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CBS AudiMAX II in the sick bay

Member for

14 years 8 months
In for a recap and retube and a few modifications. It’s missing the memory module. so it will need some thinking...     Looks like a nice build so far..
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Member for

14 years 8 months

Link555 Mon, 01/25/2021 - 16:16
No idea, but from what I gather it helped introduce the 10 Sec recovery time constant.
https://groupdiy.com/index.php?topic=914.0
https://www.radioworld.com/miscellaneous/the-cbs-audimax-and-volumax

The unit I have on the desk is not stock it has added switch with some sort of filter circuit, I will need to trace it out to figure what it is doing.

Member for

19 years 11 months

Davedog Mon, 01/25/2021 - 22:59
Is this some sort of box that was intended for timing speaker banks in a fixed in place PA system in a hall of some sorts? Many years ago when I was young and relatively stupid, I worked some shows at the Mabee Center @ Oral Roberts U. in Tulsa. They had a center pod and the room was in round. There were banks everywhere and down a manhole walkway was a room full of CBS Laboratories delays, switchers, compressors, companders, etc.... and racks and racks of power amps. Computer floor. 1970's This looks like something from that era

Member for

14 years 8 months

Link555 Tue, 01/26/2021 - 06:52
I am struggling to see how the JLM mod works? I am sure he is right, but I can't help but think we has the 6AL5 wired backwards. Does the current flow through a vacuum diode opposite to a regular diode?
 
To me it looks like he has his positive voltage source connected to the Cathode?
http://www.jlmaudio.com/Audimax II JLM Mod.pdf
What I am missing here?
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Member for

15 years 7 months

Boswell Tue, 01/26/2021 - 08:13
Yes, I read about the supposed origins of the CBS gain control units in the groupdiy threads. My guess is that the two different compressors (Audimax and Volumax, for operation in series) were originally designed and built for the US AM radio market. It's then possible that a military comms engineer spotted the products and wondered if they would work in the appalling acoustic conditions found inside a tank. He approached CBS Labs, who responded by adapting the threshold levels and time constants for those conditions. I still shudder at valve (tube) products being used in critical applications inside the first large passenger aircraft, let alone a tank.

Regarding the 6AL5, it's a rectifier, so, in signal terms, a lot of the time the cathode will indeed be more positive than the anode, and this will only reverse for input peaks greater than the threshold. In d.c. terms, the polarity is all down to what's inside the "memory unit" plugin. If that is unchanged from an original CBS potted unit, pins 3 and 9 are not shown with any connection inside the compressor, so it's hard to guess at how the wired mod is supposed to work.

I find the CBS circuit diagram hard to follow. It was probably drawn by a graphic artist who made it his job to get everything on one page without any understanding of the way signals should be represented flowing across a schematic.

The Physics and Radio Electronics diagram is about as misleading as you could find. The purple arrows flowing into the anode (plate) and round into the +ve terminal of the power supply are clearly meant to show electron movement, but labelling it "Electric current flows" is opposite to what is normally meant by a current.

Member for

14 years 8 months

Link555 Tue, 01/26/2021 - 08:52
Boswell, post: 467077, member: 29034 wrote:
Yes, I read about the supposed origins of the CBS gain control units in the groupdiy threads. My guess is that the two different compressors (Audimax and Volumax, for operation in series) were originally designed and built for the US AM radio market. It's then possible that a military comms engineer spotted the products and wondered if they would work in the appalling acoustic conditions found inside a tank. He approached CBS Labs, who responded by adapting the threshold levels and time constants for those conditions. I still shudder at valve (tube) products being used in critical applications inside the first large passenger aircraft, let alone a tank.

Regarding the 6AL5, it's a rectifier, so, in signal terms, a lot of the time the cathode will indeed be more positive than the anode, and this will only reverse for input peaks greater than the threshold. In d.c. terms, the polarity is all down to what's inside the "memory unit" plugin. If that is unchanged from an original CBS potted unit, pins 3 and 9 are not shown with any connection inside the compressor, so it's hard to guess at how the wired mod is supposed to work.

