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Old 26th May 2019, 4:05 pm   #1
sparkey58
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Default Buzz on phono input of radio.

Hi all, help needed please. If you look at the attached schematic, I am having trouble with buzz with the volume control in the mid position. At fully open or closed the normal low level hum is heard at minimum volume, at full volume again louder but normal hum/noise, but mid volume a pronounced buzz. Tried changing the pot and grounding the pot case, but can't stop it.

Anyone any ideas please?
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Old 26th May 2019, 4:33 pm   #2
boxdoctor
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Default Re: buzz on phono of radio.

It looks like a problem relating to the phono/radio switch to me.
Does this happen on radio and phono, or is it happening on one and not the other. There appears to be a separate volume control on the phono circuit. Does adjusting this affect the buzz in any way?
A careful look at the phono /radio switch may reveal some loose earthing connection, or other physical (dry joint?) problem with the wiring or the switch itself might have failing insulation due to age. Tony.
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Old 26th May 2019, 6:56 pm   #3
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Default Re: buzz on phono of radio.

Is that input really connected to one side of the mains?
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Old 26th May 2019, 7:42 pm   #4
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Default Re: buzz on phono of radio.

Looks like a AA5 radio for 120v, economy of components was perhaps more important than safety. Then again I could be doing them a disservice.
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Old 27th May 2019, 9:44 am   #5
Lucien Nunes
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Default Re: Buzz on phono input of radio.

With the volume control in its mid position the AF stage grid has a higher impedance to the set's ground rail, so it is more likely to pick up hum. What it picks up from will depend on the power supply configuration you are using to feed it 115V. If its signal ground is on the high side of the 115V supply, it will be more likely to pick up hum from true earth (i.e. all around it). What is your stepdown transformer earthing arrangement?

I assume they get away with not isolating the cartridge from the mains because there are no user-accessible connections, i.e. the phono input socket is not really an input, just where the pickup arm plugs into the chassis. For anyone not used to having mains on the arm leads it could be a nasty surprise.

E2A there's something odd about the radio/phono switch wiring. The top two contacts switch the AF stage input to the pickup and the IF cathode off when selecting phono, but I don't understand what the bottom one is doing to the motor power. In the radio position the motor seems to run all the time, in the phono position it runs when the rest of the set is switched on. Am I missing something or is this wrongly drawn?

Last edited by Lucien Nunes; 27th May 2019 at 9:58 am.
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Old 27th May 2019, 12:24 pm   #6
sparkey58
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Default Re: Buzz on phono input of radio.

Thanks for replies so far. It is an AA5 design hot chassis set, have it fed via an isolation transformer and variac, have temporarily by-passed the radio/phono switch, the problem remains. I have tried everything I can think of, but the buzz remains, only gone at full volume or at minimum.
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Old 27th May 2019, 12:35 pm   #7
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Default Re: Buzz on phono input of radio.

UK radios tended to use screened lead for the lead from the detector/volume control/ G1 of the first amp to guard against him pickup in this low signal high impedance area, presume this set doesn’t?
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Old 27th May 2019, 12:53 pm   #8
sparkey58
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Default Re: Buzz on phono input of radio.

No it does not.
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Old 27th May 2019, 1:01 pm   #9
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Default Re: Buzz on phono input of radio.

There is a lot of info on the AA5 on this site.
http://www.angelfire.com/electronic/...bes/AA5-1.html
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Old 28th May 2019, 10:16 am   #10
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Default Re: Buzz on phono input of radio.

If you're using an isolation transformer, try earthing one side or the other of its secondary to see what happens. Having the whole lot floating may be the problem with this set.

What happens if you use just the variac with its common at neutral and connected to the the set's switch so that it ends up as B-?
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Old 28th May 2019, 1:06 pm   #11
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Default Re: Buzz on phono input of radio.

Quote:
Originally Posted by Lucien Nunes View Post
E2A there's something odd about the radio/phono switch wiring. The top two contacts switch the AF stage input to the pickup and the IF cathode off when selecting phono, but I don't understand what the bottom one is doing to the motor power. In the radio position the motor seems to run all the time, in the phono position it runs when the rest of the set is switched on. Am I missing something or is this wrongly drawn?
I think the circuit is correct but the drawing isn't too clear due to the lack of resolution.

So far as I can make out the contacts on the switch bank are shown in the radio position, the bottom three contacts are not making contact with each other, the motor isn't in circuit and the radio will work when switched on via the switch on the radio's volume control, when in the phono position the bottom three contacts all make with each other enabling the valve heaters to be in circuit whether the radio is switched on or not by the switch on the radio's volume control.

Lawrence.

Last edited by ms660; 28th May 2019 at 1:35 pm.
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Old 29th May 2019, 6:01 pm   #12
sparkey58
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Default Re: Buzz on phono input of radio.

Well thanks to all who have replied, I've got it down to an acceptable level now. I think it's down to just poor design of the chassis. I removed the internal wiring for the phono between the pot and leaf switches, fitted screened wiring one end tied to chassis, fitted a grid stopper to the 12AT6 grid, shortened every component lead and tidied up the layout. Nothing I can do about the exposed leaf switches and that seems to have improved it 90%. Still not perfect, but best I can get it.

Last edited by Station X; 29th May 2019 at 6:04 pm. Reason: Demorsed.
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