Conn Multi-vider (Silverbox)

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
JD0x0
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Post by JD0x0 »

I recently bought a Multivider from a famous musician. The unit works fairly well except some issues.

The 'soprano' circuit doesn't seem to work. If I turn it up I get a slight increase in hiss. If I mix the signal so it's only soprano I get a nasty weak, gated sound from the instrument. I'm told it's supposed to be a fuzz, not an octave up, but it doesn't sound like a (working) fuzz.. Also, I'm not sure what the knob does in the top left corner. It doesn't seem to effect the signal much except when at '0' the octave no longer works. Is this the 'effect level' control?

The third possible issue, is I noticed with the 'bass' or first octave down, sometimes when I hit notes it'll seem to jump UP an octave. At one point I thought it was the soprano circuit functioning intermittently but it doesn't seem to be affected based on where the 'soprano' switch or level control is set

The unit was repaired in 1976 from a sticker on it. That may have been the last service.

I opened the unit and noticed some electrolytics, and also what looked like some transistors.. I'm assuming germanium?? My guess is they went bad which is causing the malfunction(s) but does anyonewith more experience know what's up?
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Nocentelli
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Post by Nocentelli »

The bass and sub-bass on many analog octaves jumps around, e.g. even boss OC-2 which is fairly well regarded us not immune.

Could you take some close up pictures of the board? It may provide some hints as to the soprano problem, and maybe even a trace could be attempted.
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Post by JD0x0 »

Nocentelli wrote:The bass and sub-bass on many analog octaves jumps around, e.g. even boss OC-2 which is fairly well regarded us not immune.

Could you take some close up pictures of the board? It may provide some hints as to the soprano problem, and maybe even a trace could be attempted.
Sure, I'll just have to find my good camera.

One thing I did notice, was some nasty green corrosion on one small area of the PCB. I carefully cleaned it to expose the traces, but I couldn't tell what would cause that one spot to corrode so heavily. (I assumed it was an e-lytic cap that leaked, at first, but couldn't find evidence...no other areas were like this in the pedal) There's also some old black foam stuck to the PCB in some spots, which I will need to clean better. I gave it a quick cleaning and touched up some joints to see if it would affect the issue(s) I was having, but it looks like I'll have to pull this apart a bit more so I can inspect the actual components better.

Thanks for the input!

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Post by JD0x0 »

What's the preferred filehosting site for this website? I can't directly upload pictures because the file size is too large.

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Post by bajaman »

reduce the file size of any pictures to 1Mbyte or less and you are good to go :wink:
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Post by JD0x0 »

bajaman wrote:reduce the file size of any pictures to 1Mbyte or less and you are good to go :wink:
That kills the precious resolution. I'll give it a shot, though

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Post by JD0x0 »

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Post by bajaman »

if you are using a digital camera make sure you set it to macro for close up pictures - my eyes are hurting after trying to view these images :(
Nowadays i find that most smartphones especially iphone 5 onward take the best pictures :wink:
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Post by bajaman »

some pictures o the other side of the board are good to help anyone wishing to trace the circuit too :thumbsup
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Post by pinkjimiphoton »

these were designed for brass... there was a pickup and preamp (i think) assembly before the multivider.

the soprano is indeed an octave up and should be fairly farty sounding. its basically like a division down organ, it fuzzes and gives ya various octaves.

always wanted to fuck around with one of these. its likely not broken.

first thing i'd do would be replace every single electro and tant cap on the board.
then it will probably work.
if not, use an audio probe and multimeter. the ge q's don't just go bad, and i think these were using silicon by the time they got this far
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Post by JD0x0 »

bajaman wrote:some pictures o the other side of the board are good to help anyone wishing to trace the circuit too :thumbsup
Now that I have a good camera I will try to get decent pictures of both sides of the unit

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Post by JD0x0 »

pinkjimiphoton wrote:these were designed for brass... there was a pickup and preamp (i think) assembly before the multivider.

the soprano is indeed an octave up and should be fairly farty sounding. its basically like a division down organ, it fuzzes and gives ya various octaves.

