Dead Vox 1902 Flanger

[McClure]

Hi Guys,

Finally got a question that I couldn't find the answer to in the archives!

A friend learnt that I had got into DIY pedals in a big way and gave me his lifeless Vox 1902 flanger to look at, it bypassed ok but Zero sound when the effect was switched in.
Apparently he tried to plug in a power supply of some description and it died, his words.

I put some batteries in, got the MM out and started probing around for obvious stuff. The obvious stuff was a 3rd degree burn on my arm from an unhappy battery. Found a dead diode and replaced that.

Now here's where I'm way out of my depth. The unit now "works" as in I can get sound from it when the effect is engaged but there is no effect at all on the guitar's signal
BUT
The wierd thing is that when I deaden the strings, I can hear the effect quite clearly in the background hiss. When I alter speed/depth I can hear it change in the hiss yet as soon as I play there is zero efffect on the guitar's signal.


Here's the link to the schem, I was hoping you might be able to give me some tips on what to look for, how to see if the IC's are dead etc.

http://www.voxamps.co.uk/images/circuits/1902flan.jpg


Thanks,

Dave.

[Mark Hammer]

There are a couple of critical trimpots on the board whose electrical continuity or setting might have become altered over the years.  Problems with either of these might result in an otherwise potentially functioning flanger, except that the signal is boxed up and can't get out.

R17 is a 10k trimpot that adjusts the level of the delayed signal coming out of the BBD.  Normally, maximum notch depth (and effect intensity) depends on a 50-50 balance of delayed and non-delayed signal.  Some flangers let you play with this as a chassis control, but not all do.  Some will aim for a 50-50 balance with fixed resistors and leave it at that, while others will stick a trimpot on the board and permit optimal adjustment, compensating for chip-to-chip and resistor-to-resistor differences.  If the wiper is not making contact with the resistive element on this trimpot, or if someone moved it around, then it is possible the signal coming out of the SAD1024 just isn't loud enough to be heard, hence no audible effect.

R54 is also a trimpot and is perhaps more critical, or rather small adjustments to it CAN be critical.  ALL BBD's require a DC bias signal for AC (audio) signals to ride on.  Think of it like the adult needed to get a teenager into restricted movies.  If the person *posing* as adult cannot demonstrate their age, then the teen with them can't get in.  Similarly, if the DC bias voltage isn't set right, then the BBD will not pass audio signal.  There is usually a critical zone in adjustment of this trimpot.  Being dead on gets you good delay tone, being a little off gets you a delay tone with poorer audio quality, and being further off than that gets you nada.  Play around with that trimpot, and perhaps you can restore the delay signal.   You cannot destroy the BBD by doing this, so rest easy.  It is the passage of signal that relies on settings, not the functioning life of the chip per se.

It is clear that the LFO has no problems, since there is an audible indication that it is sweeping.  However, just for curiosity's sake, I'm going to suggest that you briefly tack on a 1000pf cap in parallel with the 33pf cap (C19).  This sets the clock range.  Dramatically increasing the cap value will lower the clock frequency so that it should be heard as a VERY annoying whine that goes up and down.  If you can hear that then you know the LFO *and* clock are working just fine.

There IS a possibility that the BBD is healthy, but if if troubleshooting suggests it isn't, you can find replacements at Small Bear electronics.

[McClure]

Cool, thanks Mark. I've printed out your post and I'll report back Monday.

[mr_doyle]

i come a bit late

btw - i had a similar prob too with that device, and it was the Reticon SAD-1024. Replaced it and unit started working again.

hope this helps,

D.

[Mark Hammer]

If it is the SAD-1024, then you may have to look elsewhere than Small Bear for one.  Apparently it is popular enough that Steve's supply is exhausted for the time being, exhausted enough that he may have to provide refunds to people for chips he was certain he was going to have.  Hopefully, he may be able to pull a miracle from the hat, but it just may be that the sorts of stashes that justify listing it as a catalog item (thus inviting requests) are a thing of the past.

I'm just hoping it isn't THAT chip in your case.

The person who writes a how-to-stick-an-MN3207-in-place-of-an-SAD-1024-and-make-it-work-right paper and makes it readable....with a PCB layout for a small retrofit board, will most assuredly have children, hospitals, buildings, and highways named after them, and live on long after their time in legend and song.

