Looking for a schematic please, DOD 1642 XLR II mixer

[stagekraft]

Hey All,

Here I am again, looking for a schematic for a piece of vintage audio gear...

Seems I keep coming up with old stuff that I can't find info on with a simple internet search.

Now, I'm looking for a schematic for a 80's/90's vintage DOD 1642 XLR II, 16 channel rackmount mixer.

Digitech/DOD did not freely distribute schematics, unless you were an authorized repair center.
Now, they are pretty much defunct...
So, I'm hoping someone here has this archived...

Can anyone out there help?

Regards,
JohnR

 

[stagekraft]

hey all, just giving this a bump...
certainly someone out there has some info on this mixer?

JohnR

 

[JohnRoberts]

I'm not sure I'd call that vintage, just old.

Most value mixers back then used pretty similar designs and mixers follow familiar patterns. VE sum bus, etc

JR

 

[stagekraft]

Well John,

Thanks for the reply there, and I am very familiar with the typical designs of the day,
I worked for a "then prominent" sound manufacturer back in the 70's to mid 80's,
where I designed a couple of mixers and other products (does that make me vintage, or just old?).

One of my mentors there, went to Electro-Voice for a while, then moved on to Digitech/DOD.
He tells me that this mixer was one of his first projects there, but unfortunately, he doesn't remember
much about it.

What I do know about it, is it used the Analog Devices SSM2017 for the mic preamp, then a typical
3-Band EQ, followed by Pre/Post sends, Pan/Sub routing etc (and this is just by tracing what I can see from the solder-side)...

What I really want to know about at this point, is the Main slider on each channel, this is not the typical
"thru the pot fader", but rather, the fader controls some sort of VCA or something (8-pin SIP IC).
And of course, all of the Input components are on one big PCB, and I will have pull all the pot knobs off to get the
PCB out and actually see the components (and then hope they didn't wipe the numbers off the chips).

Now, it seems these VCA's (or whatever) are all working, but turns out I have an open slider on one channel, and when I first powered the mixer up it had an "ocean-roar" like open loop gain noise, I soldered a 10k resistor across that slider which brought that channel to about mid-slider level. and again, I'll have to pull all the knobs and such to replace the slider.

So the main reason for wanting a schematic is to clarify what this Main slider circuit is, and also, it has solder-bridge jumper pads, that allow you to change the Pre/Post attribute on some of the sends, so I would like to see documentation on that.

Regards,
JohnR

 

[JohnRoberts]

well if you designed mixers you know....

See if you can identify the 8 pin sip... that corp puts 2181 VCA in SIP package but I don't know why DOD would be putting VCAs inside a small rack mixer.

1642 is a very popular mixer number (16 inputs, 4 subs, stereo mains ).

The 8 pin sip might be a dual op amp.

JR (aka JohnR)

 

[stagekraft]

I bought this off ebay a couple months back, was taken out of an old church install, and I was hoping by it's age that it would
be built a bit more robust (individual input cards with real pots and such)...

And the nostalgia, that a buddy of mine was involved in it's design 30 years ago...

My intention was to use it in a small rental PA (in place of a Behringer 1224 that has some dirty pot issues).

It was missing the external power supply (+/-15, +12, +48, with a 7-pin DIN connector),
I built up the needed supplies, and found a connector to match...

I used it for a little "Cinco" gig last week, and was happy with it, now I guess I'll take it back to the shop in dive in...
Was just hoping for some documentation before I did, instead of going in blindly, mostly I just need to replace the bad slider,
and hopefully my exploratory surgery will tell me a little more about it..

JohnR (aka Stagekraft)

 

[JohnRoberts]

I used it for a little "Cinco" gig last week, and was happy with it, now I guess I'll take it back to the shop in dive in...
Was just hoping for some documentation before I did, instead of going in blindly, mostly I just need to replace the bad slider,
and hopefully my exploratory surgery will tell me a little more about it..

JohnR (aka Stagekraft)

It is pretty common to put a +10dB gain stage following most channel faders. If that sip is an op amp gain stage you can suss that out by resistors connected to it. It is also common to make the topology of that gain stage differential referenced to the fader gnd to assure good fader kill.

JR

 

[Analog_Fan]

My intention was to use it in a small rental PA (in place of a Behringer 1224 that has some dirty pot issues).

one asked me to open a small Behringer mixer, maybe it was that one and see why it hums.

it must have been designed in China by some secret copy and paste agent.
literally every PCB trace on the board had hard 90º angled turns.
Apart from including propertary coolaudio ic's in it, but Uli has your money.

 

[stagekraft]

I decided to jump in and see what I have, knobs came off easier than I expected (push-on D-shaft), no pot nuts to deal with...
a dozen small screws and the whole PCB dropped out...

I have not traced it all out yet, but the SIP is indeed a dual op-amp, signal comes into first half which is an inverting amp with the slider in the feedback loop, kinda like the Input Gain I would do at the Mic pre-amp, (so yep, open slider = open loop gain = noise), although I would have paralleled a resistor with the pot to limit the gain if the pot went bad...

