TNC Tachometer mantainence

Since someone brought it up - I obviously like and very highly recommend the TNC tachometer. The electronics design and the workmanship of the board and component layout is truly aerospace quality. But as good as it is, there are some significant shortcomings in the mechanical and case design that lead to the occasional problems.

First problem I found was after my tach got blown off the test stand. The LCD no longer lit up, and it looked to be dead as a doornail. I was pretty bummed out, but of course I had to open it up and look. Turns out that the LCD display is pushed into a socket, and dropping it dislogded one end of the pins, and therefore stopped working. There's plenty enough clearance between the LCD and the case to allow this, and there's no positive control that prevents it. It's held in solely by the socket tension on the pins. So, I just pushed it back in and it once again worked perfectly. Since then, mine had dislodged itself several times, and I have seen two others do the same.

Second problem is that the switches for on/off, and for 2-blade/3-blade, are just suspended on fairly long pins and not connected to the case in any way. The leads on the switch are soldered to the pins. This puts all the stress of switching it back and forth on the pins and so what sometimes happens is that the solder joints fail. The result is that the unit either won't turn on, or the 2-blade-3-blade switch won't work anymore. In both cases I have seen, it wound up stuck on 3-blade. I don't know of that's necessarily always true, but if there's a problem, this is a place to look. Of course, solder it back with a small pencil-type soldering iron ($7 at a hardware store) and ROSIN CORE ELECTRONICS SOLDER - not plumbers solder or Sta-Brite.

Problem three is that the sensor is also suspended on it's leads and just sort of pokes through the hole in the case. When replacing the battery, make sure that it gets back in the hole, otherwise it probably won't work. And it seems certain that eventually the flexing will result in it breaking the leads or solder joint.

I might also add that the "min-bit switch" that changes the display from 100 RPM to 10 RPM on the PT-10s model also looks very suspect and is almost certain to fail at some point. Once again, the microswitch itself is very nice. But the mechanism that makes the button in the case actuate the switch inside is very crude. I haven't looked at any repairs or modifications to get around this, since mine hasn't broken, yet. I would also add that despite the gee-whiz factor of having your 5 PPM tachometer display 10 RPM, I have found it to be fairly useless for stunt, and not worth the extra $10 or whatever it costs.

I have considered how to modify the unit to get around some of these issue. The loose LCD problem could be easily fixed by adding a bezel glued inside the case to take up the excess space so that the case holds the display in the socket. I would put a soft face on the LCD side so it doesn't crack.

The switch and sensor problem is a bit more complex. I'm probably going to remove the switches from the pins, and attach them straight to the case, then run flexible wiring from the board to the switch, with enough slack to allow the battery to be changed, and a removable connector like an R/C servo to allow it to be taken apart completely if necessary. Something similar would work for the sensor, and that would make the whole thing darn near bullet-proof.

To summarize:

If LCD stops "lighting up", has missing segments, or otherwise acts funny or intermittent - remove back, remove board, and ensure that the diplay chip is properly seated. Be gentle, they can crack if you put too much point load on it.

If switches seem to not work, or are intermittent, remove back, remove board, and inspect solder joints between switch leads and the pins on which it is supported. Resolder carefully with ROSIN CORE ELECTRONICS SOLDER and a small (15-25 watt) pencil iron.

When reassembling from any cause, carefully bend the sensor leads to that the sensor gets back into the sensor hole, instead of getting pushed back into the case.

If I do get around to modifying mine and it's easy and safe enought to be done by following instructions, I'll post the mod here or in SN. Don't hold your breath - I have about all the projects I could possibly stand for the next 2 months or so!

The switches seem to have a high failure rate. I've replaced the slide switches with little toggle switches.

LAST EDITED ON Jun-08-04 AT 03:33 AM (CDT)
 
WHAT???? The Holy Grail of Tachometers has issues? With the Stinkin Switches? Such Quality that the switches and display are just sorta, HANGIN THERE???

Now, it's quite true with many of the Electronic Geniuses I've known that there physical layout abilities blow. Most have made up for that by havin someone else design the layout of the boxes while the electronics in principle are spot on. So I really shouldn't give this guy much grief for his layout problems.

But the TNC tach has the mythological background of the Alpha 4 charger, the 'Holy Grail' of the RC Charging world that once you read on it, you realize that it is still a 15 year old design that was pretty good at it's time, but falls way short of even cheap chargers of today. There's still at least a 6 month waiting list on it and all the owners say, 'Look at my Alpha 4' Then get mad at you when you ask if you can charge your 27 cell battery with it, or worse yet, charge a 10 cell pack at 5 amps or so. "No, why would you WANT to do that?" "ell, so it'll get charged sometime, TODAY!"

Well now we have "The TNC Tach is the only tach to have. It'll tell you within 3 Fly Heartbeats the RPM of your engine" but they leave out "If you can get the switches and display to work"

I dunno, maybe when I get to the place where I own a PA stunt engine system that's run me around 5-600 bucks, or more, I'll feel the need to pay extra for the TNC tach, then stand and wait for it to be delivered. But while I'm running mostly Foxes and the occasional OS this Thunder Tiger tach will do just fine. Tells me when it's at the same RPM and the engine sounds right, and flys the plane the same, so it must be close. At least the switches and display work even with all the less than nice ways that I use it.

Now, granted, we're not talking an overpriced unit in the TNC. It's still under 100 bucks right? Where the Alpha 4 charger is Way way WAAAAAYYYYY overpriced for it's functionalities. Actually, that seems to be one of it's main selling points. But it does hit me kinda funny as everyone was telling folks that the TNC is the finest tach around, cept the switches are just sorta 'Hangin there'

Rant off, I'm still convinced it's a great tach, I'll probably get around to buying one sometime, then we'll have to fix the switches.