[puretube]

I heard that... :wink:

[Dirk_Hendrik]

The person who writes a how-to-stick-an-MN3207-in-place-of-an-SAD-1024-and-make-it-work-right paper and makes it readable....with a PCB layout for a small retrofit board, will most assuredly have children, hospitals, buildings, and highways named after them, and live on long after their time in legend and song.

Heard that too.

Within a week or two I'll be getting an old DOD delay with the request to replace the (missing) SAD4096 with a MN3205, most likely using an adaptor board. Although I can imagine difficulties I never saw the "impossible" LED start to flash? I Cn't imagine the 1024 to 3207 would be more difficult?.... But.. if you say so Mark,... could it be that there is something which I overlooked.?

[Fret Wire]

The person who writes a how-to-stick-an-MN3207-in-place-of-an-SAD-1024-and-make-it-work-right paper and makes it readable....with a PCB layout for a small retrofit board, will most assuredly have children, hospitals, buildings, and highways named after them, and live on long after their time in legend and song.

lol! :lol:   Lucky me, I'm only looking for a workable sub for the SAD 512D, so I can clone the MXR Micro Flanger.

[Mark Hammer]

I don't think a 512D replacement is going to be all that easy to do.  The 512D includes its own clock divider.  An MN3207 has twice as many stages, which means you'd need to double the clock rate to produce the samedelay time range.  If the existing clock circuitry ony puts out a single phase clock, then you'll need to come up with some kind of clock division arrangement (a flip-flop will do nicely) to stick between the existing circuitry and the BBD.  At that point, I'm not sure there is anything so terribly special about the original design to warrant going to such lengths.

[puretube]

MN3102 (DIP8) will provide 2 phases from ext. clock...

[McClure]

Hi guys,

Been dragging my heels a bit but I've finally done as Mark suggested and here's the results.

Adjusted trimpots R17 & R54 with no luck. Removed them from the board to make sure they were still functional, which they were. R54 turned out to be 250K.

Tacked a 1000pf cap in parallel with C19. Initially there wasn't much of a whine but I adjusted R54 and it soon became a very audible, annoying whine.

Took out the 1000pf cap and then only adjusted R17 to see if any effect came through.

Not a hint of it, the guitar signal still comes through without the effect. The only difference now is that there isn't even any effect on the hiss? I've been keeping track of where the trimpots were set but I couldn't get the effect to re-appear on the hiss.

I quizzed the guy further on what he did EXACTLY to kill the unit. He's still cagey but he plugged in a 20V? wall wart used to power his scanner. I don't know whether it was AC or DC or whether the polarity was wrong, I'm waiting for a response on those points.

Off the top of your heads, anything else I can check out?

Thanks for your help, much appreciated.

Dave.

[Mark Hammer]

Starting to look like a new chip may be called for, unfortunately.

From time to time, smells from other parts of the building (especially the guy in the office above me whose wife makes him some amazing lunches) come wafting through the ventilation in my office.  Swear to god that as I wrote this, there was a brief smell of burnt silicon chip in the air! :lol:

[McClure]

No worries, thanks Mark.

I'll hand this puppy back and get on with the building stuff

Stay well,

Dave.

[mr_doyle]

what the f@#k, that pedal is broken again

it doesn't sweep anymore

could be the CD4013? i'll try to replace it and see what happens

btw - this pedal is really bothering me, its reliability is a vague concept, and it always produces too much noise

D.

[mr_doyle]

it doesn't sweep anymore

could be the CD4013? i'll try to replace it and see what happens

nope :-(

could anybody please point me in the right direction?
the effect turns on, pots actually work, but i can't hear any sweep.

any help appreciated!

Thank you,

D.

[puretube]

If it is the SAD-1024, then you may have to look elsewhere than Small Bear for one.  Apparently it is popular enough that Steve's supply is exhausted for the time being, exhausted enough that he may have to provide refunds to people for chips he was certain he was going to have.  Hopefully, he may be able to pull a miracle from the hat, but it just may be that the sorts of stashes that justify listing it as a catalog item (thus inviting requests) are a thing of the past.

I'm just hoping it isn't THAT chip in your case.

The person who writes a how-to-stick-an-MN3207-in-place-of-an-SAD-1024-and-make-it-work-right paper and makes it readable....with a PCB layout for a small retrofit board, will most assuredly have children, hospitals, buildings, and highways named after them, and live on long after their time in legend and song.

bump...