Carmel 838A Intermittent Horn

Hello everyone. I recently was gifted some vintage audio equipment from a relative and just got some time to get it setup. The speakers are a set of Carmel 838A. The amp is an MC250 and the preamp is an MX113. It looks like both the amp and preamp were last refreshed in 1994 by McIntosh. My main issue is that one of the speakers has an intermittent horn (804A). After about 15 minutes of listening, it abruptly turns off completely. The rest of the speaker continues to work. After letting it sit for a few minutes it will work again. I tried switching the channels on the amp and the problem remained. I checked the horn with a multimeter and got an impedance of 11.5. This led me to believe that maybe the crossover was causing the issue. Its an N800-F which unfortunately is riveted shut and from a few other references I found is tar filled.

If you feel up to it, try swapping the crossovers from left to right and see if the problem follows the crossover or stays with the speaker.
Either way, once you know what needs work, you can get this sorted out... there are options.

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I got a chance to switch the crossovers last night and the issue followed with them so I guess I'm building new crossovers. I did find a schematic of the N800F, but I'm a little confused as this is my first time building anything audio related. From what I read, this circuit used a 25 ohm pot instead of an L-Pad. I did find a few references stating that a 16ohm L-Pad could be substituted, but I wasn't sure if I then should remove the 20ohm resistor or is that should be included still. Here's the part list I put together so any advice would be appreciated.

Dayton Audio PMPC-4.0 4.0uF 250V Precision Audio Capacitor

Dayton Audio PMPC-10 10uF 250V Precision Audio Capacitor

Jantzen Audio 3.5mH 18 AWG Air Core Inductor Crossover Coil

Dayton Audio DNR-20 20 Ohm 10W Precision Audio Grade Resistor

Parts Express Speaker L-Pad Attenuator 100W Mono 3/8" Shaft 16 Ohm

1) might want to pay attention to the DC resistance of the inductors (e.g. 1.1 ohms vs .36 for an ERSE iron core also at Parts Express).
For some crossovers, this makes a significant difference. Schematic doesn't explicitly show iron core inductors, but I'd guess they were.

2) leave the 20 ohms in... otherwise you could accidentally shunt the entire HF signal to ground.

Perhaps overkill, but old-school knob included? https://www.amazon.com/dp/B012S844AS

16 ohm L-pad could "work", but you could end up using a smaller useful portion of the rotation and the value would be non-linear with position.

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Originally Posted by mhershey84

I got a chance to switch the crossovers last night and the issue followed with them so I guess I'm building new crossovers. I did find a schematic of the N800F, but I'm a little confused as this is my first time building anything audio related. From what I read, this circuit used a 25 ohm pot instead of an L-Pad. I did find a few references stating that a 16ohm L-Pad could be substituted, but I wasn't sure if I then should remove the 20ohm resistor or is that should be included still. Here's the part list I put together so any advice would be appreciated.

Dayton Audio PMPC-4.0 4.0uF 250V Precision Audio Capacitor

Dayton Audio PMPC-10 10uF 250V Precision Audio Capacitor

Jantzen Audio 3.5mH 18 AWG Air Core Inductor Crossover Coil

Dayton Audio DNR-20 20 Ohm 10W Precision Audio Grade Resistor

Parts Express Speaker L-Pad Attenuator 100W Mono 3/8" Shaft 16 Ohm

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Quote:

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Originally Posted by mhershey84

Hello everyone. I recently was gifted some vintage audio equipment from a relative and just got some time to get it setup. The speakers are a set of Carmel 838A. The amp is an MC250 and the preamp is an MX113. It looks like both the amp and preamp were last refreshed in 1994 by McIntosh. My main issue is that one of the speakers has an intermittent horn (804A). After about 15 minutes of listening, it abruptly turns off completely. The rest of the speaker continues to work. After letting it sit for a few minutes it will work again. I tried switching the channels on the amp and the problem remained. I checked the horn with a multimeter and got an impedance of 11.5. This led me to believe that maybe the crossover was causing the issue. Its an N800-F which unfortunately is riveted shut and from a few other references I found is tar filled.

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I had an intermittent problem on one of my JBL speakers, it turned out that the nut holding the binding post was loose. Check the nuts inside the horn drivers before doing anything drastic.