My fz88n tach has a larger diode marked Semicon XS-17. I can find little on this. I cant tell if its a tunnel, zener, or other. Best I could find is a Vishay cross ref that says 1N4004 while not the same, should work in most applications. Has anyone confirmed a replacement for this diode? Thx
If the XS-17 is physically larger than the 1N4004, it would more likely be a higher current diode than the 1N4004 (1 amp rating). Do you have a circuit diagram for the tach? It should be easy enough to work out the diode's function.
Thx. I cannot find a circuit diagram so Im in the process of reverse engineering one. The metal diode is shown in the 2nd picture.. The tach signal comes in, goes thru the resistor and feeds the diode. I assume it clips the signal to a set voltage?
Datasheet for the XS17 shows: Cross-ref 1N610 200V max forward voltage 200V max reverse voltage 750mA max forward current 25mA max reverse current 25C max temp 1500mA max surge current Forward drop at full load: 1.6V Technology: Silicon So, the 1N4004 differs in that it has better current handling characteristics and better temperature rating, but it's also much more efficient, having only 1.1V forward drop at full load, compared to 1.6V so depending on the circuit that may throw things off. However, you may be able to adjust that bias out. Phil
I did a little Googlin' and found the DATA cross reference booklet from 1961, which has the XS17 listed. Technically you could put 2x 1N4004 diodes in series and get just about the same characteristics. That would undoubtedly be easier than trying to find either an XS17 or a 1N610. Phil
Phil, here is the circuit best I can figure. I question if I put caps on correct pins that are in parallel to 3.3k resistor. So far I replaced all 4 capacitors and the transistor. Resistor meas are shown and some are a little off, but to get nothing at all to the tach? there are three diodes. Xs17 you addressed. Another looks like color bands violet or white, green, red, red. Best I can figure its either 2.7v or 7.5v. I bought both 1N5223B and 1N4737A respectively. The third is either ZD212 or SD212 rectifier I believe. I got 1N4004 to replace it. Seems odd. When I put 12v to it I get 7.5 v across the first resistor to grnd. I get nothing at tach + to grnd BUT I get 12v from the tach + to the 12v +?
Yeah, the two regular silicon diodes 1N4004 would work; the Zener is gray green red red which is 8.5V at 200mA just to buck any transients from burning the needle voice coil. Put the 7.5V one on- worse case the needle won't reach full deflection. You won't get anything at the tach output unless you have a pulsed signal coming in from the breaker points contact through the transistor. That's normal, the DC is blocked by the two capacitors top right. Without those caps, the tach would peg the needle if the engine is stopped with the points open. That's by design so all good there. Phil
Thank you. I thought it was 8.5v but had a hard time distinquishing violet, white or gray. I assumed 7.5v because thats what I got across the resistor. Ive been studying the circuit and cant figure how it works. If someone showed me this I would think it were wrong based on curreny c to e thru the resisstor goes to grnd without the neg side of the tach coil tied in Before the transistor. Just cant see what the current leg thru the transistor is actually doing. Thx again
Circuit makes sense. It's a basic voltage divider with a DC blocking function. The base will draw current from the ignition circuit when the points are open and the transistor will conduct. That makes the potential difference from the +12 supply via the resistor across the transistor high. Vice versa when the points are closed. The change that occurs when the points close and open is what will be translated, as an AC element across the two capacitors and that is then indicated as a current flows through the meter winding. Basically the circuit only actively does something when there is a change (ie the engine turns and the points are opening and closing), else the most it does it sink current through the transistor when the ignition is on but the engine isn't running. Drawn slightly differently you'll see it forms a standard voltage divider circuit. If you were to poke it with an oscilloscope while it's running you would see this at various points: So the very noisy input signal from the points is cleaned up and rectified to something that won't blow up the transistor. The transistor switches cleanly at 0.7 Volts, and that change traverses the capacitor pair and drives the coil in the meter to make it read. The faster the engine runs, the more pulses hit the meter, the higher up the scale it reads. The diodes are simply there to protect the transistor. Hope that un-muddies the water a little Phil
Me ha, Phils the man. I know just enough not to throw broke stuff in the garbage. But apparantly just short of enough to fix it! More diodes coming this week to try Phils suggestion.
