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  #16  
Old 01-03-2018, 07:07 PM
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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
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  #17  
Old 01-03-2018, 08:41 PM
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Hi Henry ,

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 !
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  #18  
Old 01-17-2018, 11:17 PM
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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:



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:



The controls at the back of the chassis:



A quick before/after picture:



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
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  #19  
Old 01-18-2018, 12:10 AM
old_coot88 old_coot88 is offline
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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).
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  #20  
Old 01-18-2018, 12:51 AM
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I'll check that as soon as I get up the energy to pull the chassis again. Thanks.
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  #21  
Old 01-18-2018, 11:24 AM
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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
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  #22  
Old 01-18-2018, 11:43 AM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by jr_tech View Post
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.
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  #23  
Old 01-18-2018, 03:56 PM
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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
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  #24  
Old 01-18-2018, 04:53 PM
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Originally Posted by Winky Dink View Post
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 just gently push the cone in and out and listen for rubbing noises, being careful to not distort/mis-center the cone.

jr
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  #25  
Old 01-18-2018, 05:59 PM
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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.
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  #26  
Old 01-18-2018, 07:58 PM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by old_coot88 View Post
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).
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.
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  #27  
Old 01-18-2018, 09:25 PM
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Quote:
Originally Posted by Winky Dink View Post
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:



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:



The controls at the back of the chassis:



A quick before/after picture:



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
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.
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  #28  
Old 01-19-2018, 09:52 AM
dieseljeep dieseljeep is offline
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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.
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  #29  
Old 01-19-2018, 10:48 AM
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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.
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  #30  
Old 01-19-2018, 11:57 AM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by Winky Dink View Post
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.
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