Hi there,

my tachometer is not working. Yes I know , I can buy a spare printed circuit.
But the problem of my non working tach is a stupid Zener Diode ( less than $1) that I can't find.
look at pics : the diode is marked M ( motorola) 5204cc. I can't find it in the net.

Does anyone have the value of components of original tach circuit board? The only thing necessary to know is the voltage drop of that stupid zener diode. Then you can repair every tach at $1 cost ( not $60)

thanks

Aldo

p.s. I have also replaced the capacitor on the left

[IMG]local://upfiles/6949/DAE0475C08834EAD8E219B4455458F85.jpg[/IMG]

[IMG]local://upfiles/6949/1163DF577A2F48A5B03D5C9FDB97ECC6.jpg[/IMG]

 

Thought that I'd hit paydirt here: http://www.4starelectronics.com/part..._Number=5204CC
Then noticed to $300 min order.
Kept looking and found this: http://www.mouser.com/Search/Product...qYIohrCA%3d%3d
Don't know if it's the correct one. But it does say it's 36V. Good luck with your search.

Not sure why 36V in a 12V system unless it's for spike suppression.

 

"THIS IS ONLY A COMMENT & SEEMS THAT THE FIRST TWO POSTS HAVE GIVEN YOU THE INFO NEEDED."

Just thought; that a "zener diode" being in the tach circuitry in our 'Vettes was something I never imagined. I remembered my brother replacing the "zener diode" on one of his 650cc TriumphBonnevilles back in the later '60s. It was located under the triple tree. If I remember correctly, the "zener diode's" purpose was to get rid of excess voltage (like a voltage regulator of sorts) to dissipate as heat from the zener diode. It was finned and was made of an alloy of some kind. Out of curiosity, I need to check this out. Your picture, Aldo, is very different from what I knew. Thanks for the interesting info.

 

73shark, you sure are good at finding stuff, PG.

 

Hi Texas Jim!

A diode is like a one way gate for electricity, it lets thecurrent flow in one direction but won't let it go backwards, like a check valve in plumbing. If you try and force electricity backwards through a diode it will blow up. A Zener diode is the exception to the rule. It works like a regular diode in one direction, but will allow a very high voltage in the reverse direction without being destroyed. The item you are describing above sounds like some kind of a heat sink, usually found on a transformer or some other component that gets hot. In most cases diodes look a lot like resistors do.

[sm=icon_cheers.gif]

 

I must say I enjoy loging on to this site every day just so I can learn something new. I had an idea of what a diode was but DeeVeeEight explained it in very basic terms. Thanks for the info

 

Just the power of Google.

One other difference that a zener has is that it breaks down at fixed voltage so it can be used to reduce a DC voltage down to whatever it's rating is.

 

Hi,

thanks to you all

I already replaced the burnt zener with 40V one ( I somewhat realized that it should be about that value) but no way. There's possibly some more damage at microchips.
So I think I going to replace the whole circuit board ( mine is in too bad conditions)
Anyway , my suggestion to you all is : if your tach starts to go wrong , don't replace it , but try to replace just those 2 -3 zener diodes : they are very fragile because undersized and tends to burn. Lets say that this Corvette tach has a limited life.
The tach in Vette ( 79) has been replaced in 1985 ( just 6 years working) and after it blew again( idon't know when , since I got my car just 1 month ago)

Aldo

 

DeeVee,
Thanks for the info. I am familiar with diodes as I've had, in the past, some formal education in electricity, however I haven't run into any "zenor" type in a very long timeand honestly was surprised to hear that I had one in my tach. Is this a commonly found component in cars since more electronics have been introduced?

 

One of the common uses of a zener in today's electronics is as a protective device. For instance if your circuit has components that can't sustain voltages over 40V, then you can put a 36V zener on those lines and it will clamp any voltage excursions to 36V. If you remember in the early days of hand held calculators, one of the common failures was caused by ESD (electro-static discharge) which is the spark you feel [:@] when you walk across a room and touch someone or a grounded object. This can be several thousands of volts like 30K to 50K. Needless to say that fried unprotected electronics. This is the reason that one wears a grounding wrist strap when working on electronics like say your motherboard in your PC. Today most of your electronics are a lot more robust and some microchips actually have zener protection right on the chip so there's no need for any additional zeners.

I'm guessing that Aldo's tach got fried by someone messing around with the wiring and put 12V where it shouldn't be. Or maybe a bad jump start.

Sorry, a little long winded. Probably too much info.