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Fixing Up Audimax III - Very Little Gain Reduction

A

AMRadioGuy

I just acquired a classic CBS Audimax III to pair with my Volumax 400. However, I can't get the Audimax to do much - the needle hangs right around the bottom end of the scale consistently. I'm feeding it with a Russo broadcast console, so I doubt it's an impedance issue - feeds the Volumax just fine when I hook it directly there and bypass the Audimax.

The Audimax is passing audio through just fine and is doing what it should be with AGC - just very little. I haven't been able to get it to hit -5 or anything higher.

Any chance someone here knows what I should be checking for? I know this guy uses a little nuvistor tube - any chance that's all this needs to be working correctly again?
 
Tubes don't typically fail from storage. Electrolytic caps do though, and are always the first things to suspect. Unless it's already been done, replace them all. There aren't all that many.

Otherwise, troubleshooting and line-up instructions begin on page 34 of the manual.
 
dc2bluelight said:
Tubes don't typically fail from storage. Electrolytic caps do though, and are always the first things to suspect. Unless it's already been done, replace them all. There aren't all that many.

Otherwise, troubleshooting and line-up instructions begin on page 34 of the manual.
Click to expand...
Thanks - that does make a lot of sense - tubes definitely don't usually fail in storage.

I know the chief engineer who pulled this unit out and he said it had been pulled because the AGC stopped responding properly - the issue I'm having with it. I doubt he knew there was a tube in it that could fail, so it's pretty improbably that he would've even checked for that. It did fail while in service, so that could be a tube burning out.

It definitely could be a cap that's the issue, too. It looks like this unit has been recapped at some point in the past - the caps in there certainly don't look 60s to me. Considering that definitely didn't happen after it was pulled from service, it would appear that it was a successful recap job. Either way, a recapping is likely in order to bring this unit back to prime operating spec.

I was hoping someone here would know specifically what happens when that tube fails and whether the symptoms line up with what I'm experiencing.
 
The nuvistor tube has a lot to do with gain. There is a simple test procedure for the tube in the manual. I don't think the tube is the problem here. The input and output pots are notorious for getting noisy and dirty. The Volumax needs a lot of level, you might not be feeding it enough. I would start there.
 
Way back in the 80's had the nuvister fail in the Audimax (used in a production studio). Stuck a FET in there and was back in business for a while.
Since the FET was working a little above its voltage rating it would occasionally fail.
Found a new tube and back to normal before selling the Audimax/Voulmax pair to 570 in Alturas.
 
So I got a new tube in the mail today... popped it in and the unit appears to be working flawlessly.

Yowza - it's amazing how much more crush-capable the CBS system is when both units are working. Even with the Volumax set to live in "normal" reduction with occasional peaks into "excessive," it's pretty crushed. And I absolutely love it. There's something incredibly pleasing about the sound of these units. Audiophile? No. Fun? Absolutely!!! Especially with 60s 45s, it imparts some real magic.

Just me... but I'd absolutely take these in an airchain over the Orban 9100 that's in the chain of the oldies station I work for that I set up a couple years back...
 
radiofan2023 said:
Can you make a recording of how it sounds compared to the Orban 9100?
Click to expand...
It would be difficult for me to, unfortunately, because I can't take the 9100 out of the airchain nor can I put the CBS equipment into the airchain - I'm not technically the engineer there and set up the 9100 under previous ownership. There's also other equipment in the chain with it, so recording it directly off the air wouldn't give a very good comparison, either.

The big difference in sound comes from the CBS units being single-band processors - they don't separate the audio into different bands for compression, etc. As a result, they have a very distinct sound. It's a sound I find extremely pleasing. A lot of those 60s records were mixed with this kind of airchain in mind, too. From a "technical" perspective, the Orban is light-years more advanced. But it just doesn't sound quite "right" with 60s/70s tunes, to my ears at least.

All that said, if I do end up getting the opportunity to compare the units, I'd absolutely post a comparison here. But unfortunately, I'm not holding my breath that I'll get that opportunity.
 
I've got another question here... As I understand it, the Audimax/Volumax both want to see 600 ohm inputs and outputs. The input shouldn't be a problem - I've got a Russo 5-channel broadcast board that feeds a 600 ohm output. That gets me into the Audimax III. That then heads into the Volumax 400. So far, so good.

For the output of the Volumax... I don't have an AM transmitter haha. Instead, I want to run these through a regular RCA input on an amp (typical 70s/80s receiver with line-level inputs). That's how I've been listening to them for the past couple of weeks and they sound great that way. I want to make sure I'm not going to damage anything with this setup, though! Am I okay to run these units this way?
 
The Volumax & Audimax have transformer outputs so going to an un-balanced input is not a problem.
Putting a 600 ohm load (620 ohm resistor) on the Volumax output going to the high impedance input could give you a flatter frequency response. You're matching the transformer's impedance and designed response.
 
boiseengineer said:
The Volumax & Audimax have transformer outputs so going to an un-balanced input is not a problem.
Putting a 600 ohm load (620 ohm resistor) on the Volumax output going to the high impedance input could give you a flatter frequency response. You're matching the transformer's impedance and designed response.
Click to expand...
Thanks; I appreciate the answer!

Just a couple of clarifications: When you say to put a 620 ohm resistor on the Volumax output... do you mean to connect it to the + & - wires, bridging them together through the resistor?

The other question is if a flat frequency response isn't a big deal (seems to sound alright as-is) am I going to damage anything running it without the bridged resistor?
 
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