Moog Opus 3 Multivibrator needs a kick start to start

[codystein9]

Im working on a Moog Opus 3 that Im having issues with. I recapped the whole machine and cleaned all the tar crap off. For some reason the multivibrator will not start up on power cycle. I found out I can couple my oscilloscope to the output and it seems like if there is a small spike when it connects, the oscillator starts up and the unit works flawlessly..until another power cycle. I replaced the 100pF caps thinking maybe it was something with the timing, but Im still getting the same thing.

Im mostly a tube amp guy, trying to expand my knowledge base and work with other stuff, this seems to be above my pay grade. Id love some where pointers.
schematic.pdf

[Enzo]

Hi

Um, multivibrator. I admit I am old and don't see so well, but where in the schematic is this, I don't see multivibrator. DO you mean the 2MHz oscillator? It has a couple 100pfcaps associated with it.

Just idle thinking. TO me +5v supply for TTL is canon. I see they have 6v on the print. SO much as I dislike that, is the supply 6v? I had a arcade video game a year or two ago that just loved to glitch and reset itself. I got the +5 dead on. Then we found a note in the bottom of the game, someone specified this one must be at 4.5v to work. Sure enough. SO supply can be important.

On the other hand, it could just be your chip is tired, have you tried another 74221?

And is the input to that IC doing its job?

[codystein9]

Hey Enzo, I dont think Ive talked to you on this forum yet, but I feel like youve helped me before. haha

Yes Im talking about the 2MHz Oscillator. They use a Dual Shot Multivibrator 74221 coupled together as the oscillator. I didnt know what that was before yesterday. Sounds dirty. The supply voltage is dead on 6.055VDC.

Looking at the input, I thought it was okat right around 4v but looking at the datasheet of the 74221 it looks like the minimum is 4.5V. Ill investigate that further.

[codystein9]

Yeah, even with the input voltage right at 5 volts, its not doing anything. I did replace the 74221 as well. Just confused why it works fine after a kickstart.

[Enzo]

The pitch input signal comes from the master tune pot/. U14 has a 7.5v reference on pin 3, does yours?

[codystein9]

Yeo I have almost perfect 7.5V on pin 3

[codystein9]

So Im still working on this. Im still stuck on the thought that this has to be the multivibrator/oscillator. I do find it weird that, as Enzo mentions, I feel like the TTL voltage should be 5V and not 6V even on the datasheet. The schematic is a little hard to read there, but I traced the pcb back to the 6V supply and I dont see any voltage dividers or anything. If I cant figure anything else out, I might try lowering it to 5 and see what happens.

The only other thing I can see is that its supposed to have some initial pulse to trigger the oscillator, right? I dont really see where this would come from. I just see the control voltage, which is running super smooth. I feel real bad keeping this synth from this dude.

[codystein9]

Heres the datasheet, so you dont have to look.

https://datasheetspdf.com/pdf-file/3...ctor/74LS221/1

[dmeek]

A bad bypass capacitor could cause problems. Try replacing C56

[g1]

I feel like the TTL voltage should be 5V and not 6V even on the datasheet.

Seems to be a bit of variance depending on manufacturer.
ON semi wasn't around back then. The TI datasheet (attached, pg.5) shows max. VCC of 7V.

[codystein9]

Oh yea, ok that makes sense. I think I was looking at the TI datasheet before. Thats the chip I tried replacing the old one with. So the supply voltage should be just fine.

I replaced the bypass capictor but there was no change.

[Enzo]

On semi was known as Motorola then, same company and continuous product lines.

Don't confuse max supply voltage with operating conditions. That 7v means the chip can withstand up to 7 volts without sustaining damage. But TTL is designed to use regulated 5v supply. Look just below that 7v spec to see the operating condition recommendations For the 74221 it is 5v +/-0.25v

Now Moog was always weird, I hated working on their analog stuff as it depended on precision parts and matched transistors and such. SO he may have found running the ICs at 6v got him somewhere he wanted to go, but it would be unusual.

Also not the 74LS221 had a 7v max, but plain old vanilla 74221 was limited to 5.5v

Logic levels in TTL are set at specific voltages. Contrast CMOS logic where the logic levels are just percentages of supply. And there is a CMOS version, the 74C221.

When I was training techs on digital arcade stuff, they learned to set power supply right at 5v, but if you set the SMPS to 5v, by the time it got through the connectors and wiring to the logic board, it might be only 4.5, so we taught them to always check the supply right on the board. So I thought maybe he set the PS to 6v so it would not be low at the ICs, but it doesn't really look like the case here.

Looking at the schematic, it specs a LS221, not a plain vanilla 221, that detail can matter.

[g1]

I replaced the bypass capictor but there was no change.

How about C52?
Also, R111, R112, C53 and C54 are critical values for setting duty cycle so they could be factors as well.
And have you checked the DC at U14 pin6 ?

[codystein9]

Yea I did check and this is the one I pulled from the board and replaced with the exact same chip. https://www.mouser.com/ProductDetail...TFfT7bDQ%3D%3D
I'm going to try lowering the Vcc to 5 volts with a voltage divider and see what happens. It seems the chip is designed to work at 5V, but all the divider chips run on 6vs, so maybe that was the thinking originally?

I did try replacing C54 and C53 (although with 5% mica capacitors) and R111 and R112 are within .05 ohms. Depending on the master tune and R109 the DC at U14 is right around 5V +/- about a volt. No setting would trigger the oscillator.

[dmeek]

I breadboarded the oscillator to see how it works. I needs a minimum 1V on the opamp pin 6. 1V to 15V gives 260 kHz to 4 MHz. An important point is the power supply
has to rise to 6V quickly and cleanly in less than 100uS by my experiments, or the oscillator won't start.

Your problem could be a dirty or pitted power switch or a bad 1000 uF main filter cap.