The slider is made by Alpha, 50K, D-taper, low profile, 60mm, I don't have a clue what the part number would have been 30 years ago, but going on the TaiwanAlpha website, I decoded their numbering system into part number; RA6043F-20 15C1 D50k
Although they don't list the D-taper, but I'm guessing an A-taper would be close...

But so far, a search for this pot has been fruitless...
I sent an email to [email protected], we'll see if I get a response...

attached is a pic of the business side of the PCB with the bad slider removed....

JohnR (aka Stagekraft)

 

[JohnRoberts]

I have not traced it all out yet, but the SIP is indeed a dual op-amp, signal comes into first half which is an inverting amp with the slider in the feedback loop, kinda like the Input Gain I would do at the Mic pre-amp, (so yep, open slider = open loop gain = noise), although I would have paralleled a resistor with the pot to limit the gain if the pot went bad...

+1 not only failed pots, but wiper bounce can cause scratchy noise as it modulates the op amp gain. Besides a WTF resistor, a cap across the whole thing can damp wiper bounce noise.

The slider is made by Alpha, 50K, D-taper, low profile, 60mm, I don't have a clue what the part number would have been 30 years ago, but going on the TaiwanAlpha website, I decoded their numbering system into part number; RA6043F-20 15C1 D50k
Although they don't list the D-taper, but I'm guessing an A-taper would be close...

But so far, a search for this pot has been fruitless...
I sent an email to [email protected], we'll see if I get a response...

attached is a pic of the business side of the PCB with the bad slider removed....

JohnR (aka Stagekraft)

Have you tried Mouser? they apparently carry some alpha slide pots.

JR

 

[Analog_Fan]

is this the one?


https://www.jameco.com › z › RA6043F-20-10EB1-A50K-Taiwan-Alpha-Slide-Potentiometer-Audio-Taper-50K-Ohm-100mW-20-60mm-Slide-Travel_2268759.html

RA6043F-20-10EB1-A50K: Taiwan Alpha : Slide Potentiometer Audio Taper ...​

Order Today Taiwan Alpha Slide Potentiometer Audio Taper 50K Ohm 100mW 20% 60mm Slide Travel. Authorized Distributor - Products in stock ready to ship. RA6043F-20-10EB1-A50K: Taiwan Alpha : Slide Potentiometer Audio Taper 50K Ohm 100mW 20% 60mm Slide Travel : Passive Components

datasheet: https://www.jameco.com/Jameco/Products/ProdDS/2268759.pdf

https://www.mouser.com/ProductDetail/Alpha-Taiwan/RA6043F-20-10EB1-A50K?qs=Dd62CwBrIOAznsWkuPvP0g==
datasheet: https://www.mouser.com/ProductDetail/Alpha-Taiwan/RA6043F-20-10EB1-A10K?qs=Dd62CwBrIOAi4PqILykqWg==

at $2.49 a piece, you can afford changing them all.
$2.07 at 25 pieces.

: )

nothing shows up with: 15C1

check the drawings if it matches with the leads your pot/fader.

that's why mixer are expensive, that much "hardware" and placing it.
not alot silicon stuff or is that on the underside?

 

[JohnRoberts]

is this the one?


https://www.jameco.com › z › RA6043F-20-10EB1-A50K-Taiwan-Alpha-Slide-Potentiometer-Audio-Taper-50K-Ohm-100mW-20-60mm-Slide-Travel_2268759.html

RA6043F-20-10EB1-A50K: Taiwan Alpha : Slide Potentiometer Audio Taper ...​

Order Today Taiwan Alpha Slide Potentiometer Audio Taper 50K Ohm 100mW 20% 60mm Slide Travel. Authorized Distributor - Products in stock ready to ship. RA6043F-20-10EB1-A50K: Taiwan Alpha : Slide Potentiometer Audio Taper 50K Ohm 100mW 20% 60mm Slide Travel : Passive Components

datasheet: https://www.jameco.com/Jameco/Products/ProdDS/2268759.pdf

https://www.mouser.com/ProductDetail/Alpha-Taiwan/RA6043F-20-10EB1-A50K?qs=Dd62CwBrIOAznsWkuPvP0g==
datasheet: https://www.mouser.com/ProductDetail/Alpha-Taiwan/RA6043F-20-10EB1-A10K?qs=Dd62CwBrIOAi4PqILykqWg==

at $2.49 a piece, you can afford changing them all.
$2.07 at 25 pieces.

: )

nothing shows up with: 15C1

check the drawings if it matches with the leads your pot/fader.

that's why mixer are expensive, that much "hardware" and placing it.
not alot silicon stuff or is that on the underside?

if the taper does not match, changing them all is not crazy...

JR

 

[Analog_Fan]

if the taper does not match, changing them all is not crazy...

JR

i knew about the A and B, but not D, did a quick search, but no result.

https://eepower.com/resistor-guide/resistor-types/potentiometer-taper/#
what is it?

 

[Analog_Fan]

the types of pots.

https://www.diystompboxes.com/smfforum/index.php?topic=64369.0
source: http://part.freelab.jp/image/VRcave.gif

D = log taper ; 10% (of total resistance at mid position)
A = audio taper; 15% to 20% (of total resistance at mid position)

In the old days there were a lot of D's. Perhaps around the late 70's A's started to become the norm.
Both were always available.

