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).
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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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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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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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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. |
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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. :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.
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