Got the tach running again!

[Paul Workman]

Thanks to WVZR-1 who sent me his old LT1 cluster board, AND THIS REGISTRY SITE, I was able to harvest the differential op amp from the LT1 board and install it on my ZR-1 cluster. Thanks very much, Dave!

Once I had the "new" chip installed, it only took a couple minutes to get the tach to track the rpm on my scanner. But, I thought I would confirm what I and others thought might be the case - the compatibility question re using the LT1 cluster as a potential source for parts. From measurements I did, all the components on the LT1 cluster are the same values as the ZR-1. Hope that helps.

Anyway....I just wanted to thank everyone that responded to my call for help. Once again the "Brotherhood" comes thru. Owning a ZR-1 without this site and the people here would be a whole different experience when things go wrong!

P.

[Bill]

Did you end up with about 190K Ω to get the tach to track the scanner?

[Paul Workman]

Originally Posted by Bill

Did you end up with about 190K Ω to get the tach to track the scanner?

In my case the "magic number" was 225k.

P.

[tomtom72]

Okay I never had my cluster out so this Q is from ignorance.

It would seem if all the LT1 cluster components are electrically the same could one just swap cluster faces?

TIA for tolerating my endless Q's


Tom

[Paul Workman]

Originally Posted by tomtom72

Okay I never had my cluster out so this Q is from ignorance.

It would seem if all the LT1 cluster components are electrically the same could one just swap cluster faces?

TIA for tolerating my endless Q's


Tom

That is a good question. I was tempted to do just that. However, except for the damaged IC chip, my board was in like new condition, whereas the donor board had significant "frosting" on the connector pins especially - possibly leading to the reason it was a "donor"(?).

I would offer to write the whole calibration process up. But, in truth there is such an excellent, long-standing procedure and writeup with photos, etc, over on the CF forum, I don't know that I could add anything of significance to it. "If it ain't broke...Don't fix it!" (in other words).

However, I did learn a few things not mentioned in that writeup: For one, the CF testimonials for resistance value favor 300kΩ, ±. In discussing this with Marc, he tells me in his experience the value to result in re-calibration for the ZR-1s seems to be generally half to 2/3 that value. (In my case it was 225kΩ)

Note: Coincidentally(?), if you install a 300k Ω (±) on the board w/o first removing the old resistor, the net resistance of the two in parallel falls (more or less) within that range of the values Marc typically finds to recalibrate the tach with the failing resistance removed from the circuit. The writeup (I saw) on the CF board is ambiguous w/ regard to removing the failing resistor from the circuit. I, and obviously SOP for Marc too, opted to remove the failing resistor from the circuit, replacing it with "fresh" parts - which coincidentally results in a huge increase in wattage capacity over the lazer-cut resistance on the stock array. Well, at least that resistor will never fail again.

The hard lesson learned was to take steps to insure good connections to the board and the test jig. Should the resistor circuit be or become open - due to a bad solder joint, or connection on the jig itself, the negative feedback loop to the op amp is lost. Bad things happen next! The amp gain will literally go "open loop" - for about a second or so before it fries an internal transistor. Then you're hunting up a 9907A IC to replace the one that just fried.

According to Marc, it appears that in some cases there may be other resistors (on the board) or other factors at play, resulting in a non-linear tach response. Such does not appear to be the case, in my instance, but Marc says he sometimes has to split the difference between the resistance needed to calibrate the tach at 6000 rpm and what it takes to dial it in at 2000 rpm.

Hmmmm..... Maybe I should write it up.

P.

[Torchred96]

Good news for sure. I envy your lectical skills. My tach is completely dead ..it spikes once in a while to full scale and then back to 0. but, lacking any real knowledge of lectriks, I'll probably send it out.

Is the tach section seperate from the speedometer and everything else? What I'm getting at is, if I send it out, is there a way to remove just the tach? and reinstall so I can have a speedo?