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I have an old ignition module from a 1978 W116 model Mercedes-Benz, and I'm trying to "reverse engineer" it. The original circuit diagram is not available to mere mortals, so I have made one based on the PCB itself. I believe it is, beside being ugly, electrically correct. Thus far I have identified all resistors, caps and transistors (bar one). What I have a huge problem with are the diodes, and any help would be appreciated.

Schematic: Schematic

PCB Layout: PCB layout

The known diodes are: ZD1 - 6.8V Zener (5 Watt?) ZD2 - 2.7V Zener ZD3 - 27V Zener

ZD4, ZD5, D1 to D8 are unknown. ZD4/ZD5 are not shorted, but I'm getting nowhere with these with my 30V power supply, so they're probably rated above >30V. D3, D5 and D6 are fried.

D4 is just beside R4. The two left-most black diodes are D1 and D8 (both marked "703"), the one underneath them is D7 (marked "1849" and "714"). The missing T5 transistor is a TO3 packaged power transistor marked "1120008" and "7/32". I haven't cracked this one myself, but have been told a BU426A is a direct replacement for it. T1 to T3 are BCY58IX, T4 is a BSV15-16K.

The power supply is a standard 12V car battery.

Edit: The entire ignition circuit looks like this: enter image description here (higher resolution)

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The entire ignition circuit looks like this: link –  ptashek Jan 18 '13 at 2:19
    
Just in case anyone's not aware, a 12V car battery will tend to run about 13-14V when engine is running (if that is of any help). –  Shamtam Jan 18 '13 at 2:33
    
Make that schematic easier to read, and it might be possible to infer what the components are, or at least what might be a suitable replacement, by their function. See Rules and guidelines for drawing good schematics. –  Phil Frost Jan 18 '13 at 2:38
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1 Answer 1

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Partial answer : for any diode with yellow/brown/yellow/grey bands, 1N4148 is a pretty safe guess! I see at least two - D5 and D6.

D3 I can only see a yellow band - aka "4". Is there any sign on the solder side that it is a replacement? If so, 1N4148 may be OK, otherwise keep looking. Possibly 1N914 though the spec is so similar to 1N4148 (they are often interchangeable. Just checked in a Philips databook : 1n4148 handles 200ma, 1n914 only 75ma; other specs identical) I don't see any reason to use both.

Can you get better photos esp around ZD4,5? They look colour coded but I can't make them out. I suspect they may be fried, rather than >30V.

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Brian Drummond: D3 has only a single yellow band, and there are no other markings on or around it. It has definitely been on the PCB from new - no traces of re-soldering, and the whole PCB was factory sealed with a silicone gel. ZD4 has a green cathode marker, and two bands - black and green. ZD5 has an orange cathode marker, and also two bands - white and green. Phil Frost: I'll see what I can do about making the schematic better quality. Would a parts list be of assistance? link –  ptashek Jan 18 '13 at 12:35
    
Then I suspect D3 is different for a reason... ZD4 : 5 0 5 ... 50V (non-standard) 5%? ZD5 : 3 9 5 ... possibly 39V (standard) 5%. This is a ropey diagnosis (unless the black band is really brown, 51V) Worth checking with a higher voltage supply and a fairly high value resistor. –  Brian Drummond Jan 18 '13 at 12:42
    
It is positively black and green. The question of what D1, D7 and D8 are remains, but I suspect they're just standard rectifiers. –  ptashek Jan 18 '13 at 17:44
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