Roland jet phaser debug

Fender3D · August 21, 2015, 09:22:22 AM

Quote from: Luke51411 on August 21, 2015, 08:23:43 AM
How can I check it?

Simply lift it...

Luke51411 · August 21, 2015, 09:42:36 AM

Quote from: Fender3D on August 21, 2015, 09:22:22 AM
Quote from: Luke51411 on August 21, 2015, 08:23:43 AM
How can I check it?

Simply lift it...

In that case I'll probably just replace it.

anotherjim · August 21, 2015, 12:28:42 PM

I wouldn't get carried away by what the fet's may be doing. If they short to ground the amp + input is ground (the reference here) and the amp is unity gain inverting - should pass signal. If the fet is open circuit, the phase shift cap gives signal to the + amp input. Same signal on + and - inputs of an op-amp gives unity non-inverting gain - again should pass signal. If the fets gate is blown through to the channel, you might see the gate voltage putting a nasty offset on the opamp outputs or letting the LFO swing through. The signal level should be weak compared to other circuit types because fet phasers don't like a lot of peak-peak signal swing, but it should be consistent throughout.

LM324's are so cheap compared to the value of a working Jet-phaser, I'd change IC1. If that's not the fault, you still have a good amp you took out.

Fender3D · August 21, 2015, 01:08:18 PM

Quote from: anotherjim on August 21, 2015, 12:28:42 PM
...should pass signal.

Yes,
but you'd check with an o-scope, not a cheap signal probe, if you really wanna know what's going on...

Anyway, I agree with Jim, in 3 days you could have swapped more than 3 simply ICs...

Luke51411 · August 21, 2015, 01:47:12 PM

Quote from: Fender3D on August 21, 2015, 01:08:18 PM
Quote from: anotherjim on August 21, 2015, 12:28:42 PM
...should pass signal.

Yes,
but you'd check with an o-scope, not a cheap signal probe, if you really wanna know what's going on...

Anyway, I agree with Jim, in 3 days you could have swapped more than 3 simply ICs...

Ok I'll just go that route then. I only have 1 lm324 on hand so I need to order a few more. The ICs that are in there are LM2902 which from everything I've gathered are the same thing as LM324. My dad has an o-scope that I haven't used yet but this seems like a good opportunity to do so.

Luke51411 · August 21, 2015, 02:44:12 PM

I ordered some more ICs. I'll probably put this project on hold until they come in. I have a few other things I want to work on in the mean time anyway.

Luke51411 · August 26, 2015, 06:40:41 PM

Fixed it, it was IC2

armdnrdy · August 27, 2015, 10:23:53 AM

Good to hear!

This troubleshooting was a bit misleading at times.

It seems that the signal was being fed back though the phase stage resistors giving false readings of signal where signal wasn't passing.

Looking back now, it would have been a good idea to check the circuit with the Resonance pot fully CCW, and lift one side of R23. (phase signal mixing resistor)

We learn...we move on to the next one!

Luke51411 · August 27, 2015, 10:42:00 AM

Quote from: armdnrdy on August 27, 2015, 10:23:53 AM
Good to hear!

This troubleshooting was a bit misleading at times.

It seems that the signal was being fed back though the phase stage resistors giving false readings of signal where signal wasn't passing.

Looking back now, it would have been a good idea to check the circuit with the Resonance pot fully CCW, and lift one side of R23. (phase signal mixing resistor)

We learn...we move on to the next one!

Yeah I didn't even think to dial in the controls other than volume, This is a pretty cool sounding phaser! After playing with it a bit I think I need to dial in the bias trimmer just a bit more but it sounds pretty good!
Up next: A Mutron Phasor ii with a drifting rate but I'll post a new thread for that when I get around to the full teardown.

anotherjim · August 27, 2015, 12:03:57 PM

Just goes to show eh?

The op-amps have a real, hard, ground reference. I think this hid the chip failure. We expected ins and outs close to ground and they were! Probably, if it were a single supply design with a voltage divider providing reference, the chip fail might have been obvious.

As I said earlier though, it really bothers me that this design has no reverse power protection. It's really easy to try to snap a battery in the wrong way around and "Phttt" - new op-amps please!
If it were mine, there would be a medium power Schottky diode added in series with each battery connector.

I do have a set of K30A jfets from a long ago scrapped AP-7. They all test good, so one day I'll build one. As said, it's a very nice Phaser!

Luke51411 · August 27, 2015, 12:37:49 PM

Quote from: anotherjim on August 27, 2015, 12:03:57 PM

I do have a set of K30A jfets from a long ago scrapped AP-7. They all test good, so one day I'll build one. As said, it's a very nice Phaser!

Nice! If like to build one sometime but with modern features of polarity protection and use with a power supply

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Roland jet phaser debug