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MPX 19khz amplitude question

Carbsandcaps

Carbsandcaps

Super Member
I was wondering if anyone knows at what amplitude the 19khz tone has to be at to “activate” the mpx. I’m trying to get a relay to work in a fisher mpx unit but I can’t get a good mpx signal where I’m at so I’m using a signal generator. I can get voltage at the relay coil but it’s varies pretty wildly with different amplitude of the 19khz signal.
 
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I'm still a bit new at this so somebody might have a better take on this than I do. 19kHz pilot modulation is usually 10% of the carrier level but that has to do with bandwidth, not signal level as I understand it (could be wrong). The idea is you want the MPX to kick on when the signal is strong enough to get good sound and below that level you'd rather just have mono. It might depend a bit on the receiver but ballpark, maybe with a 50uV signal you'd want it to kick on.
 
Ok I should clarify a little better. This would be the input to a standalone MPX unit. So it would be the output from a non MPX tuner from the MPX output rca jack. So it would be amplified but I don’t know the round about voltage coming out of that port.
 
Pilot levels should be about 10% of the composite output signal from a tuner's detector. Typical composite out levels are 200 to 500mV AC rms (full modulation). So pilot will be 20-50mV rms - these are typical levels that should easily engage stereo, with everything correctly adjusted in the mpx unit. Depending on the MPX, it may work at much lower pilot levels.
 
Ok thanks for the input. So the relay has a 10k ohm coil, I at least know that from the schematic. A 19khz signal at 40mv into the input gives me about 30v across a 10k ohm resistor in place of the relay coil. When I hook up this 10k relay I found on eBay I can get it to latch but needs some help. I believe it’s a 48v relay and I think if I can stretch the return spring on the relay I can get it to latch normally at 30v. IMG_1563.jpegIMG_1562.jpegThanks again guys I’ll report back.
 
Might need to look for a relay with a coil at the right current more than the right voltage. If thats being pulled by something like a 12au7 or whatever its only going to flow 10ma or so.
 
Agree ^^^
You likely need a 12V coil with 10K coil resistance. This would trigger on 1.2 ma across the coil. Be careful working there, there is 220V DC across that 10K relay coil. It's the plate load for a 6AB4, which is 1/2 of a 12AT7. The data sheets says they can run between 4-10ma across the plate - you measured 3 ma at a healthy MPX level, so I would go lower.
It is critical to use 10K there, but you could put a 5K resistor in series with a 5k relay coil if needed.
 
so once I got the original relay out I found the part number which is nowhere close the the fisher number. It’s a 5v relay which makes sense with 215v on one side and 220v on the other. Luckily I found an nos relay on eBay for $20. However I think it would fun to try to find a newer replacement but most of the 5v relays I found were super low resistance. IMG_1574.jpeg
 
Not a 5V relay.
P&B RS5D relays.png
2.5mA x 10kOhms = 25V
25V x 2.5mA = 62.5mW ("60MW" in table)
SPDT with 2A contacts
$3.80 each back then
 
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Wow that is interesting. I was just going by the eBay description. I don’t k ow there was such a thing as a current relay.
 
It was marketed for use in plate circuits, for which the input voltage would be less important than the current required to activate it. Possibly it is just a 24V relay in disguise?
 
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