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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 7:20 pm 
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FIXED (again)!!! :D

I put the phono circuit back in and it's working great again! No rumble. I think the radio input wire may have been a little too close or slightly touching the ground shielding at the radio/phono switch.

Thanks again everyone!!!

EDIT: The rumble started coming back so I poke around inside with it on and it looks like C10 cap is bad. When I poke it I hear noise through the speaker and the rumbling comes and goes.

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Last edited by sooner on Jan Fri 11, 2019 7:36 pm, edited 1 time in total.

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 7:25 pm 
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You're welcome.

Given this uses negative bias on the first audio tube be sure there's no ground going directly to the volume control.


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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 7:39 pm 
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Tube Radio wrote:
You're welcome.

Given this uses negative bias on the first audio tube be sure there's no ground going directly to the volume control.


The shielding is grounded at the volume pot. It may have been unintentionally grounding at the phono plug too where there was too much solder on one of the connections. Should I disconnect the shielding or connect it somewhere else?

But note my edit above. It looks like a bad C10 cap?

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 8:15 pm 
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Steve, I would put an ohmmeter between ground and the center lead of the phono plug (or volume pot), radio turned off of course, and wiggle wires and such around while watching the ohmmeter for movement. Put the ohmmeter range on one of the higher scales. I always find analog meters better for this purpose than DMMs, as you can more easily see slight twitches of the meter needle. This only tells if you're getting inadvertent grounding with the voltage of your ohmmeter and not actual circuit voltages, but it's an easy first test.

Also, on C-10, is it the cap that is bad or its leads to wherever it goes?

-Bryan


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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 8:35 pm 
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Old Rad Lab wrote:
Steve, I would put an ohmmeter between ground and the center lead of the phono plug (or volume pot), radio turned off of course, and wiggle wires and such around while watching the ohmmeter for movement. Put the ohmmeter range on one of the higher scales. I always find analog meters better for this purpose than DMMs, as you can more easily see slight twitches of the meter needle. This only tells if you're getting inadvertent grounding with the voltage of your ohmmeter and not actual circuit voltages, but it's an easy first test.

Also, on C-10, is it the cap that is bad or its leads to wherever it goes?

-Bryan


The wires and solder connections on the C10 cap look good. It's only when I tap the cap itself that I hear noises and the rumbling comes and goes. Right now it's working fine so I'll leave well enough alone but will order a new C10 cap in the meantime.

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 8:37 pm 
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sooner wrote:
Tube Radio wrote:
You're welcome.

Given this uses negative bias on the first audio tube be sure there's no ground going directly to the volume control.


The shielding is grounded at the volume pot. It may have been unintentionally grounding at the phono plug too where there was too much solder on one of the connections. Should I disconnect the shielding or connect it somewhere else?

But note my edit above. It looks like a bad C10 cap?


C10 is most likely bad.

The shielded cable should not be grounded to the volume control if the RCA jack is grounded to the chassis as that will short out the grid bias unless the shielded cable is not grounded to the RCA jack.

The safest way is to ground the shielded cable at the RCA jack only. That way there is no potential difference between your source and chassis.


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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Fri 11, 2019 9:24 pm 
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Tube Radio wrote:
The shielded cable should not be grounded to the volume control if the RCA jack is grounded to the chassis as that will short out the grid bias unless the shielded cable is not grounded to the RCA jack.

The safest way is to ground the shielded cable at the RCA jack only. That way there is no potential difference between your source and chassis.

The two inputs on the RCA jack are isolated going through the chassis. Inside the chassis, the outside pin of the jack is connected to the shielding that connects to the back of the volume pot. I get zero ohms between the back of the pot and the chassis so I assume the back of the pot is grounded. I don't understand why grounding the shield again at the RCA jack would cause a problem if it's already grounded at the pot.

FYI. The inside pin of the RCA jack goes through the shield and connects to the left side of the volume pot. In the phono mode the switch breaks the R6/R7 connection and connects the phono input to R7, the volume pot instead of the R6 resistor.

Seems to be working now though so no worries. :D

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sat 12, 2019 3:18 am 
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Now if it is grounded to the pot frame itself then that's ok.

But it should not be grounded to one of the three variable resistor terminals.

I'm assuming theres only one version of this chassis though which has negative bias voltage on the first audio tube as that's the only schematic I found.


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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sat 12, 2019 4:57 am 
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Tube Radio wrote:
Now if it is grounded to the pot frame itself then that's ok.

But it should not be grounded to one of the three variable resistor terminals.

I'm assuming theres only one version of this chassis though which has negative bias voltage on the first audio tube as that's the only schematic I found.

Yes it's grounded to the pot frame, not one of the three terminals. This is the schematic without the phono switch and circuit thsat mine has. Two other discrepencies are that can't find the "mute switch on lever tuner" that's noted on the speaker and there's no C19 on mine.
http://www.nostalgiaair.org/pagesbymode ... 002295.pdf

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sat 12, 2019 5:46 am 
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Oh ok that's a decent ground then, but that depends on the tabs that hold the pot cover on making good contact to the rest of the pot that mounts to the chassis.


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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sat 12, 2019 6:16 am 
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sooner wrote:
Old Rad Lab wrote:
Steve, I would put an ohmmeter between ground and the center lead of the phono plug (or volume pot), radio turned off of course, and wiggle wires and such around while watching the ohmmeter for movement. Put the ohmmeter range on one of the higher scales. I always find analog meters better for this purpose than DMMs, as you can more easily see slight twitches of the meter needle. This only tells if you're getting inadvertent grounding with the voltage of your ohmmeter and not actual circuit voltages, but it's an easy first test.

Also, on C-10, is it the cap that is bad or its leads to wherever it goes?

-Bryan


The wires and solder connections on the C10 cap look good. It's only when I tap the cap itself that I hear noises and the rumbling comes and goes. Right now it's working fine so I'll leave well enough alone but will order a new C10 cap in the meantime.



Try tapping on the chassis in the vicinity of C10--if you still hear the rumble, there's a good chance you have a microphonic tube

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sun 13, 2019 8:17 pm 
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I put the old original C10 back in and that fixed the rumbling noise problem.

Now a question about using these 6V6s instead of 6F6s. I like the higher gain with the 6V6s. My guitar now has just the right volume level when running through the phono input. The radio however is so loud now that only about 1/4 of the rotation on the pot is usable. Anything over 1/4 turn and it gets too loud. I'm afraid if it were accidentally turned up to max it would damage the speaker. Is there any way to reduce the gain on the radio without reducing the gain on the phono input? For example, could I add a resistor between the R6 and R7 or just increase the value of the R6?

http://www.nostalgiaair.org/pagesbymode ... 002295.pdf

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sun 13, 2019 10:58 pm 
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The section around R6 is for the AVC line. Any changes there will effect the action of AVC. The set was made to use 6F6 tubes. Now it's changed to higher gain 6V6's. Put it back to original 6F6's That way the volume level is the way it was intended.


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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Sun 13, 2019 11:23 pm 
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thunderbird281 wrote:
The section around R6 is for the AVC line. Any changes there will effect the action of AVC. The set was made to use 6F6 tubes. Now it's changed to higher gain 6V6's. Put it back to original 6F6's That way the volume level is the way it was intended.


That's what I was afraid of. Thanks! :-)

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 Post subject: Re: Volume Jump on Warm Up
PostPosted: Jan Mon 14, 2019 1:38 pm 
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I would suggest putting a resistor in series with the volume control at the end the audio is fed into in such a way that the resistor is only used when the switch is in the radio position, but with the way the first audio tube uses negative bias you cannot as that might affect the bias or AVC voltage or both.


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