Where can I find Peavey 87478 ICs for a Phasor circuit?

[QdB]

U12 is the phase LFO (with U11). it is a low freq oscillator, yes. The LFO runs all the time, the phasor is switched in and out of the signal path. So turning phase on and off won't affect U12. Note that LFO runs through R64 to all the pins 5 in the bucket brigade of 478s. Got that LFO on each pin 5?

There is no LFO at U12 pin6 nor at of the pin 5s on the bucket brigade 478s.

At the end of channel A is U17, which is a buffer to drive the bucket brigade. Got signal ther? With phasor on, look at the output of each 478 along the brigade. Pin 6 in each case. Is the audio signal moving along the chain?

I am not sure how to check for audio signal in the circuit. Looks like maybe you need to use an oscilloscope? I don't own, or have access to an oscilloscope so if that is the case, I will have to try to track one down.

I did try to trace audio signal with an improvised audio probe but that proved to be futile as all that I could detect was 60 cycle hum

[QdB]

I reworked my audio probe.

I got signal as expected through the treble pot, bass pot, U15 at pin 6, U16 at pin 6, C73, Post pot. There was signal on one end of R106 but not on the other.

Then I found out the hard way, don't probe pin 2 on U17.

Then I found no signal at U17 pin 1 or pin 7, no signal on any pin 6 of the the bucket brigade ICs and also no signal at C41.

[QdB]

rookie strikes again

so it has been a few days since I tried to tackle this thing.


I got signal as expected through the treble pot, bass pot, U15 at pin 6, U16 at pin 6, C73, right leg of the Post pot...

And My newbie-ness reared its ugly head.

Initially (a few days ago) when I followed the signal to the post pot I noticed that I was only getting signal on one leg. That shouldn't work.
I DID have the post turn all the way down.
So today, I gave it a just little bit of volume, enough to hear the tone that I was passing in, in the connected speaker cab. Checked with the audio probe again. still couldn't hear anything but I figured that was due to me being next to the cab passing the tone. So I turned up the probe amp, and checked the second leg of the post pot and whaddya know, the signal was there. Which, when I look at it now should only be common sense.

the signal in the second leg of the post pot was very faint since I had it turned down very low.

I looked at the schematic again, it seems very clear to me now , that U17, that Enzo had asked me to check for signal, was never going to show any signal since it was in line after the post pot and the post pot was turned all the way down.

I rechecked U17 pin 7 and all of the pin 6s on all of the 478s in the bucket brigade, and lo and behold, with the post pot turned up just a little, there is a signal, faint but clear, all the way through.

Look at U9, see the little 2uf cap at the output, C41?

C41 is also passing signal.

I also double checked pin6 on U12 which should be LFO'ing, it is not LFO'ing, it sits at a steady -14.2Vdc and 0Vac.

So, My last 2 post entries were entirely incorrect due to me being a greenhorn. Any next steps are greatly appreciated.

TL;DR -
There is signal all the way through the U17, the bucket brigade, and C41.
U12 is not LFO'ing but steadily sitting at very near the negative rail voltage.

[Enzo]

You will never get any phasing without the LFO. U11 and U12 are the LFO oscillator. Note the output of U12 feeds back to the input of U11.

You now have the rest of the amp working? Pick a 478 you know is working, and substitute it into U12. Does the LFO wake up? At this point we don;t care if audio passes anywhere else, we are ONLY looking for the missing LFO. If that makes no change, put the U12 back and sub the good one into U11. Any help there? if you feel strong, sub both of them with two ICs.

Also I also suspicious of the two little tantalum caps.

[Enzo]

You will never get any phasing without the LFO. U11 and U12 are the LFO oscillator. Note the output of U12 feeds back to the input of U11.

You now have the rest of the amp working? Pick a 478 you know is working, and substitute it into U12. Does the LFO wake up? At this point we don;t care if audio passes anywhere else, we are ONLY looking for the missing LFO. If that makes no change, put the U12 back and sub the good one into U11. Any help there? if you feel strong, sub both of them with two ICs.

Also I also suspicious of the two little tantalum caps.

[QdB]

Pick a 478 you know is working, and substitute it into U12. Does the LFO wake up?

Still no LFO

If that makes no change, put the U12 back and sub the good one into U11. Any help there?

Still nothing

Also I also suspicious of the two little tantalum caps.

C46 & C49?
Can the tantalum caps be replaced by electrolytic caps? That is all that I have on hand at the moment. I suspect that tantalum caps are required, since there are very few spots on the schematic that explicitly specify tantalum.

If that is the case, I will have to order some in and report back.

[Enzo]

Electrolytics should be fine, I never stocked tants.

[QdB]

Electrolytics should be fine

I replaced C46 with an electrolytic 1uF 50v and C49 with 10uF 50v. Still the same results. No LFO.

[Enzo]

Did we swap good ICs into BOTH 11 and 12?

[QdB]

Did we swap good ICs into BOTH 11 and 12?

Hey Enzo, I did swap out both U11 and U12.

Baffling, no?

[Enzo]

Oh well, just the off chance both were bad...

[R.G.]

Here is a possible step sideways on the ICs. The CA3094 was a CA3080 and an emitter follower in an 8-pin DIP. It was used in other phasers, notably the Small Stone. The LM13700 dual OTA is a 16 pin IC that contains the equivalent of two CA3094s in one chip, so two LM13700s could replace all four of the 8-pins on that board.,

The problem is the package of course. the pins on the 13700 are nowhere near being able to plug into 3094 holes, so the only effective way would be to mount it upside down, dead bug style, and run wires to the matching holes on the PCB. If you can get real (not counterfeit) 3094s, mounting would be much easier.

[QdB]

Does this help at all?

I don't know if this helps or has any meaning at all but I measured all of the Vac and Vdc on all of the pins of U11 and U12. The only one that stands out to me is the Vdc on pin 1 of U12, not only because it is a bit lower, but also because while I taking the measurement, it started at < -14 Vdc then steadily increased until it leveled off at around -12 Vdc, as if taking the measurement was charging(discharging?) a cap somewhere.

	U11		U12
pin #	Vac	Vdc	Vac	Vdc
1	0	-13.31	0	-11.98
2	0	-13.43	0	-13.44
3	0	-13.37	0	-13.38
4-	0	-14.78	0	-14.79
5	0	-14.24	0	-14.25
6	0	-13.31	0	-13.44
7+	0	15.07	0	15.06
8	0	15.07	0	15.06

[Enzo]

What strikes me is that all your voltages are slammed over to the negative rail.

Any chance your RATE pot is open? Power off, measure resistance from pin 5 to 6 of U12.. WIth rate maxed you should see about 2k or so. WIth rate at minimum, add the value of the pot to that.

With both ICs removed from their sockets, power up and take those readings. Pins 8 and 4 should have power supply, but to those other voltages go away?

[Enzo]

What strikes me is that all your voltages are slammed over to the negative rail.

Any chance your RATE pot is open? Power off, measure resistance from pin 5 to 6 of U12.. WIth rate maxed you should see about 2k or so. WIth rate at minimum, add the value of the pot to that.

With both ICs removed from their sockets, power up and take those readings. Pins 8 and 4 should have power supply, but to those other voltages go away?