258V VCO problem

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jamb
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258V VCO problem

Post by jamb » Wed Jan 04, 2017 11:36 am

Seeking a helping hand here:

I have a 258V that has been showing some problems for the last few months. Basically the bottom VCO is fucked up... it sends out a garbled, wavering pitch accompanied by white noise. If you lightly tap the panel it sometimes it cuts in and out. (take a look at the vid) Top VCO is perfectly fine.

https://vimeo.com/198069227

I sent it back to Mark Verbos who said he'd fix it, but when he sent it back, it was still broken. In fact it's far worse now. I haven't been able to get a reply from him for a couple of weeks, I know he's moving to Berlin, but this is one of four Buchla modules I own and is the core of my system so I really would like to get it back in shape asap. That said, hopefully there is someone else stateside who might be able to help. Anyone have experience working with these modules? Any help would be greatly appreciated and I'm obviously willing to compensate if you're able to fix. THXX

:despair: :bart:

negate
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Post by negate » Wed Jan 04, 2017 2:09 pm

Hi jamb.

Can't help you much because I don't have Buchla boats or this particular module. But people around here usually advise moving the module to a different place in the boat. From what I know the 258v is shallow so it shouldn't be a problem, but try it if you already haven't.

Cheers.

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Post by jamb » Wed Jan 04, 2017 2:31 pm

Hey thanks for the input, negate. I've had it in a few different spots in my little boat and tested it outside of the case entirely, doesn't seem to effect it. I would assume it's a bad solder joint, since tapping it causes intermittence-- someone messaged me that it could be a trimmer adjustment issue, so I'm going to take a closer look at those in a bit...

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J3RK
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Post by J3RK » Wed Jan 04, 2017 2:46 pm

There could be a few edge cases on the wave shaping trims that could cause some strange behavior I think, but it sounds a bit more like maybe something shorting or disconnecting at the panel, or maybe a bad ground connection somewhere.

I haven't looked at the V version before. Is it two VCOs on a single PCB, or two separate PCBs?

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Post by jamb » Wed Jan 04, 2017 3:04 pm

J3RK wrote:There could be a few edge cases on the wave shaping trims that could cause some strange behavior I think, but it sounds a bit more like maybe something shorting or disconnecting at the panel, or maybe a bad ground connection somewhere.

I haven't looked at the V version before. Is it two VCOs on a single PCB, or two separate PCBs?
It's a single PCB. I don't have have the best means to diagnose a bad solder joint or loose connection, but I suppose if I knew where to look I may be able to solve it. Mostly confused why it was sent back this way, if maybe he didn't test it or something. I did noticed he replaced the fine tune pots on the front panel with longer pots...

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Post by J3RK » Wed Jan 04, 2017 3:11 pm

It's possible that checking and replacing those pots just happened to also bump something into place long enough for it to appear fixed. I can't imagine him sending you something back that he didn't check or knew was broken. There must be something loose in there. You may want to see if you can localize it by carefully wiggling wires going to the panel, maybe gently pushing down on the top of ICs or any other headers that plug into the board. Maybe check the solder joints on pots and jacks visually very carefully to see if maybe a stray strand of wire didn't get soldered into place, and could be floating around back there. If you can get a closeup of the back of the module, I could probably suggest a few areas to check.

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Post by negate » Wed Jan 04, 2017 4:49 pm

Don't know if jamb will be able to take pictures so here is one from Mark Verbos' blogspot.

Front and back of the PCB (double sided), angled but most is visible (click link for higher resolution).
Image

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Post by J3RK » Wed Jan 04, 2017 5:10 pm

Nice!

I didn't realize (for some reason) that these had board-mounted pots. For some reason I thought they were flying. Anyway, then I would take a quick look at the wiring for the jacks, maybe press on them a bit, while connected, to see if any one connection seems to make the problem occur more or less.

The pots are probably fine, unless there's a bad joint, or the pot itself is bad.

I wouldn't touch the scaling trim. It's probably not the issue. If it is a trimming thing, you'll want to figure out which ones are in the wave shaper. The very ends of the range on the crossfade trim (at least on the 258J) can have some pretty strange results.

