Phaser woes on Peavey Classic VTX

[woodmedic]

Also if I find a replacement and get that working, it’s not necessarily going to fix the sweeping part is it?

[dmeek]

To test the control circuit you could connect 2 red LEDs as in the schematic at U5 pin 13 and 14. You will be able to see the brightness modulate if everything is working.
You don't need to disconnect the LDRs since they are open anyway.

[woodmedic]

Interesting. I don’t have any LED’s laying around. Anyway to test that with a DVM? If not I might just wait for my parts to come in and see what happens at that point.

[dmeek]

The output of the LFO is at U5 pin 8. There should be a triangle wave -10V to +10V with the pull switch in. Hard to see with a DVM.

Here is a Vactrol I took apart years ago. The more difficult thing is to find a suitable replacement LED. I guess I never did for this one.

https://music-electronics-forum.com/...1&d=1582982567

[woodmedic]

I have access to an old oscilloscope but I have never used one so not sure how to set it up. Looking closer at the amp this morning, I can see that the LFO's have been changed before as they have VTL5C3 on them. I am a little worried about what someone else has done in here. I have new LFO's, some new IC's and some 2.2/50 np caps to replace the one. I am going to just do some point to point continuity testing and make sure there are no open traces any where in that circuit. You guys are awesome and I greatly appreciate your help so far. I feel like I will get this problem corrected with your support. Thanks again and keep the ideas coming!

[Enzo]

A nitpick:

LFO means low frequency oscillator. In this case it refers to the circuit that makes the sweep signal to control the Vactrols. The Vactrols themselves are not called LFOs, the LFO is what DRIVES them..Vactrols would be called opto-couplers or opto-isolators. A lot of us shorten that to opto for discussion.

As to seeing it with a DVM, I think the issue is usually is it oscillating or not. So on a meter we either see a steady DC, or we see the meter moving around unable to settle.

[woodmedic]

Yes I am still trying to wrap my head around what drives what. I just replaced U4 and U5 with new TL074 ic's. There was no change. I measured U5 pin8 with the knob pushed in and measured -.03 and it was steady.

[dmeek]

Post a photo of the front of your oscilloscope. I can tell you how to set it up.

[woodmedic]

Don't laugh!

[dmeek]

I started off with a scope a lot worse than that (Eico 425) This one is 7MHz bandwidth - more than enough for audio. You might need to spray the pots with cleaner once in a while.

Check U5 pin 8, you should see a triangle wave with the Range knob in and centered.

https://music-electronics-forum.com/...1&d=1583008057

[woodmedic]

I’m seeing nothing but a flat line. If I touch a spot where I know there is voltage, the line will go up or down I assume with negative voltage but I see nothing on U5pin 8.

[dmeek]

It should be oscillating. C47 or C62 could be bad. Check R89 22K and R90 33k.

[woodmedic]

C47 has a resistor that measures around 13k instead of a cap. That’s weird. Also I noticed that C37 seems to have blown up at some point. R89 and R90 check ok. C62 has been changed to a polarized cap. I have new NP cap on the way to replace it.

[dmeek]

Leave C47 disconnected, it should run without it. If not, what are the DC voltages on U7 pins 5,6 and 7 and U5 pins 8,9 and 10?

[woodmedic]

U7 pin5 -.15 ,pin 6 0, pin 7 .09.
U5 pin 8 -.66, pin 9 0, pin 10 is also 0. Cutting the cap loose at C47 did cause a thick wave form on the scope but not what I would call a triangle wave.