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CBS Volumax 400

This baby came in for a recap and few mods....Most Maintainable build I think I have ever seen.  Its dream to work on. 

Front Panel

Inside

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Profile picture for user Link555
Member for
14 years 2 months

Link555 Wed, 06/09/2021 - 22:37

In reply to by Kurt Foster

Starting to look at a few things:  The balance or ratio between the attack and release times seems to  fixed because of the driver and buffer impedances. Its strange how they drove the meter without a separate buffer.

 

but it looks like I can Vary C14 values to affect both the attack and release times.  So maybe a switch with selectable caps. 0u5, 1u,2u2, 4u7,10u  something like that....

The other thing I want to try is bringing a switch out to the front to switch between symmetrical and asymmetrical side chain detection.

I also want to try add an adjustable threshold control.  Right now I am looking at two possible controls.  Moving R62 to the front panel and putting a 10k trim pot in for R61.  And/Or bringing R65 to front panel.  I am not sure which one will work best, as they both will affect the gain.

 

Other than that I will move the input and output T pads to the front plate, added a grounded power plug, change the VU meter lamps to LED, and recap the unit.

 

Profile picture for user Link555
Member for
14 years 2 months

Link555 Wed, 06/09/2021 - 22:42

 I have now recapped the unit, and started in on the mods.  I am adding a threshold control.  I added 1K pot to the front panel and wired to where the R62 trim pot was.

I changed R61 to a 10k trim rheostat.  It gives me a bit more control over the threshold. 

I cleaned and lubricated the input and out attenuators, and moved them to the front panel. 

Output attenuator Before:

Output attenuator After:

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I added a switch to change the side chain signal for symmetrical to Asymmetrical.

I have ordered a SP6T selector switch and plan to change C14 with 6 different values.  I will try 0.1uF,0.47uF.1uF,2.2uF,4.7uF and 10uF start and see how the circuit responds. 

I also planning to add a switch to remove the side chain signal from the amplifier, to defeat the limiting action.

I also still need to remove the input and output terminal block and add two XLR connectors. This will allow to reduce the internal wiring for the input and output signals, which will brings signals directly to their respective cards. Also it will allow me to not have to wire the input signal cable across the signal input 120VAC power input.

The owner is also looking for a new faceplate for this unit as well.  To do this I will  remove the service hinge and put a new fixed panel on the front. I will move the power switch and fuse to the back panel, as well change the ungrounded power cord to a chassis grounded cord.

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Profile picture for user Link555
Member for
14 years 2 months

Link555 Wed, 06/09/2021 - 22:44

Had time last night to put the new XLRs in.  The old line input via the terminal strip had an internal cable tied the the AC hot for at lease 8" of cable.  The new connections go directly to the their respective cards.  I was surprised that this new shorter connection really improved the signal quality, I was not excepting and audible difference.

"

';

I also star grounded the pin 1 on the xlr connectors.

Profile picture for user paulears
Member for
7 years 4 months

paulears Wed, 06/09/2021 - 22:44

I'm afraid I can't accept this as the physics means that if you experienced signal quality changes, you must have removed compromised cable. Cables can have resistance, capacitance and some might have inductance. Other than that, a bit of metal is a bit of metal. If you hear a difference then probably the old cable had much higher capacitance that it should have. Easy with adaptors, damp cable, or cable that has water ingress and has started to tarnish or rust internally. Unless the interconnect is running at very high impedance, than a few inches vs a few yards should be impossible to detect. I don't doubt you hear something, but it's not shortening the cable that did it, it is replacing faulty cable with good stuff. The inside of old gear can be a real mess by contemporary standards.

Profile picture for user Link555
Member for
14 years 2 months

Link555 Wed, 06/09/2021 - 22:47

I got my fresh order of WIMA caps and the SP6T rotary switch yesterday. Digikey is sure taking longer to deliver these days. I started in on the variable cap perf. board. I can't wait to see how far I can push the response time of the Volumax. I am ok to start by trying 0.1uf, 1uF, 2,2uf(current value), 4.7uf, 6.8uf and 10uf.

Profile picture for user Link555
Member for
14 years 2 months

Link555 Wed, 06/09/2021 - 22:54

Well let's try this again, My last post got lost in the shuffle. THe unit is completed now was picked up last night. Its a pretty transparent limiter in the end. If you push it hard it breaks up, in not such a nice way. But if you keep it in the green its pretty useful.  

I grounded the chassis and made a star point ground for all the cards.  The thick green wires are the new grounds.

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I cleaned up the wiring, and tested the unit. 

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I also added the true bypass relay and switch. I modified the original face plate to allow for some some testing.

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Then the new front panel Arrived. 

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Profile picture for user Boswell
Member for
15 years 2 months

Boswell Wed, 06/09/2021 - 22:56

In reply to by Link555

Really good job, Link.

It's great to see people taking a genuine pride in what they do. I hope that showing your skills by posting in these forums inspires others to engage in the rewards of practical work. It's on it's way to becoming a lost art, otherwise!

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