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Audimax/Volumax tuneup

B

Bitman

Guest
You guys have been a great help here...so I thought I'd come back for more! I just got an Audimax 3-444 and a Volumax 4400. They've been sitting around quite some time, so I'm guessing they need a tune up. If I can't find a broadcast engineer, what would I tell any other sort of other audio/radio type person to do to them? Also, which of the two units would be the final output?

Thanks

David
 
Other than an engineer, not sure who would be qualified to align these units. Any good electronics tech would probably work as long as you had complete manuals available for the individual. Always the Audimax in line first, then the Volumax. Good Luck!
 
Bitman said:
You guys have been a great help here...so I thought I'd come back for more! I just got an Audimax 3-444 and a Volumax 4400. They've been sitting around quite some time, so I'm guessing they need a tune up. If I can't find a broadcast engineer, what would I tell any other sort of other audio/radio type person to do to them? Also, which of the two units would be the final output?

Thanks

David
Click to expand...

Audimax = initial AGC, Volumax = final peak limiter.

Even when they were new, the THD (distortion) measurement of these beasts was nothing to write home about. My 1965 Audimax III manual says "Below 1% from 50 cps to 15,000 cps at +16 dbm output." Guess that was part of their 'sound.' ;) (For reference, in 2000 our analog Ariane was spec'ed at 0.02% THD, and its noise level was 15 dB better than the III!)

For any forty+ year-old piece of audio gear, my suspicion would be that the prime suspect for any poor performance or even outright failure in these old pals will be coupling capacitors. IIRC, CBS often used tantalums for coupling. Yech!
Put some nice new electrolytics in their place. If you want to go crazy, buy 105-degree rather than the usual 85-degree rated capacitors. If you REALLY want to go crazy, parallel the electrolytics with film capacitors.

The input and output transformers on the Audimax III have a rather poor low end response. Since the output transformer is part of the gain feedback system, unless you are willing to do major surgery you will have to live with this. But once again, for better or worse, that is part of its 'sound.'

Don't forget, the Audimax III has a TUBE (8056 Nuvistor) in its detection circuit. This little puppy may be dead, and finding a replacement could be tough.

If you Google, I believe there's an Audimax III schematic online somewhere.

The Volumax is, shall we say, not the best FM processor ever made. Better than anything that came before it, but that's not saying much! Pre-emphasis processing was greatly improved with the first Optimod. However, the Volumax limiting stage BEFORE the pre-emphasis is pretty sweet (I used one for a mike processor once. Real punchy!). Since IIRC you aren't going directly to air with this unit, If you could figure some way to get that signal out of the box cleanly it might be worth a try. But I really don't know your capabilities or how much time you have on your hands!! ;D

When all is said and done, you may encounter some frustration getting what you want of of the boxes. Quality-wise, their day is long past, even if they work perfectly.

But...
As a kid, I grew up listening to CBS processors on my favorite Top 40 station (AM). To me, THAT was the sound of radio...

Kind Regards,
David
 
Then there's those magic potted modules to deal with. Has anyone been able to unpot them and get a decent schematic of them? If they have bad caps a guy doesn't know how to build a new one, then it's GAME OVER. The CBS stuff always sounded rough and muddy to my ears. I suppose for the time it was about the best out there, but I agree they are WAYYYY past their prime, even restored to "perfection".
 
OKCRadioGuy said:
Then there's those magic potted modules to deal with. Has anyone been able to unpot them and get a decent schematic of them? If they have bad caps a guy doesn't know how to build a new one, then it's GAME OVER.
<snip>
Click to expand...

Yeah. I haven't got a clue exactly what's inside them. I am curious, but since I'm relatively well-versed in how the 'magic' of their platforming/gating control concept works, I really don't need to know. ;)

BTW, for those who haven't seen their schematics, only the Audimaxes have the secret modules. The Volumax is a pretty open design.

Kind Regards,
David
 
I've fired up the audimax and the sound is everything everyone here said it would be...and less! It's old, antiquated, mono...and I love it! Just what I was looking for. I'll probably have it recapped just for the sake of doing so...but it's going to make the oldies show sound great! Next up...the volumax...

David
 
If you parallel coupling caps, wouldn't you want them to be 1/2 the capacity or would the 2X capacity help with the bottom-end response?
Wouldn't there be some places where doing so would change the attack/release timing?

I haven't looked at the schematic, but it would seem you sure wouldn't want 2X capacity everywhere......

Increasing capacity in cathode bypasses of audio chain tubes doesn't seem to cause boominess, but sure does extend low-end response.
 
Tom Wells said:
If you parallel coupling caps, wouldn't you want them to be 1/2 the capacity or would the 2X capacity help with the bottom-end response?
Wouldn't there be some places where doing so would change the attack/release timing?

I haven't looked at the schematic, but it would seem you sure wouldn't want 2X capacity everywhere......

Increasing capacity in cathode bypasses of audio chain tubes doesn't seem to cause boominess, but sure does extend low-end response.
Click to expand...

I'm not sure if you're referring to my suggestion or not. If so...the idea is that a number of sources say the Dielectric Absorption of the capacitor influences the sound of audio passing through it. Electrolytic capacitors typically have a relatively poor DA measurement.

The better-quality-DA capacitor types are simply not available in the relatively large values (>2 uf) that would normally be needed for medium-impedance (1k-10k Ohm) audio inter-stage coupling purposes.

Fortunately, (so the story goes) dielectric absorption audibility seems mostly to be a problem with higher frequencies. So the suggestion is to use a large-value, low-ESR electrolytic cap which will pass the lowest frequencies, paralleled with a smaller (though large as possible) film capacitor that has a better DA (such as polypropylene, styrene or Teflon), to 'shunt' the higher frequencies across the electrolytic.

it's a bit of an audiophile type thing, but I can't see any downside, other than time, money and complexity.
:D

PS: I would not change any of the timing capacitors.

Kind Regards,
David
 
David Reaves said:
<snip>the idea is that a number of sources say the Dielectric Absorption of the capacitor influences the sound of audio passing through it. </snip>
Click to expand...

Whoops... AFTER I wrote the above, I went to check my sources... I should have been referring to DF (dissipation factor) rather than DA (dielectric absorption), a separate characteristic.
Got corn-fused...Just replace all the DAs with DFs. ::) Thanks!

Kind Regards,
David
 
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