AR-3a pot orientation

[TimmyTonga]

I'm just putting these back in the cabs. Mindful of the advice in the restoration document, I noted the indents in the base-board that receive the tabs on the pots.

What I don't understand though, is how, on the mid-pot, all the tabs seem to be 180 degrees from where they should be, given how the soldering tags need to line up.

There's only one way to put these pots back together, right?

This is the same on all four speakers.

i know the way the pots should line up is this;

but if I put the mid-pot tab in its original place, I get this;

 

I just can't figure what I'm doing wrong....

Thanks in advance for putting me straight,

Tim

[lARrybody]

Not only do I make a drawing and photograph of a crossover before disassembly. I take note of the tab indents, but also label each pot with a T and M, plus make marks where the bales line up on the board. To my knowledge there is only one way to assemble the ceramic and metal sections. 

Maybe the pots from the other speaker are mixed up. What would it hurt to rotate it to the correct position?

[genek]

Your best bet is to use a VOM to set the pot in its correct mid position and then orient that to the white dot on the cabinet. There's always the possibility that the pots weren't originally installed correctly in the first place, that their resistance was originally out of spec or that their resistance has changed due to corrosive loss of material in the windings, especially if you cleaned them using an abrasive tool.

[TimmyTonga]

2 hours ago, larrybody said:

Not only do I make a drawing and photograph of a crossover before disassembly. I take note of the tab indents, but also label each pot with a T and M, plus make marks where the bales line up on the board. To my knowledge there is only one way to assemble the ceramic and metal sections. 

Maybe the pots from the other speaker are mixed up. What would it hurt to rotate it to the correct position?

 

It would hurt nothing that I can see Larry; I'm just paranoid. I took photos, and wrote down a schematic, labelled each one H and M, but I confess I didn't make a note as to where the tab was when I removed the pots. All I know is my top picture is how they were all orientated when I removed them. It has to be that way, otherwise the yellow interconnecting wire is not long enough. And the tab is in the same place on all pots anyway.....

so I'm just going to shrug, orientate them correctly, and press them in very hard.

Thanks for the reassurance

Tim

[RoyC]

Tim,

The "correct" position of each pot is the result of placing the tabs in the stamped indents on the board. The tweeter and mid pot positions are different by design.

Roy

[stupidhead]

great question and I too have often wondered the same thing. Like Larry, I would photograph and mark internally where the OEM placement is/was but never found them to be set in placements that made sense given the dial pointer on the stem in relation to the outer graphics with the white dots. After much fussing I have come to the conclusion for my own projects that the more important piece of this is if the pots work properly. After all the actual setting is done, at least by me, entirely by feel and ear and wherever the pointer ends up so be it.

My default now is to spin wiper to halfway point, I match that with the halfway point (white dot) on the external graphics and secure. They both end up being exactly the same orientation this way (if two pots) and whether right or wrong it is how I have chosen to do it.

I have to wonder if there was an ohm setting on the dial which equated to "flat" for each of the various drivers these pots controlled, which maybe accounts for the what appears random orientation of OEM.

[RoyC]

3 hours ago, stupidhead said:

My default now is to spin wiper to halfway point, I match that with the halfway point (white dot) on the external graphics and secure. They both end up being exactly the same orientation this way (if two pots) and whether right or wrong it is how I have chosen to do it.

I have to wonder if there was an ohm setting on the dial which equated to "flat" for each of the various drivers these pots controlled, which maybe accounts for the what appears random orientation of OEM.

Not only does the indentation provide secure seating of the pot, it is positioned to set the rotation for a specific resistance setting in relation to the white dot. The "white dot" settings varied for each model. For example, the white dot settings for the AR-3a are 1.75 ohms in series with the tweeter and 3.25 ohms with the mid. The arrow on the shaft of an original pot is therefore in the proper position only when the indentations are used to reinstall it. The AR-3 and AR-5 have pointer knobs over the shafts which can be adjusted to be more precise.

Roy

[owlsplace]

The first set of AR speakers I restored (AR-5) one of the mid pots had a detent setting. Not sure it it was intentional or not but one loop of the nichrome coil was pressed in at the white dot setting forming a detent stop. These had never been apart so it was factory. Spec for the AR-5 is 6 ohm for the mid and 4 for the tweet.

Roger

[RoyC]

Hi Roger,

Based on the hundreds of pots I've seen, I doubt AR deliberately put an indentation in the nichrome coil. It was probably caused by previous users pushing on the shaft as the pot became troublesome.

Roy

[stupidhead]

13 hours ago, RoyC said:

Not only does the indentation provide secure seating of the pot, it is positioned to set the rotation for a specific resistance setting in relation to the white dot. The "white dot" settings varied for each model. For example, the white dot settings for the AR-3a are 1.75 ohms in series with the tweeter and 3.25 ohms with the mid. The arrow on the shaft of an original pot is therefore in the proper position only when the indentations are used to reinstall it. The AR-3 and AR-5 have pointer knobs over the shafts which can be adjusted to be more precise.

Roy

Thanx Roy, I thought my comments might open the door for more insights on this topic. As I suspected the placement of the pot in relation to the "white dot" was a calculated placement from the factory. Is there a data base that has the various set-points for the various vintage era AR's that had these pesky pots?

40+ odd years later is it still a calculation that is relevant? With the degradation of drivers I suppose the only real loss is in driver volume, not an alteration of tonal characteristics. Not sure if that statement makes sense and I hope you can understand my intent.

With the pointers on the bakelite (sp) knobs, it would be easy to set the pointer in such a way as to match the calculations being suggested here, and not necessarily in line with the actual pointer on the stem.

Thanx again Roy and others for bringing up and elaborating on this topic as I have devoted a great deal of grey matter to it.

Geoff

[Carlspeak]

I did a survey some years back in preparation for the introduction of the Super-Mod kit. The question was asked of 3a owners: "where do you have your pots set". I got a 1/2 dozen or so responses. Most said the the tweeter pot was maxed out and mid pot was set about mid way or a little higher. Certainly not a scientific sample size, but an indicator, at least, that the original 3a dome tweeters were showing their age and need some 'help' to balance the sound better. There was even a poster here who espoused bypassing the pot to eek out a dB or two more! :-)

[owlsplace]

32 minutes ago, stupidhead said:

Is there a data base that has the various set-points for the various vintage era AR's that had these pesky pots?

It is spec'd on the factory crossover schematic diagrams.

11 hours ago, RoyC said:

Hi Roger,

Based on the hundreds of pots I've seen, I doubt AR deliberately put an indentation in the nichrome coil. It was probably caused by previous users pushing on the shaft as the pot became troublesome.

Roy

Yeah, I imagined that also but I don't think the contact feeler has enough resistance to dent the coil wire -- it was probably just a coincidence but interesting to know you could create a detent point at whatever resistance value you want by putting a dimple in the wire coil.

Roger