 

[stagekraft]

I've been to Mouser, Digikey, Newark, Allied and yes I saw that one at Jameco....
I don't think the difference between A + D taper is that significant, but the "15C1" is the Lever code,
thin metal, 15mm tall that matches my knobs. where-as the "EB1" is a short fat plastic with a marker in the center,
for use without a knob...

The next thought is; have they changed the mechanics of the design over the years, or could I take the element
off the new and put it on the old shell? I've done crazier things...

Also, I dis-assembled the bad one, and the "open" is on the carbon trace right before where the solder lug is riveted on.
Perhaps I can fix it with a conductive pen, but of course, mine is dried up...

Now, back to the schematic, while I don't need it for this repair, it is nice to have if just to "see how they did that"...

Which brings me to my next "Things that make you go Hmmm..."

There is a little daughter board mounted on a pin header, it is a dual 10 segment LED Meter.

And the first thing my brain picks up on is; Hmmm, I only see one LM3915...

 

[JohnRoberts]

I've been to Mouser, Digikey, Newark, Allied and yes I saw that one at Jameco....
I don't think the difference between A + D taper is that significant, but the "15C1" is the Lever code,
thin metal, 15mm tall that matches my knobs. where-as the "EB1" is a short fat plastic with a marker in the center,
for use without a knob...

The next thought is; have they changed the mechanics of the design over the years, or could I take the element
off the new and put it on the old shell? I've done crazier things...

I wouldn't feel lucky about trying that... Those are mature very high volume components so have probably been re-tooled over the years with succeeding generations to cut cost or improve feel. They were not designed to be taken apart and reassembled like P&Gs.

Also, I dis-assembled the bad one, and the "open" is on the carbon trace right before where the solder lug is riveted on.
Perhaps I can fix it with a conductive pen, but of course, mine is dried up...

if it conductive path opened because of flexing, an ink patch might just open up again.

Now, back to the schematic, while I don't need it for this repair, it is nice to have if just to "see how they did that"...

Which brings me to my next "Things that make you go Hmmm..."

There is a little daughter board mounted on a pin header, it is a dual 10 segment LED Meter.

And the first thing my brain picks up on is; Hmmm, I only see one LM3915...

It is possible to multiplex*** LM3915s while that does not seem wise in the middle of a mixer master section (clicky noises). The 3915 is not expensive enough to justify the effort, check that there isn't another one hidden somewhere nearby.

JR


*** back last century on my bench for chuckles I tricked a single LM3915 into simultaneously displaying a "peak" dot and continuous "VU" bar display. That trickery required some extra glue circuitry to alternate inputs etc.

 

[Analog_Fan]

I've been to Mouser, Digikey, Newark, Allied and yes I saw that one at Jameco....
I don't think the difference between A + D taper is that significant, but the "15C1" is the Lever code,
thin metal, 15mm tall that matches my knobs. where-as the "EB1" is a short fat plastic with a marker in the center,
for use without a knob...

is the pcb footprint the same?
what about the total length of the plastic tab of the EB1 versus yours.
Would it penetrate the cover/case if you would mount them?

I don't think you want to attach wires to pots in the middle of a sound path. wires could and probably act as antenna's.
maybe you could widen the slots of the faders in the case/cover carefully a bit with plastic masking tape, a "dremel" and sweet/soft file to be able to reach them new faders.
it's an old device, but you overhauled it, is it for personal use or selling it?

 

[stagekraft]

Ahh yes, the old Penney & Giles sliders, you could take them apart and wash them out...
We did some custom consoles back in the 70's using them...

But we are talking about cheap "china-doodle" sliders here... and I somehow doubt that the physical aspects have changed
much in the past 30 years..
Regardless, risking a couple of dollars on an assumption that I can Frankenstein a part is do-able..
Not like the P&G sliders that were $30 each back in the 70's, lord knows what they would go for now.....

Yes this one is for personal use, and if I was gonna replace all 18 sliders I would be up for a change, but right now I just wanna fix this one channel...

My first thought about the 3915 VU meter was some sort of multiplexing, I'll have to dig deeper to satisfy my curiosity.
Years back I made a compressor circuit using a 3915 in two different modes, but that was with a physical switch.
in one mode it showed the signal level in normal bar mode, then flip the switch and it was in dot mode (with inverted signal) showing the compression...

 

[Analog_Fan]

Yes this one is for personal use, and if I was gonna replace all 18 sliders I would be up for a change, but right now I just wanna fix this one channel...

wel, mouser will charge 20€ for the envelop to put it in, unless you spend 50€ and bit (25 pieces) and it's free.
so one will cost you almost 23€

Alpha is taiwan, i believe.

if it doesn't petrude enough, try what i said and widen the slot in the panel a bit.

 

[stagekraft]

I went ahead and ordered the ones at Jameco (RA6043F-20-10EB1-A50K), got 20 of them for $1.50 each...
I will figure out how I'm gonna use them when I have them in my dirty old hands....