Denon PMA-250 amplifier - volume issue - diyAudio
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Old 1st April 2013, 04:22 PM   #1
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Default Denon PMA-250 amplifier - volume issue

Iv lifted this description of the issue from another post as I cant put it any more succinctly myself.

Quote:
Denon PMA-250 amplifier -Volume knob not responding correctly, high volume on low setting, sometimes crackly and sometimes only works through one speaker, can improve by manipulating the knob.


Basically iv tried contact cleaner into the pot with no success. I'm wondering weather it would be worth de-soldering the pot from the board and attempting to service it? If so how would I do this?

If its not serviceable could I replace the pot with a new one? If so how would I find out which type to buy? Iv seen replacement pot's in values from 1k ohm to 1m ohm, and linear mono, linear stereo, logarithmic mono, etc. I know absolutely nothing about electronics so these values unfortunately mean very little to me.

The pot is quite easy to access to I have no issues with de soldering it from the board.

Any help would be greatly appreciated. Cheers.
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Old 1st April 2013, 04:59 PM   #2
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Just looking at pots and most seem to have 3 pins or 2 rows of 4. The one Im looking to repair/replace has 6 pins. Perhaps Im searching for the wrong thing?
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Old 2nd April 2013, 11:11 AM   #3
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Anyone?? I just want to know how to go about finding a suitable replacement pot if they can't be repaired.
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Old 2nd April 2013, 11:35 AM   #4
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Location: West Australia, straight over the road from the beach, natural ambient sounds only.
Have you resoldered the pot ?....the symptoms describe intermittent earth connection.
I have repaired pots in the past by dismantling them and squashing the pins that connect to the track pcbs with pliers...restore the compression connection.
Silver conductive paint may be needed also...at the pins.
Dan.

Last edited by Max Headroom; 2nd April 2013 at 11:38 AM.
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Old 2nd April 2013, 11:36 AM   #5
pilli is offline pilli  France
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Hi,
I'll try to help you, but I'm afraid that it could be a long explanation, based on the questions you ask...
The symptoms are probably from a faulty potentiometer indeed, and it is indeed easy to replace, all you need is a close enough value, and a geometry that fits in the mounting of the current one.

The potentiometers are marked most of the time.
Can you spell us, or take a picture of the markings on yours, then we can tell you what it is.

There's tons of info about this on the web.
For example here: Potentiometers (Beginners' Guide to Pots)

Cheers,
g.


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Old 2nd April 2013, 07:13 PM   #6
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Click the image to open in full size.

This is the pot itself. Hope you can make out the writing. Thanks!
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Old 2nd April 2013, 07:13 PM   #7
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Quote:
Originally Posted by peacocksuit View Post
Click the image to open in full size.

This is the pot itself. Hope you can make out the writing. Thanks!
pot denon.jpg Picture
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Old 2nd April 2013, 07:35 PM   #8
pilli is offline pilli  France
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...well...
when I said they're marked "most of the time"...
this is a case where I can't really make sense of the numbers...

However:

- There may be other numbers on the side of the pot (although it looks skinny)

- You can always measure it with an ohm-meter (why didn't I think of this before...) This is easier to do If you get it out of the board, but you can make a good guess even when mounted.

As you see, there are six pins, which makes two potentiometers, one per stereo channel. (Some pots that are less easy to replace would have an additional pin per channel, used for "loudness control"...)
The shape of this pot is not the most common to buy as a spare.
You could also get one with different pinout, that fits in the front panel hole and then connect it to the board with short cables. This then depends on what you can fit on the front side...

As I said, all this is a bit long to explain in full detail...



And as it was suggested, you should try to check the soldering of the six contacts on the PCB before going for a replacement. Why don't you resolder them anyhow: it won't hurt and it may solve things.


_

Last edited by pilli; 2nd April 2013 at 07:39 PM.
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Old 2nd April 2013, 10:32 PM   #9
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Location: West Australia, straight over the road from the beach, natural ambient sounds only.
That style of pot has a plastic disc shaped end to the shaft that holds two wiper contacts via small peened over rivets.
These rivets break and the contacts are then loose causing connection issues.
If you are really good you can dismantle the pot and repair the contacts mountings.
Easier is to replace with a standard log pot and run connecting wires.

Dan.
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Old 3rd April 2013, 06:34 AM   #10
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Location: Coffs Harbour, on the east coast
Agreed, I had a JVC pro. amp with a similar pattern pot. - an absolute sod to dismantle and repair. There seems
to be plenty of room around the pot. and since there is no loudness tapping apparently, you could fit a standard
ALPS RK27 type of the correct audio taper (log) and value according to the label.

Just wrap the pins with solid core hook-up wire and fit to the relevant PCB holes. Unless you have schematic,
you will need to be certain of what channel and which connection, either start, end or wiper it is you are
dealing with by simple, logical checks with a multimeter on the original pot. Use a suitable resistance range to
encompass the value of the pot. Just remember; you'll be looking at transposed start/ends from the rear and
you need adequate shaft length and the right splined or plain shaft.

Of course, you can fit any old brand log pot. of any quality and it will work OK but I guess you will want to lavish
top quality parts on it and the 8 quid or so +post on Epay might be only a small price?
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regards

Last edited by Ian Finch; 3rd April 2013 at 06:39 AM.
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