CBS Volumax 4000

[mitsos]

Hi, anyone have a schematic for this thing? It's the later 1U solid state version...

thanks!

 

[RuudNL]

Something similar like this?

 

[mitsos]

Thanks, I've seen that one, but from what I read online, this version has no input transformer or T-pads (so maybe other differences?).  I have yet to open it as it's all riveted together   

I'll drill those out later today and have a look around.

Here are some photos of one that looks very similar.

http://www.flickr.com/photos/31603983@N05/sets/72157625257688095/

Haven't had a chance to study the pics, but I noticed one shows an MC845 flip flop, which is not on the 400 schematic, so I'm not sure how similar/different these might be from the 400.  I read somewhere online that the 4000 were newer (possibly cheaper, hence no input TX or T-pads) versions of the 400.

thanks!

 

[RuudNL]

From the internet:

"The first units were designed with tubes; but few were sold, as they cost nearly as much as a contemporary Ford or Chevy automobile.

These were updated by the successful, more cost-effective 400 series. Using early solid-state silicon transistors and a balanced amplification scheme, these boxes achieved level control by changing the bias on an intermediate stage's transistor bases, essentially varying the amp gain.

Well designed and built, they were easy to service, with each subsection on its own PCB. Input and output were transformer-coupled. The overall gain was sufficient to make up for all but the loss of the worst telco audio circuits.

The Achilles heels of these units were the 600-ohm, T-configuration wire-wound variable attenuators on the input and output. They were noisy from day one. Replacements are nearly impossible to find today.

The AM Volumax had a nifty but primitive asymmetrical diode clipper scheme to achieve higher positive modulation.

The FM Volumax was probably the first limiter to level the pre-emphasized portion of the upper end audio above about 2 kHz in a separate sidechain gain controller. The result was a more natural audio leveling that was not bottomed or "plunged" by a random high-end audio component like a record pop (remember records?).

A little lazy, the FM Volumax attenuated any peaks that got by via soft clippers made of complimental zeners on the line-level output.

The 400 series gave way to the functionally similar, yet smaller, single-rack-unit 4000 series. For this compactness, the T pads and input transformer were eliminated. "

 

[mitsos]

Yes, that's what I had found. Thanks for posting, I couldn't find it when I searched again.

So, I found out that the rivets don't need to come out. The thing just slides out of its case. They used extruded aluminum pieces for the sides, and the power and I/O just slides in and out of connectors fixed to the rear. Pretty neat. 

It looks almost identical to the pictures in the link in my previous post, just a slightly different power trafo (attached to the bottom plate instead of the rear like in that unit). 

Uses 5K pots (allen bradley type W) instead of Tpads and has only one audio transformer.  It's going to need some clean up and there are some obvious "modifications" (jumper capacitor, added resistor, and one resistor that was cut and later attempted to be repaired). 

I'm surprised there isn't a schematic online seeing as its old broadcast equipment.  It hasn't been turned on in a long time (missing power connector) but I might try my luck later.  8) 

 

[PRR]

> From the internet: .... achieved level control by changing the bias on an intermediate stage's transistor bases, essentially varying the amp gain.

FWIW: the transistor amplifier stages are fixed-gain. Gain control happens by varying bias current in CR7 CR9 CR10 CR8, which shorts-out the signal coming from R19 R20 R21 R23.

Signal rectifier is CR11 CR12. High output level pulls C14 more negative. Q14 Q15 buffer. R25 biases the gain-control diodes. There's some bias-compensation through R25 so Q4 Q5 don't thump so much.

The multi-level design and push-pull drawing make it hard to work-out nominal volts/amps. Looks like the maximum possible diode current is 10V/5K or 2mA. Each diode will be 15 ohms, the string is 60 ohms, source impedance around 4K, so 36dB ultimate attenuation. Diode signal voltage must be under 10mV for low distortion, four series is 40mV, there's voltage gain near 500 to the OT, so 40V output, both sides of a push-pull fed with 20V... the diode voltage is acceptable.

Source of 4K is about 1.4uV broadband hiss at idle. 40mV max voltage means 89dB S/N *minus* however much gain-reduction you need to take.

We shouldn't have pushed these things more than 10dB, but I did test to >20dB GR. So output S/N less than 70dB. Entirely ample for my old lo-watt AM station; and indeed for most pop-music in any decade. Not a stunning number next to some rather low-price limiters today.

> an MC845 flip flop

Has to be for Polarity Detector.

In AM modulation, you can NOT go to zero carrier (100% negative) but you can in theory go UP any amount you want (>100% positive modulation). If audio is symmetrical, both sides must be equal. And on-average, audio is symmetric (moments are asymmetric but either way, averaging symmetric). But the special case of Male Speech, audio is consistently asymmetric (for a given DJ). While it will change with DJ and with mike (we didn't fret the polarity), it isn't hard to sense and self-flip the audio on sustained asymmetry. Some radio-cowboy operators cranked-up the pos-side to 150%, 200%, got a LOUDer sound, but also used-up all the reserve in their modulators and clipped it. Eventually the FCC set a 125%(?) pos-mod limit. Which is just enough to be worth the cost of a flip-flop.

