Phaser woes on Peavey Classic VTX

[woodmedic]

I have signal at U5 pins 1 and 7, and U7 pins 1,7,8 and 14

[dmeek]

I'm totally confused now, except that C47 seems to be bad. Here's an extract from the manual. The range knob has two functions: Pressed in it is a auto-sweeping rate control, pulled out it is a manual sweep control.

Read and study:

https://music-electronics-forum.com/...1&d=1583084665

[woodmedic]

Right now with a jumper across C47, the paser is kind of working the same with the knob pushed in or pulled out. With it pulled out it seems to be working like it is supposed to. With it pushed in it is almost the same as pulled out. There is no sweeping action unless I turn the knob. Im sorry to be so confusing. This is a little hard to explain. Nothing that I do at this point causes that phaser to sweep back and forth unless I manually do it with the knob whether it is in or out.

[dmeek]

Just remove C47, don't jumper it.

Looks like your LDRs are OK.

[woodmedic]

Removed the jumper and nothing really changes. Works fine pulled out but won’t sweep on its own pushed in.

[dmeek]

Are you seeing a waveform at U5 pin 8?

[woodmedic]

Yes

[dmeek]

There should be a lower amplitude waveform on U5 pins 12 and 14, if not check R99 4.7K and R104 47K.

[woodmedic]

There are. Good to that point.

[dmeek]

U5 pin 14 is the LED driver. at that point in manual mode the Range pot should vary the voltage from 0V to+4V. In auto mode there should be a waveform.
The only other thing I can think of is maybe the pull switch is slightly conductive even when open. Lift one end of R87 to eliminate the switch.

[g1]

Are you seeing a waveform at U5 pin 8?

Yes

How about a pic of the waveform here. I think earlier you said you did not have it, now you said yes. Let's see what you have.
It's possible there are triggering issues or different types of waveforms being discussed.

[Enzo]

Way back like post #41 or thereabouts, you told me you had a waveform at the output of the LFO, and that you also had it on teh LEDs on the optos. I then suggested a way to check the operation of the photocells. Plus I suggested looking for guitar signal at the output pins of U4 and U5 upper right on the schematic. You returned with waveforms back in the LFO.

Please clarify.

[woodmedic]

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?

Placing the leads of my meter on each led one at a time in diode test mode a get a beep in one direction and not the other. I also have a wave form present at both legs on each led. Measuring across r61 r62 r63 and r64 i am getting 3.7 to 3.8k on all 4 and it is not fluctuating. I have guitar signal at U4 pin 7, U4pin 1, U5 pin 7 and U5 pin 1.

[woodmedic]

Here is a picture of the waveform.

[dmeek]

Do you remember the Volts/Div and Time/Div settings. Or can you take another photo including the controls so we can see the settings?

Here is a simplified schematic of the oscillator. It's a basic textbook design except that U7-pin6 would normally be grounded. The addition of C47 and R88
slows the square wave rise and fall times so it doesn't cause noise in the power supply. The triangle wave feeds into the diode matrix which is a simple
lin to log converter, then to the LED driver op-amp.

https://music-electronics-forum.com/...1&d=1583193310