Need Some Electronical Advice

[Calvert Dayton]

This may be a little OT, but I figure the percentage of people logging in here who know more about analog electronics than I do likely runs in the high 90s and I could really use some help with a hobby project. I've been mixing bands in a local bar just for jollies/free beer/something to do on weekend nights in Putnam CT. The owner of the place, whom I've become good friends with, recently picked up a used EV mixer at a church auction -- presumably having grown tired of listening to me cussing his POS Behringer.

I figure this EV is probably 15-20 years old but it looked to be in very good condition. So we put it in and tried it out and I find I really like mixing on it. It's dead quiet and decent sounding and I like the feel of the controls. The problem is it has dead mic trims on 4 channels and it's only a 12-channel mixer to begin with. Everything else on it seems to work just dandy but 8 fully functional Mic level inputs is a little light even for this place. So I'd like to try and fix it.

I found the manual for it online and it includes a full schematic and parts list. I've uploaded the relevant pages as JPEG files at the links below:

Input board Schematic (http://rationalacoustics.com/calvert/BK-42_Series_Input_Board_Page_19r.jpg)
Input Board Parts List (http://www.rationalacoustics.com/calvert/BK-42_Series_Input_Board_Page_24.jpg)
Input Board Parts List (cont.) (http://www.rationalacoustics.com/calvert/BK-42_Series_Input_Board_Page_25.jpg)

The symptoms are:


On all of the channels with non-working trims except one, turning the trim pots has no effect whatsoever.
On the channel that's the one exception to the above, the trim pot has no effect until it's turned almost all the way up. Past that point the level jumps up like 20 dB in a one leap.
The output levels on the channels with non-working trims (including the one above) are 6 dB lower than the fully working channels, with all trims turned all the way down and all faders set at the same level.
Lifting either pin 2 or 3 on the input signal drops the signal another 6 dB in all cases though, so whatever is causing the level difference, that ain't it.
All affected channels seem fine otherwise with no noticeable noise or distortion. They work OK as line inputs.

That's pretty much everything I know about it at this point. It would be pretty nice if I could possibly have the parts to fix it in hand before I go taking it apart though, so any help I can get narrowing down the list of likely suspects would be much appreciated.

Thanks!

[Ferrit37]

Hey Calvert,
This circuit is a "Long-tail differential-pair" using Q101-102, with the gain controlled by VR101 this then feeds the TL702.
Interesting Diode clamping circuit D101-104.
Check the pots VR101 if they've gone open/imtermittent that would affect the gain and given the age of the unit check the caps around that part of the circuit.(c105-c107).
Also check the supply voltage to this area, it needs the +15v -15v to get the correct gain.
It seems unlikley that the TL702's have gone bad, you say the line inputs work okay yet they use the same circuit ???? maybe you came in on the insert point, that bypasses the mic pre and comes in at the Eq section.
I have some TL072's here if you want them.
Hope this helps...............Arthur?

[Andy Peters]

The symptoms are:


On all of the channels with non-working trims except one, turning the trim pots has no effect whatsoever.
On the channel that's the one exception to the above, the trim pot has no effect until it's turned almost all the way up. Past that point the level jumps up like 20 dB in a one leap.
The output levels on the channels with non-working trims (including the one above) are 6 dB lower than the fully working channels, with all trims turned all the way down and all faders set at the same level.

On the three channels where the trim pot has no effect at all, I suspect that the wiper connection is broken. The gain law is basically larger resistor value means lower gain, and lower resistor value means higher gain. So when the pot is at its minimum resistance setting, the gain is set by the 27-ohm series resistor. (That resistor sets the maximum gain; without it, the preamp might oscillate, especially if the wiper goes off the track at the end.)

If the wiper gets disconnected, the circuit gain is at minimum, as set by the 10k end-to-end pot resistance (which swamps out the 27-ohm series resistance). You might see higher gain on the inputs that work, because in those cases the wiper might never get to the far end so the resistance set by the pot isn't maximum.

