Seek CBS FM Volumax Schematic
Old 23rd May 2012
  #1
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Thread Starter
Seek CBS FM Volumax Schematic

I'm looking for a schematic for the CBS FM Volumax. This was a solid state dual broadcast compressor from the 1960's. I have a clear schematic for the single channel non FM unit (which has some major differences), and a poor quality FM Volumax schematic that almost unreadable.

A clear copy of the FM Volumax schematic would be a great help. Thanks in advance.
Old 24th May 2012
  #2
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mjrippe's Avatar
 

Hi David,

If it is the model 4100/4110 I have the full manual w/schematic. Send me a PM.

Mike
Old 24th May 2012
  #3
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Thread Starter
It is the model 411. (Sounds like there were 4 models altogegther -- the 410, 411, 4100, and 4110.)

Someone else has kindly sent me the schematic but if by chance you have the text from the 411 manual that would be a help, and please PM me.

Thanks...David

Quote:
Originally Posted by mjrippe View Post
Hi David,

If it is the model 4100/4110 I have the full manual w/schematic. Send me a PM.

Mike
Old 24th May 2012
  #4
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The 411 was actually two 410's in a shared chassis to make a stereo unit. A pair of 410's could be strapped in the field to help convert a mono FM station to stereo Back in The Day.

Bri
Old 25th May 2012
  #5
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Thread Starter
Quote:
Originally Posted by brianroth View Post
The 411 was actually two 410's in a shared chassis to make a stereo unit. A pair of 410's could be strapped in the field to help convert a mono FM station to stereo Back in The Day.

Bri
Brian, are you sure that's true? I thought the 410, besides being a single unit, was the "non FM" version, without HF pre-emphasis compensation. But I'm not sure.

BTW, I'm now finishing the 411 I have on the bench, and it's turning out really nicely. I'm not usually a "mod guy" but this unit is pretty unsuitable for music recording as stock. I padded the input level by 18 db, added a Threshold pot and 3-position Release Time control, added a stereo link switch, moved the input and output pots to the front panel with knobs, and disabled the HF pre-emphasis and "symmetry" circuits. Very happy with it now -- it sounds and works great.
Old 25th May 2012
  #6
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The manual, referring to the 411, says "This instrument consists of two Model 410 FM Volumax coupled mechanically and electrically in a single case."

Best,

bri
Old 26th May 2012
  #7
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12ax7's Avatar
 

Old 29th May 2012
  #8
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Ike Zimbel's Avatar
 

Wow, thanks! I just had a 410 come in today!
best,
Ike
Old 29th May 2012
  #9
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Ike Zimbel's Avatar
 

Just got it going. Man, that is one cool box!
Cheers,
Ike
Old 29th May 2012
  #10
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12ax7's Avatar
 

Quote:
Originally Posted by Ike Zimbel View Post

Just got it going. Man, that is one cool box!
Yes, VERY cool (in my opinion).

Mine is modded in two ways:
1) I added switches to lift the clipper diodes when I want "clean".

2) I made the 75 microsecond pre-emphasis switchable (in/out).
.
Old 21st June 2012
  #11
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Thread Starter
I noticed a very strange thing about this unit that I want to pass on. In final tests I saw that on both channels, turning down the output level increased the amount of compression. This was not good, and initially made no sense at all!

Long story short, it turns out that the output amplifier in this unit is not a fully buffered low impedance driver, as one expects in solid state gear. The load on the output of the amp reflects back to its input. The output control is a constant impedance attenuator between the output amp and the output terminals, but it's only constant impedance if the output is terminated with 600 ohms. Which mine wasn't.

With no output termination the amp sees 600 ohms with the output pot all the way down, but with the pot full up it sees a much higher impedance. Therefore, when the pot is full up the output level of the output amp increases, as does the input to the output amp (strange, but true). The input of the output amp is connected to the input of the sidechain circuit. So turning the output pot down increased level to the sidechain circuit, causing more limiting.

The solution was simply a 649 ohm resistor across the output terminals. I use 649 because, in parallel with a bridging 10K load, the total is close to 600 ohms. I soldered it to the inside of the unit so that it wouldn't inadvertently be removed. This totally resolved the problem. Very strange, I must say! In 35 years of tech work, I had never seen a limiter do this before.

I'm filing this one under "design error".
Old 21st June 2012
  #12
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12ax7's Avatar
 

Quote:
Originally Posted by David Kulka View Post
I noticed a very strange thing about this unit that I want to pass on. In final tests I saw that on both channels, turning down the output level increased the amount of compression. This was not good, and initially made no sense at all!

