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Magneti Marelli voltage regulator

Discussion in 'Technical Q&A' started by MJO, Nov 22, 2009.

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  1. MJO

    MJO Rookie

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    #1 MJO, Nov 22, 2009
    Last edited by a moderator: Sep 7, 2017
    I’m working on a Magneti Marelli Model RTT 101C voltage regulator. This is one that has two NPN transistors (other MM regulators that I have worked on employ three transistors).

    A component connected directly between the emitter and collector of what I call the “input” transistor was zapped almost beyond recognition (the other transistor is connected to, and outputs directly to the Field terminal). The mystery component suffered major heat damage and the part ID is not visible. Initially I thought it to be a small electrolytic capacitor. After closer examination I’m pretty sure that it’s a diode. If so, is it a rectifier or Zener diode?

    The component is the one located at the lower right of the attached photo. Its connected between the emitter and collector of the adjacent transistor.

    Can anyone please supply the part number of this component?

    Thanks,

    Mike, Edinburg VA
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  2. RGigante

    RGigante F1 Rookie
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    #2 RGigante, Nov 23, 2009
    Would you have a picture of the damaged component?
     
  3. MJO

    MJO Rookie

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    #3 MJO, Nov 23, 2009
    Last edited by a moderator: Sep 7, 2017
    Here are two photos. Like I said, at first glance it appears as a small electrolytic but based on the glass shell, its more than likely a diode. Also attached is my schematic diagram sketch of the voltage regulator. The mystery component is the one with a question mark, immediately to the right of Q2.

    FYI, I found another RTT101C in a 275 Ferrari at the restoration shop where this one came from. But, when I removed the cover I found that it had been gutted. The entire circuitry had been replaced with a modern, sealed module regulator unit.

    BTW, My original post incorrectly described the transistors as NPN, they're actually PNP.
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  4. Jeff Pintler

    Jeff Pintler Formula Junior

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    #4 Jeff Pintler, Nov 24, 2009
    I am not an electrical engineer and I do not play one on TV. Kinda looks like a diode or zener diode. Notice that D1 is in the same relative positition. FWIW. You might find a voltage regulator for an old Alfa or Fiat?

    Jeff Pintler
    89 348tb, 86TR
     
  5. RGigante

    RGigante F1 Rookie
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    #5 RGigante, Nov 24, 2009
    It does look like a diode or zener diode, but it's going to be hard to find the reference or voltage ... I would try to find someone with a complete unit and find the reference.

    Wouldn't hurt to try the official Magneti Marelli website and ask them ... you never know!

    But also don't forget that component is damaged for a reason. Are you sure the other components are ok?
     
  6. MJO

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    #6 MJO, Nov 24, 2009
    I’ve found the other bad components. The original type numbers are easy to cross to easy to find replacement parts (NTE). For example “ZD1” on my schematic is a BZZ28. This is a 22 volt/10watt Zener and cross references to a NTE 5197A.
    The problem with the mystery component is that its type number has been burnt away.

    I have already e-mailed Magneti Marelli. No response yet though.

    Thank you,

    Mike
     
  7. MJO

    MJO Rookie

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    #7 MJO, Nov 30, 2009
    Last edited by a moderator: Sep 7, 2017
    I found a schematic (attached) of the regulator in a 365GTB4 service manual. The mystery component is an ordinary rectifier diode. A 1N4007 will do as a replacement.
    The diode obviously serves as a limiter/protector.

    Thanks to all for your help.

    Mike
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  8. Paul_308

    Paul_308 Formula 3

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    #8 Paul_308, Nov 30, 2009
    I concur with the exception of recommending a 1N4007.

    The 1N4007 is a 1000v diode, very lightly doped and hence has much less reliability than a 1N4004 400V PIV diode. Actually, a 1N4001 is adequate for the circuit.

    With any silicon devices, a higher voltage isn't a better choice unless it is mandated by the application. To accomodate higher voltages, the substrates are substantially compromised. Which may be why the original diode failed, poor choice.
     
  9. Emanuele

    Emanuele Rookie

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    #9 Emanuele, Dec 1, 2009
    Since it has a glass case, it looks like a germanium diode. It may be a AA118 - AA119 - OA95.
     
  10. Paul_308

    Paul_308 Formula 3

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    #10 Paul_308, Dec 1, 2009
    Good thought Emmanuel...you would have best knowledge of local (euro) components and I've given it more thought to the circuit...(after many years of disuse). Note that I cannot find a datasheet on an AA118 (comes back as an SCR)

    Forward voltage of silicon is 0.7v and germanium is 0.3v. That is the voltage at which the current starts to flow in a diode.

    The diode in question is across CE of the php transistor T1, and will start to conduct when it's anode (T1's collector) is more positive than T1's emitter. Normally that's not the case as normally the emitter of a php is more positive than the collector. But alternators produce nasty spikes, and to protect T1 from reverse spikes which could blow it, use of a germanium diode with it's lower Vf of 0.3v will protect the spike from going through T1 which conducts at 0.7v. I.e. reverse voltage spikes will flow through the diode instead of the transistor because it starts conducting sooner.

    OK...I change my vote.
    Don't know what you will find in Radio Shack in germanium besides a 1N34A. The lowly 1N34A might work, it's been around since 1960...data sheet here: http://pdf1.alldatasheet.com/datasheet-pdf/view/42334/SEMTECH/1N34A.html It's smallish 150Ma Ifm is bothersome but may be adequate. It's primarily a signal diode, not a power diode.

    Note that the circuit will work without the diode in place. How long is another matter since it's there for reliability.

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  11. MJO

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    #11 MJO, Dec 3, 2009
    Thanks for your inputs guys. Very much appreciated. Good point about the diode glass case being related to a germanium component. Not sure that it would make that much difference though. The transistors are both germanium types. The other two rectifier diodes, BYX38/300 & BYX38/300R, are silicon. I’ll keep looking to find the actual type number of the diode in question. This is a spare unit so there is no hurry to complete the repair.
    Thanks again,

    Mike
     
  12. MJO

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    #12 MJO, Feb 28, 2010
    I have access to another RTT 101C regulator. The mystery component is a diode as suspected. The designation is a P100 which crosses to an NTE 116 (Silicone, 600V, 1A). A 1N4004 is also equivalent.

    Thanks,

    MJO
     
  13. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
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    #13 Mitch Alsup, Mar 1, 2010
    I count 8 components on the photo in original post

    I count 15 components on the schematic.

    Thus, we need to see a photo of the back sides of the regulator to determine what device blew.

    I concurr that what blew was a diode of some form: zener or regular, si or ge not determined.
     

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