Coronado 575 Volume Control Issue
 

I just finished the electronics on a Coronado Model 575 (Gamble-Skogmo, Nostalgia Air http://www.nostalgiaair.org/Resources/072/M0007072.htm). I powered up the set and immediately got good reception, but the volume control doesn't work right. At the lowest setting, where I should have zero volume I get low volume. Then, as I turn the control the volume level stays the same until I reach a particular point at which the volume jumps to a very high level.

I assumed the problem was the 75K ohm volume control. In the first quarter of its range it went from 70 to 800 ohms, the jumped to several Megohms.

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Mechanically, it appeared to work properly. So I tried a good, used 100K pot, and I got the same result as before. A third pot also gave the same result.

This is the volume control.

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All electrolytics and paper caps were replaced. The remaining components and tubes were tested and replaced as needed. Continuity through all coils was checked.

So, I must have done something wrong. I hope there's an obvious answer to this problem, and I'd appreciate any advice on the matter.

Thanks,
Henry

That would appear to be normal operation. Volume is a misnomer here...It really is an RF input level control....As such capacitive and inductive qualities of the part will allow some RF leakage past it. Any missing shields in the set may render the control inoperable and leave you with a fixed volume output.

In all my TRF sets with 'volume' as a front end RF level adjustment the muting position or lack thereof depends on the strength of the station...Often the controls don't have a very linear response either....There was a reason later sets moved the 'volume' knob to the audio section.

The set shown is a non-avc superhet and they used the same scheme, similar to a TRF. They were a pain to locate a distant station, as you had to crank up the volume/gain control and then readjust it when the station was found.
AVC sure was a godsend. :thmbsp:

Wow, you're right! Glancing at it I totally missed the IF and osc. markings and saw a schematic that looked too simple to be a superhet. I can't recall if I've worked on any superhets with only RF stage 'volume control'...If I have I could probably count them on one hand.

:) 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. :scratch2:

:thmbsp: 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.

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.

RF Gain Issue!
 

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.

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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.

https://farm5.staticflickr.com/4643/...5c66cd0bb3.jpg

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.

https://farm5.staticflickr.com/4739/...638e7a62e0.jpg

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.

https://farm5.staticflickr.com/4729/...a050efb8_b.jpg

Many Thanks,
Henry

:) 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 .

Thanks. I'll follow up on your suggestions. As for "breadboarding"--In this kind of situation I put little alligator clamps on all the leads until everything is satisfactory.

I hooked up both the RF control and the audio control yesterday, and everything worked as expected. Used a 500K pot for the audio, and had to drop R9 down to 100Kohms to get the plate voltage to where it was before. Today I put the components into their permanent positions, did a temporary hookup again, and now I get nothing audible. I've checked almost everything I can think of, so tomorrow I may put it back in it's original configuration and start over.

Have you checked your new volume control pot for leakage from the 3 terminals to the body of the pot? Sometimes a leaky pot will work great when breadboarded, but will short out the signal when grounded to the chassis... :scratch2:

jr

Good call! I disconnected all the leads and sure enough, only 5 ohms from the sweep to the body. Hence, chassis-grounded. This pot was at least 60 years old, and it was the only one I had that was switched, had reasonable resistance, and a two-inch shaft. I have to go shopping. Thank you.