Refurbishing the tuner: If you're planning on trying this yourself, if you have some experience working with electronics and mains voltage electrical appliances, it's a fairly straightforward job - but even if you are, please be careful. Later model 400s included an internal mic, but this one and this schematic do not include one, the input connector also does not supply any sort of bias voltage or phantom power. The audio processing goes through an optional low pass filter (the image clarifier switch) which I like to leave on, and then to the lamp driver board - to get that relatively small signal to drive neon lamps which have a 400V power supply. To control motor speed, the tuner uses a high stability oscillator (at least, for the time) that drives a bipolar stepper motor, and each note on the selector has an individual resistor to trim the speed exactly. The schematic even shows which board each part is on (dotted outlines), which is especially useful because there is no silkscreen or part markings on the boards, and it includes some of their design considerations - using low current noise resistors on the audio input line and using high stability, low leakage capacitors on a few key components used in keeping tuning frequency stable. It helps in understanding the design and troubleshooting or refurbishing to have the schematic available, and it is available online, including here. The reference pitch vernier is marked in cents, but you can use it to play with a non A=440 reference pitch, for example, if you set it to be just over 7 cents high, you will be effectively playing with an A=442 reference pitch. Conversely, if you set the vernier to 12 cents low, when you play your not a major third below and see the ring stop moving, you'll be playing the note in tune with equal temperament. This is an artifact of equal temperament and the harmonic series, so if you play your note 12 cents sharp, that ring will stop moving, because you will be fitting in the just intonation of the major third. Interestingly, when playing the major third below the note the selector is set to, you will see one ring telling you that you're playing flat, even if you are playing the correct pitch. Because the motor speed has to be precisely controlled to measure against a fixed reference, you have to change the note dial (centermost large dial) when tuning other notes, but because the harmonics show, you can still get good tuning information on the perfect fourth and fifth of whatever note the tuner is set to - the patterns will still be visible and will stop moving when the note is in tune. The rings on the disk match up to different sounding octaves, so the higher the pitch being measured, the higher up on the disk the pattern will seem to slow or stop moving, and because the tuner measures all of the incoming audio, you will typically see multiple rings looking active - higher harmonics in the sound will show, usually more faintly, on the other rings of the tuner disk. If the markings are moving to the left, your note is flat, if they are moving right, your note is sharp, and if they appear but are not moving at all, you are perfectly in tune. You set the note you want to play, make sure the vernier knob is set to the right reference pitch, and then play your note - when close to the right pitch, you will see some of the markings on the disk get darker. It sounds complicated, and I suppose it is, but the circuitry involved is comparatively much simpler than other electronic tuners, so when electronics were comparatively primitive, it could be an extremely accurate instrument without too much electronic complexity. This is the same sort of effect you can see watching a video of a driving car's wheels, where the wheel seems to rotate quite slowly, but in this case, the designers have done all the math needed for the speed of the motor and the patterns on the disk to look like they stop moving when the incoming audio frequency is in tune. When the rotation speed of the disk, which has its speed precisely governed by a separate circuit, and the speed of the flashing of the light match up in the right way, it looks like the markings on the disk are moving very slowly, or not at all. What that means is that a strobe tuner converts an audio input into a blinking light - neon lamps in this one - and shines it on a wheel with patterned markings that a motor is spinning at a constant speed. How a strobe tuner works: A strobe tuner, at least traditional ones, is an electromechanical tuner that relies on the strobescopic effect (surprise surprise!) for its display. I replaced a few parts, made a couple modifications, and added an internal mic to make it quicker to setup and use. In developing my own precision tuner and being interested in strobe tuners, I found a working-but-needs-refurbishment Peterson Model 400 inexpensively on ebay, so I bought it with the intent to fix it up, calibrate it, and use it.
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