It could be a bad FET. They tend to be a bit sensitive to things like heat, static electricity, etc. If you're not comfortable or don't have a way to check though, it's probably best to get a tech to look at it. (I'd offer this, except with my schedule the way it is right now, I'd probably have it sitting around a lot longer than you'd want).

On a completed module, are there only three trims like pictured, or is there maybe a fourth one mounted on the back side? Based on the J (which is actually quite a bit different) there would be an initial frequency trim, a scaling trim, a crossfade trim, and a sine trim. (four total)

(sorry, once again, I've never seen a V in person)

Edit: Duh!!! sorry, the fourth trim may be on the panel on these. (unless it's fine pitch...) That would explain three on the PCB.

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Post by J3RK » Wed Jan 04, 2017 5:28 pm

I can't see Vimeo links at work, so I'll take a look at the video to see if the actual behavior rings any bells here when I get home tonight.

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Post by jamb » Wed Jan 04, 2017 6:34 pm

wow, thanks for the help J3RK and negate. I just took the module out and while powered toyed around with it for a bit. It definitely seems like a connection issue... if I pull back on the corner circled in red (towards me) on the bottom right of the PCB it fizzles for a sec and kicks in with what sounds like normal operation. I can't seem to tell with a naked eye if a joint looks bad but I've pushed on a few with a wooden cuticle pusher.

I lightly tweaked the trim pots on the bottom side and those had no effect. Put those back where they were at as best I could judge.

I could take a soldering iron to all the contacts on the back board and just touch the whole thing up I suppose. I will do a closer inspection shortly...

Image

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Post by jamb » Wed Jan 04, 2017 6:57 pm

better higher res photos of bottom VCO here:

https://www.flickr.com/photos/kreemyffu ... 5020999853

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Post by djs » Wed Jan 04, 2017 9:14 pm

very highly unlikely it's a trimpot setting. Your pushing on the corner definitely makes it sound like a loose wire or funky solder joint. You could try heating up your iron, and just hit every joint in that area to see if reflowing them improves the situation.
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Post by J3RK » Thu Jan 05, 2017 11:46 am

I agree. Reheating may just do the trick. I would be careful of the FETs/Transistors (especially FETs as they are heat sensitive) (the small black three-legged components)

I do see something a little weird though. One the pot retention leads (left one) on the pot just to the left of your red circle in the image above looks a little off. It looks like it's pulled some of the plating out of the board. If the plating is loose in the hole, or maybe touching something else, that could be a problem. It looks like there's only one nearby component, but it's something to look at.

Also, if you look straight up from the red circle, there is a row of four pads, with only one of them soldered to something. That joint could be better. Maybe heat it up and/or add a touch of no-clean solder to it.

There are a few messy spots, and with the flux all over the place, it's hard to see some of the smaller gaps between pads, but I don't see anything else that immediately jumps out at me. It doesn't look like the ICs are sockets (IMO a good thing) so I wouldn't worry too much about them, unless you see a bad joint on one of the pins.

If you need to, you can use isopropyl alcohol (90%ish is best) with a Q-Tip or cloth/paper towel, and wipe some of that flux away. You may need to gently scrape it a little bit to break it up, so I would do it sparingly. If it's obscuring a potential issue though, that would work.

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Post by jamb » Thu Jan 05, 2017 12:05 pm

Thanks for taking a close look at these pics! I have my soldering iron at my sound studio and my synth at home, so I'm going to go at it with the iron tonight. Last night I prodded around a bit more and scraped some of the flux away and got a more consistent pitch to come out of it. I'll let you know how the iron goes... some of these joints are so damn sloppy. where was the QA check??

Is this the rev 2 version of the 258v?

