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Are Replacements Available for the ARC LS5 MKII Motorized Control Pot and Board?

I recently picked up an LS5 MKII. It has not been modified. I found a few issues that the seller didn't mention. I didn't question this as I paid very little and I really enjoy restoring old gear, any kind.

I have replaced all the electros, obtained 10 NOS Amperex 6DJ8 tubes from Brent Jessee, and replaced the output caps with 3.3 uf Vcap ODAMs. I am burning all this in right now. The sound is great!

However, there's more left to do. The Source/Monitor function does not work. I replaced the switch (gee, that was hard) but the problem persisted. I haven't done a detailed test of the control board yet. A problem for later as I have a work around right now.

The biggest problem is the motorized volume control. It doesn't work at all. I have the AR remote. The mute function does work, but not the volume control. I talked to Audio Research about this. After thinking a bit, AR said repair would be very expensive as they would have to re-engineer this function since replacement parts are not now available. AR didn't say which. Right now I'm using a Fosi P4 as volume control. I also use a GEQ, so the path is Preamp -> P4 -> GEQ -> Power amp (ML #27). I'd like to use the tape loop for these functions as I've done with the AR LS22.

Ideal would be a direct drop in replacement. I've seen the large variety of motor control pots and boards on Ali Express and eBay. I really would like a cleaner solution.
 

kevemaher
kevemaher's avatar
kevemaher OP
419 posts
 

@fiesta75 

Thanks for the suggestion.

I've already done this. As I turn the knob up and down, there is no AC or DC voltage or change in voltage apparent with my voltmeter.

I have not yet applied voltage to the motor. I need to do this.

I've also activated the remote while monitoring the voltage. No change.

This means it may be the board. But I still don't know if the motorized pot is good or bad.

So I still don't know what's defective. I could buy a replacement motor. It is not expensive. But disassembly will be extremely challenging.

kr4's avatar
kr4
3,782 posts
 

Not sure if you disconnected the wires from the control or not but:

1.  If connected and no power (ac or dc), it could be the power input at fault or a shorting in the control.

2.  Disconnect the 2 power wires and measure across them while manipulating the remote control.  If nothing, it ain't the pot.  It it responds, it is likely the pot motor.

4.  I have one of those pots in a drawer somewhere, I will try to find it and the specs.

kevemaher's avatar
kevemaher OP
419 posts
 

@kr4 

I'll do this test tonight after the listening session.

Great suggestion to isolate the problem to the board or motor.

kevemaher's avatar
kevemaher OP
419 posts
 

@kr4 

I just did the test. There are two leads from the board, red and grey. With my voltmeter, red lead was connected to red wire from board and black lead connected to grey wire from board.

There was -0.5V DC across the leads, which was slowly decaying. I assume this is a capacitor somewhere slowly discharging. There was 0 AC volts.

With power on, the -0.5V remains.  As I activate the remote volume control up and down, there is no change in either the AC or DC voltages. The remote was placed in front of the IR sensor.

This test indicates that the board is to blame. Thanks for the tip!

There's not much of a chance I can find a replacement. I will need to see if I can cram an aftermarket board and sensor in there. I'll need power from somewhere. I could use a battery to power the control board if it will supply enough current to move the motor shaft. 

Another project. 

fiesta75
1,535 posts
 

Bad transistor from whatever powers the control board?  Just check for bad BJT in the physical area.  Can't be that many?

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