I find the CBS circuit diagram hard to follow. It was probably drawn by a graphic artist who made it his job to get everything on one page without any understanding of the way signals should be represented flowing across a schematic.

The Physics and Radio Electronics diagram is about as misleading as you could find. The purple arrows flowing into the anode (plate) and round into the +ve terminal of the power supply are clearly meant to show electron movement, but labelling it "Electric current flows" is opposite to what is normally meant by a current.
Thanks Boswell, In the JLM mode he removed the Memory module and replaced it with this...
http://www.jlmaudio.com/Audimax II JLM Mod control box.pdf

Adding the above circuit in place of the memory module as per JLMs mods :
http://www.jlmaudio.com/Audimax II JLM Mod.pdf

I think has a max 61.48VDC threshold voltage applied to the R36 and R37 which through R10 and R11 is connected to the Cathodes of Vacuum diodes. Where the anodes go back the center tap of the input transformer. So I am correct in my understanding that any peak above the threshold voltage will be applied to one side of C8 and C9? The other side of the caps essentially tied to the output transformer. The primary center tap of the output transformer is tied to 265VDC?

Oh forgot to include that I inspected the J2 plug in module which shorts 1 to 4 and 2 to 3. This was removed for stereo operation...

Member for

15 years 7 months

Boswell Wed, 01/27/2021 - 05:11
I had a better look at the JLM modifications last night. One thing I really like about the original CBS design is that it is a fully-differential circuit from input to output, and this means that common-mode points (such as the centre-tap of the input transformer secondary) can be used to set d.c. operation points separately from the a.c. signal passing through. It's a bit like using phantom power on a microphone XLR input.

What JLM have done is (a) unhitch the input transformer secondary CT from ground and make it available to the compressor control voltage, and (b) feed the transformer secondary signals (d.c. coupled) straight to the grids of the 6386 vari-mu triode, bypassing (and removing) the 6DK6 input pentodes. If you look at the transfer characteristics of a cascode-connected 6386 (shown below), you can see how much the transconductance can be made to vary with the first-stage grid voltage. What the JLM mod does is add the control voltage to the signal, so that the gain of the triode stage is controlled by the amount of (negative) grid voltage applied to the input transformer CT. The 6AL5 dual rectifier is wired to produce these negative voltages, which are modified by attack and release time constants in the plug-in box.

Neat.

 
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Member for

14 years 8 months

Link555 Wed, 01/27/2021 - 06:30
THANK YOU Boswell, that was exactly what I was struggling with. I was stuck on the negative control voltage for some reason... I really don't know why...most compressors I have seen used negative control voltage... anyway thanks Boswell this really helps!

I received the cap and tubes in record time. Ordered Saturday arrived Tuesday...that's quick for my area. Started to sort out the other mods done to this unit. It looks like the last person working on this removed the memory Module and changed the 6386 to a 5670. Then added a switchable set of RC filters to 6AL5 anodes. The yellow caps in the picture are the added filters.

 
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Member for

15 years 7 months

Boswell Wed, 01/27/2021 - 06:57
I'm not surprised it had a 5670 instead of the 6386, as 6386s tested for matching halves have not been available for many years. The 5670 is about as close a replacement as can be found these days, but apparently does sound different at higher compression levels.

Here's an interesting piece on the Manley site about the 6386 availability problem, and the lengths that some will go to overcome it!

Member for

15 years 7 months

Boswell Wed, 01/27/2021 - 07:36
Looking back at my notes from perusing the schematic yesterday, I had jotted down a further couple of minor points that I'll mention here for completeness.

1) Bypassing the input pentodes changes the overall signal polarity from inverting to non-inverting, so either the wiring on the input or the output transformer will need to be changed to correct the input-output polarity. It's probably part of the JLM mods anyway.

2) The resistance values are strange in the R14, R15 meter adjustment. The other circuit half has 240R as the cathode resistor, but the meter feed section has 300R paralleled with 1K2 to 2K2 plus the meter resistance. This can only get down to 240R (to match the other half) when the pot is shorted out and if the meter has zero resistance. I would have thought that resistive balance at the 6386 cathodes was critical, as it determines the transconductance matching of the two sections. Prior to this schematic, I had not before seen the self-bias voltage of a valve being used to set the 0dB compression level on the meter. I think you should squeeze in a FET to buffer the cathode voltage to the meter and then change the cathode resistor to match the other half. Not.

