UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 26th May 2019, 4:05 pm   #1
sparkey58
Triode
 
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 25
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?
Attached Thumbnails
Click image for larger version

Name:	ristaucrat radio.jpg
Views:	137
Size:	111.5 KB
ID:	183903  
sparkey58 is offline   Reply With Quote
Old 26th May 2019, 4:33 pm   #2
boxdoctor
Heptode
 
boxdoctor's Avatar
 
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 665
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.
__________________
The job "couldn't be done".
He didn't know that,
So he did it.
boxdoctor is offline   Reply With Quote
Old 26th May 2019, 6:56 pm   #3
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,226
Default Re: buzz on phono of radio.

Is that input really connected to one side of the mains?
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is online now   Reply With Quote
Old 26th May 2019, 7:42 pm   #4
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 6,742
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.
__________________
Frank
Nuvistor is online now   Reply With Quote
Old 27th May 2019, 9:44 am   #5
Lucien Nunes
Octode
 
Lucien Nunes's Avatar
 
Join Date: Apr 2008
Location: London, UK.
Posts: 1,699
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?
__________________
Three anodes good, six anodes better!

Last edited by Lucien Nunes; 27th May 2019 at 9:58 am.
Lucien Nunes is offline   Reply With Quote
Old 27th May 2019, 12:24 pm   #6
sparkey58
Triode
 
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 25
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.
sparkey58 is offline   Reply With Quote
Old 27th May 2019, 12:35 pm   #7
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 6,742
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?
__________________
Frank
Nuvistor is online now   Reply With Quote
Old 27th May 2019, 12:53 pm   #8
sparkey58
Triode
 
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 25
Default Re: Buzz on phono input of radio.

No it does not.
sparkey58 is offline   Reply With Quote
Old 27th May 2019, 1:01 pm   #9
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 6,742
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
__________________
Frank
Nuvistor is online now   Reply With Quote
Old 28th May 2019, 10:16 am   #10
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 13,160
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-?
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline   Reply With Quote
Old 28th May 2019, 1:06 pm   #11
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 7,728
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.
ms660 is online now   Reply With Quote
Old 29th May 2019, 6:01 pm   #12
sparkey58
Triode
 
Join Date: Jun 2010
Location: Weston Super Mare, Somerset, UK.
Posts: 25
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.
sparkey58 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 6:30 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.