always wanted to fuck around with one of these. its likely not broken.

first thing i'd do would be replace every single electro and tant cap on the board.
then it will probably work.
if not, use an audio probe and multimeter. the ge q's don't just go bad, and i think these were using silicon by the time they got this far
It's definitely broken, as far as the soprano switch. E-lytics will be the first to be changed. Are those stripped caps tantalums? I assumed they were old Poly's but I could be wrong, plenty of ceramics(which typically tants are use in place of), which I may replace with other compositions as an 'upgrade' (The unit is a bit hissy, on certain settings, that could also be the carbon resistors, too.)

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Post by JD0x0 »

So I replaced the electrolytics. Still no dice. I decided to mess with the trimmer pot for the soprano circuit. If I turn the trim pot up, I can get a little more volume out of the multivider when the soprano is engaged. If it's turned up quite a bit, it seems to start taking on an overdrive tone, but this could just be the front end of the amp distorting, at this point. The clipping sounds much like a low gain tubescreamer type pedal, and not the square wave fuzz I assumed this would produce. Not a bad sound, but I assumed a more extremely fuzz-like clipping.

Perhaps something in the clipping circuit has died? Since I'm getting signal out of the soprano circuit, just not the desired clipping.. How exactly is the signal clipped for the soprano fuzz effect? Is it the transistor doing most of the work?

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Post by marcao_cfh »

I use postimages.org to upload images. It's nice, free and doesn't need registration, so you can easily upload better resolution pictures. Also, you get an "ready to post" code with the image url, so you just need to copy and paste here.

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Post by JD0x0 »

marcao_cfh wrote:I use postimages.org to upload images. It's nice, free and doesn't need registration, so you can easily upload better resolution pictures. Also, you get an "ready to post" code with the image url, so you just need to copy and paste here.
Thanks!

Let's see if these work...

https://postimg.org/gallery/2cyeffce4/

P.S. had to parallel those 2 nichicons, until a 10uF value arrives..

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Post by grizzlytone »

Here's a link to my homedrawn schematic and my notes regarding the soprano circuit. There's no clipping stage. I had some trouble with the soprano circuit as well when trying to use it as a guitar effect. Apparently I worked at balancing the soprano circuit frequency doubler (which is what it really does) and ended up replacing the FET's w/ matched 2N5457 and balancing the doubler circuit (trimmer provided) for a nice full wave rectified output at the oscilloscope.
Also notice that input threshold for getting any signal at all out of the doubler circuit is -28 dBu with the sensitivity control at max (compared to - 48,4 dBu for the octave down effect). I always take it that a "nominal" guitar output level is about -30 dBu although humbuckers produce a bit more output than this and the attack transient on a normal output may be say -10 dBu. So this might be your problem as well. Try using a booster ahead of the multi-vider to confirm that this is the cause of the missing effect.

https://postimg.org/image/9ahaua4bp/
https://postimg.org/image/opgbx9zpx/

Good luck
/M

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Post by grizzlytone »

Also, the input impedance is extremely low, only 1,5 kilohm, killing even more of the already weak signal from a passive guitar pickup. The input impedance is in fact typical for mic preamp inputs, designed for standard professional microphone source impedance 50 or 200 ohms. If you were to plug your guitar "direct inject" into the mic preamp input on a studio recording console you would have to use an impedance transforming DI-box in order not to severly load the guitar pickup and loose all treble but this would also cost additional overall signal level. However, a professional recording console mic preamp would have gain enough to bring the signal level back up. There's no such preamp stage in the Multi-Vider. One solution would be to insert any Boss/Ibanez type pedal in between guitar and Multi-Vider. These pedals are non "true bypass" so they will have a low output impedance buffer driving the output even when in bypass.

