it's not so much the lines, it's the missing pixels/information when looking at the values for the resisters and capacitors ect: :'(

Some of the values are in the description of operation. Also, if you reverse colors in paint to make out values. Pull the white paper on the LM1815 and more values could be had.

Clean schematic

tach amp wire colors for wires 2,3,6?

New design amp

Designing a Better Tach Amp

Agreed, many pixels missing.

If you enlarge the image, scoot back from the screen, and use your visual closure skills (aka imagination) then you might be able to make out the numbers and letters.

I got:

***************************************************************
Note: 'Unless indicated otherwise'
* All resistor are 1/4W 5%
* All capacitors in microfarads

R1 32 1/2W, R8 360
R2 360 2%, D1 IN914 , C1 100 15V Electrolytic, Z1 5.5V 0.5W, R9 470k
G1 BC327, C5 0.001 110V
R3 830
C3 0.0033 16V Polyester
C2 2 2 25V Tantalum
R4 1.9k
R6 4.7k
R5 1.8k, R10 27k
R7 1.2 M, C4 4.7 10V Tantalum

*************************************************************

I don't know if my guesstimates are accurate, but that's my read of the values.

Edit: Of course a clean schematic is infinitely better.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

The "Clean schematic" may have one or more errors. The inductor shown across pins 2 & 3 should be a capacitor.

I have repaired many of these tach amps. It was always problems with poor solder joints, especially where 12 V comes in on pin 6 to the 37 ohm resistor. Due to heat and poor solder joints, that connection becomes intermittent, or that resistor is burned out.

Scrape away the potting compound, check and replace the 37 ohm resistor if n.g. (use a 1/2 watt instead of 1/4), re-solder the entire board and you may be in luck.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I'm not an electrical engineer and truthfully have very little knowledge about circuits but I managed to make this tach tester out of really cheap common parts. It will test the wiring from the socket and the tach itself only. On my 1982 300sd it moves the tach from 0 to 1000 rpm with key on and engine not running. And No, i can't explain how it works. LOL

Attached Thumbnails

 

I got the tach working again. Found a crack on the circuit board in the tach amp. I repaired the solder trace and it's good to go.

Attached Thumbnails

 

The 555 timer circuit you built creates a square wave signal that simulates the output of the tach amp. It goes from zero volts to 5 volts and back to zero at a speed that simulates the single pulse from the pickup on the crank being amplified and cleaned up by the tach amp. If this is truly your first go at electronics, well done! And even if it's not, still well done.

Also, good find on the broken trace. Did you span the break with small piece of wire? Just bridging it with solder will work fine but a stronger fix when the PCB cracked is to bridge the crack with solder and a bit of wire. It can be tricky as the wire tends to want to stick to the soldering iron.

I consider solder joints like the one near the top of your picture as being "bad". You can see a ring or circle around the component lead. Some would call it a cold joint, some call it broken. Either way, the component lead isn't making the best contact with the circuit board trace. It wouldn't hurt to remelt that joint and any others that look like it, adding just a touch of solder which will put fresh flux on the joint. That will let the solder flow into the joint, solidifying the connection.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

Next project....My electronic speedo also has problems....it does this: when i go 20mph, the needle fluctuates between 20 and 0....constantly, not randomly.....when i go 30mph, it goes from 30 to 0 constantly, not randomly...Any ideas why it constantly bounce back to 0?

I've looked at the diagram for the '83 300SD which also has an electronic speedometer. Based on the diagram, the function that the tach amp provides, meaning taking the vaguely sinusoidal signal from the pickup and converting it to a square wave, is done in the speedometer rather than a separate circuit board.

As a first step, I would take a very close look at the speedometer circuit board, looking for cracks and dry or broken solder joints. Dry joints have a dull or crusty look to them and may have a ring around the component lead that looks odd. Broken joints can look like the area you fixed on the tach amp or be a just visible ring around the component lead.

Next, I would suspect any electrolytic capacitors. Electrolytics don't age well, especially in the hot environment of a car instrument cluster. It's been a year or so since I had the speedometer out so I don't remember if the capacitors are axial (leads come out opposite ends) or radial (leads come out one end). Radials tend to bulge or end up with a domed shape top when they fail which makes them easier to spot. Axials may leak a bit but don't seem to bulge as much. You could "shotgun" it, meaning just replace all electrolytics. That's what I tend to do when repairing old radios.

After capacitors, I'd wonder about what is driving the needle mechanism. It might be transistors or FETs or maybe a chip. I'd check to see if anything is getting hot. It might be that some component is leaking which could cause too much current to flow. That will heat things up. The excess current could make the circuit shut down, dropping the needle then recover. I'm not sure about this one since I doubt it would recover instantly which is what it sounds like it's doing.

I'd also take a look at the output from the pickup. You don't need a fancy oscilloscope, one of the cheap handheld ones from eBay or Amazon would work for this. I used an almost 25 year old RadioShack ProbeScope to look at the signal from the tach amp. It's not ideal since the display is only .75" but I could see the signal. The hard part with looking at the speed pickup is that the car needs to be moving so this is a 2 person job unless you have a lift and the guts to put it in drive while in the air. This type of pickup rarely fails, it's a coil of wire around a metal core. but the wire harness can get pinched or the connectors can corrode which will cause signal loss. I don't know what the pickup is looking at, it could be a single bump or slot on a disc or a tone ring as used in modern ABS brakes so I can't tell you how many pulses you should see. The important thing is that the signal is steady and consistent.

And finally, it may be the cruise control that's causing the issue since it gets it's speed reference from the speedometer. You could try unplugging it if you can find it. Just pulling the fuse may not be enough, you need to isolate it from the speedometer.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120

Great tips Michael. I’ll add that you can hook up your 555 signal generator to the Speedo and see if it works. This will allow you to see if the problem is on the pickup side or the instrument side.

Here is my thread on repairing my 82 300SD odometer. Maybe some of it may help you. My problem turned out to be mechanical. Too much endplay in the stepper motor that drives the odometer digits. You may have another issue as it sounds rather electronic in nature. Best to get it apart and see what is going on…inspect gears, capacitors, board etc.

I recall looking up the chip that does the signal conditioning. It was a purpose built pulse amp like the tachometer amplifier circuit.

Fixed 1982 300SD odometer slip at high speed

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles