HI

i recently got this SSI passive summing mixer, 8 channels with a Melcor 1731 DOA makeup gain output.  there are some mods I would like to do with it, but first I'd like to get it functioning as it should stock before I mess with it.

For 1 thing, the panners are all dying, they use these Ohmite 10k type AB panning potentiometers, its a double pot, with one for left and one for right, I don't see any slugging resistors, but there is a significant drop in level when at center, and thre signal cuts out randomly when turning the panner, so I'd like to replace them, maybe with a single center dent pot,. can anyone recommend a pot that would be compatible? I believe the input transformers are wired for 4k input impedance and 600ohms output to the channel gain pot( is 600 ohm), and they seem to work fine.

there is no master gain, but I do notice that there is a difference in level between the left and right channels, but there are no trim resistors in the master section, so I was wondering if I can add one, or replace a certain resistor with a trim pot resistor.  I cant figure out the value of the feedback resistor, the 1st color is a cream colored bar, it says 1k under where both feedback resistors are but they are not.  also there is a 47k resistor tied between the Trim of the 1731 and the V+, and a 320uf 6.4v cap between the output of the DOA and the output.  the input from the 1k buss resistors go to the negative input of the DOA, and the positive input is tied to the COM and then to ground, i'm assuming then that the black is the HI out and the red the LO?  ive attached a couple photos for reference, any help I'd appreciate it.  thanks!
« Last Edit: June 13, 2013, 02:53:23 PM by Fuccimain »


and the drawing..
« Last Edit: June 13, 2013, 09:43:43 PM by Fuccimain »

whole board

upon further studying I. guessing that the output amps are inverting, since the + is tied to ground and the - is from the mix buss, the input transformers are wired out of phase as well, from reading the type on the transformer it says( if I'm reading it correctly) for 4k: Connect 1&4 and join 2&3, 1 would be the positive and 4 the negative, but as elsewhere, the red cable goes to 4"-" and the black the to 1"+", on the barrier terminals in and outs, the red is to "2" the Black is to "1" and G to ground..

so I'm thinking the signal inverts at the input transformer and then inverts again at the  DOA, so I should wire the unit as normal, with the hot to pin 2 on barrier and low to 1, on the in and outs..I believe that's correct.

So i've learned that this is an Active mixer, not a passive.. so the flip on phase at the input transformer, allows the inverting amplifier to flip the phase back to the original phase.

i've never seen anyone use the "Trim" pin on a DOA, so I'm curious what the 47k resistor is doing, and wondering if that is somehow regulating the V+ and if I could add a variable resistor/trim there, as the L&R gains are uneven.

EmRR

Pull the FB resistors and measure them.  If they don't match one another, replace with ones that do.  See if gain matches.  I just love that particular cream color WTF? that shows up from time to time, we clearly have all the rest of the color code, a multimeter should tell you the rest.  Any reason you want a different type pan pot?  The mixing loss will relate to the panner value.  I'd probably buy new dual linear 10K's and add the appropriate slugging resistors. 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

abbey road d enfer

so I'd like to replace them, maybe with a single center dent pot,.
Don't do that. Single-element pan-pots f**k the level diagram big time. You lose 10dB at least, that you have to recover with extra gain, which adds noise.
Quote
can anyone recommend a pot that would be compatible?
Apart from the performance aspect, it's a significant reworking of the circuit that would involve a lot of track-cutting and little green wires. Anyway, the actual pots are Allen-Bradleys, if I believe my eyes. These should last a lifetime or two. If one became faulty, I would get to the manufacturer, who probably would have it replaced for free.
Quote
there is no master gain, but I do notice that there is a difference in level between the left and right channels, but there are no trim resistors in the master section, so I was wondering if I can add one, or replace a certain resistor with a trim pot resistor.
Definitely you need to check the value of these FB resistors. You can replace one of them with a fixed resistor and a trimmer in series. What level difference are you experiencing? Anything more than 1-2 db points to a serious fault or mistake.
Quote
I cant figure out the value of the feedback resistor, the 1st color is a cream colored bar,
Can't you control them with a meter?
Quote
also there is a 47k resistor tied between the Trim of the 1731 and the V+,
I'm just guessing here, but it's educated guess...this resistor sets the operating current of  one or several stages in the DOA, in order to optimize performance. Typically it could control the input pair current for noise optimization (more current=>better noise performance at low impedance source), output capability or operating voltage.
« Last Edit: June 15, 2013, 05:35:30 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

I don't see any slugging resistors, but there is a significant drop in level when at center,
It seems that these 10k pots are loaded with 1k bus-injection resistors! No doubt there would be a huge drop at center. What drop are you measuring?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

thanks for all this info!  it's going to help me out a bunch. I'm going to take some measurements with my meter and report them here.

