Moog Source no Ramp wave VCO 1

[rekem1000]

Hi all,

as the title says I have no Ramp waveform on VCO 1, only the triangle will sound when ramp is pressed. I believe the membrane is working as the light appears when the ramp is pressed.
Anyone seen this problem and know a fix. Any help is appreciated.

thanks

[Technician Larry]

The SAWTOOTH waveform is likely present at pin 3 of U32B since the SAWTOOTH wave is produced first by a functioning VCO. If the other waveforms are present, SAWTOOTH has to be there. If the other waveforms are switching OK, the most likely problem is a bad U32.

Many things can go wrong when working on a Source. If you are not proficient and practiced, the Source is not the place to start learning. On the other hand, there's nothing like making a mess of things to give you a heads up on what not to do!

[Technician Larry]

Another thought: Keep in mind how easy it is to get fooled and to chase down the wrong path when component level troubleshooting a circuit. Use an oscilloscope to verify the presence and / or absence of all the signals involved in the circuit. If you don't understand the function of the circuit when it's working properly, see if you can figure it out from studying the schematic and making observations with an oscilloscope. Making the assumption that my advice is OK and blindly swapping out U32 escalates the possibility, even probability, of getting fooled.

[rekem1000]

Yes I thought that was the case so i swapped U32 with U23 on OSC 2 to see if the problem would switch to OSC 2 however it didn't, do you think this could mean the membrane even though the light indicates it is being pressed?

[Technician Larry]

No - the membrane switch has to be closing if the indicator turns off and on. CPU looks at that single switch closure and sends signals to the indicator AND switching circuit in response. This is where you need a scope to see what is going on. If this was on my bench I would start by verifying your observations so far. Are you sure Pulse and Tri waves are turning off and of properly? U22 is common to both VCOs. I have not looked close enough to have more than an educated guess, but if U22 is bad there would be other switching problems. That leaves a broken or open resistor, trace / connector problem or perhaps an observation error. Wish you were here - we'd probe that out and know for sure in short order. Should be an easy fix.

[roginator]

i have similar problem

My VCO one dont have PW!!

so I checked 4016 is working fine and panel is sending signal to CPU
also Opamp on PW out is working cause i swapped it to VCO 2 and i have same issue on VCO 1

now i tried to mess with digital board BUT!!

I just notice i might have cold soldering point cause when i touch pin 7 of U31B (3290) looks like PW is there for few seconds then go away !!!

I will update you on my service !!

I bought source 3 months ago: (cheap with manny problems)

- replaced membrane switch panel with one from Ebay (works perfect)
- Cleaned beast
- recapped PSU and rest of electronics
- Volume pot was broken so i made new one from old Prophet 600 pot//and combinated with original one) now it works perfect
- set it to work on 220v... (auto trafo is easy just connect black and white wire)
- I got myself set on nicer black keys cause original ones were slightly bended

2 things to to ... FIX PW on VCO one .. and check calibration of unit !!!!

SO FAR IT SOUND PERFECT!!!

[puggle]

Hello, I am having exactly the same problem as the OP described. There was a saw wave in VCO 1, now there isn't. When I press the VCO 1 pulse button I get pulse, tri I get tri, but when I hit saw, it stays tri. Any ideas on what to do? This sounds like an easy fix, but I don't know how. Love the source, sounds great, can anyone help me get it back without taking it to the shop? Thanks!

[thealien666]

Sounds like it might be part of U32 (on the analog board) that's defective (quad analog switch chip CD4016 or CD4066). Easily found and inexpensive chip to replace.
Either that, or a cracked solder joint somewhere...

[puggle]

hi, thanks so much! since I'm really new at trying to fix anything, any way you could be a little more specific on how to get this up and running again, like how I would locate the defective part inside the board, source a replacement, etc? many thanks!

[thealien666]

Maybe you could start by having a look at the schematics of your synth. And reading the service manual for it, too. And if after that you're still lost and don't understand much of any of this, find a qualified tech that could ?

[puggle]

I've got the schematic, and it does list the parts pretty clearly as you described them, which is good, didn't realize it was that easy. I could just try to take each one out and see what that does, but could I do any damage that way, and how would I know which one was bad if I don't see any broken connections? Would I need an oscilloscope or voltmeter to go any further? Alternately, I could just source all three parts (but how do I know what to order, the service manual doesn't say, does each part have a label on its somewhere) and replace them one by one, but is that sensible? Any help would be appreciated, this could likely save me a few hundred bucks in repair. Thanks!

[thealien666]

... Any help would be appreciated, this could likely save me a few hundred bucks in repair. Thanks!

I understand that a qualified tech isn't usually cheap. But poking around inside a synth that is also not cheap, runs the risk of damaging something when you don't exactly know what you're doing.
And as much as I would like to help you, with my own somewhat limited electronics knowledge but long experience as a hobbyist, from your answers I sense that you might be wise to let someone else have a look at your beloved and precious synth.

That being said, if the chip that I'm suspecting and was talking about in an earlier message is on a socket (some models are and some others not, depending on year or month of production), then you could order an exact replacement for that chip (inexpensive and easy to find), and plug in the new one and this could maybe fix your issue. But in the case that it doesn't, any further diagnostic work should be left to a pro with proper tools and equipment. If however that chip is soldered directly on the PCB and you're not properly equipped or experienced at desoldering then, again, maybe you should let someone else who is have a look.

P.S. Removing U32 and powering up the synth would result in not having any waveforms coming out of OSC 1, since that chip switches the different waveforms on, or off.

[puggle]

many thanks, that helps just enough for me to see if this is something I should handle or not. Yeah, I've definitely gone inside synths before and caused more problems than I fixed. Ok, thanks!

[thealien666]

You're welcome, and good luck.
Sometimes, it is as simple as a cracked solder joint or bad contact somewhere, but sometimes it's failing parts due to age or heat, or both.

[puggle]

ok, I went inside the source, and the only chip that I think it could be is in U32 and is labelled cd40168E r049. I googled the first part and it came up in electronics supplies, so I could order this. I also noticed that this chip was sticking up out of its socket last time I opened the source, I think because I accidentally was resting the top panel on it (!) and it came out of the socket a bit. I put it back in the socket, I'm not sure if the problem had started before this or not, but I'm guessing I may have damaged the chip somehow?

I can't seem to find a chip with the names you gave me, and the schematic also says 40168. Am I doing something wrong, or do you think that's the right one? I'm guessing I should just order the chip monday, and see if that fixes it.