Peterson Strobe Tuner (Model 400) - Page 1
Forum section: electronics / repair

author: AndyLanderdahl
author_rank: Newbie
timestamp: July 15, 2018, 09:48:13 pm
content: I grabbed this awesome tuner off eBay, and loved it thoroughly for a day or two. Then one day I turned it on and it blows a fuse. No problem right? Well it blows the replacement instantly, so I investigated further. It appears the rectifier diodes on the power board are bad (all four test bad, one has visible damage to the package). I've been able to roughly figure this based on the schematic.The problem is, the diodes aren't marked, so I can not figure out a replacement. I'm also not sure if anything else might be damaged, the motor does not turn when it powers on as it normally would, but that could be all down to the blown fuse. If anyone can provide a bit of insight, please speak up! Let me know if any more information is needed.

author: floobydust
author_rank: Super Contributor
timestamp: July 15, 2018, 11:05:10 pm
content: The rectifier diodes, one is 1,000PIV like a 1N4007 for the +400VDC, and the 50VDC 4-diode bridge is another 1A affair so I would put a 1-2A 200PIV part like 2KBP002M (2A 200PIV)Given the age of the tuner, I would check for a shorted electrolytic capacitor in the power supply or motor.It would be best to replace them all, maybe a dozen parts. I know professional piano tuners who need reliability when they are doing a job and $25 investment is worth it.Check the two motor drive power transistors, if they have shorted it would overload the rectifiers and you will just keep blowing fuses. You can put a 40-60W light bulb in series with AC mains power to the unit while checking for shorts, it will just light up if there is an overload.

author: james_s
author_rank: Super Contributor
timestamp: July 16, 2018, 01:52:59 am
content: That's an interesting contraption, I've never seen one before, how does it work?The above advice looks like it pretty well has you covered, the diodes are not critical and lots of different types can be substituted but it is possible something downstream has shorted and caused the diodes to fail.

author: DaJMasta
author_rank: Super Contributor
timestamp: July 16, 2018, 02:29:34 am
content: I refurbed one a year or two ago and replaced the main filter caps as well as earthing the chassis - the default cord just floats the chassis between filter caps and when I touched it on a concrete floor (though not on wood) I could feel the tingle... Mine was reading a fair bit off what it should, turned out one of the leads on the polyester caps that formed the main oscillator had broken (it was like 4 caps bundled together), so reconnecting it brought it back much closer to calibration.I also put in a built in electrolytic mic with a switch so I could use it without an external mic, though other model 400s sometimes had a built in one.As for how it works, basically it amplifies the incoming sound signal and flashes a neon light with it, then it has a disk with a whole bunch of black and white segments in rings that correspond to octaves at a fixed, though switchable, frequency using a stepper motor and a low drift oscillator. When the flashes of the neon light correlate to the speed the marks on the disk are going around, you get the same strobe effect you see with cameras and synchronized moving objects - the marks on the disk slow down as you get closer to being in tune until they stop moving. Slight perceived movement to the left or right means the incoming tone is flat or sharp, respectively.Some notes on what I did to mine and some views of what was in there:http://medpants.com/strobe-tuner-refurb

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: July 16, 2018, 04:56:38 pm
content: Someone who was a tuner gave me one of these some time ago. Need to grab it, and see if it still works, maybe tune up the Wurlitzer. Nice Teardown, and photos. Thanks!

author: AndyLanderdahl
author_rank: Newbie
timestamp: July 23, 2018, 08:02:29 pm
content: Quote from: floobydust on July 15, 2018, 11:05:10 pmThank you greatly, I was at a loss as to where to even begin finding specs on those diodes. Considering I have to buy the diodes and replace them anyways, it really makes sense to go ahead and do the caps. I use it to set up and tune various stringed instruments, and I too can appreciate the small investment into reliability.Quote from: DaJMasta on July 16, 2018, 02:29:34 amI came across your write up when trying to find some more technical data, it's kind of funny to see how many little changes they made to the model. Mine is almost identical to yours, except the power cord and mic input are on a little cut out on the side and the power cord is detachable. I imagine it would take quite some experience to date the individual revisions. I wasn't crazy about the Astatic mic that comes with them usually, but I couldn't bring myself to drill into the face plate. Mine is fairly stationary, and it's actually nice to be able to run the mic closer to the sound source of whatever you may be tuning. One thing I may do in the future, particularly if I can find a spare face plate or second tuner, is add a line input for tuning in louder environments.Quote from: james_s on July 16, 2018, 01:52:59 amThey really are neat little devices, and so precise. Peterson is pretty much one of only two names in the precision tuner game, with a long standing history. I don't know that the strobe device is necessarily still the best display for it, digital readouts giving you fractions of cents is probably more concise, but there is just something warm and comforting of the neon indicators and that disk whirling around. Changing from note to not isn't as easy as your standard chromatic tuner, but come on, this thing is worth it.