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Fritzy Tachometer  Rating:  Rating
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 Posted: 12-29-2005 07:54 pm
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edward_davis
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My speedo had the ol' weeble-wobbles, so I went to the trouble of pulling out the rusted-in dash pod to check out the cable connection at the back.  After painting the dash pod spike and lubing it with some bicycle gear grease (good all-purpose stuff), I can now take out the dash pod easily.  I opened the speedo and dusted some graphite inside and put everything back together, wiping off the grime with a damp cloth. 

Now my tachometer doesn't work.  At first it was reading too high, by about 500 rpm.  Then when the weather cleared up and I took her out for a spin, the tach gave up completely as I wound out second gear.  There's power coming in on the hot lead, and the electricity is making it through the tach and to the ignition, since I have no trouble with the engine.  The ground connection seems to be good, too.

All of the other gauges work, and I've reduced the weeble-wobble in the speedo (still there, but smaller), but I'd like to know if there's any chance I can fix this myself.  If not, who is a good fixer of these things?  How much can I expect it to cost?

Thanks, Edward Davis

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 Posted: 12-30-2005 12:32 am
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Mark Rosenbaum
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The British car club sites have quite a bit of information posted on Smiths tachs, and much of it is pertinent to the JH.  Here's part of a post made back in August 1994 by Mark W. Olson, who apparently was one of the first LBC owners to 'reverse engineer' the Smiths tach:





"[...]  For those of you interested in the tach itself, the Smiths tachs of that  vintage have an interesting design. The ignition wire loop acts as the  primary winding of a transformer. On the inside of the tach is a coil that picks up the current pulses from the ignition wire. Each ignition pulse triggers a one-shot made with two germanium transistors. The output of the one-shot drives an ammeter. The more frequent the pulses, the higher the duty cycle of the one-shot output, the higher the reading on the ammeter. The tach is calibrated by varying the one-shot's pulse width via a pot. They tend to calibrate well at low and high RPMs, but they tend to read a couple of hundred RPM high when the needle is nearer to vertical.

"The design was obviously originally done for a positive ground car, as the circuit is referenced to the 12V and it floats off the ground through a resistor. [...]

"The most common failure mode I've found so far is failure of the main timing capacitor, 
which can lead to erratic behavior or total failure. Mechanical failure of the ammeter is the second most common failure. [....]"




If the above makes sense to you, then you can probably fix the tach.  Other options are to find someone who can fix it, or who can replace the electronics with something more modern; here, the internet is your friend.

Finally, try eBay: most Smiths RVI-type tachs will work without changes, and Smiths RVC-type tachs will work with only minor wiring alterations.  Often you can even exchange tach faceplates so the replacement will look original.  In any event you should re-calibrate the new tach by comparing its reading to a known accurate tach or tach-dwell meter.

Good luck.

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 Posted: 12-30-2005 04:04 pm
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Mark Rosenbaum
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For what it's worth, here's a schematic of the tachometer circuitry, from classictiger.com:

 

Attachment: Tach Schematic.jpg (Downloaded 161 times)

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 Posted: 01-05-2006 05:43 pm
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edward_davis
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I've disassembled my tach, and it turns out to have much simpler circuitry than the old Sunbeam Tiger tachometer that the diagram comes from.  There's only one transistor and one capacitor.  All of the individual elements seem to work in my tach, but the capacitor is no longer connected to anything.  There's a place for a connection in the circuit board, but nothing there.  I didn't find any loose parts, either, but I might have lost the bit the first time I took the gauge out of the dash.  I didn't realize how easily the bezel comes off, so it slipped off while I was looking at it; could have lost the circuit element then.

I've bought a natty old '76 MGB tach on e-bay to "reverse engineer".  If it really is the same sort of design, hopefully I can make one working tach from the two units.  Once I get it worked out (if, I should say), I'll post the circuit diagram.

There is a site online that has a circuit diagram for a DIY conversion to a voltage trigger, so that the tach can be used with an MSD system.  If I can't get the old-school system to work, I might try that instead.  It apparently works fine for old Jags and MGB's.  The big problem with MGB's is that the tach has the ignition warning light in the lower part of face, interfering with the placement of the new circuit board.  Good ol' Jensen folks, putting our ignition light on the console...

Does anyone out there have a photo of the insides of their original Jensen tachometers?  If I saw a working unit (or a unit broken in a different way), it would help with troubleshooting mine.

Mark, thanks for the pointers; they were good leads that will hopefully lead to a happy DIY resolution to my problem.

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 Posted: 01-05-2006 10:08 pm
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edward_davis
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Here's another quick question:

Can an old MGB speedometer work in a JH?  I've seen some on ebay that look just like my unit except for the silver bezel.  Would they have to be recalibrated?  Is that sometihing I can do myself?

