Potentiometer Replacement

[JustRamsey]

Hello friends! Long time lurker, first time poster.

Im currently rehabbing a pair of Marantz HD77’s and I’m going to need to replace the potentiometers for the mid, tweeter, and “super-tweeter”. Their values are 10, 5, and 3ohm respectively. The oem pots are wire wound. I’ve done a fair bit of searching around and finding pots in those values is close to impossible. This lead me to a thought. My understanding of pots is that the value associated with any given pot is its max resistance value and the minimum is 0ohm. So the 10ohm pot that came with the speaker would have a max impedance of 10ohms (turned all the way down as to decrease mid-bass?) and then would offer no resistance if turned all the way up as to increase mid-bass, 0ohm. If my assumption is correct, would I be better off finding audio grade resistors of the median value of each pot so as to simulate the pot in the “flat” position? The ability to adjust the levels from the speaker really has no appeal to me. But if replacements were a viable option I would do that for originality sake.

Thanks for the help.
Ramsey

[AEIOU]

Hello friends! Long time lurker, first time poster.

Im currently rehabbing a pair of Marantz HD77’s and I’m going to need to replace the potentiometers for the mid, tweeter, and “super-tweeter”. Their values are 10, 5, and 3ohm respectively. The oem pots are wire wound. I’ve done a fair bit of searching around and finding pots in those values is close to impossible. This lead me to a thought. My understanding of pots is that the value associated with any given pot is its max resistance value and the minimum is 0ohm. So the 10ohm pot that came with the speaker would have a max impedance of 10ohms (turned all the way down as to decrease mid-bass?) and then would offer no resistance if turned all the way up as to increase mid-bass, 0ohm. If my assumption is correct, would I be better off finding audio grade resistors of the median value of each pot so as to simulate the pot in the “flat” position? The ability to adjust the levels from the speaker really has no appeal to me. But if replacements were a viable option I would do that for originality sake.

Thanks for the help.
Ramsey

Are you sure they are potentiometers? Could they be variable L-Pads, which look similar to L-Pads? Yes, I'd replace them with resistors.

[Psycoacoustics]

Where were the pots set as you were listening to the speakers? If they were in their “off” position maybe you could just remove them.

Have Fun! Mark

[tomzarbo]

You could use this rotary switch from PE: https://www.parts-express.com/rotary...rting--023-666
and solder in some resistors in the range of where you normally leave the knob.

So, have the knob where you normally use it on eat pot, cut the leads, measure the resistance, and then pick resistors that are a few ohms above and below where you normally like it. You could use 2, 3, 4, or however many you want of the available switch positions and just wire the ones you don't want to use together so they are the same as the previous selection.
That gives you the rotary look of the original, without the hassle of finding pots in the original values. I imagine a shot of detoxit for switches would probably be in order every few years or so, but I think it would work.

Some volume potentiometers on preamps use many multi-stepped resistors to do the attenuating in lieu of true variable pots and some think it works very well for that.

Could be a bit of work since you have three per speaker, but I think it'd be fun. I think I'd use the 'shorting' switch that I linked to so you wouldn't have 'pops' when switching between selections which might be annoying.

TomZ

[Millstonemike]

Here's a schematic for the HD-77s. The pots carry the full power to XO/driver so have to be rated (watts) accordingly. You can certainly use a 5 Ohm for the 3 Ohm, a 10 Ohm for the 5 Ohm, etc.

What Pots have you found?

Also, have you given each pot a mega-dose of CRC contact cleaner (HD) while vigorously working their range. That cures many, many ills ...

[Chris Roemer]

I find it interesting that it's a 4-way speaker with (basically) the top 3 drivers all overlapping (NO low-pass filters to be seen, not even a 1st order coil on the mid). SEEMs messy, phase-wise??

Your initial idea (to shoot for mid range fixed resistors) doesn't seem like a bad idea to me, but instead of going w/: 5n(ohm), 2.5n, and 1.5n, EYE would probably get a pair of
8n, a pair of 4n, and a pr of 2n (for EACH cab). Odds are that you wouldn't need MAX attenuation, and if you wired in the 8, 4, and 2n (and any of them seemed too quiet), you could piggyback the "other" (giving you 4, 2, and/or 1ohms). That would get you within +/- 1-2dB in most instances.

[Millstonemike]

I find it interesting that it's a 4-way speaker with (basically) the top 3 drivers all overlapping (NO low-pass filters to be seen, not even a 1st order coil on the mid). SEEMs messy, phase-wise??

Your initial idea (to shoot for mid range fixed resistors) doesn't seem like a bad idea to me, but instead of going w/: 5n(ohm), 2.5n, and 1.5n, EYE would probably get a pair of
8n, a pair of 4n, and a pr of 2n (for EACH cab). Odds are that you wouldn't need MAX attenuation, and if you wired in the 8, 4, and 2n (and any of them seemed too quiet), you could piggyback the "other" (giving you 4, 2, and/or 1ohms). That would get you within +/- 1-2dB in most instances.

+1

Me, I'd create a simple list of many resistor combos (MS-Excel) to optimize my variations since shipping costs typically equal the cost a half dozen or more resistors. You don't want to order again for one value you missed the first time. Also, you could replace each pot with a simple switch to add an additional resistor in parallel with the fixed one(s) and retain one alternate adjustment for each driver - different placement or room and the speakers will sound different.

The HD-77s are rated at "250 Watts" and the pot assembly actually has a heat-sink on it. What minimum wattage resistors will be needed here? Thermal Paste?

But I'l reiterate my first suggestion as the best first step: If the pots are scratchy, lose contact at certain settings or even most settings, I'd spend $8 bucks on CRC Contact Cleaner (Home Depot). Most likely these pots are oxidized from age, not worn out from use (unless they're burnt up). I've had similar situations where the CRC brought back like-new performance. You just keep injecting it into the pots while working their full rotation and it cleans them up.