C106, but I'm not sure if that's to be expected, and the capacitor measures fine..

Any help would be greatly appreciated!


Ibanez_PT9_service_manual.pdf

[Helmholtz]

Check 4.3V zener D101 and parallel cap C120.

[dmeek]

Check R143, make sure it is 470 ohms

[clarisso11]

Thanks both!

Check 4.3V zener D101 and parallel cap C120.

C120 measures fine, I replaced this with a new 47uf capacitor (double-checked orientation).
D101 looks to be undamaged, component tester recognizes it as a diode (Vf = 0.7V, C = 0F, Ir = 11uA)

[clarisso11]

Check R143, make sure it is 470 ohms

I pulled R143, measures as 460 Ohms

[Helmholtz]

See if 9V reaches the upper end of R143.

The zener voltage can be measured out of circuit by wiring a 1k resistor in series and connecting it to a 9V battery, positive to cathode.

If the zener is good something must be shunting it in the circuit. With zener out measure resistance across C120.

[clarisso11]

See if 9V reaches the upper end of R143.

The zener voltage can be measured out of circuit by wiring a 1k resistor in series and connecting it to a 9V battery, positive to cathode.

If the zener is good something must be shunting it in the circuit. With zener out measure resistance across C120.

R143 is getting 9V, 1.58V at the other end.

I rigged up everything like you described, to test the zener voltage, using a fresh battery. I'm only getting 1.3V, should this be 4.3V with a working diode?

[clarisso11]

With the zener removed, resistance across C120 is about 6k ohms, with the pedal turned off.

[clarisso11]

I also measured resistance across C120 with the zener removed, with the pedal turned on, and that's 0 ohms (hope I didn't damage anything doing that, it only occurred to me afterwards that that might be a bad idea)

[Helmholtz]

I rigged up everything like you described, to test the zener voltage, using a fresh battery. I'm only getting 1.3V, should this be 4.3V with a working diode?

Yes, looks like a bad zener.

[clarisso11]

Yes, looks like a bad zener.

Thanks for you help finding that, very cool trick with the battery! I ordered a new 4.3V 0.5W Zener, will report back once it arrives.

[clarisso11]

Looks like this forum hasn't disappointed, once again! I swapped in the new zener diode, and the voltage is restored, plus the effect is back!

Thanks everyone for the great help!