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Old 23rd April 2008, 10:17 PM   #1
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Default Adcom preamp - scratchy pot

I've got a scratchy balance pot on my Adcom GTP-450 preamp. I'd probably be happy enough just to take the pot out of the circuit completely. Can anyone point me towards a schematic for this unit?

Has anyone opened one of these up? I took off the top cover, and unfortunately the daughter-board for the tuner/front panel display is blocking easy access to the pots.
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Old 24th April 2008, 07:44 PM   #2
gain is offline gain  United States
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hi Ty. yeah im waiting on a volume pot in the mail to replace the one in a power amp ... a very common problem as the wiper starts to intermittently break contact with the resistive material inside the pot, hence the scratchy sound. the volume control is often the one to fail is it is the most heavily used.

fortunately you don't need a schematic in this case to get the parts right. disconnect the pot from your circuit. if its stereo there will be six wires to desolder, if mono there will be three. often the pot will have its value stamped on the back. if not, break out your trusty VOM and find the two pins where the resistance does NOT change as you crank the pot up and down by hand (usually the left and right pins, as the middle pin is usually the wiper but always check to make sure). this value you read on the VOM is the value of the new pot to order/replace with.

most (but certainly not all) voulme controls are 100k dual ganged pots.
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Old 24th April 2008, 07:44 PM   #3
gain is offline gain  United States
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hi Ty. yeah im waiting on a volume pot in the mail to replace the one in a power amp ... a very common problem as the wiper starts to intermittently break contact with the resistive material inside the pot, hence the scratchy sound. the volume control is often the one to fail is it is the most heavily used.

fortunately you don't need a schematic in this case to get the parts right. disconnect the pot from your circuit. if its stereo there will be six wires to desolder, if mono there will be three. often the pot will have its value stamped on the back. if not, break out your trusty VOM and find the two pins where the resistance does NOT change as you crank the pot up and down by hand (usually the left and right pins, as the middle pin is usually the wiper but always check to make sure). this value you read on the VOM is the value of the new pot to order/replace with.

most (but certainly not all) voulme controls are 100k dual ganged pots.
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Old 26th April 2008, 01:26 PM   #4
Muff is offline Muff  Australia
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You could give the pot a squirt of CRC 2-26 you don`t need to dismantle or even remove the pot, they almost always have a tiny gap somewhere around the solder tags which is where the dust enters,
use the supplied extension nozzle and apply the CRC at that location and give the pot a dozen or so full rotations, if the resistive coating is still serviceable the noise problem should be fixed.
As it happens, I finished restoring two old portable car radio`s circa 1950/60`s last night, they both had very noisy volume pots which are now absolutely silent.
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Old 26th April 2008, 03:41 PM   #5
rob3262 is offline rob3262  United States
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Quote:
Originally posted by Muff
You could give the pot a squirt of CRC 2-26
I've done the same with WD40 to rinse out the pot, works fine.
-Rob
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Old 27th April 2008, 12:49 AM   #6
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I've got a can of Caig D100 (DeOxit) which I shot in the pot as best I could. Unfortunately, as I mentioned before, access to the balance pot is nearly impossible due to the daughterboard in the way.

I only put two short shots in it so far, but it hasn't solved the problem yet. I'll open it up and hit it again.
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Old 27th April 2008, 05:36 PM   #7
rob3262 is offline rob3262  United States
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I would suggest really saturating the pot, taking Muff's advise on rotations. Catch overflow dripping out the bottom of the control.
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Old 28th April 2008, 04:50 AM   #8
TerryC is offline TerryC  United States
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I've worked on several of these, and they are a bear. It's definately doable but tedious. You will have to get under the daughter board to access the pots.

I found that the easiest way is to remove the outer front panel completly, then remove the inner front panel with the daughter board remaining attached to it. You will have to disconnect the ribbon cables running from the daughter board to the main board, but you should be able to access the pots without disconnecting the remaining wires.

I suggest using plastic bags and labeling all the screws and small parts so you know what goes where when you put it all back together.

I have purchased several ADCOM manuals, including the 450 here...

http://www.vintageaudiomanuals.com/-A-.htm

However, I don't think the manual is going to help much with this particular repair.
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Old 28th April 2008, 01:27 PM   #9
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Quote:
Originally posted by TerryC
I've worked on several of these, and they are a bear. It's definitely doable but tedious. You will have to get under the daughter board to access the pots.

I found that the easiest way is to remove the outer front panel completely, then remove the inner front panel with the daughter board remaining attached to it. You will have to disconnect the ribbon cables running from the daughter board to the main board, but you should be able to access the pots without disconnecting the remaining wires.
That's what I was afraid of. Maybe I was hoping someone knew some easy trick to get at the pots without taking it all apart. Oh well...
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