So first check the PCB to make sure that the solder joints are good. They probably are, so the pots are toast.

Now with the channel that has no effect until the pot is turned to near max gain: again, a pot problem. It sounds like the resistive element is toast and the wiper doesn't contact it. When you get near the max gain setting, the wiper does make contact. Since that's where the pot-circuit resistance is minimum, small changes in position/resistance mean large gain changes.

Actually, it sounds like some well-meaning but unknowing person sprayed some kind of cleaner into the pots and destroyed them.

Good luck finding replacements. ;)

-a

PS: Good to hear you're out there mixing. Fun, ain't it?

[FILO4PRES]

Actually, it sounds like some well-meaning but unknowing person sprayed some kind of cleaner into the pots and destroyed them.


Like maybe grape juice? or even holy water.

[Calvert Dayton]

Hey Ferrit,

Thanks for weighing in on this. I'm quite sure that I was coming in on the line input, not the insert point. I was pretty systematic about it. Basically I set all the input and output faders to the unity position, then ran pink noise through and took the transfer function of every channel. I tested all the EQs and mic trims and everything looked just ducky except for the trims and the gain difference on those four channels.

The gain difference looks to be exactly -6 dB as best I can tell, so I made a cable with pin 2 open and used that and a polarity swap adapter to re-check the four channels in question. When I lifted either pin 2 or pin 3 the gain dropped another 6 dB in all cases though. So I spent a while gazing at the schematic, but nothing really jumped out at me and said "hey! I'm your -6dB." But I don't really know a lot about this stuff, so I was hoping it might speak louder to one of y'all.

[Calvert Dayton]

Hey Andy,

Many thanks for the speedy reply. Damn, I was hoping you wouldn't say the pots though. I take it you don't need me to tell you those are likely the toughest parts to match. I didn't really see any evidence that the board had ever been taken apart, but of course some folks might be more careful about not dinging up screw heads and such than I apparently am. Also couldn't really tell from the schematic it a bad pot would explain what looks to be a precisely -6 dB level difference.

I was really hoping for silicon. I'd already found a good match for the transistors, thinking the 6 dB difference might suggest looking for anything that came in pairs. The op amps are still in production and everything else is just parts... so I guess it would just about have to be the frickin' metric, reverse log taper pots, now wouldn't it.

Anyway, I suppose I'll have to break down and pull out one of the channel strips. The pots are obviously easy enough to test once I have a board out and if those are the problem, I'll need to see how they're packaged before I can really go looking for replacements. Yikes.

I've been having a lot of fun mixing though. I did a battle of the bands this past weekend. 14 bands in two days, three songs per band, all original material. I got to see some pretty decent local talent and really dust off my dial-in-a-mix-in-one-song chops. Got quite a few compliments on the sound, which was kinda nice since almost everyone in the room was a musician.

[Ferrit37]

Hey Calvert,
Everything points to the pots then :(

[Calvert Dayton]

I believe the technical term would be damned pots then. (grumble, grumble)

Thanks again though!

[gungho]

Did I understand correctly that the trim pots are working when you use the line inputs ? The trim pots control both mic & line gain.

[Calvert Dayton]

The trim pots on the 4 channels in question don't work at all. Turning them produces no gain or attenuation, irrespective of which jack the signal is coming in on. Ferrit was just making sure I wasn't coming in on the insert jacks which, given a balanced signal, might produce exactly the symptoms I'm seeing on those channels. But alas, this was not the case.

[gungho]

Assuming you can find pots that would physically fit, the 10k log taper pots should be easy to find and cheap as chips :)

[Calvert Dayton]

There are just two small problems. If you look at the parts list (see my original post for links) this calls for a 10k reverse log (aka antilog or reverse audio) pot. That's a fairly exotic item to begin with. Second problem is it's a 16mm size with a 6mm split fluted shaft.