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 Posted: 01-05-2006 11:53 pm
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John Finch
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Do you happen to remember the web address for the MSD/Tach site? I will be installing an MSD in 18309 this spring and will likely need the diagram. Thanks

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 Posted: 01-06-2006 03:47 am
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Mark Rosenbaum
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Attached is a rather poor photo of the interior of a JH tach.  Either I'm an even worse photographer than usual, or my camera isn't working all that well tonight -- this was the best of eight tries.  You're right, only one transistor, so obviously it has to be a different circuit than the one I provided.  I've spent more than forty years in electronics, yet I missed that small but vital detail in several previous examinations... aargh!

Several owners have reported what sounded like MG tachs in their JHs, that worked just fine.  Since both cars have 4-cylinder engines no recalibration should be necessary.  But then you don't know what a PO might have done to the MG tach, so it wouldn't hurt to check its calibration, if you can.

Attachment: tach interior.JPG (Downloaded 148 times)

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 Posted: 01-10-2006 06:33 pm
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edward_davis
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John,

Here is the MSD tach site I found.  It's sometimes hard to navigate, but it has a lot of information and some good circuid diagrams:

http://members.shaw.ca/tsmit/tachmod/tach_other.html#MG

You can buy the prebuilt circuit board from the folks or you can make it yourself.  I have no association with these people; I just found them on a somewhat-random Google search.

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 Posted: 01-12-2006 06:01 pm
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edward_davis
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Mark, 

Could you tell what component attached to the negative end of the capacitor in your tachometer?  I don't seem to have any components attaching there; there's an empty hole in the board and the other side of the board doesn't have any soldered connections there.  In your photo, it looks like the hole is supposed to be empty, so it could be I just traced the circuit wrong: this is only a hobby and I haven't done anything like this in 10 years.

The capacitor seems to be OK; it charged up when I tested it with my ohmmeter, and it slowly discharged when I applied the voltmeter to it.  The break in continuity in my tachometer seems to occur in the meter itself: if I test the resistence across the black and red wires that attach to the "meter" part of the device, I get an open circuit. 

I'll know more when I can get my hands on another tachometer; I don't want to pay too much since I'm just going to open it up and take it apart.

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 Posted: 01-12-2006 06:35 pm
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Jensen Healey
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I may have some tachometer guts for sale. Let me check the back shelves. Send me an email klhoush@comcast.net

 

Kurt

San Anselmo, CA

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 Posted: 01-13-2006 03:27 am
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Mark Rosenbaum
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Edward,
I've just taken a thorough look at my spare tach.  To respond to your most recent post:
(1)  The negative end of the capacitor connects to +12 volts, the positive end to a pad on the board that does not go anywhere.  This seems odd, but perhaps the cap was hooked up for some special application.
(2)  The meter movement should measure about 8 ohms, if yours reads open, that's probably the problem.

I've traced out and drawn up a schematic for my tach, hopefully correctly, though I've left out the capacitor referenced above.  This drawing is attached below.  Component values reflect item markings, or presume compliance with international color code standards.  The unknown device could be a capacitor, thermistor, or something else entirely -- I can't tell with the means at my immediate disposal.

The other thing I'm not sure about, is the transistor.  I presume that MT59/A is the part number, and that MT stands for Mullard Transistor, but I can't find any data whatsoever for the device.  Possibly MT59/A is a Smiths house number for some standard part.

The circuit makes sense if I assume that the transistor is a PNP germanium device, but this has not been confirmed.  (It's possible the device is an SCR or UJT, but I consider this unlikely.)  The pinout is also uncertain.  Pin 3 must be the collector, but it's unclear whether pins 1 and 2 are emitter and base, or base and emitter, respectively.  Arguments can be made for both arrangements.

Meter zero is a mechanical adjustment.  Because of the uncertainties about the transistor, it is not apparent whether the potentiometer is a calibration adjustment or a sensitivity adjustment.  If the latter, then tach calibration is purely mechanical as well, probably by adjusting pointer spring tension at time of manufacture.

For a large number of subtle reasons, this tach is a sophisticated and brilliant combination of mid-20th Century technologies.

Attachment: JH Tach schematic.jpg (Downloaded 122 times)

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 Posted: 01-17-2006 07:04 pm
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edward_davis
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Another link explaining Smith's Tachometers for those who care.  From the Golden Gate Lotus Club:

 

http://gglotus.org/ggtech/smith-tach/smithtach.htm

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 Posted: 01-31-2006 03:53 pm
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edward_davis
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I have replaced the insides of my original Smith's tach (RVI 1439/O3A) with those from an MGB tach I bought on e-bay (RVI 1439/O1A).  The ad said it was in working condition, and there seems to be no problem with the meter itself, like my old one was having.  We'll see when I get her all hooked up this weekend.  Interestingly, there were only two differences in the gauges, except for the design of the faceplate.  The MGB gauge was oriented upside-down relative to the JH gauge (the faceplate attaches 180 degrees).  And there was no mystery capacitor in the MGB gauge.  I did not check to see if the resistors were the same, though, and the potentiometer was of a slightly different design.  Hopefully it'll work.

