Pilot Tuner T601 and Motorola MPX HS996A

wa2ise

Super Member
Merry Xmas everyone.
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(My Xmas party outfit) :) After the Xmas activities, I decided to see if I could merge an old Pilot tuner T601 my father bought back around 1950, and a Motorola add on MPX HS996A console MPX stereo decoder. First I had to replace most of the paper and electrolytic caps, and I used a bridge rectifer in place of the single ended selenium that was there (this feeds 120Hz pulse DC to the filters, and avoids DC saturation to the power transformer, which may let me get away with the extra loading from the MPX unit on the transformer, it's not getting too hot). The MPX decoder has no power supply, so I tapped the Pilot's. The MPX unit used a neon bulb to light up to indicate stereo reception, but the avaliable B+ wasn't high enough, so I substituted a white LED from a string of Xmas lights. That drops 3V instead of 67V, so the 19KHz amp and/or 38KHz doubler tube can function, as this LED is in its plate circuit. The presense of a stereo pilot signal makes the tube draw more current, making the LED light up brighter.
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This shows that MPX units are not at all fussy about what tuner is driving them.
 

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I cut a piece of aluminium to slip under the Pilot chassis to also mount the MPX unit. It sticks out the back. But as the MPX has no power supply, this allows the two units to be moved together without wire connections being stressed (I made it easily removable to get access to the tuner's screw terminals). One thing I found out is that I needed to avoid the area under the tuner chassis where the FM local oscillator coil is located, else the alignment gets thrown off. So I cut a corner out of the aluminium. Ads a stereo tuner, it is not not that sensitive, but stereo reception requires more sensitivity, as there is roughly twice the information to be received. But this shouldn't be an issue for receivers built to have MPX units attached, not cut and pasted like I did here.
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I see that this thread has been a yawner... :boring: :D Anyway, I was hearing some distortion on the highs, so I figured that the tuner's IF might be too narrow. Stereo signals go up to 53KHz, and if the IF is too narrow, FM demodulation won't get it right at those high frequencies. On the first IF can's secondary paralleled a 33K resistor to widen its bandwidth, also added a 470K resistor to the 2nd IF prinary to widen it too. Fisher does this in their tuners. And the distortion is a lot better. Also did an alignment as well (you have to, as the stray capacitence of the resistors will throw it off).
 
It's not a yawner, just over some of our heads. Looks like an interesting project and kudos to you for taking the initiative to do it. I have a couple of mono tuners I would like to adapt to mpx myself but haven't taken the time to approach the subject. Some of the old adapters seem to sell for more than the stereo tuners did so I haven't bothered with that route.
 
Interesting application... would be nice if the pic of the equipment was bigger - like a nice close up of that Pilot. I have one of these Motorola HS996 MPX units. I plan to do something like this in one of my tuners.
 
Attached are diagrams, before modifications and after (mods are marked by colored pointers). And alighment procedures. The MPX unit sounded rather tinny until I realized that 0.005uF output coupling caps are way too small, especially as I was temporalily sending the signal into a squalid state amp for testing. Used 0.47uF caps, and I see that I need to rewire the channel output the transistor inverter stage is connected to. Right now on that channel I have an 0.05 in series with the 0.47uF cap I put in. Didn't notice the loss of bass, but the setup I'm using isn't much on bass anyway, but it would be noticeable on a serious system. I corrected the modified diagram showing this rewire. Looks like Motorola added this transistor inverter stage in case one of several models of stereo console they made needed the signal inverted... Strange. :scratch2:

I changed one of the 6AU6's in the MPX unit to a 6BH6, works just as well, and it draws 150ma less heater current, to make less load on the Pilot's power transformer. Also removed the Pilot's pilot light, and mounted the LED stereo indicator (was a white LED from an Xmas light string) where the pilot light bulb was, an extra 150ma savings off the heater winding. And I changed the selenium rectifier to a silicon bridge, less hum ripple and also avoids DC magnetic bias from the transformer core. Which may allow me to draw an extra 300ma of heater power, the transformer isn't overheating.

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My Pilot's dial has the FM FCC channel numbers (201-300) as well as the frequencies on it. Today, only the FCC cares about FM channel numbers.
 

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