Tachometer Repair

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leombayard
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Tachometer Repair

Post by leombayard »

Here once again.

Pending solution for nonfunctional tachometer, will confirm this weekend.

For those who wonder what's up with their tach, likely SAK215 chip failing on the tach board, if playing with the potentiometer doesn't work.

Chips old and more than obsolete, while it's still available via NOS, more effective to replace the entire assembly.

Attached is the pending schematic for a replacement board, documenting here for if I ever exit the merkur world.

Will be manufacturing replacement boards along with boards that'll allow V8s to retain "stock" tach assembly

Once I have confirmation board design is valid and fits, I will archive the files via this forum also.

Image

Not gonna lie, don't know if we can upload images here, but If some one wants to save this and document it, please do.
jkxr4ti
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Re: Tachometer Repair

Post by jkxr4ti »

Looks like ltspice for the schematic, no?
Did a quick resoldering on mine this year but didn’t produce much difference. Mine works but the rpm is just low and movement seems lazy.
Was going to try replace the aging caps next and see what that yields. Assume you’ve been down that road before if you’ve got this far.
If you need help on PCB layout let me know. I use cadence orcad but do have Kicad as well for a more open source approach.
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leombayard
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Re: Tachometer Repair

Post by leombayard »

I'm switching from altium to kicad right now.

Altium is nice but over complicated, and the autorouting can be a nightmare.

Definitely an ltspice model, and luckily one was available for the lm2917-8

I've got my original one "working" but lazy, to me it's better to just switch everything over to something that'll last another 35 years.

Appreciate the offer, I'll reach out regardless and share the source files when available.
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brokencase
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Re: Tachometer Repair

Post by brokencase »

I've done a few tach repairs with LM2917. Mostly on old British cars. I don't see an adjustment. I presume your pre-set values are good to go without calibration?
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andyofcolumbusmerkur
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Re: Tachometer Repair

Post by andyofcolumbusmerkur »

Image
Where did you find this? I've never seen that before. Is that a V1 capacitor? What microfarad is that? It is not a 14uF. There is some very small blurry text beneath that like Rser=0.3 or something? The resistor is a Rzener 470 is that a brand? I would check with the vendors here for a replacement. You don't need to replace anything else in the cluster just the tach so even a bad "parts" cluster will work for you. :sawzall
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Re: Tachometer Repair

Post by my8950 »

andyofcolumbusmerkur wrote: Thu Nov 10, 2022 6:36 pm Image
Where did you find this? I've never seen that before. Is that a V1 capacitor? What microfarad is that? It is not a 14uF. There is some very small blurry text beneath that like Rser=0.3 or something? The resistor is a Rzener 470 is that a brand? I would check with the vendors here for a replacement. You don't need to replace anything else in the cluster just the tach so even a bad "parts" cluster will work for you. :sawzall
I believe he drew it based on comments above using ITspice/kincaid or other.
leombayard
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Re: Tachometer Repair

Post by leombayard »

andyofcolumbusmerkur wrote: Thu Nov 10, 2022 6:36 pm Image
Where did you find this? I've never seen that before. Is that a V1 capacitor? What microfarad is that? It is not a 14uF. There is some very small blurry text beneath that like Rser=0.3 or something? The resistor is a Rzener 470 is that a brand? I would check with the vendors here for a replacement. You don't need to replace anything else in the cluster just the tach so even a bad "parts" cluster will work for you. :sawzall
As my8950 said it's nothing I found, next at the bottom left is parameters for the simulation in ltspice, which is a circuit simulation.

Can, and have checked with the vendors, like most people who own one of these cars, I'm in frequent contact with them. A working tach is a fleeting item and this will be a long term solution which will work for another 35 years.

Much better to just swap a board out than make blugg look thru a bunch of gauge clusters for one that works.
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Re: Tachometer Repair

Post by brokencase »

No need for simulation as reference schematics for LM2917 automotive tachometers are readily available.
See attached below. Note the protection Zener on power. Very important.

What I don't understand is why your LM2917 only shows 8 pins.

You will need to calibrate it. The second attached image shows a circuit that provides a simple means to calibrate using a 12v transformer.
Although it is better if you can calibrate at a lo and hi rpm range with a signal generator.

I full agree fully - no need sorting through old dash units, just replace the circuit. Looking forward to see how you make out with this.

Sorry if I have missed something here, hope what I provides helps. This is the circuit I used on an old British car.
2917_tach_schem.jpg
2917_tach_schem.jpg (124.15 KiB) Viewed 4786 times
tach_calib.jpg
tach_calib.jpg (45.99 KiB) Viewed 4786 times
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leombayard
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Re: Tachometer Repair

Post by leombayard »

Lm2917-8 vs lm2917-14

They come in both 8 pin and 14 pin arrangements, and the 2917 should have a built in zener diode

Though I'd rather spend the extra $3 and have a voltage regulator too.

Reference schematics are widely available but most are for 7v or so EMAs while the XR (and likely Scorpio) EMAs max out at 1.75v

I have a working prototype (shared in the merkur xr4ti group chat on fb) but needs more refining before I make it available.

Once completed I'll have boards made, likely a run of 25 or so, available for purchase, and source and production files will be made public for archival purposes.
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brokencase
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Re: Tachometer Repair

Post by brokencase »

I still think you will need to be able to calibrate it. The meters are not all the same. There is an adjustment pot even on the stock tachs.
It's an open frame potentiometer that often goes open from oxidation and is one of the reasons the stock gauges stop working. You can often wiggle it and the tach will start working again. This, at least, is what happened on my Scorpio.

You'll need the Zener on the front end regardless if you run a regulator or not. It's purpose is not to regulate but to protect the circuit from crazy transients that can happen in the automotive environment, like when someone jump starts the car. That's why it is rated 18-23 volts. See Intel application note AP-125. Don't use a little glass Zener, it has to be a big one rated at an amp or more.

Funny, I never noticed that they made an 8 pin version of that chip. I'm actually surprised they still make that chip. I think it was at least 20 years ago when I bought some.
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leombayard
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Re: Tachometer Repair

Post by leombayard »

Texas instruments bought the company that originally made it if I'm correct, and now they even have surface mount versions.
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Re: Tachometer Repair

Post by leombayard »

Image

Revised schematic

Switched to lm2917 14 pin

8 pin would work but not with proper conditioning and clamping of input.
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Re: Tachometer Repair

Post by brokencase »

I just remembered something. There is a regulator on the cluster circuit that I believe outputs 10-11 volts. If you run your tach circuit off that then you can probably get by without the protection zener that I mentioned previously.

On a somewhat related note, I picked up some of these...
https://www.temu.com/5-10pcs-3a-mini-dc ... 79556.html

Pretty handy to power DIY automotive circuits that you need to run at a lower voltage.
Tiny, cheap, adjustable, and can output 2 amps. However I would still put a protection zener in front of them.

The Temu site has a lot of good stuff, with free shipping.
I though these were a good deal as well.
https://www.temu.com/352pcs-boxed-autom ... 85075.html
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Re: Tachometer Repair

Post by my8950 »

So I have a cluster or 5 on my bench...Where is this potentiometer at?
I am looking at the back of the cluster, removed anti slosh board, but, I don't see potentiometer.
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Re: Tachometer Repair

Post by Ed Lijewski »

my8950 wrote: Tue Apr 04, 2023 8:50 pm So I have a cluster or 5 on my bench...Where is this potentiometer at?
I am looking at the back of the cluster, removed anti slosh board, but, I don't see potentiometer.
You have to remove the tach from the IP cluster frame to see the little screw.

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