Ancient Conn ST-2 Strobotuner - Grounding?

[the elephant]

I want to make my old ST-2 tuner grounded. The original two-prong cord is becoming junk very quickly as the insulation starts cracking all over. I am afraid bare copper will soon be showing, so a new cord is on my list of things to do to it.

Just as with a timing belt change, where you also do the thermostat and water pump ("since you're already in there, you might as well…"), when I replace the cord, I want to try to ground the unit.

I am assuming I have to screw the green wire down to the metal chassis if it is not hot. But what needs to be used to attach it? Probably not a run-of-the-mill self-tapper from Home Depot.

Also, to check to see if the chassis is hot, do I use the probes of my Fluke meter with one to the chassis and one to a prong, trying this with both prongs, one at a time? These old tuners hold a 600V charge in the cable from the transformer to the neon bulbs. I do not know whether it is held there for a long time after being unplugged or what.

Just trying to have a bit of "look-ma-no-hands" fun without killing myself. I am still considering sending it off to one of our TF members who kindly offered. I do not know much about these things, but I love learning, and this would make a fun project.

What's a microfarad or two between friends?

[the elephant]

I guess what this comes down to is — would grounding such a device be an "improvement" or is it even advisable? I don't have any kids. I do not make a habit of licking my finger and jamming it into electric devices. My cats are old and don't really GAF about sniffing around stuff like this. I don't even stick forks into toasters these days.

Grounding a strobotuner: YEA or NAY?

[gocsick]

You would have to check to make sure the neutral is not bonded to the case... otherwise you could create a ground loop which would potentially introduce hum or noise in the system.

Use your multimeter to check continuity between both prongs of the cord and the case. If it is isolated.. go for it.

For audio or RF equipment, grounding the chassis can reduce hum or interference by providing a path for electromagnetic noise to drain away. It also provides the shock protection you mentioned.. meaning a fault would trip the breaker rather than allowing the case to become live.

On newer stuff you have to be careful... because it might have been designed to be double insulated.. in which case a chasis ground would negotiate the designed safety features..

[the elephant]

On newer stuff you have to be careful...

This thing was made when Ritchie and Postie were in junior high school, heh, heh…

[the elephant]

Okay, so there is a 600V charge in there, in the cable from one of the two transformers to the neon bulbs. I have not yet discovered how to discharge this. I keep reading "don't touch this if the unit is energized," which honestly is not all that helpful.

I have it working again. The calibration is screwy, as it shows proper calibration, but when in operation mode, everything reads off. I guess I need to get a more accurate tone generator than my freaking phone…

Both cords (power and mic) need to be replaced, which I can do just fine, however, It has some sort of internal grouding (that I d not understand) so as long as I do not cross wires, touch the wrong doodad, or kiss that cable to the strobes I ought to be okay with case-closed, normal operation.

It needs to be cleaned up badly. Or goodly. It needs to be cleaned up. It needs a new "lens", which on some of these earlier machines was simply a clear celluloid square taped to the inside of the case in front of the wheel, while on others of the same era, there was a 1/16" thick square of lucite. Conn used whatever TF they had lying around, so long as it was crystal clear and heat-resistant.

This thing has not been turned on in about a decade. I got it to some on right away, once the tubes warmed up. I have been told that the oscillator will not give truly accurate readings (to within ±0.1 cent) unless the tuner has been running for about an hour. Before that and you might have to recalibrate it a tad. So as I type this, it is running happily and SILENTLY. (My ear must be within two inches of the case top to hear the motor at all.

The pitch selector switch is very gummy-feeling and not at all crisp, so I guess that also has to be looked at, as does the calibration knob, which moves nicely but is very tight. (It controls an air gap at the motor transformer, and is likely wound in all the way, which could account for some of the goofiness.

I think that once it has been carefully cleaned up and its shortcomings addressed, it ought to run like new.

Now for a couple of pics. My cat Sam is very curious about it. I think she wants to tune up her meow…





So, final verdict about the three-prong plug: I think it is set up for this to be unnecessary, but the cord *does* need to be polarized. My Fluke read one prong hot and one not. So my next question is:

Which prong goes to the hot side? The wide one?

I clearly do not understand the terminology, but I sort of understand what gets you zapped, as I have done a lot of electrical and electronics work over my life. I just never learned how to read a schematic or even the names and functions of some pretty basic parts, and my diagnostic skills are zero. I have no idea how to use my Fluke meter unless someone shows me which setting to use and then what to look for, reading-wise.

I am just happy that it has been running for about half an hour with no burning smells, no noise from the motor armature, and it actually works.

I love old tech, especially when it is better than current tech. By the way, I got this for free from our bass trombonist, who also GAVE me my upright bass. "You can fix instruments. Take this broken bass. If you can fix it, the thing is yours."