You can make a plumbing analogy. There's a tank of water, with a restriction at the top and a rotating valve that either lets water pass or not. If it can drain, there's no pressure in the middle, if it cannot there's the pressure from the cistern. That's attached to a big rubber diaphragm. The water cannot flow through, but a pressure change will cause the diaphragm to flex, and that causes the pump vanes to spin each time it flexes- that pump vane then moves the tach meter. The capacitors are the equivalent of the rubber diaphragm- they don't allow water to flow through, but a change in pressure can be translated. I didn't understand this years ago, and I made myself determined to understand just what was happening in the circuit, and the function of all the parts. Somewhere along the way I must've electrically shocked myself enough to cause some of it to sink in.
I was born too late to work for Lockheed, sadly. Would have been fun! Uhm. I might have built a few radios. I'm not going to admit to that, though.
I removed the diodes. xs17 diode 1.9Mohm fwd, OL reverse. sz212 1ohm, both directions. w,g,r,r 1kohm fwd .8 kohm rev. So, I think all are bad.
Ahha, that schematic is great. It makes sense now to see how the caps cleanup the signals at each node. You get 12,000 stars for taking the time to help! Thank you thank you thank you
Phil, my Dad wired missiles at Lockheed in Sunnyvale Calif in the early 60's and built a bunch of Ham radios. Hence the remarks LOL
Depends what voltage your meter reads the diodes at but even for a modest digital meter those readings are pretty junky. The SZ212 giving a failure mode will cause the meter to flatline. Clip one leg off and you should be able to get a small blip out of the meter by tapping the tach-in peg to +12 Phil
Well good and bad. One zener diodes was bad. So after 4 caps, 3 zeners and one transister plus glueing the trunions back on the needle coil… it worked! But.. the needle balance was off if I rotated the tach. It read about 1000 rpm high when tilted to the position I needed. Tore it all apart and used the weight to get it within 400 rpm. Shouldve stopped. Some excess glued on the needle trunion was throwing it off. Scrapped it off slowly, got within 200 rpm then, cut the damn coil. Tore it all apart and unwound the coil. Waiting on a spool of 43awg to try and wind 180 turns of .002” thick wire on a 3/8” square coil. Not gonna be easy, but we shall see. Phil has been a big help on thus project. Im not giving up at this point. Its going to work or get smashed with a sledgehammer!
Ok I made a winding jig. Used a 1/2” piece of octagon stock filed to fit the bobbin. Turned the ends on a lathe. Used a 3/4” rod for the spool. Wrapped two pieces of cardboard with strps of an old shirt to make a wire tensioner. The c-clamp modifies the tension. Used a tap handle for a crank. Wound 180 turns and got 67 ohms. Took about twenty turns off shooting for 58 ohm but got 57 ohm. Put the tach back together measured 58.5 ohm, I wanted60 ohm to match what I measured before I broke the coil. So got pretty close. Needle balanced well within 200 rpm at 90, 180, and 270 degrees. Hope it works well.
Finally seem to have it working. I changed from an 8.2 to a 9.1V zener diode to get a little more range. Even considering rebuilding the board, winding the coil, and mounting the trunions, getting the needle balance and calibration was by far the most difficult. Here is a video. I borrowed a function generator to provide the signal for calibration. https://youtube.com/shorts/SKnHMpjskYk?feature=share
Wow! You win the "Who says it can't be fixed?" prize! I have straightened out meter movement hairsprings using tweezers and a magnifying lens, but rewinding the coil takes the cake! You need a stiff drink before, during and after this job! Glad to see it sorted out.
Thanks. I have to thank Phil for pushing me to try rewinding the coil because I was shining up the sledgehammer to smash the sh!t out of the tach when I cut the coil!!!
Looks to have been worth the effort, that's a nice looking gauge. That's pretty accurate, too- by adjusting the Zener voltage you've at least limited the top end and will protect that voice coil now. That's gotta have been hooked up backwards to have damaged everything like that. Just take care when connecting it up, there's no reverse polarity protection in the circuit. Phil
The Jalopy Journal
