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Hi All.
First time poster on the forum.
I have an older Denon amplifier (PMA-525R) that I have grown quite attached too that was damaged in a move from NZ to Aus. The Moving company packed it vertically and the variable loudness knob was pushed into the unit wrecking the internal variable resister - see. I am looking for the identical replacement - but could not find any parts that fit the size required to fit onto the circuit board and into facia panel/knob.
The Denon manual refers to the part number as follows:
Ref No: VR004
Part No: 211 0830 003
Part Name: Variable resistor 100 kohm
Remarks: V06PB502
Part Number on the top of the wrecked variable resister = 104B 528D
Any help in sourcing this resister would be appreciated!
That is a nice little amplifier. Unfortunately, locating replacement parts is very difficult. This is especially true for things like switches and variable resistors.
Sorry, I can offer no direct assistance. The best I can do is recommend watching online auctions for a donor unit. But, even here you may have problems as the PMA-525R is rare as it was only produced for a short time. Ref: Denon PMA-525R Stereo Integrated Amplifier Manual | HiFi Engine.
BTW, did you see the 110th anniversary model (PMA-A110). It has striking interior with dual transformers. It’s a shame they put a cover on it.
Best wishes,
APDahlen
P.S. As a last resort you could permanently “set” the control using fixed resistors. The control could be glued in place.
The image looks a lot like a Bourns PTV potentiometer such as PTV111-2425A-B104. There are of course many variations on this depending on shaft length.
Thanks so much for the valuable input - the bourns PTV pictured seem to fit the bill - I need to now take some measurements and see if I can trace the same one.
BTW - the PMA 110 look great! - Ill need to save up a bit for that though!
The Bourns PTV potentiometer identified by @Robert_Fay is likely correct. However, in the off chance that it is not, and you decide to permanently set the loudness control, we would need to consider the construction of the variable potentiometer.
You are correct, it would take a total of 4 resistors e.g., a 25 kΩ and 75 kΩ pair would preeminently set the control to 1/4 rotation. The center tap of the resistor pair would connect to the where the potentiometer wiper was once placed on the PCB. The other ends would attach where the other sides above and below the wiper as shown in your parts layout. You would need to carefully consider the construction of the potentiometer to determine which resistor went to which side.
For a loudness (aka volume) control the potentiometer is audio taper. From a practical manufacturing standpoint it is a modified logarithmic function. You can pick the output ratio % vs. rotation % from the chart in this article.
In this case, the volume control is large and center mounted control while the loudness is a small control mounted to the side of the unit.
Either way, you raise an interesting question. Is the control linear or audio taper? Thankfully, it’s not one of those multi tap units that were common in older equipment.
HI All. Thanks for the valuable feedback so far. @APDahlen you are correct, this Amplifier has a large motorised central volume/attenuator knob, but also has a feature I have not seen elsewhere by having a variable ‘loudness’ knob as well - this feature seem to run though the Balance, Bass & Treble circuitry. It fortunately has a bypass switch to deactivate the whole circuit - but I don have any settings which I would like to have working.
I have seem to narrowed own the Bourns Model I need - see below: The top of the existing resister has a ‘104B’ printed on it, so I am hoping this is a ‘B’ type 100kOhm curve resister - not sure which one though as there are 5 different B curves available…
My thoughts are to now potentially use a rear pin out (vs. bottom pin out) configuration and use short flexible wires to re-route the pins to the correct locations on the board.
The loudness control is a popular control for audio. It effectively boosts the low and high frequencies so that the human ear perceives an equal volume at low sound levels. Ref:
Assuming I have the correct value entered, here is a simulation for the Denon circuit when the loudness control is set to 50%.