[gocsick]

The narrow blade goes to the hot side... Yeah I've screwed that up before..

[the elephant]

So, when the probes touch the chassis and the one blade that displays no current (continuity?), that one becomes the wide blade? Does the regular blade become the hot wire?

I know green is ground, and red is hot, but these are black and white wires. I assume the black is the hot one?

Oh, dummy! D'OH! Run the test with the meter with the existing cord first and mark the two wires.

So, out of curiosity, with matching blades, if it happens to be plugged in correctly and you touch the wrong thing to the case, it is safe, but if it is flipped, it can cause you to temporarily become the ground, right? This is why they have plugs that can only be inserted one way now?

This stuff I interesting and I don't know how (or why) I avoided it for so long. I guess I had too many bad experiences working on the wiring in my old Volvo, which was overrun with funky ghosts and intermittent issues. I sort of gave up, I guess. (I had a serious love/hate relationship with that car.)

Thanks for your patience.

[gocsick]

That's why the "never stick a fork in the toaster" advice was so important. Before polarized plugs you had a 50/50 chance of the exposed heating wire being +120V or 0V relative to you and the fork.

[tubatodd]

These pictures bring back memories of middle school. We had one of these on the wall behind where the band director would stand. He would use it to get us all relatively close to in-tune in rehearsal and before concerts. We did not have a band cat to make adjustments.

[tadawson]

The schematic for the ST-2 shows a fully transformer isolated power input, so ground to the chassis should be good.

[the elephant]

I found out that the later versions of this form factor had grounded cords. There is a grounding rail inside that everything seems to be connected to, and on the grounded units the green wire is soldered to that same rail. Mine has this same rail, so I am fairly sure that is what I will end up doing.

Any suggestions for a good video explaining all the symbols on multimeters so I know what to select when, say, someone says to check continuity? I have a very good Fluke meter and several also good ones of other brands, seemingly all having slightly different capabilities and options.

Honestly, I know nothing, so a self-paced text is likely more along the lines of what I need. I was shocked badly a few times back in the eighties and have been loath to explore electronics any more than necessary. It is time for this to end and for me to become more conversant in this stuff.

[tadawson]

Continuity is nothing more that a resistance check with a beeper. If you set to a low resistance range, a display of zero (or close) is continuity, and "OL" (or a very high value) is open. (On continuity, this would be beep or no beep . . . ).

At the end of the day, a continuity test mode really doesn't give you any more capability, but does give eyes off conveinience.

No idea on a video, but the manual for the meter should tell you everything you could ever want to know.

This might help as well: https://toolsgalorehq.com/wp-content/up ... -Guide.png
https://www.thespruce.com/thmb/PAPow6xB ... 8bc9a1.jpg

(Did a search for "multimeter symbols" . . . )

[the elephant]

@tadawson

In the photo, the circled "rail" keeps being referred to as an "internal ground rail" by several techs who post repair videos of these old tuners.

This is an ST-6 or ST-8, likely made near the end of the run of this form factor, and it has a three-prong grounded cord. The ONLY difference between my ungrounded one from the 1950s and this two-decade newer build is that a green/yellow wire is soldered to this so-called "internal ground rail". One end of that rail is connected directly to the chassis.

I am guessing that this actually is the only difference between these versions to accomplish grounding. If I understand correctly, the ground wire in these cords is either solid green or the green/yellow shown in this photo (shown by the arrow), and to correctly attach a new cord with grounding would be to connect the black and white wires where they currently live and then solder the ground wire to the rail as in the photo.

[the elephant]

Thanks for the two images! I downloaded both and will compare them with my meters.

[the elephant]

Well, rats. The green/yellow wire in the photo is present in my unit, so it is part of something else, and now I have no idea how to wire the ground to the chassis other than directly to it.

I will ask one of these guys who has an ST-8 to look at some pics I will take of mine, so they can figure out what I need to do. This cannot be difficult, despite being sort of mysterious…

Time for me to shrug my shoulders at myself, I guess.

[tadawson]

I looked at the schematic again, and both transformer secondary center taps *ARE* chassis ground (which would be the two cloth wires that are in your pic - yel/grn and org/grn, as I recall) so that would be the correct place to tie a safety ground.

[the elephant]

Excellent…

So, affix the back and white wires where they are now and tie in the ground wire to that rail. Got it.

Last question: What gauge of wire/cable/plug should I order?

[tadawson]

These pull almost nothing, so size based on what fits the hole in the panel. If you have a spare IEC cord (for computer monitors and such - the squarish equipment side connector) just cut the end off and use that. 18ga or so would be fine (or larger if it fits).

[the elephant]

My next project has been identified!