Coronado 575 Volume Control Issue

[Winky Dink]

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.

[jr_tech]

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

[init4fun]

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 !

[Winky Dink]

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

[old_coot88]

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).

[dieseljeep]

Originally Posted by old_coot88

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.

[Electronic M]

Originally Posted by Winky Dink

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.

[dieseljeep]

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.