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  #16  
Old 09-24-2006, 05:15 PM
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Bob
 
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Location: Paris, FR
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Ok. I went through this exercise since I now have two. After a bit of frustration I went with the known semi-working one and just did the cigarette butt trick which worked like a charm.

In the meantime, I was able to lift the chip ID off of the 14 pin DIP. It's an LM1815N which is a commonly available chip. The gold capacitor looking thing has markings on it, but I could not decipher it. The little green ball markings have also eluded me and never seen anything like it before (I'm a ChE, not an EE).

Maybe if someone else wants to get back into the game and decipher the rest we could get somewhere. If in a fit of boredom I decide to have another try at my semi de-siliconed board, I'll post back.

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  #17  
Old 09-24-2006, 05:43 PM
Brian Perkins
 
Join Date: Aug 2006
Location: Luanda, Angola
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Mine is working, so I'm not taking it apart, and without a photo, I'm not real sure what the "little green ball" might be. However, tantalum capacitors frequently look like little balls with two metal leads. If you guys would post clear, closeup pictures of both sides of the board, I'd be willing to make a schematic and offer suggestions for alternative components.

As I read the thread, I was thinking that the most difficult part would be to identify any integrated circuits (although I suspected it'd just be an opamp), but thankfully jshadows did that. Now that we know the main circuit, the rest should be pretty easy.

By the way, the LM1815N is just under $4 at Digi-Key (over 600 in stock) and here is the datasheet.
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  #18  
Old 09-24-2006, 06:27 PM
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Bob
 
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Location: Paris, FR
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Here's the board side and the big gold 'capacitor'

http://www.flickr.com/photos/85645942@N00/sets/72157594298090382/

images are of highest quality my camera can do so you can zoom in on them.

my digital camera sucks. i need a canon SLR D as I have the lenses to go with but haven't broken down to spend the money yet.

when boredom hits, I'll dig a bit deeper.
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1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
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  #19  
Old 09-24-2006, 06:46 PM
Brian Perkins
 
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Location: Luanda, Angola
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The pictures look very good, but flickr won't let me zoom. I'll PM you my email address so you can send me the hirez version.
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  #20  
Old 09-24-2006, 07:19 PM
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Joe Myers
 
Join Date: Dec 2004
Location: Alabama, near Birmingham
Posts: 46
Reverse engineer the interface?

Seems like one way would be to measure the input signal and output from a working unit. From that it should not be too hard to duplicate. It doesn't have to be the way they did it. Electronics have advanceed a lot since then.

If some one can measure signal from pickup at a couple of different RPM values and voltage going to gauge at those RPMs, between some other EE friends we might be able to design a circuit. I don't have a working one to test.

I would assume the pickup sends pulses with repetition rate (freq) proportional to RPM. Need to know level, polarity, and freq. Then need to know if AC or DC signal to Tach. Likely DC?

Another question is if pickup is supplied a voltage or does it generate output without amp plugged in.

If someone can find interface values, I am willing to work on a circuit.
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  #21  
Old 09-24-2006, 07:32 PM
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Bob
 
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well, I stuck some probes in the sensor line a few weeks back with nothing no matter the combination. There's 3 wires so maybe there is a voltage supplied to it. Would have to investigate further. As for other side, a green and brown wire to test signal off of in order to determine values to send to gauge.

Most of the stuff on the board appears to be 10 cent parts (aside from the chip), so ideally could just figure out what breaks on them and just replace. That said, getting silicone off is a major PITA.
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1979 300SD 245K miles (never ending project)
2007 Pinarello F3:13
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  #22  
Old 09-24-2006, 07:49 PM
Brian Perkins
 
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Hey Joe,

Well the deal is that the input signal is a very, very low level pulse, probably in the 50 millivolt range. (I'm just guessing, but it's an "educated" guess based on my work with electric guitars.) Looking at the MB FSM schematic, the unpowered signal travels over a shielded twisted pair to the "Solid State" amp. The amp then has a single line to the gauge module which is also "Solid State" circuitry. The LM1815 is designed for exactly this situation. Take a low-level noisy signal, clean it up, amplify it, and output a useable pulse signal.

As for designing another circuit, I really don't think we need to reinvent the wheel. All I'm suggesting is making a replacement board that is functionally identical to the original - maybe with slightly different parts. The thing is, we've got to figure out why these things are failing if we hope to make them work reliably.

Mine was just the electrical contacts. I guess I got lucky. Another possibility is that most of the failed units are because of electrical contacts, but I did something different than other people. Maybe I cleaned my contacts better. Maybe I used just the right amount of silicon (not silicone) grease. No way of really knowing unless I got my hands on a few other units.
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  #23  
Old 09-24-2006, 08:15 PM
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Joe Myers
 
Join Date: Dec 2004
Location: Alabama, near Birmingham
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more on tach circuit

The spec sheet on the LM1815 (variable reluctance sensor amp) says it operates on as little as 100 millivolts P-P. So unless they have an amplifier before the LM1815 the signal should be at least 100mv.

