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.

https://farm5.staticflickr.com/4599/...482bd69634.jpg

https://farm5.staticflickr.com/4601/...4403c80ac6.jpg

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.

https://farm5.staticflickr.com/4589/...87d00d64a3.jpg

https://farm5.staticflickr.com/4590/...b96f756c_c.jpg

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.

https://farm5.staticflickr.com/4684/...870619c4_c.jpg

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.

You might be able to clear the short (tin whiskers, dendritic growth?) by thoroughly cleaning the pot or zapping it with a charged capacitor.

jr

Hi Henry ,

:thmbsp: Very happy to hear of your success with it ! Sure , we altered the circuit a bit , but we gained in the functionality arena just as hobbyists have been doing since before even that radio was new , good job !

Light at the End of the Tunnel
 

I try to live in accordance with the ancient proverb, "A journey of a single mile begins with a thousand blunders."

With just a few more blunders I'll be finished with the Coronado 575 journey.

I installed the 1-Meg audio pot (switched) as advised. That went in the front volume control position. After much experimentation, I replaced the original RF pot with a 1-Kohm linear pot. I couldn't bring myself to drill holes in the cabinet, so it went to the back of the chassis. I also have a 20-ohm 25W line voltage dropper, and I added a toggle switch for that. I think this represents what I did:

https://farm5.staticflickr.com/4697/...5e0f960b_b.jpg

One other change was that I had a 100K resistor for R9, and I changed that to the correct 250K. The final wiring (except for the toggle switch) looks like this:

https://farm5.staticflickr.com/4722/...4af059f9_c.jpg

The controls at the back of the chassis:

https://farm5.staticflickr.com/4661/...2dfc1f36b6.jpg

A quick before/after picture:

https://farm5.staticflickr.com/4723/...39d73ce4_b.jpg

Everything works well. Very good reception of local stations with a 12-inch wire antenna, exceptionally sharp tuning at the precise frequencies on the dial, and no noise between stations.

Unfortunately, the audio quality is not so good. I can only describe it as "buzzy." The distortion is what you would expect from a damaged speaker cone, but the speaker seems to be in perfect condition. Here's a link to a 15-second audio sample:

https://flic.kr/p/22umN23

It sounds a bit worse in real life. Please let me know if you recognize what the problem might be.

If anyone is interested, this is a link to a Flickr album with 15 additional photos of the restoration.

https://flic.kr/s/aHsmbhh8ct

Regarding the sound, you might try disconnecting that auxiliary volume pot, and connecting C7 back per original. The pot is forming a voltage divider, reducing plate voltage on the 1st audio tube. That might be causing degraded audio (or might not).

I'll check that as soon as I get up the energy to pull the chassis again. Thanks.

I would break the connection between the plate of the 57 and the 1 meg pot and insert a capacitor there, to take the pot out of the DC path. Likely a .05 would be ok in that location.

Are you positive that the speaker voice coil is not rubbing slightly on the magnet?

I would also try a larger value for C-9 for a more mellow tone... perhaps experiment with different values for the most pleasing sound. A "voice/music" switch, with 2 different caps might be fun to incorporate.

What station is that on 620? Many talk/sports stations are over-processed for AM broadcast these days to overcome interference... they don't sound very good, but they can be heard better in fringe/noisy locations.

jr

A few AM stations around here went from all talk, back to playing music. The music always seems to be distorted because of the over-processing. :thumbsdn:

Well, I took reconnected C7 to the plate. I had to wait until my wife was out of the house to try it out because the volume is quite loud without the 1 Meg pot. Indeed, the plate voltage increased about 40 volts, from 175v to 215v. I rigged the set to switch back and forth with the same loudness, tried various stations, and listed with and without hearing aids. And I can't tell the difference. Furthermore, I don't have a critical ear, so it doesn't sound that bad to me.

I considered the possibility of the speaker magnet rubbing, and I know how to check that out. I've recentered a cone successfully before, but I'd be hesitant to put that effort into this set.

I was getting ready to put in a line filter/safety cap. After that it will interesting to try dieseljeep's suggestion.