Long story short, it turns out that the output amplifier in this unit is not a fully buffered low impedance driver, as one expects in solid state gear. The load on the output of the amp reflects back to its input. The output control is a constant impedance attenuator between the output amp and the output terminals, but it's only constant impedance if the output is terminated with 600 ohms. Which mine wasn't.

With no output termination the amp sees 600 ohms with the output pot all the way down, but with the pot full up it sees a much higher impedance. Therefore, when the pot is full up the output level of the output amp increases, as does the input to the output amp (strange, but true). The input of the output amp is connected to the input of the sidechain circuit. So turning the output pot down increased level to the sidechain circuit, causing more limiting.

The solution was simply a 649 ohm resistor across the output terminals. I use 649 because, in parallel with a bridging 10K load, the total is close to 600 ohms. I soldered it to the inside of the unit so that it wouldn't inadvertently be removed. This totally resolved the problem. Very strange, I must say! In 35 years of tech work, I had never seen a limiter do this before.

I'm filing this one under "design error".
I never noticed that before.

...But then again, I've always had mine terminated with 600 Ohms. (I'm old enough to remember a time when most pro gear was meant to observe true 600 Ohm balanced and floating x-former operation).

...Not sure I'd call this a "design error", though:

The truth is that almost ALL "vintage" pro gear (of this age) SHOULD be terminated in this way when patched into "modern" gear. (In some cases it makes a MAJOR difference if you want to really get that "rich" sound we expect from such gear!)

NOT doing this can do all kinds of wanky stuff (like eating up headroom, etc.)

You ARE correct that a 649 Ohm resistor IS a wiser choice, but I WOULD caution against doing this INTERNALLY, though:

If (for instance) you end up patching in another true 600 Ohm piece AFTER another (like an old Pultec or something), you can end up with the wrong load again!

My solution was to put a couple of termination loads on a couple of jacks in my patchbay, and then using one of the mults on the patchbay to add one of them to specific gear when needed.
.
Old 22nd June 2012
  #13
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Thread Starter
We always add 649 ohm resistors to vintage transformer-output gear that needs them, I just hadn't yet gotten around to doing it in this case. I'm kind of glad actually, because it brought this issue to my attention.

And I basically agree with your comment about wiring them internally. In all other cases I connect these resistors on the outside of the unit; in this case the risk of them accidentally being removed seemed greater than the customer double-loading the output by patching it to something with a 600 ohm input, and the customer is aware of this.
Old 22nd June 2012
  #14
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brianroth's Avatar
 

One issue comes when that vintage gear is patched into a bay with all sorts of OTHER gear...some may or may NOT have a input 600 Ohm termination.

IF you patch into a piece of gear...and some new gear qualifies...with an existing 600 Ohm input impedance AND you randomly add a 600 Ohm loader at the source, then you are double terminating....loading into 300 ohms.

Bri
Old 25th June 2012
  #15
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12ax7's Avatar
 

.
Just thought I'd add that the 410/411 model was made for FM radio, and in addition to employing HF pre-emphasis, it also has a HF limiter after the main limiter (but before the clippers).

This is one reason why it sounds so good on bass and toms.

When working with digital, it almost aproximates what happens when you're tracking those instruments to analog tape (and print it "hot").
.
Old 14th October 2012
  #16
Gear nut
 

Hi. I just want to add to this discussion that there is yet another version of the 410/411 for which very little info exists on the net. It's called the "Recording Volumax" and like the 410/411 it came in two models, one mono (Model 420) and one stereo (Model 421). I own a mono version (and the manual), and it is very similar in design to the 410/411, but was designed for "disc, tape and optical recording capability with no risk of overloading or distortion". From a quick comparison of the schematics for the 410 and 210, it looks like the main differences are some values in the frequency compensator circuit and at the input and output. I'll try to get a scan of this and post very soon.

@David Kulka - is there any way you could provide some more specifics on your mods? I would love to be able to use mine for actual recording - but as is, they are just noise boxes.

best - DT
Old 14th October 2012
  #17
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Old 14th October 2012
  #18
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Ike Zimbel's Avatar
 

Thanks for posting that!
best,
Ike
Old 9th December 2012
  #19
Gear nut
 

This is a bit of an aside in reference to Brian's double termination post above. I was just looking at the Urei LA-4 manual (the one freely available online by JBL), and it states that "the LA-4's inputs are suitable for use with virtually any source impedance, low or high." (PDF page 21).

So, if I have termination on the output of one LA-4 and then feed that output into a second LA-4 (with stated 40k input impedance) is it correct that I don't have to worry about double loading, since 300ohms is acceptable to bridge to the 40K input of the next device?