The most common domestic variety of potentiometer has a solid 1/4" shaft (cylindrical or half-round) with no flutes. The knobs on the mixer wouldn't fit them. Metric pots are a generally a tad harder to find than domestic types, so basically I'm looking for an uncommon taper in a hard-to-find package. I spent at least a couple of hours searching online last night but I was only able to find 10k reverse log mini pots with 1/4" shafts from any domestic source. That was pretty much what I expected though, hence all the moaning and groaning.

I did however 20K linear taper pots and 10k dual-gang log pots in the correct form factor. You can put a 20k linear pot in parallel with a 20k resistor to make a 10k reverse log pot. Depending on how you connect the resistor you can even "reverse" the direction of rotation. I believe that taking apart some 10k stereo log pots and swapping the wafers around should also work, although it might be a little more of a project.

Anyway, I'll figure something out. The bigger question is, why the heck can't anything be easy?

[DennisA]

I may have missed it, but did you contact EV tech support? While this mixer is an old product that they likely do not have parts for, there is a chance they have a few left over sitting in a bin someplace OR have a source for. These won't be the cheapest parts in town but so what?

Dennis

[Calvert Dayton]

I thought about that, but since they were thoughtful enough to provide a full schematic and parts list in the user manual (© 1991), I think I've got this one. Soldering an extra resistor to a 20k pot should really work just fine. It also sounds relatively painless compared to tracking down an EV service center, calling them up and waiting while they contact EV to see if it's an orderable part, most likely find out it's not and then soldering an extra resistor to a 20k pot.

[Calvert Dayton]

So, post-mortem on this: It was the pots.

Wasn't hard to diagnose once we actually opened up the mixer and started pulling out boards. Three of the trim pots literally fell off the channel strips when we pulled them out, having broken right off at the pin base. I really couldn't see anyway this could have happened as a result of end-user abuse so my guess is they were cracked at some point during manufacturing and just didn't separate completely until some point after the board passed QC.

I have managed to find some 10k, 16mm, stereo anti-log pots with 6mm split fluted shafts at a place called Mojo Music down in NC. I only need singles but these should work. (The closest place I found pots in exactly the right package and taper was in Australia and they wanted AUD 10 apiece for them!)

Also figured out (in case anyone was wondering), once I had one of the broken pots in hand, that something more like a 50k linear pot with a 12.5k resistor in parallel would have made a better match for proper 10k reverse log pots than 20k/20k. Anyway, the board is back together now with all mic preamps working. The 20k pots with 20k resistors don't provide enough taper for a smooth control feel but the new pots from Mojo are on the way.

Thanks again to everyone who chimed in on this! I can't tell you how much I appreciated the advice and morale support!

[Harry Brill Jr.]

Mojo is a hip place. All kinds of replacement parts for classic guitar amps.
That mixer is most likely a DDA in disguise if it's individual channel strips. I just used one a few weeks ago oddly enough. It was in perfect condition, all 12 channels. The PSU was rather large for a 12ch. DDA may know where to get any future parts. http://www.ddaconsoles.com/

[Calvert Dayton]

I definitely thought Mojo was a good find. Other places I saw that specialize in vintage amp parts were selling pots (but not the ones I needed of course) for like 3, 5 even 10 bucks in one case -- just plain old Alpha carbon pots, the same ones you can buy from Futurlec (http://www.futurlec.com/) for 50 cents (if you don't mind waiting a week and a half for them to arrive by mail from Bangkok).

Incidentally, I also picked up some of their Mojo brand TRS connectors for 2 bucks each and some bulk 24 Gauge VTG mic cable for 40 cents a foot, which was one of the better prices I have seen on shorter lengths. I was pretty pleased with the connectors when they arrived. They're made with nice heavy gauge stock and seem pretty substantial. I think I'll order some more of them.

I don't think this console is a DDA design though. EV was still Mark IV when this beast was made. My guess is it's a homegrown design from EV's glory days. Once upon a time they made everything PA, from the microphones to the speakers and all the parts in between. 20 years ago, mixers with separate boards for each channel were also probably still more the rule than the exception.