I found a very simple circuit for a variable reluctance sensor interface at
www.dainst.com/info/circuits/VRS_interface.gif. It is supposed to work from 200 mvolts.

Given the difficulty of removing the goop from the module, thought a replacement might be considerably easier and longer term fix.

I certainly am all for fixing it the easiest way and something to be said for keeping it original.
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'83 300D (light blue)
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  #24  
Old 09-25-2006, 12:24 AM
Brian Perkins
 
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Location: Luanda, Angola
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Hey Joe, I like that circuit. Simple, yet elegant and if additional gain is needed, say from less than 200mV, it would be easy to add a driver transistor to the LED. Good alternative I think.

Coincidentally, a friend of mine here in town has a 300D with a bad tach and called me tonight to see if I'd look at it. He's tried cleaning the contacts and inserting a piece of foam and it still isn't working, so I'm gonna take a look at this week and I'll get back to you on what I find.
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  #25  
Old 09-25-2006, 08:40 AM
Brian Perkins
 
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Location: Luanda, Angola
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Ok, Joe (oops, sorry Bob) sent me some pictures. From those and the other text he was able to get off the components. I think the goldish-green thing is a mylar capacitor. The green ball I would have guessed is an LED from the picture, but it has 3 leads and reads 25V, so now I'm thinking maybe a dual Tantalum capacitor. It's hard to tell. I had to zoom 200% to fill the screen with just the board and leads and it's already pixilating.

As for the alternative amp circuit, I just realized it's not likely that circuit will work for a signal as low as ours because the forward turn-on voltage of an LED is about 1.2V, so unless we bias the LED or pre-amp the signal with another transistor, we're not likely to get anything at the output. This all means more experimenting. Now, I'm leaning back toward the original idea of discovering which component is the likely failure in these units and replace that with a more reliable version.
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Last edited by PerkHouse; 09-25-2006 at 09:51 PM. Reason: credited wrong person
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  #26  
Old 09-25-2006, 10:11 AM
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Bob
 
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actually that was me, Bob (aka jshadows) that sent them. The green ball which is solid only has 2 leads. The black chip next to it has 3 which I'm guessing is a transistor. Then there's the light green/lime green flat capacitor like chips.

I'm still doing surgery on the items and I haven't taken a dremel to the top side so I don't scrape off the markings (mistake made on someone elses previous attempt).

I need 2 things to get a better look at the components, 1. less silicone, and 2. a magnifying glass. will work on it as time permits. If anyone else wants to start taking a part their dead amp to speed up the quest, feel free.
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  #27  
Old 09-25-2006, 03:00 PM
Slow Attack Submarines
 
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Could the amps be failing due to the stress put on them by being right above a spring? Mine works intermittently, often coming in or out when driving over a bump. Maybe this could be fixed by mounting the amp longitudinally.
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  #28  
Old 09-25-2006, 03:55 PM
Registered User
 
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Mine works all the time (on the '85) but I think it is consistantly low (1700rpm at 60MPH) but I havent looked into it at all yet.
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  #29  
Old 09-25-2006, 09:58 PM
Brian Perkins
 
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Quote:
Originally Posted by pleiades View Post
Could the amps be failing due to the stress put on them by being right above a spring?
Oh, of course! Stress fractures can easily occur in the solder joints or even the copper traces on the board.

I'm not a mechanical engineer, but one once told me of an experiment he was involved with where his team cut a 24 gauge steel sheet with a nylon "knife" by vibrating them at the right frequency. This came up because we were having an unusual number of failures in one of the transformers we manufactured. After further investigation, we determined that only the lots shipped with a particular trucking company were failing prematurely. We changed shippers for that product and our failures practically disappeared. We didn't care to spend the resources to investigate further, but we speculated that it may have been differences in the suspension systems used on the different brands of trailers.
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  #30  
Old 09-25-2006, 10:16 PM
Brian Perkins
 
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Quote:
Originally Posted by jshadows View Post
actually that was me, Bob (aka jshadows) that sent them. The green ball which is solid only has 2 leads. The black chip next to it has 3 which I'm guessing is a transistor. Then there's the light green/lime green flat capacitor like chips.
Man, I'm sorry. I messed up your name and misunderstood what you were saying too.

Ok, since the green ball has 2 leads, I think it is probably an LED. Without seeing the traces on the board, I can only speculate why they would have it there, but LEDs do make fairly decent 1.2V "regulators".

The black thing with 3 leads is almost certain to be a transistor, but not necessarily. If it has 2Nxxx or 2Sxxx (where xxx is a number of 3 or 4 digits), it's a transistor. If it has 78xxx or 79xxx, it's a voltage regulator. That is possible since the LM1815 is only capable of a maximum supply voltage of 12V and as most everyone knows a car's system is usually right around 14V when the car's running.

The lime-green thing is probably some sort of metalized plastic (mylar, polyester, polystyrene, etc) film capacitor. See if this link looks like it.

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