What's confusing is that the manual also seems to indicate that it wants 600Ohms at the input (noise specs on PDF page 14 are listed in regard to 600Ohm in and out).

Likewise, the 1176LN manual (also via the JBL site) states 20K input impedance and output load of 150 or greater... which would seem to imply that it can work in a bridging scenario (though maximum output is in reference to 600Ohms). On PDF page 11 of this manual, there is a section on Impedance and Termination, but no discussion of terminating the output with 600Ohms (just the input to match 600Ohm gear).

Further compounding my confusion… If you have two pieces of gear strapped together and you terminate between them… is this an output termination, input termination, or both at the same time?

Sorry if I'm not articulating this well….

best DT
Old 26th December 2012
  #20
Gear nut
 

Quote:
Originally Posted by 12ax7 View Post
Yes, VERY cool (in my opinion).

Mine is modded in two ways:
1) I added switches to lift the clipper diodes when I want "clean".

2) I made the 75 microsecond pre-emphasis switchable (in/out).
.
12ax7,
How this is made?? could you help me with that mod in more detail please??

Thanks
Old 30th December 2012
  #21
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12ax7's Avatar
 

Quote:
Originally Posted by opacheco View Post
12ax7,
How this is made?? could you help me with that mod in more detail please??

Thanks
Been away from GS for a few days, so I just caught this request...

...Anyway, in the next couple of days I'll try to pull the Volumax from the rack, open it up and snap a couple of pictures - and mark the schematics as well.

(Might take me a few days to get it together.)
.
Old 31st December 2012
  #22
Gear nut
 

Quote:
Originally Posted by 12ax7 View Post
Been away from GS for a few days, so I just caught this request...

...Anyway, in the next couple of days I'll try to pull the Volumax from the rack, open it up and snap a couple of pictures - and mark the schematics as well.

(Might take me a few days to get it together.)
.
12ax7,

Ok, I will wait all necesary for your explanations!
Thanks
Opacheco
Old 1st January 2013
  #23
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Quote:
Originally Posted by opacheco View Post
12ax7,

Ok, I will wait all necesary for your explanations!
Thanks
Thanks - I'll get it to ya as soon as I can.

... In the meantime, I thought I might point all interested parties to this little ditty from RadioWorld about the Audimax/Volumax combo (and its place in radio history):
http://www.radioworld.com/article/the-cbs-audimax-and-volumax/17232
.
Old 1st January 2013
  #24
Gear nut
 

12ax7 - thanks for doing this for us. I'm looking forward to getting the details so I can actually use my 411.

I think someone mentioned (but didn't link to) this thread in another post. It has some relevant details (but please still spell it out for us by marking up the schematic... I'm probably not the only total hack with electronics on this forum

Volumax

I still intend to scan and post that recording volumax manual....

best - DT
Old 2nd January 2013
  #25
Gear nut
 

Quote:
Originally Posted by 12ax7 View Post
Thanks - I'll get it to ya as soon as I can.

... In the meantime, I thought I might point all interested parties to this little ditty from RadioWorld about the Audimax/Volumax combo (and its place in radio history):
http://www.radioworld.com/article/the-cbs-audimax-and-volumax/17232
.
12ax7, that is a interesting link!!!.....very clear expalnation about of performer for the CBS Volumax Equipments.....you know what??, the Juticalpa radio station named in this link is almost 3 hours where I live here in Honduras.

Thanks and I will wait your own notes
Opacheco
Old 28th January 2013
  #26
Gear nut
 

12AX7 - have you had a chance to get those pictures? I would love to see them.

Thanks - D
Old 28th January 2013
  #27
Gear nut
 

12ax7, I am waiting for your notes too!!....

I will appreciate your help and time so much!!

Opacheco.
Old 31st January 2013
  #28
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12ax7's Avatar
 

.
Hang in there folks.

I have not forgotten you.

...Just need to get the camera, the time, and the Volumax (out of the rack) in the same room at the same time.

(Should happen soon.)
.
Old 8th February 2013
  #29
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12ax7's Avatar
 

.
Okay, folks:

I FINALLY have a camera that will work for this.

(...Now, if I can just find the proper USB cable for it.)
...Hang in there...
.
Old 9th February 2013
  #30
Gear nut
 

Quote:
Originally Posted by 12ax7 View Post
.
Okay, folks:

I FINALLY have a camera that will work for this.

(...Now, if I can just find the proper USB cable for it.)
...Hang in there...
.
Ohhh no!!!......jejejeje....Man I will wait for your info!

Thanks
Opacheco
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