Strange noise when turned off

9/21/2011 6:41:07 PMBob

RCA Model 88K 1937, Rider 8-132 radio receives excellent, but when I turn power off kind of an echo or twanging sound from speaker for about 2 seconds as talking or music fades out. Any ideas as where to l@@k??

9/21/2011 7:07:20 PMcodefox

Best wild guess for the day, a microphonic tube responding to the click of the volume control. Probably the first AF amplifier. Tap it with a dry chopstick while operating to confirm.

:RCA Model 88K 1937, Rider 8-132 radio receives excellent, but when I turn power off kind of an echo or twanging sound from speaker for about 2 seconds as talking or music fades out. Any ideas as where to l@@k??
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9/24/2011 5:11:52 PMBob

Hi; Checked all tubes & still after turning power off music or talking fades out. What else could be checked??

9/24/2011 5:49:51 PMEdd

Sir Bob. . . . .

One would certainly want to logically suspect either that 6Q7 or 6F5 tube.

Like its possibly some shrinking /compression /stretching of a tubes internal infrastructures elements on onset of thermal cool down.

To see how far forward it is occuring, could you initiate an initial test of those two stages by starting at the 6Q7 and take a 10 ufd @50 V rating ( or upwards voltage) electrolytic cap and have its POSITIVE terminal to ground and its NEGATIVE goes to the 1st grid of the 6Q7.

Now, that should DRASTICALLY change the tonal response of anything coming into that stage, almost to a "low burp" so DON'T expect any normal sound at all , but after the usual warmup time, turn off the set and see if the effect is now gone or just being a subtle "blurp".
If the effect is unaffected and is as was, move that cap to the Firstus gridamus of the 6F6 AF outputsy tooberator and perform the same test.

(My next suspicion would be the filters associated with the sets power supply . . . . . even new ones . . . if having adject ESR.)

Standing by and waiting . . . . .with bated breath. . . . . I 'jes LUV's these WIERD 'lil mysteries.

73's de Edd

Text mesage left on mobile phone . . .

370HSSV 0773H

? ? ?

(You're looking at it upside down dummy !)

:Hi; Checked all tubes & still after turning power off music or talking fades out. What else could be checked??
:

9/24/2011 6:51:55 PMWalter

My National NC-125 would increase volume momentarily after turning it off. This was due to power supply filter caps that were much larger in value (50 mF) than needed.
With a 10 henry filter choke, only 8 mF caps were needed.
If it isn't a microphonic tube and the set has been recapped, make sure that the filter cap values are close to what was originally called for.

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:Sir Bob. . . . .
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:One would certainly want to logically suspect either that 6Q7 or 6F5 tube.
:
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:Like its possibly some shrinking /compression /stretching of a tubes internal infrastructures elements on onset of thermal cool down.
:
:
:To see how far forward it is occuring, could you initiate an initial test of those two stages by starting at the 6Q7 and take a 10 ufd @50 V rating ( or upwards voltage) electrolytic cap and have its POSITIVE terminal to ground and its NEGATIVE goes to the 1st grid of the 6Q7.
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:Now, that should DRASTICALLY change the tonal response of anything coming into that stage, almost to a "low burp" so DON'T expect any normal sound at all , but after the usual warmup time, turn off the set and see if the effect is now gone or just being a subtle "blurp".
:If the effect is unaffected and is as was, move that cap to the Firstus gridamus of the 6F6 AF outputsy tooberator and perform the same test.
:
:
:(My next suspicion would be the filters associated with the sets power supply . . . . . even new ones . . . if having adject ESR.)
:
:
:Standing by and waiting . . . . .with bated breath. . . . . I 'jes LUV's these WIERD 'lil mysteries.
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:
:
:
:
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:73's de Edd
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:Text mesage left on mobile phone . . .
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:370HSSV 0773H
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:? ? ?
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:(You're looking at it upside down dummy !)
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::Hi; Checked all tubes & still after turning power off music or talking fades out. What else could be checked??
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9/24/2011 7:08:41 PMBob

Edd; before this test 6Q7 1st grid are we talking about the top cap or pin 4 or 5 ?? & on the 6F6 pin 5 or 8?.

9/24/2011 7:24:35 PMMitch

:Edd; before this test 6Q7 1st grid are we talking about the top cap or pin 4 or 5 ?? & on the 6F6 pin 5 or 8?.
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Bob,

Do make sure the Filter capacitors are of the correct value. Tube 6Q7 grid is top of tube, 6F6 grid is pin 5.

9/25/2011 6:35:51 PMBob

Thanks for all the input- somebody since 1937 changed & installed 2, 30 uf caps when all thats required was a 10 & 16 uf. Over the years never had this issue, turns off correctly now.

9/26/2011 7:38:51 PMRay

Nice to see a follow up as help from other radio people solved his problem .