I don't necessarily "want" to change the panniers, but since they are faulty (cutting in and out) and I'd like to have center dent ones ideally, so I'm looking for options.  I'll keep them as dual linear 10ks and try to figure out the appropriate slugging resistors as I'd like a 3db drop at center.

one thing i'm curious about is the phase of the unit, assuming Red is supposed to be HOT "+" and Black COLD "-", if it's inverted at the input of the  transformer, is it inverted again at the output of the transformer? if the COLD output of the transformer is used as the HOT? that's 2 flips of the phase correct?  and the output amplifier flips it again, which makes 3 total which means the unit is running at 180 reverse.  I'm just curious at the master output if I should take the output of the DOA as the HOT and the COLD off the NFB path, cause right now the HOT is taken form the NFB path and the COLD from the output of the DOA.


I also noticed there seems to be a resistor missing on each channel, although it isn't obvious what it was tied to as both ends are tied to ground I believe.  Some tech must have removed them, but perhaps they were jumped somewhere else on the circuit, perhaps this is the missing slugging resistor placement, although since dual linear pots were used, there would have to be 2 slugging resistors per channel correct? you can se the place for the missing resistors in the photo.

peace

ok, so I had to wait for my new digital multimeter to arrive as my old analog one bit the dust.. and I wanted to be more accurate with values anyway ..

the feedback resistors are measuring as 12k7 for one and 12k5 for the other,

the buss resistors are 1k1


the capacitor from DOA output to the out of the mixer says 320 uf 4v, but they are measuring as 550 uf for one and 530uf for the other. should I replace them with new 320uf caps? or should I change the value of them? curious what would be the best type of cap for this, since it's in the signal path.

i'll measure the db losses at center and between the outputs late tonight or early tomorrw as I'm off to a job, but i'm curious how I go about finding out what value slug resistors I need  to have around a -2.3 db drop at center assuming everthing is working correctly,.. since right now the 10k pan pots are going to 1k1 buss resistors..

thanks for all the help...

sean


abbey road d enfer

ok, so I had to wait for my new digital multimeter to arrive as my old analog one bit the dust.. and I wanted to be more accurate with values anyway ..

the feedback resistors are measuring as 12k7 for one and 12k5 for the other,

the buss resistors are 1k1
Are they really? They should be about 10x more. Have you measured them in-circuit or have you lifted one leg?
Quote
the capacitor from DOA output to the out of the mixer says 320 uf 4v, but they are measuring as 550 uf for one and 530uf for the other. should I replace them with new 320uf caps? or should I change the value of them? curious what would be the best type of cap for this, since it's in the signal path.
same question; measured in-circuit or one side disconnected?
Quote
i'll measure the db losses at center and between the outputs late tonight or early tomorrw as I'm off to a job, but i'm curious how I go about finding out what value slug resistors I need  to have around a -2.3 db drop at center assuming everthing is working correctly,.. since right now the 10k pan pots are going to 1k1 buss resistors..
If that was really the case, the "slugging" resistors should be about 200 ohms and the panoramic law would be quite strange. First do the measurements as is, and measure the resistors with one leg lifted. Only a very poor designer would use a 10k pan-pot with 1.1k injection resistors.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

I lifted the whole resistor out of the circuit both legs ( for both feedback and buss resistors)

I did not remove the caps when measuring, should I do the same and take out of the circuit? Or is jut one leg ok? Positive or neg leg lifted is better?


I don't see any slugging resistors, but there is a significant drop in level when at center,
It seems that these 10k pots are loaded with 1k bus-injection resistors! No doubt there would be a huge drop at center. What drop are you measuring?

