Sun tachometer schematic internal board repair

This would be for a marine Sun Tach from 1977 for a v8 engine
I have 4 of these and one of them when powered with 12 volts, the meter needle goes all the way to max 6000 rpm with or without out the distributor sending wire hooked up. Another on does not respond but remains at zero. If you momentarily connect it like scratching the wire against the terminal the needle slightly jumps. These were the 2 up on the flybridge that have failed.
The meter parts funtion fine applying voltage to the meter causes the needles to move.

I have it all apart and was wondering if any electronic guru has any knowledge of the simple circuit board which has a few resistors, capacitors and two transistors.

My plan was to replace one electrolytic and one paper capacitor.
one is a .02 mfd 200 vdc, othere I need to unsolder to read.
Then replace the transistors 2n3566.

It also has one diode cr1 and one zener diode cr2. For the zener I have to unsolder to read and the diode has no numbers. Any thoughts on these parts? What is the chance of either diode being bad? zener regulates voltage, diode might be for reverse wire protection??

 

I checked the resistors they are all in spec except R2 which reads slightly too high for the tolerance level.
Both diodes I unsoldered one leg and they seem to work block one way flow the other.
Two capacitors, also seem to work, charge and discharge on the digital multimeter. Blue and yellow ones.

So I am left with the transistors as likely being a problem.

 

Unless you cleaned the board before making the pictures this doesn't look like components damage.
The electrolytic cap may have dried out, but even when removed, it wouldn't cause the symptoms you described. R9 could be the culprit because it is an open type where the resistive layer may degrade, corrode and the wiper makes a poor contact with it.

I would heat all solder joints to make sure there is no "cold joint" where the solder never made good contact with the wire.

 

Board is as it come out of tach, no cleaning done.
I checked R9 pot and it is working and adjusts ohms smoothly.
the axial capacitor, c3, says printed on the blue case

25V
015 FA
10 /

Do you think this is a 15uf ?
such as this here?
http://www.alltronics.com/cgi-bin/item/15A100/search/15-%B5F-100-V-AXIAL-ELECTROLYTIC

the yellow capacitor, c1, .02 MFD is a metallized film capacitor which means likely robust. Just like this one here.
http://www.ebay.com/itm/50pcs-Paktr...aultDomain_0&hash=item2c5c1f1caf#ht_780wt_915

the transistors are 2N3566 which I can buy from alltronics
http://www.alltronics.com/cgi-bin/category.cgi?item=2N3566

and supposedly also can substitute NTE123 for this part
http://shop.vetcosurplus.com/catalog/product_info.php?products_id=1841

The zener diode has printed on it

1N5
236
B
TTT

Any idea what the voltage rating is or a source?

 

C3 is 10 or 15 uF 25V for noise suppression.
The Zener is 1N5236B, 7.5 V 0.5 Watt
2N3566 is a universal NPN transistor, max 30V 0.2A, exchangeable with 1000 others.

None of these parts is subjected to high voltage or current, except CR1, which blocks the 200 V spike when the breaker contacts open.

Once again, the most common cause of circuit failure is a bad solder joint. These boards are solder dipped for a few seconds; over the years the temperature differences sometimes cause an open circuit. A tachometer in the sun can get scorching hot inside.

 

cr1 seems to be ok, passes current one way, blocks the other way and
has no numbers printed on itself. Any chance that could be bad and what would you put in, a common high watt diode?

R2 reads 136K and 160K and should be reading 120K, they are the only out of spec resistors. Look to be 1 watt gold tolerance.

I will certainly resolder the connections. Since one of the tachs goes to max rpm when powered on makes me think something is shorted like perhaps a transistor.
the other one does not react to the distributor pulse. These broken ones were up on the flybridge, exposed to sun, water, wind,electrical storms ,and EMP from terrorists.

The interior cabin ones still work well.

 

Unsoldered 2 transistors and one leg was snapped off who knows when or how. Maybe I did it.
I found a datasheet for the 2n3566 which is an obsolete case design called a TO-105

The replacement transistors are NTE123
I notice they have the same pinout so easy to R&R
first picture is for 2N3566, second for NTE123

It took a while to find this information!
If you know the case design number for a transistor, then you can find out what each pin is.
http://www.jedec.org/standards-documents/focus/registered-outlines-jep95/transistor-outlines-archive

 

Tachs are working now, very good.

The failures I found were several things.
Both tachs the meter ground needs a wire soldered to the adjuster for the clock spring and run to the meter negative on the board. Internal high resistance of the meter due to manufacturing where they rely on metal to metal contact for the current return. Likely a typical SUN tach failure from 35 years of use. On both tachs R2 out of tolerance high.

One tach the R1 was open so no signal processed. Strange.
other tach which went to 6000 rpm when powered on, the circuit board trace on one of the circuits for the transistor was completely eaten away by corrosion. Looking at it it looked ok, but there was no connection.
Was kind of discouraging to replace those parts and it was still not working. So started comparing meter readings between boards and found the resistance difference on that copper trace.

I replaced all transistors, a zener, an electrolytic and 2 metalized caps likely uneeded. But the new parts are better than the old ones.

I was wondering do any of you coat circuit boards in a clear coat spray? It might help prevent board damage, but might make it harder to fix later.

To take apart sealed gauges like this, grind the backside crimp off the bezel. Leave a lip to lock and center bezel back on the housing.
Then to reinstall the bezel use black permatex gasket maker. Glue the glass face directly to the bezel, consider discarding the old rubber sealing ring which likely will or is leaking.
Then after it is set, glue the beezel back to the housing with black rtv. Perfect strong waterproof seal wont easily come apart.

If you try prying the chromed bezels crimped lip up, you will distort the appearance and maybe crack it.

I ordered parts from here. the tech was knowledgeable when picking the new components, meaning he knew this stuff well.
https://www.alliedelec.com/default.aspx

 

I always do, but I use CRC, Plasticote 70, Clear protective lacquer. It insulates, protects and seals electronic parts. If you think that your clear lacquer will do the same, use it. There is no problem in lateron soldering a part out of in.
Bert
note: It is not the first time that I used thinners to clean a pcb after coating with clear lacquer and thereafter clean it up with methelated spirits by doping it into the spirits and shake it nicely.

 

My company used a German lacquer spray that made future repairs easier by acting as a flow agent for solder. For outdoor circuits we used a 3M product that left a durable, flexible skin covering the board and its components. It was applied after the board had been thoroughly washed in MEK. Once cured, the board was moisture-proof, but repairs were almost impossible.

 

That 3M product has a similar effect as potting. CDK, what the hick is MEK, I see so many abbreviations, that maybe one day we will have a abbreviations conversion list. MEK? More Electronics Kaput ??
Bert

 

MEK is Methyl-Ethyl-Ketone....

 

You mean Butanone, you confuse an old men like me. It sits in my CRC Plasticote, sweet smelly smell.
Bert