In my unit the polarity-flip was a relay. It held on sustained signal, but if there was asymmetry, the wrong way, and then a pause, the relay clacked. It happened rarely on records, fairly often when a new DJ took the mike, and occasionally throughout the day. (Once I understood it, and the limits of my low-power rig, I disabled it.)

> the rivets don't need to come out.

NO. It is Righteous Gear, not a Chevy fuel-pump. The 1u Volumax is so serviceable I used to tinker with it "on-air".

 

[topcat]

Just posted the 4000 manual in the tech docs area...  Nice sounding unit....worth restoring
Enjoy

tc

 

[mitsos]

I love this place..  thanks topcat, though the manual is for the audimax, not the volumax, do you happen to have the volumax as well by any chance?

PRR thanks for the breakdown. Do you know if this is specific to the 400? I'll have to pull the 4000 out again and compare, didn't have time to mess with it yesterday like I had hoped...

cheers!

 

[emrr]

Long ago I had the final Thomson-CSF versions of Audimax and Volumax.  I did some path shortening mods to the Volumax that I liked.  I know I have seen both manuals in PDF, somewhere out there.  You really need the whole manual to grasp what's going on, for adjustment.  I recall a pseudo T pad input with no input transformer, and an output transformer.  Setting the Volumax modulation to greater than 100% makes for some interesting forward sounding results on some sources, does something similar to what the AM-864 tube guy does with regards to asymmetry. 

 

[mitsos]

hmmm, pulled it out just now, and it may be the same, just mono.  For one it uses the same transistors but has the extra PCB with the flip-flop.  I haven't traced it at all, just cleaned off some dust (lots of that too!) to read the part numbers.  Full of those same bullet type (tantalum?) caps, marked 475M (4.7uF?). 

Interestingly, on the front panel, to the side of the output trim rotary switch, there is a "positive limit %" 3-way toggle switch, choices being 100, 120, and 300 (in red!).  These controls are on the flip-flop PCB by the way.

Will take a closer look later. thanks again topcat!

 

[emrr]

Those tants are the sound here. 

 

[mitsos]

cool! won't touch them for now...
I'm waiting for my friend to see if he has the full manual but will probably solder a power cable to it soon and see what happens!  ;D

 

[topcat]

I love this place..  thanks topcat, though the manual is for the audimax, not the volumax, do you happen to have the volumax as well by any chance?

PRR thanks for the breakdown. Do you know if this is specific to the 400? I'll have to pull the 4000 out again and compare, didn't have time to mess with it yesterday like I had hoped...

cheers!

Doh...one day I'll learn to read!...just about to post the 410-411 VOLUMAX manual in the tech Docs section.
There is an interesting if rather scathing review of the Volumax published by the BBC ref  L-063 1965/47 which can be found on the web.
Enjoy

tc

 

[mitsos]

you and me both!  Thanks for posting it anyway, the schematic looks about the same though I still didn't look too much into it. Going to hook it up to power today and hope nothing blows up.  Anyway, the manual you posted is just missing the polarity/flip-flop etc part of the volumax (the PCB on the right side of the case in the link I posted up above).  If you see it anywhere, let me know!

thanks again!

 

[emrr]

I think the only difference between the last CBS and first Thomson version is layout, with the later being a single board. Could be wrong....

 

[mitsos]

Hi Doug,

I looked for the thompson manual when you mentioned it but came up empty so far.  Thanks for the tip though, will come up eventually.

cheers!

 

[plumsolly]

I have a hard copy of the manual. Let me know if you need anything.

Best,

Ben

 

[mitsos]

oooh, I knew it was out there somewhere! Well, I need the manual but you probably don't want to get rid of it. (On the off chance that you do, I can send you an address and some paypal funds or... something else? ).  Or, can I maybe bother you for a scan of the schematics and whatever else you think might be relevant to testing/calibrating/modifying this thing for studio use? Not sure how many pages it is, but if you want to scan it and post to the tech docs, it will prbably help a lot of people. 

It looked to me that the manual topcat posted for the audimax has the same I/O PCB (but doesn't have the other PCB) though I only looked at it for a few minutes, I didn't have enough time to check it and am on the road atm, so won't be able to for another week or so.

Anyway, let me know what you can be bothered to do, it's not a super huge rush, but if I could have this thing working in a month or so, that'd be great.

cheers!

 

[Mike Cleaver]

If your Audimax or Volumax came from a radio station (where most of them went,) you'll probably find it was modified (slightly or majorly) by some very talented broadcast engineers who may or may not have documented those changes.
For their time, they were pretty good out of the box compared to what else was available but most engineers felt they could make them "better." 
Some were successful, others, not so much.
You'll probably be hard pressed to find any two that are exactly alike, unless they were being used for FM Stereo.

 

[mitsos]

Thanks mike, yes this one has been modified in several places.  No idea where it came from originally, but a couple tant caps have been jumpered, there are some mica caps that dont look stock, one attached between test points, for example, some added resistors, and one cut resistor.