Volume Potentiometer - RCF ARTxxx

[Tone Meister]

Having no luck sourcing a volume potentiometer for an RCF ART200A powered loudspeaker. Does anyone know where to source these, or has anyone taken the time to source a retrofit replacement for these pots?

[Jazz P Bass]

P1. 47K.
Well that helps.
Are there any markings on the component?
Post a picture?

[Fragger]

You might try these guys:

RCF USA Inc
110 Talmadge Rd. Edison, NJ 08817
Tel +1 732-902-6100 - Fax +1 732-902-6105
e-mail: info@rcf-usa.com

[Enzo]

What he said^^^


But it would be immensely helpful if we had a photo of what you are looking for. I can;t tell from the schematic the size and shape of it.

[Tone Meister]

Contacting RCF is a non-starter. We've been RCF dealers since the 90's and now many of these powered speakers are showing up with worn out volume pots. The edited version of the story is simply that component-level parts are unavailable for these "legacy" units from the current RCF corporation.

I'll pull the pot and try to get some detailed pictures and info to post in this thread. I was just hoping that some of you here had experienced the same parts unavailability and had located a suitable replacement.

NOTE: There was a parts list in the RCF schematics thread (post #6 in http://music-electronics-forum.com/t19416/) but I was unable to open the Word doc. That may be a possible short cut in determining a suitable retrofit replacement part.

[Tone Meister]

OK, I finally got around to pulling one of the amps and yanking one of the 47K pots. Here are the details:

• The plastic shaft is knurled, shaft diameter is ~6mm, length ~26mm
• Thread mounting depth is ~ 8mm, diameter ~10mm
• Pinout, 3 pins one side, 2 mounting lugs opposite

[Jazz P Bass]

That is an odd duck.

[Tone Meister]

That is an odd duck.

That it is indeed. We have literally sold hundreds of these ART Series powered speakers and this is a problem whose time has come to solve. When RCF was bought out by Mackie parts became very hard to source, then impossible. This volume pot issue has been brewing for several years and now all parts in the supply chain have dried up. There are many of these speakers that are still going strong except for failing volume pots.

I am beginning to think that my only option is to spec a solid 50K pot that will mount in the chassis and then wire flying leads to the circuit board. The idea is to come up with a solution to apply as a standard repair.

[flyingdutchman]

IIRC in the ART200 the pot is mounted on the pcb only, the panel bushing is not used - the panel hole is approx 6mm dia. and the pot shaft pokes about half way through. Correct me if I'm wrong.

I'd use a standard 16mm 50k linear taper pot e.g. ALPS or similar (in european pots "A" is lin taper, "B" is audio taper, unlike ALPS etc. where it's the other way round), ream out the panel hole to 7mm, and fit the pot to the panel with short lengths of solid wire connecting to the pcb. If the replacement pot has a knurled shaft, the original knob will fit.

(BTW, hi all, new here, thanks for havin' me )

[Tone Meister]

Welcome to the forum flyingdutchman! Being you are from Italy, you don't happen to be an RCF employee, do you? And yes, you are correct on the mounting procedure and details for this pot. What I plan to do is establish a standard solution for repairing these failed pots, so even if the knob must be changed to accommodate the new pot, then that is well enough.

The technique you described is pretty much what I had in mind to do. Any particular reason why you'd spec a linear pot versus an audio taper?

[flyingdutchman]

Being you are from Italy, you don't happen to be an RCF employee, do you?

Nah, I pride myself on not being affiliated with any brand

Any particular reason why you'd spec a linear pot versus an audio taper?

The original pot should be a linear taper.
As you can see in the schematic, the pot is not wired as a standard attenuator, instead it is part of the feedback network around U1B. An audio taper pot might not yield the expected result in this type of circuit.

As a matter of fact, in many power amps and powered cabs even if the volume pot is wired as a standard attenuator, many designers prefer using linear pots and "bend" them a little by applying a resistor from wiper to ground. The result is something halfway between linear and audio taper, and this gives a reasonably precise control with a range of 30dB or so.

[Tone Meister]

Very well, linear taper it is then. Here is a shot of the PCB to help sort out the correct pinouts.

[flyingdutchman]

The three pads in top center of the pic are, from left to right, CCW, wiper, CW.

The other two pads should be for mechanical purposes only, but they do have tracks attached - these are probably connected to ground. You might want to insert a wire link between the two just in case

As you might have noticed the pcb is quite fragile. If you use solid wire to hook up the pot, put a little s-bend in each to absorb mechanical stresses, and apply a bit of hot glue to the top of the pcb where the wires come out. Otherwise you stand a good chance of the customer coming back in two weeks complaining of intermittents...

[Tone Meister]

I've settled on a potentiometer and matching knob and plan to build a handful of retrofit kits to have on hand. The pot is a long-life Bourns, linear taper (mfr part # PDA241-SRT02-503B0), and has a solid shaft with which I can use a knob with a set screw. Here is the spec sheet on the pot--see any problems?

[flyingdutchman]

Here is the spec sheet on the pot--see any problems?

The only possible snag I can think of is that the pot's bushing is a bit on the long side, which might require a spacer or counternut between the pot and the panel.