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 Posted: 03-17-2020 11:12 pm
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Tom Bradley
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cjwilson wrote:
The only issue I see is the wires in the male bullet connector are starting to fray. I'm guessing that 50% of the wires are still connected. It's at least a good enough connection for the car to run. I'm guessing that's not my issue.

If there is fraying, it often is worse than it appears. To be on the safe side, I would replace that connector. Better to waste time doing that than go to the expense of replacing, repairing or upgrading the tach to find out you have the same problem.

The only other relatively easy fix I can think of is that the internal calibration trim pot connection may be bad. This happens sometimes on these types of potentiometers. If you have an ohmmeter, you can measure the resistance between points 1 and 2 on the pic shown below. It should be approximately 20 ohms. If the resistance is very high, then try rotating the orange knob back and forth to try to clean off the connection. Also check to make sure the small nut on the back is tight.

If neither of these work, my guess is that you will have to replace or modify the tach.

Attachment: tach internal.jpg (Downloaded 47 times)

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 Posted: 03-17-2020 11:20 pm
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Tom Bradley
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cjwilson wrote:
I do have 12V DC into the tach. Someone told me that I need to check the signal wire into the tach for AC voltage. It could have up to 90V AC on it. That will confirm if I have signal to drive the tach. And if I do have AC voltage present, the tach is the problem.

Can any of the much more experienced people here confirm this?


No way. If this is the original, unmodified tach, then it works by measuring the AC current, not voltage. It is only if the tach has been converted to modern voltage-pulse system (which requires some rewiring) that you will be able to measure a voltage signal.

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 Posted: 03-17-2020 11:31 pm
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cjwilson
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I am pretty sure it's original. So I should measure AC Current into the tach? My meter goes up to 10A. Is that enough? Do you know what amount of current I should be looking for?

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 Posted: 03-18-2020 01:30 am
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Tom Bradley
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cjwilson wrote:
I am pretty sure it's original. So I should measure AC Current into the tach? My meter goes up to 10A. Is that enough? Do you know what amount of current I should be looking for?

The amount of current will vary depending on the RPM, but will certainly be less than 10A. But measuring this is a waste of time. The current has to be flowing for the engine to run. So if the engine is running OK, then there is no problem in that part of the circuit. The problem would instead be either the +12V or ground connections or internal to the tach.

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 Posted: 03-18-2020 01:49 am
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cjwilson
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And I've checked both the 12v and ground. They both are good. Are there any good tutorials on the tach? I'm pretty good with a soldering iron and enjoy tackling things as opposed to shipping them off. Worst that can happen is I destroy it and have to buy another.

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 Posted: 03-18-2020 04:31 am
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Tom Bradley
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I don't know of anything that explains the internal circuitry much. You might try googling "Smiths RVI tachometer" and see what you can find. On this particular model you might also check the 39 ohm carbon composition resistor on the board. These are notorious for going bad when they age. Also carefully examine the small coil wires for breakage or bad solder joints. Otherwise, I don't see anything that should go bad readily.

Since you are good at soldering, you might consider ordering an RVI to RVC coverter board from Spiyda. This is what I used and am happy with the results.

https://www.spiyda.com/smiths-rvi-rvc-conversion-board.html

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 Posted: 03-18-2020 09:50 pm
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Jim Ketcham
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If I recall correctly, the tantalum capacitor in the rvi tachometers is a common failure point as they had about a 20 year life expectancy. An equivalent electrolytic should work (2.7uf?).
I converted to RVC using the Spyda kit to work with my Pertronics Ignition system. It was easy to install and works well.

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 Posted: 03-18-2020 10:11 pm
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cjwilson
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I put a new 2.5 mf 20v cap in about an hour ago and it still doesn't read anything. The old one was reading zero mf. Double checked power and ground. I checked the 39 ohm resistor and it's good. Don't see any broken solder joints anywhere. I haven't checked ohms through the pot. I'll do that next and report back.

Thanks to folks that are pitching in.

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 Posted: 03-19-2020 01:59 pm
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cjwilson
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I checked ohms through the pot and it's working well through the entire range. I also desoldered the 39 ohm resistor and it's actually out of spec. It's reading about 50 ohms.

I'll run into town and get one to see if that helps.

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 Posted: 03-21-2020 11:36 am
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cjwilson
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My electronics store is closed due to the virus... I've ordered the resistor on line and will post back once it's arrived and soldered in.

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 Posted: 03-25-2020 05:37 pm
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cjwilson
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So I've put in a new 39 ohm resister and a new cap. Pot reads 12-83 ohms. Still no reading. Car is running great, and when unplugged at the tach, car dies instantly, so I know the through loop is good. Not sure where else to go in attempting to self diagnose and repair.

Anyone else that can share wisdom??

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 Posted: 03-25-2020 07:12 pm
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cjwilson
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Since my tach has never functioned since I bought the car, I decided it was probably stupid of me to trust resoldering replacement parts into the same holes they were desoldered from.

Now that I am looking at the tach with cynicism, I see that the red wire feeding into one of the coils is not connecting to anything via the board. It is soldered to the board, but nothing else is soldered to the same contact. And there is an empty hole at the other end of the solder point.

Does anyone have a few good pics of what is to get soldered where on their tach?

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 Posted: 03-26-2020 07:37 pm
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cjwilson
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It's working now.

Along with several fried parts, the previous owner also soldered in a transistor to the wrong points on the board. But it's all sorted and seems to be accurate.

Thanks to those who pitched in!

Attachment: Tach 1.jpg (Downloaded 14 times)

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