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Post by djs » Thu Jan 05, 2017 12:06 pm

J3RK wrote: If you need to, you can use isopropyl alcohol (90%ish is best) with a Q-Tip or cloth/paper towel, and wipe some of that flux away. You may need to gently scrape it a little bit to break it up, so I would do it sparingly. If it's obscuring a potential issue though, that would work.
What i usually do is just use a sharp object (like tweezers) and lightly scrape away flux, especially if it's between two close solder joints- in case there's some sort of hairline connection between them. Wipe the tweezers clean with a paper towel, and keep going. Make sure not to accidentally scrape on PCB traces or anything like that.

However, given the symptoms you are describing, I would be more inclined to think it's a solder joint that needs reflowing, or maybe a component that has cracked/broken somewhere that is making intermittent connections. Hard to tell however, since you can't see the component side of the module.

Can you read a schematic? There are a few 258 schematics out there that may not be exactly like this one, but close enough to follow to check certain areas for proper function.

This one might be a good start:

http://pugix.com/synth/wp-content/uploa ... 258mod.jpg
"Noise is what the Earth is made of" - David Bowie

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Post by batchas » Thu Jan 05, 2017 2:09 pm

Seeing the state of the pcb it is mostly certain it's a cold joint somewhere.
I hope you get it back to life soon.
Last edited by batchas on Thu Jan 05, 2017 2:31 pm, edited 1 time in total.
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Post by batchas » Thu Jan 05, 2017 2:20 pm

Okay. I unmounted mine to have a look at it. It's indeed green. And the solder joints don't look better on mine.
I was like sure the pcb was grey. If not grey, then blue :doh:
I should have remembered, as I moded some out on the top half.
Well...

Obviously as Mark did send it back to you as repaired, the cold joint appeared during transport/shipment back to you.
Last edited by batchas on Sat Jan 14, 2017 12:08 pm, edited 1 time in total.
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Post by djs » Tue Jan 10, 2017 4:30 pm

Any luck with this? Any reply from Mr. Verbos?
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Post by jamb » Sat Jan 14, 2017 12:00 pm

I spent a bit of time with it and my soldering iron. Touched up 20 or so joints, and found one in particular that was problematic which made a huge difference. However, there are still issues with the 2nd from the left CV input control pot. Lightly pressing this pot will cut out signal from the bottom VCO entirely. I've resoldered the joints to that pot repeatedly to no avail. I'm wondering if the pot itself has crapped out. The middle of the row that I circled in particular was the primary source of the wavering I think.

Mark did finally get back, and said it's undoubtedly a bad solder connection and at this point I'm debating whether or not it's something worth sending to him in Berlin for complete touch-up.

Thanks again to everyone for taking a look at this. I'm going to keep touch this guy up little by little...

Image[/img]

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Post by Peake » Sat Jan 14, 2017 3:59 pm

Sounds like that pot might be bad. Anything mechanical is prone to failure. Get this working again and it'll probably give you sonic pleasure for a long time.
This is not the place I'd imagined it to be.

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Post by Don T » Sat Jan 14, 2017 4:39 pm

Also, do not overlook the possibility that a solder pad may have broken away from its trace. Use a magnifying lens and carefully watch the affected area while you press on the pot and/or flex the board. If you find one, just run a jumper from the lost pad to its destination. Alternately, you can scrape some of the solder mask away from the trace, and bridge the gap with a bit of resistor leg.

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Post by emp » Sun Feb 12, 2017 3:52 pm

Any luck with this so far? I'm experiencing similar problems with both of my 258v:s.

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Post by emp » Wed Feb 15, 2017 3:14 pm

emp wrote:Any luck with this so far? I'm experiencing similar problems with both of my 258v:s.
For the record, i think i found and fixed the problems on mine now. It was indeed bad solder joints. On one of them it was the trimpot for the waveshaper that was loose and on the other the wire from the grey banana cv input had a bad connection with the pad on the pcb. :party:

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Post by J3RK » Wed Feb 15, 2017 5:34 pm

Nice! :party: (well, nice on the finding and fixing of the problems, not that there were problems in the first place :hihi: )
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Post by jamb » Wed Feb 15, 2017 9:03 pm

Mine was fixed for a few days and then the problem started happening again. I fixed a few other questionable joints and it seems to be back in full force. There must just be a batch of these things with some solder issues.

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