Member for

14 years 8 months

Link555 Wed, 01/27/2021 - 07:56
Boswell, post: 467104, member: 29034 wrote:
I'm not surprised it had a 5670 instead of the 6386, as 6386s tested for matching halves have not been available for many years. The 5670 is about as close a replacement as can be found these days, but apparently does sound different at higher compression levels.

Here's an interesting piece on the Manley site about the 6386 availability problem, and the lengths that some will go to overcome it!
I managed to get 6386 with no issue,maybe the supply chain is better now.

1) Bypassing the input pentodes changes the overall signal polarity from inverting to non-inverting, so either the wiring on the input or the output transformer will need to be changed to correct the input-output polarity. It's probably part of the JLM mods anyway.
I don't where the polarity was swapped on the JLM mod schematic, but it could be easily done via the J2 adapter.


2) The resistance values are strange in the R14, R15 meter adjustment. The other circuit half has 240R as the cathode resistor, but the meter feed section has 300R paralleled with 1K2 to 2K2 plus the meter resistance. This can only get down to 240R (to match the other half) when the pot is shorted out and if the meter has zero resistance. I would have thought that resistive balance at the 6386 cathodes was critical, as it determines the transconductance matching of the two sections. Prior to this schematic, I had not before seen the self-bias voltage of a valve being used to set the 0dB compression level on the meter. I think you should squeeze in a FET to buffer the cathode voltage to the meter and then change the cathode resistor to match the other half. Not.
That's a good thought, I am sure I can figure out a way to buffer the meter impedance from the cathode voltage.... Thanks for that!

Member for

14 years 8 months

Link555 Thu, 01/28/2021 - 06:51
I had a good look at the input transformer last night, mainly because I got confused ;)

The Freed 34776 input transformer has 4 isolated windings according to the schematic. The wiring looks correct for pins 1,3,4,6,7 and 10.
Checking with ohmmeter I confirmed connectivity between pins 1 and 3 and pins 4 and 6. I had trouble measuring the secondary coils.
 
Then I noticed something odd, pins 8 and 9 are not, and have never been connected?
Bad picture but.... no solder on pins 8 and 9 ....

 
I checked for conductivity between pins 8 and 9 and confirmed there was none? Did this thing ever work? What am I missing?
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Member for

15 years 7 months

Boswell Thu, 01/28/2021 - 08:19
Odd, indeed. All I can think of is that the model of transformer used does not have a CT secondary, and the circuit is relying on the junction of the two 30K pots (strange value!) to form a common-mode ground point in the original. If that's the case, then the mod of lifting the ground connection and applying the control voltage to this point will work just as well. I know there were many different versions of this box over its production lifetime.

I see you got a reply from Joe himself on the JLM forums!

Member for

14 years 8 months

Link555 Fri, 01/29/2021 - 06:13
Confirmed the secondary is a straight through coil, no center tap.

I started the mods last night. This box has clearly seen a few repairs and mods along the way. I almost feel like completely rewiring it.

I am very slowly working my way from the input the output investigating all of the changes made, this will take time.

With JLM mod the original input threshold circuit is bypassed. I have managed to remove it in one chunk, it can easily be put back in needed. But for now it is out the way.
 
I started to recap the main boards.
 
 

and I found this....the main power supply dropping resistor caught on fire at some point... its funny to me that someone used the burnt hole to feed wire through...
 
Good to see that somebody upgraded the resistor to a higher wattage ...
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Member for

14 years 8 months

Link555 Sun, 01/31/2021 - 17:50
I have preformed most of the JLM mods now. The caps are done, a much of the failing wiring has been replaced. I have removed all not needed parts and the found mods in the hope to simplify future maintenance. The unit was a bit of mess. Lots of clean up and double checking to do before I retube and test the unit... but progress non the less.    
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