Coronado 575 Volume Control Issue

[dieseljeep]

Originally Posted by init4fun

Hi Henry ,

As a funny bit of side info here , in my radio experiments I learned that even though it may sound tempting , it's not a good idea to attempt to circumvent the original "volume control" with fixed resistors and to relocate the control to the first audio stage where we're most used to seeing it . Since as Tom and Dieseljeep said , it's more of a gain/tuning control than a volume control it must be adjusted for each different station received and what's good for one station won't be good for a different one . I have had success with leaving the front end circuit as designed , using the "volume control" more properly as a gain/tuning control , and then putting a "real" volume control in the first audio stage where it belongs . That way you can use the two original controls to tune the radio for best reception of each station and then adjust the volume to a tolerable level with the added in control .

The audio gain control could be easily added around the C-7 audio coupling and installed where the existing volume control is and the RF gain control mounted elsewhere. Just a thought on my part.

[init4fun]

Originally Posted by dieseljeep

The audio gain control could be easily added around the C-7 audio coupling and installed where the existing volume control is and the RF gain control mounted elsewhere. Just a thought on my part.

Exactly what I was thinking !

Now were this an "heirloom" set with the value of a Walton or similar of course I'd want to see it remain all original , warts & all , because that's how it was built . But where this isn't a set of any great monetary value , and because tinkering to make it better was a common thing back in the day when most radios were owned by technically minded/inclined folks , I'd say go for it .

I always found the lessons I remembered best were those where improvements like this were attempted . Most were successful , some weren't , but I'd say I learned something with each attempt that I may not have learned had I just left the set alone in it's stock configuration .

[dieseljeep]

Originally Posted by init4fun

Exactly what I was thinking !

Now were this an "heirloom" set with the value of a Walton or similar of course I'd want to see it remain all original , warts & all , because that's how it was built . But where this isn't a set of any great monetary value , and because tinkering to make it better was a common thing back in the day when most radios were owned by technically minded/inclined folks , I'd say go for it .

I always found the lessons I remembered best were those where improvements like this were attempted . Most were successful , some weren't , but I'd say I learned something with each attempt that I may not have learned had I just left the set alone in it's stock configuration .

I see the schematic shows a lot of the components were in modules. It must've been fun to restore this set.

[Kevin Kuehn]

That circuit relies heavily on using a period long wire(outdoor) antenna along with a good(Short)earth ground at the chassis. Now days folks tend to think of the antenna as a few feet of wire strung indoors and letting the ground float. Of course the signal cant' go to zero if there's a good portion of rf riding on the chassis ground reference. In some situations it's possible to have more rf on the ground than at the antenna. Even the wire between your chassis and earth ground acts as an antenna to the rf signal.

[Winky Dink]

The restoration was fairly easy. Two of the modules were already gone, and other than deciphering what the last guy had done, it was just a matter of putting in terminal strips to replace the modules.



I have a few TRFs. This is different from anything I've experienced. However, beyond the unwanted reception, the issue is inability to control the RF gain. The original pot was defective, so I tried several others with different resistances and tapers to no avail. The outcome is represented by the blue line.



The red line is what I got after restoring the original pot--removing bumps and adding graphite to the resistive element, resulting in a roughly linear 95K. The original spec was 75K with a fixed 100 ohms at the high end.



I have found one workable configuration--connecting the 40-foot indoor antenna and lowering the line voltage to 80VAC. I know that can be done. You guys showed how to do it when I was doing an AA5 with the wire-wound resistor in the line cord. But assuming I don't want to do that, I'll try your recommendation for separate RF and audio gain controls.

Regarding adding an audio gain control "around the C-7 audio coupler," what kind of potentiometer would you suggest. I have a couple of switched pots with a long-enough shaft to use where the current RF gain control is.

Exactly how would I wire the audio control--where to connect the high, low, and sweep terminals?

In case it's helpful, here's the voltage and resistance data I've collected.



Many Thanks,
Henry

[init4fun]

Hi Henry ,

Were this mine to try , here would be my starting point ;

I would disconnect C7 from where it connects to C12 , R9 , and the #57 tube's plate , and then would connect a 1 Megohm audio taper pot with what your calling the "low terminal" to the place where you disconnected C7 from . C7 itself would go to the "sweep" terminal and the "high" terminal would go to ground . And that would satisfy the pot's three required connections . I believe before I started I'd take a measurement of the #57 tube's plate voltage , make note of it , and then check it again with the 1 Megohm volume control pot connected , it'll likely be lower due to the additional 1 Megohm load on the circuit so to preserve the correct plate voltage the resistance of R9 will need to be decreased somewhat , it's a 250 K ohm now , maybe in the neighborhood of 220 K or so for a starting point should get the plate voltage back up where it belongs .

And remember , "breadboard" this first , make no final changes till you see that the circuit actually tolerates the changes (no weird effects to the operation of the #57 tube with the additional 1 meg plate loading) and if it performs to your satisfaction in bench testing then go ahead and actually mount the new volume control .