By the way, if you have to point out to someone that they need to use Rosin core solder and an appropriate sized soldering iron for this, they really need to stay out of the back of a 'precision instrument'. I know it makes sense to you and me, but I've trained plenty of guys to solder and when they show up with the attitude that acid is a good thing, cleanup is un-necesary and the like, well, it's best that they don't go soldering in places where a little practice and technique is necessary. Even a 25 watt pen-iron can lift a trace if you do it 'Right'.

If your TNC Tach, or any device like your glow plug ignitor charger, is having an intermittent, cold solder or flexed joint problem, find the electronics expert in your club, we're all over, and have them fix it for you. Heck, if I have my toolkit in my van I can fix it for you at the field. But you lift a trace, cook a resistor or let the smoke out of an IC and it's back to the manufacturer for you boy.

Well, I can fix the resistor and probably work around the trace, if the IC is standard, we can probably figure out a replacement for that too. But I are a genus, others ain't necessarily so. (humor)

I would agree and acknowledge that the TNC packaging is very much a proto or limited production implementation. The battery kinda flops around inside.

Its the design and operation that sets it apart. It really will read a three blade at 12000RPM in bright sun from a couple of feet out.

>By the way, if you have to point out to someone that they
>need to use Rosin core solder and an appropriate sized
>soldering iron for this, they really need to stay out of the
>back of a 'precision instrument'.

Great theory, but people are going to try to do it themselves anyway, and might as well have the warning. If nothing else, I can't be accused of not giving full instructions.

The thing really does work well, and if you want to know what RPM the engine is running on a reliable basis, this is the only game in town. Problems like switches wearing out or LCD's coming loose after getting dropped on concrete or 6 years of heavy competition use really seem pretty minor. It's sort of like complaining the back of the case wears out - because after you slide it across the blacktop after the engine starts, 1500 times, it gets ground down!

one other "nit": the battery doesn't fit very well...

I was all hot to buy a TNC tach until my brother gave me my birthday present. He made a crystal controlled tach checker. It generates flashes up to 200k and is good to within 0.1% That's plenty good enough for me.

Now the interesting part is that EVERY tach at the field that I checked is right on the money! They must ALL be crystal controlled to achieve that kind of accuracy. There were about 5 different brands that we checked. Cheap $18 up to about $35.

The only differences seem to be the ability to pick up the signal from the prop. Is that worth $100+ ?

My brother, btw, has both his Bachelors and Masters in electronics from Caltech. I expect his device is what he says it is.

It is if you save sticking your fingers and or a tach into a spinning prop even one time.

Or not getting a reading at all because of the daylight.

Whuh???

NOW THATS MARKETING!!!

The TNC Tach will keep you from cutting your fingers off. If you buy a half priced tach, you'll loose half your finger and a dirt cheap Tower special will probably loose your hand up to the nub!

You didn't have anything to do with the latest Atkins diet commercials, did you? You know, Atkins, it'll help you find chicks. Or the Atkins diet can get you a dog.

If on the other hand you mean that the TNC tach will keep you from loosing a finger because you'll push it into a prop before your finger hits, I think you're using it wrong.

Tolerances are what make the TNC a better unit. When they put these other tachs together quick and dirty, they don't use the best fo the best in parts. Now, while this doesn't mean that the readings will be off, and everyone that I have checked with the flourescents have read 36, the ability to see a bright flashing light as compared to a grey prop against a slightly darker grey asphalt will make the differences. This is where the tolerances of materials comes in. When it says 300 ohms and is 300 ohms not 294, it can make a difference.

Even your high falutent Caltech Cousin will tell you that.

>I was all hot to buy a TNC tach until my brother gave me my
>birthday present. He made a crystal controlled tach
>checker. It generates flashes up to 200k and is good to
>within 0.1% That's plenty good enough for me.
>
>Now the interesting part is that EVERY tach at the field
>that I checked is right on the money! They must ALL be
>crystal controlled to achieve that kind of accuracy. There
>were about 5 different brands that we checked. Cheap $18 up
>to about $35.

Uh, that is a very atypical experience. The old square black Hobbico tach was notorious for giving variable readings from second to second, and no two were EVER alike, and not by a little, either - sometimes 1000 rpm off. Even if you calibrated them every single morning. The Globee tachs are a lot more consistent from moment to moment, but not remotely consistent from unit to unit. They all give reasonable readings on flourescent lights and tach checkers - but in practice in the field, they are a complete crap shoot.

I don't care whether anyone buys one or not, really. But I have gone down this road enough to know that buy one good one, once, would have been a very cost-effective move.

I too had the LCD socket problem. Same fix.

One mod that I made was to wire a "momentary on" push-button on the side of the case inline with the "on" switch. So even though the on/off switch is left on (there isn't any auto off capability) it won't run down the battery. Taching the engine, I just hold down the momentary on button.

But, this is still a superior tach. It works in low level light conditions. It will tach and engine from about 5 feet. The update rate is faster than most of the $29.95 tachs (the display jumps around less).

Frank

OK, so I want a Mark II version of the TNC. The one with the innards glued down, supported and stuff.

I agree that cheap tachs don't get a signal well at all.

I did have an entertaining experience with tachs some years ago. I kept hearing from a famous Free-Fligher how hot his engines were. Quoting 27K with a full 6x3 on a Tee Dee .049, 21K on the Medallion, and such. One day at the field, he was running an OK sounding Medallion and told me that it was the one that turned 21K. I DON'T THINK SO!! I had just checked my two tachs against our calibrated GR strobe at work, and brought them over. Turns out the guy was using an old Heathkit tach, and it was 8,000 rpm high. OOPS! Haven't heard from him in a wile.....

This was why I asked for a tach checker.