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 Posted: 02-06-2006 05:08 pm
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edward_davis
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Well, it all seems to work.  Once again, the club has saved me money while providing me with good guidance for tinkering.  Thanks, everyone!

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 Posted: 12-15-2006 09:45 am
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jls
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I have just fixed the tacho for my 1970 MG Midget which is the same as yours.  The fault was with the component you have omitted from your circuit which was a 2.5uF 16V electrolytic connected between pins 1 and 2 of the transistor (+ve end to pin1 which is the + rail of the circuit).  The original component was an axial type which went from the pin2 (base?) connection to the +ve rail, with the +ve connection looped under the edge of the board.

The fault on mine was that the capacitor had gone short circuit (old age).  I replaced it with a 2.2uF 63V aluminium electrolytic in a radial package which fitted the holes available in the PCB nicely.

I tested and calibrated it with a pulse generator.  The 50 ohm pot is for calibration.  The repaired instrument seemed to be quite accurate and surprisingly insensitive to pulse width and to ignition circuit current.  I haven't fathomed how it works or exactly what the feedback winding of meter current to the transformer winding does.

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 Posted: 01-01-2008 07:58 pm
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d.dewdney
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Hello gents, this is my first venture into the message board. I live in the North....way North than most of you, King City Ontario Canada. I'm new to the Jensen family but not new to british autos (1st car was an MGB and I never looked back. Now to my problem.

I bought a 1973 Jensen Healy in June (Just as I blew one of my own valves) I have yet to put her on the road but have managed to tinker here and there. One of my problems is the Tach, On start up it looks good and as she warms up the tach does not return but stays at 2000 rpm, If I rev it to 3000 she stays there. When the engine is shut down it again stays at 3000, if I leave it for period it eventualy goes back to zero.

 

Any thoughts you may have would be greatly appreciated.

Thanks Dave

In King City

Last edited on 01-02-2008 12:48 am by d.dewdney

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 Posted: 08-10-2008 08:15 pm
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dwalls1
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K den, I can assume that my dead tach needs repair and that the signal from the ignition is most likely present and is not the problem. Have used Mr. Rosenbaum's excellent schematics in the past, but on this one I am lost as to how the tach signal gets from the coil to the tach and the factory schematic is tough to follow. So, tell me where to start. Should I begin by removing the tach from the dash? The speedo R&R took three visits to the chiropractor to recover from. Maybe I'll wait 'til I've got time to remove the seat first.

My apologies for confusing Mr. Kimbrough and the late Mr. Rosenbaum. I am a dolt.

Last edited on 08-10-2008 08:41 pm by dwalls1

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 Posted: 08-11-2008 03:06 pm
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edward_davis
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The factory tach gets its signal through the input wire for the coil.  The wire runs from the ignition to the tach and then onward to the coil.  The tachometer gets its reading by measuring the changes in current in the ignition wire: inside the unit is a little transformer, with the ignition wire wrapped around one side and another wire with many more windings on the other.  This is not at all how modern tachs work, so going to an aftermarket tach requires a bit of rewiring. 


When you pulled the speedo, did you get the whole gauge cluster out?  I pulled mine out with great difficulty the first time, but after sanding down and painting the "spike" that holds everything together, and adding a bit of grease when I reassembled it, I can now pull it out with two fingers and get easy access to the back of the gauges.  See my above post from 7-05.

Cheers!
Edward

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 Posted: 08-11-2008 07:12 pm
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dwalls1
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Thank You for responding with an alternative to working upside down with my back bent in the wrong direction. No, I did not remove the entire cluster when I repaired the odometer. I will definately look into doing that to get at the tach. BTW and FWIW I found with the speedo that the sealing ring from the oil filter canister on my TR7 makes an excellent replacement for the o ring under the bezel when mounting the gauge back into the dash cluster.

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 Posted: 08-15-2008 03:23 am
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dwalls1
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Got the tach out without doing the Dashboard Limbo. Didn't see any fried components so that about exhausts my diagnostic capability. Since my tach just suddenly quit altogether is there a component that most likely is the culprit, or would it be just as well to replace the capacitor, the resistors and the transisters. (what, no vacuum tubes?) Should I be able to source these components at Radio Shack? In reading the posts on this thread it seems that erratic readings are more common than total failure. If it comes to it can I send it somewhere for repair without replacing the whole circuit board for $165.00 plus? Then again now might be the time to upgrade to a unit that will work with the inevitable pointless ignition though my ignition is OK now. I'm glad I checked here first as I was able to figure out that I needed to plug the white and white/grey wires together to run the car without the tach from info on this thread. Guess I'll bring my multimeter home this weekend and make like I know how to use it and check some resistance readings. Would appreciate any thoughts you anyone might have on my ramblings and Thanks for allowing me to ramble. Dale

Could anyone help me identify the main timing capacitor? I think I may just replace it and not try to test it.

Last edited on 08-15-2008 03:26 am by dwalls1

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