/M

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Post by JD0x0 »

grizzlytone wrote:Here's a link to my homedrawn schematic and my notes regarding the soprano circuit. There's no clipping stage. I had some trouble with the soprano circuit as well when trying to use it as a guitar effect. Apparently I worked at balancing the soprano circuit frequency doubler (which is what it really does) and ended up replacing the FET's w/ matched 2N5457 and balancing the doubler circuit (trimmer provided) for a nice full wave rectified output at the oscilloscope.
Also notice that input threshold for getting any signal at all out of the doubler circuit is -28 dBu with the sensitivity control at max (compared to - 48,4 dBu for the octave down effect). I always take it that a "nominal" guitar output level is about -30 dBu although humbuckers produce a bit more output than this and the attack transient on a normal output may be say -10 dBu. So this might be your problem as well. Try using a booster ahead of the multi-vider to confirm that this is the cause of the missing effect.

https://postimg.org/image/9ahaua4bp/
https://postimg.org/image/opgbx9zpx/

Good luck
/M
Wow! Awesome! Thank you so much for this.. Would J113's work by chance for the FET's? The 2n5457's are cheap enough, but I've should have the J113's arriving in a couple days for another repair.

I don't seem to have any trouble triggering this pedal with single coils. It seems to trigger better than my Parasite audio CMOS based effects that I've built. They have trimmers to set the sensitivity, they definitely seem to function ALOT better with a compressor up front. Hard to find a setting, where they don't oscillate, but also don't cut out too early. The Multivider doesn't seem as temperamental, with my guitars compared to those pedals. I think that's one of the reasons I like this pedal so much. That, and, with the mix on the clean/octaves set right, it works surprisingly well, polyphonically.

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Post by grizzlytone »

JD0x0 wrote:
grizzlytone wrote:Here's a link to my homedrawn schematic and my notes regarding the soprano circuit. There's no clipping stage. I had some trouble with the soprano circuit as well when trying to use it as a guitar effect. Apparently I worked at balancing the soprano circuit frequency doubler (which is what it really does) and ended up replacing the FET's w/ matched 2N5457 and balancing the doubler circuit (trimmer provided) for a
Wow! Awesome! Thank you so much for this.. Would J113's work by chance for the FET's? The 2n5457's are cheap enough, but I've should have the J113's arriving in a couple days for another repair.

I don't seem to have any trouble triggering this pedal with single coils. It seems to trigger better than my Parasite audio CMOS based effects that I've built. They have trimmers to set the sensitivity, they definitely seem to function ALOT better with a compressor up front. Hard to find a setting, where they don't oscillate, but also don't cut out too early. The Multivider doesn't seem as temperamental, with my guitars compared to those pedals. I think that's one of the reasons I like this pedal so much. That, and, with the mix on the clean/octaves set right, it works surprisingly well, polyphonically.
Glad you're happy with you Multi-Vider and glad I can help! I haven't tried mine in ages and I need a pickup for using it with my saxophone and clarinet. For guitar and bass I have a number of other octavers to use. I bought it on a whim just out of curiosity when the opportunity arose like one could do in the golden days of ebay. At the time I was very interested in the largely undocumented evolution/chronology of musical gear inventions.

I wish my memory was better...
I seem to recall that my particular FET substitution didn't improve the subjective sound much, although the output signal waveform improved from a technical objective perspective. I think I just took whatever reasonably low Vgs cutoff FET I had at hand at the time and tested 2N5457's on my homemade FET tester until I had a matched pair. The lower half of the paper w/ the notes just shows the measurements on the original MPF103's in that tester. I had no info on the pinout of the MPF103 at the time (maybe available on the internet today?). The gate pin was obvious. I ran some simple tests to confirm that Source and Drain are completely interchangeable for all practical purpose just like on about any other TO-92 JFET that I've experimented with - That is what I call "completely symmetrical" in my own made up terminology. I guess the J113 might work if the Vgs cutoff are reasonably low. Just try them! I figure that the number of JFET types to choose from were very limited in 1967 so there's a good chance of finding a better candidate today.
The low input impedance is established by the 2k sensitivity control. If you like -try substituting with a 100k potentiometer and see what happens.

/M

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pinkjimiphoton
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Post by pinkjimiphoton »

if one of you gents could kindly repost the hand drawn schematics, i'd be very grateful!
thanks!
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