10k dual pan pots (ohmite type AB)
600ohm level pots
« Last Edit: June 22, 2013, 05:03:45 PM by Fuccimain »

Curious if it's possible the slug resistor is what's missing from the underside photo from like 4 or 5 posts ago... Wondering what was there... Guess it can't be th slug cause both holes are tied together... And ground....Curious..

abbey road d enfer

I lifted the whole resistor out of the circuit both legs ( for both feedback and buss resistors)
So it looks like a major design fault. That would lead to a center attenuation of about 15dB, which is ridiculous. Typically, with 10k pan-pots, you should see 10k injection res and 3.3k "slugging" resistors.
Pls note that the latter are to be connected between wiper and top.
Quote
I did not remove the caps when measuring, should I do the same and take out of the circuit? Or is jut one leg ok? Positive or neg leg lifted is better?
One leg is ok, doesn't matter which.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Ill check the caps tonight.
When you say typical,  what would the drop at center be?



Out of curiosity, if I wanted to use 47k buss resistors... What would I need to do regarding the 10k pans and 3.3k slug resistors and I guess 600 ohm level pots to make the circuit work? A mod I'm thinking of doing to the master section calls for 47k buss resistors...

Thanks again Abby road


I lifted the whole resistor out of the circuit both legs ( for both feedback and buss resistors)
So it looks like a major design fault. That would lead to a center attenuation of about 15dB, which is ridiculous. Typically, with 10k pan-pots, you should see 10k injection res and 3.3k "slugging" resistors.
Pls note that the latter are to be connected between wiper and top.
Quote
I did not remove the caps when measuring, should I do the same and take out of the circuit? Or is jut one leg ok? Positive or neg leg lifted is better?
One leg is ok, doesn't matter which.




When u say "top" , the slugging resistors get connected the wiper and the top of buss resistors??

I lifted the whole resistor out of the circuit both legs ( for both feedback and buss resistors)
So it looks like a major design fault. That would lead to a center attenuation of about 15dB, which is ridiculous. Typically, with 10k pan-pots, you should see 10k injection res and 3.3k "slugging" resistors.
Pls note that the latter are to be connected between wiper and top.
Quote
I did not remove the caps when measuring, should I do the same and take out of the circuit? Or is jut one leg ok? Positive or neg leg lifted is better?
One leg is ok, doesn't matter which.

I wish I could find the schematics for this... I highly doubt these are the original values as looks like it been modified, especially due to those missing components .. Plus Neil Muncy (the designer), from what I've read, seems to be pretty picky with components and values and was a highly regarded engineer, so obviously somethig was changed at somepoint.   I wish I had been able to speak with him, as he passed away last year , to get the schematics. Apparently he was a really nice person and would dig out the tech docs for DIyers working on his old gear. I wonder who is handling his estate, I wonder how to find that out.

There is someone selling a whole lot io SSI tech docs on eBay for $40... I really want them, but I feel like that sort of stuff should be made available for free on the net, especially when the designer was willing to give them out.

Anyways, sorry for the rant.
« Last Edit: June 23, 2013, 01:21:41 AM by Fuccimain »

abbey road d enfer

Ill check the caps tonight.
When you say typical,  what would the drop at center be?
Read my previous post.
Quote
Out of curiosity, if I wanted to use 47k buss resistors... What would I need to do regarding the 10k pans and 3.3k slug resistors and I guess 600 ohm level pots to make the circuit work?
3.3k wouldbe right. In fact for 10k bus resistors, 2.2k is the right value.
Quote
A mod I'm thinking of doing to the master section calls for 47k buss resistors...
How can that be?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

abbey road d enfer

When u say "top" , the slugging resistors get connected the wiper and the top of buss resistors??
Between top of the pot (the point(s) where the signal from the level pot's wiper is connected, and the wiper(s).
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Ill check the caps tonight.
When you say typical,  what would the drop at center be?
Read my previous post.
Quote
Out of curiosity, if I wanted to use 47k buss resistors... What would I need to do regarding the 10k pans and 3.3k slug resistors and I guess 600 ohm level pots to make the circuit work?
3.3k wouldbe right. In fact for 10k bus resistors, 2.2k is the right value.
Quote
A mod I'm thinking of doing to the master section calls for 47k buss resistors...
How can that be?

Yeah say 3.3k resistors, but I was curious what drop in db that would be at center.. Didn't see that in post, I see 15db cause if the bad 1k1 Russ resistors

You now saying 3.3k would be good for 47 k buss resistors and 2.2 k good for. 10k buss resistors

As far as the mod, the value components it comes with is based on the mix bus resistors being 47k:
http://classicapi.com/catalog/product_info.php?cPath=22_46&products_id=81



 I pulled a leg on each cap, measuring 470uf for one and. 450uf for the other


 

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