Phaser woes on Peavey Classic VTX

[g1]

I can see that the LFO's have been changed before as they have VTL5C3 on them.

For clarity, VTL5C3 has 4 leads. VTL5C3/2 has 5 leads. Which do you have in your unit?

[woodmedic]

It has the VTL5C3/2

[dmeek]

Something's wrong with U7. Try replacing it with another 4558 or TL072 or other dual op-amp that you have.

[woodmedic]

I don’t have any other. I can swap it with another inside the amp. I have actually done that already. I’ll try swapping with another one and report back.

[dmeek]

Make sure you have +15V on U7 pin 8 and -15V on pin 4

[woodmedic]

Swapped it with U2. Pin 5 was -.2 pin 6 was 0 pin 7 was .2.

[woodmedic]

Yes I have both 15 on p8 and -15 on p4.

[dmeek]

Something is not making sense here. There should be a full amplitude square wave on U7 p7 and a +/- 10V triangle wave on U5 p8. There must be more than one fault which is confusing things.
Leave C47 out, it's not needed for now. Check that the ICs aren't in backwards. If they ever were in backwards they are burnt, throw them out. Check for solder bridges. Maybe if you can post a photo of
this section of the circuit board it might help. I notice the voltages have changed, re-check the 6 readings.

[woodmedic]

Ok so I’ve been messing with it some more and now I am getting some wave forms but they change depending on where the push pull knob is located. They stretch when it is counter clockwise and the opposite for clockwise. It all depends on the position of the range switch/knob. I’m sorry for all of the trouble but could you tell me again where things should be set on the amp for this battery of tests please? I’m tired tonight so I’ll probably stop until tomorrow. Thanks again for all of the time and effort you are giving towards this project. Please don’t give up on me.

[woodmedic]

[Enzo]

The push switch switch should turn the sweep off and on. When it is on, the sweep oscillator works, that is what you are seeing. The sweep rate determines the speed of the phasor. So turn the control one way and it slows down - stretches the waveform. Turn it the other way, ot speeds up - compressing the waveform. It sounds like it is acting like it should. So U5 pin 14 ought to have that active waveform driving the two LEDs there on the schematic. These LEDs are the ones inside the Vactrols - LDR1, LDR2.

look at LDR1 and LDR2. The diodes withion are the same ones down in the other part of the schematic. They are duplicated in this part of the schematic just as reference. Note the ABCD on them and also on the diodes in the oscillator.

I would check the L:ED in each Vactrol with your meter diode test. And then with it running see if you can detect the waveform on those LEDs.

Note the Vactrols are dual - two photo resistors on one LED. And a 1meg resistor in parallel with each photoresistor. With it running, measure resistance across each 1meg resistor and see if it seems to be varying with the oscillator. Also look at the five op amps just above. Parts of U4 and U5. Got signal at each output pin?

[dmeek]

I agree, from your description it sounds like the oscillator is working. It only runs when the range switch is pushed in and turning the
range knob changes the frequency which is the stretching you see on the scope.


Here is a simple test you can do that assumes one LED is good and the other is open.

1 Set Range control to center and pushed in, Depth control on full.
2 Play music or white noise from an FM radio through the amp so you can hear any phasing effect.
3 Short the LED leads on one opto and then the other opto.

You should hear a weak phasing when shorting one of the LEDs. The one you are shorting is the bad one.

The phasing effect will be weak since there will be only 2 stages working. Like an MXR Phase 45

[woodmedic]

With the rate knob pushed in, I am seeing the wave form on U5 pin 14 and on both LED's (LDR1 and LDR2). This signal is present now that I have cut C47. If I jumper it back together the signal goes away. The sweep is not varying however. The waveform changes only when I turn the knob back and forth. I then shorted the LED in LDR2 and with that shorted I could hear the tone change but it still isnt sweeping. The same holds true if I jump across the LED leads on LDR1. I think that both of the Vactrols have a problem. With the LED jumered on either one, the phase sweep will work in manual mode with the knob pulled out. It just wont sweep by itself with the knob pushed in.

[woodmedic]

"Also look at the five op amps just above. Parts of U4 and U5. Got signal at each output pin?"

What signal should I have at these? Regular guitar signal?

[Enzo]

The way the phasor works it it takes the guitar signal, what we call the dry signal, and delays it, then mixes that back in with the dry. The difference in timing causes the phased effect. SO the signal at each step ought to look more or less normal, your scope won't know that each stage is a trifle delayed from the previous one.