Thanks, All
Henry

I just gently push the cone in and out and listen for rubbing noises, being careful to not distort/mis-center the cone.

jr

I did a beginners' series on this a couple of years ago. The Flickr album is:
https://flic.kr/s/aHsksL3RPk
The results were impressive.
For a quick listen, the "Before" audio is here: https://flic.kr/p/Ee3Z9M
And the "After" audio is here: https://flic.kr/p/F8K7nW

I checked the speaker, and it doesn't seem to be the problem.

The way you had the volume pot connected was semi-correct. You need to use a cap .02 to .05mfd between the plate of the 1st amp tube and the high end of the volume control. The wiper would go to C7 and the low end to ground, as you had it. It should correct your distortion problem. You might have distortion after this fix if you are tuning to a 50KW station during the day. Then you would have to back off on the sensitivity control.

The cabinet pic jarred my memory. I have that model! two of the cap blocks on mine are original and IIRC it sounded better last time I powered it up.

Coronado 575
 

Nice job on the cabinet!
The cabinet was originally intended for a battery farm set. The chassis on the battery sets were really narrow, explaining for the closeness of the volume and tuning controls.
I thought the cabinet was re-purposed but after examining the under chassis layout, they intended to use the same cabinet for both models. :scratch2:

The dual use of the cabinet--battery and AC chassis--may explain why I had to raise the "floor" 1/4" to get the dial and control shafts aligned with holes. I'd bet the chassis of the farm set sat 1/4" higher than this one.

It looks like the chassis had some kind of rubber cushion grommets, that crumbled off through the years. :scratch2:

Yes. It was easy to replace the grommets.

Turning to electronics,

Previous suggestion:
"I would break the connection between the plate of the 57 and the 1 meg pot and insert a capacitor there, to take the pot out of the DC path. Likely a .05 would be ok in that location...I would also try a larger value for C-9 for a more mellow tone...”

My reply:
Replacing C9 with .01uF did improve the tone, and replacing R9 with 100Kohms restored the original plate voltage. I also tried putting a cap between the plate and the volume pot, but that seemed to have no effect except to lower the sound level. At this point, the audio is satisfactory.

A subsequent suggestion:
"The way you had the volume pot connected was semi-correct. You need to use a cap .02 to .05mfd between the plate of the 1st amp tube and the high end of the volume control. The wiper would go to C7 and the low end to ground, as you had it. It should correct your distortion problem."

Is this the same as the previous suggestion? The blue is how I wired it before, but noted no improvement and removed the capacitor. So I humbly ask if there is something "semi-INCORRECT" in this picture?"

https://farm5.staticflickr.com/4756/...c6d739d8_b.jpg

Finished.
 

The final configuration is diagrammed above. I don't remember specifically what I used for C-15, but I'm not gonna pull the chassis again to find out. This is what I ended up with after a lot of experimenting. Here's a link to a 13-second audio sample:

https://flic.kr/p/23F6oQ6

I think that's fairly good for a budget radio with a 4-inch speaker.

Thanks again. It's a nice-looking set and it'll be great for listening to the ball game, but maybe not so good for Toscanini. More importantly, it makes me happy.

That's the way I would've done it!
How does it sound with the gain control "maxed out"? The improvements you incorporated is still a great way to increase the enjoyment of this set, even though it's no longer "pure".

It works just as explained to me here. Max gain is too much the stronger stations, in which case lowering the gain will reduce the distortion. In fact, the audio is the sample is not the best--it probably could have been improved a bit by tweaking the gain. It is a more enjoyable toy with the extra gimmicks to play with.

:thmbsp:

When you crank up the "RF Gain" control at night can you receive many of the western US powerhouse stations such as KSL, KEX, KOA?... in other words, is it a fairly decent DX set, since it has a RF stage?

jr

Wonder if the two IF trimmers are fully 'on peak'. Unless peaked on a weak signal, a tiny fraction of a turn can make a big difference DXing.

At this time, it's not a decent DX set. I found KOA, and it was not easy listening. Not having the patience to wait for a station ID, I found KOA online to confirm what I was listening to. However, as old coot88 points out, I haven't redone the alignment since I got the set working properly. That sounds like a worthwhile endeavor. Maybe I could work on that Friday morning. What... I have a dental appointment Friday? I'll just have to cancel that.

With just two (not 4) IF trimmers, it shouldn't need more than a a tweak to be sure they're 'on the peak'. But peak it on the weakest signal that you can tune in.