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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 17th May 2007, 10:54 am   #1
scruffygit
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Default Stereo balance pot

Hi. I've got a 70's vintage Rotel RX 303 receiver amp here at the mo, which I've repaired (O/C bias resistor on the output stage of the left channel), but have another problem. I'm trying to source a suitable balance pot for the old gal. The one thats fitted in shot, even more so since I removed it ! ( I was having a bad day). Anyway I've tried to look at the type, but the print on the body is pretty poor. It looks to be 50K and being a balance pot it has 2 seperate carbon tracks , for left and right with a single wiper. Anyone know of a source for a suitable replacement? I've seen a post on here about Omega....but I really don't want to buy 10 pots! Any help gratefully received. Thankyou.
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Old 17th May 2007, 12:00 pm   #2
Hermit6345
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Default Re: Stereo balance pot

Hi,

Is this the sort of thing you are after?

http://www.maplin.co.uk/module.aspx?...C=SO&U=Strat15
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Old 17th May 2007, 1:57 pm   #3
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Default Re: Stereo balance pot

A word of caution here.A balance pot often is almost always a special version with no resistance beyond the middle position.This is done to prevent unnecessary signal loss.You'll have to consult the schematic.This part of the pot is often shown black,whilst the other is white.Sometimes the pot is marked BAL(not to be confused with B,which indicates a logaritmic curve).
You CAN use a linear dual-gang pot,but it will cause signal loss!
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Old 17th May 2007, 3:36 pm   #4
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Default Re: Stereo balance pot

Yep,
I'd take it apart and clean it up, re grease it and put it back in.
You'll find it'll work fine and keep the item 'propper' original

Dave W
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Old 18th May 2007, 9:32 am   #5
scruffygit
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Default Re: Stereo balance pot

Hi guys, thanks for the advice so far. I don't have a schematic for the Rotel, but it's virtually indentical to the RX-403 and very similar to the RX-400, all of which I've worked on, but I don't have schematics for those either! I'd love to be able to dismantle and clean etc, but the pot is physically broken, so it may be the dual gang option is the only way to go. I can't seem to get the link to the Maplin item to work. My guess is that I have to use a dual gang pot it would mean that each gang would be used for each channel ? Regards Scruff
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Old 18th May 2007, 10:39 pm   #6
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Default Re: Stereo balance pot

As Roffe stated there is usually no resistance past the middle position, I fitted my own balance control to a project home here, in which I painted one half of each track (left half of track on left channel pot, right half of track on right channel pot) with a good layer of conductive paint. It still works fine, (probably not recommended for a customer repair) If your pot is of this type and with a 50K value I guess you would need to buy a 100K dual pot as painting over half the track in this way will reduce the resistance by half. Shoving in a normal dual pot will cause the sound to be at a medium level at the centre and as you turn toward say left, the left channel will get louder as the right channel gets quieter! Not what you want at all.

Hope this helps,
Matty
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Old 19th May 2007, 12:25 am   #7
'LIVEWIRE?'
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Default Re: Stereo balance pot

I have just tried the Maplin link in Hermit 6345s post and it works fine-bings up a page showing a range of dual gang pots, both lin and log, of various values. If a dual pot is wired so that the 'hot' and ground connections are reversed for one channel vs. the other it should work fine as a Balance Control
As a general comment many designs use a single linear pot' as abalance control, but I'm not familiar with the Rotel.
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Old 19th May 2007, 10:52 am   #8
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Default Re: Stereo balance pot

I have a Rotel 820b in which the balance control was damaged i have replaced it with a standard linear pot however I placed a 10k resistor between the center pin and the live outer pin in opposistion 1 for the right and again for the left !
this appears to work very well
regards Trev
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Old 19th May 2007, 11:20 am   #9
scruffygit
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Default Re: Stereo balance pot

Well I tried that Maplin link and yep it's working now. Ta. I've had a good look at the Rotel's pot, and measuring the carbon track gives me approx 250K. Looking at the carbon track there's a bit missing in the middle at the top of the travel (centre position), so the wiper at this point isn't contacting the carbon track but is connected to a 4th pin on the pot which is wired to the rotels main board with a flying lead. I think I'm gonna experiment with dual gang pots....but at the mo I'm just confusing the hell out of myself!
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Old 19th May 2007, 1:02 pm   #10
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Default Re: Stereo balance pot

The fourth pin could be some kind of centre tapping on the track which would short one half of it out maybe? The arrangement I've used myself for basic balance controls is the pot across L - R with an earthed wiper, but I'm pretty sure that some older amps (the Leak Stereo 30 for example) have one track for each channel which has no resistance past the halfway point as Roffe has pointed out, so that the volume in (say) the left channel won't increase any more between centered and fully left.

A standard dual gang pot would probably be better than nothing. Where does the flying lead go?
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Old 19th May 2007, 8:57 pm   #11
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Default Re: Stereo balance pot

Now I get it.What you have is a special single gang balance pot with four connections.Take a look at the Luxor schematic below,and you'll see how it is connected between the volume pots.
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Old 1st Jun 2007, 8:37 am   #12
scruffygit
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Default Re: Stereo balance pot

Hi Roffe. I've now taken a closer look at the Rotel and the connection is just like on the schematic for your Luxor.....it's now becoming as clear as mud! I'm going to experiment today. Thankyou all for your help.
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