Testing components (Diodes) in-circuit

Discussion in 'Technical Repair' started by Standisher, Mar 20, 2017.

  1. Standisher

    Thread Starter Member

    Jan 16, 2015
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    Not sure if is the right board for this thread so happy for moderators to move if necessary.

    Background - feel free to skip

    I acquired a faulty Function Generator (Instek SFG-1013) for a very low price and am hoping, despite my quite basic skills, to repair it. The unit powers up and generates a display of frequencies. The TTL output seems to be functioning also and the set frequency corresponded with the frequency measurement on my oscilloscope. The main output however delivers NO signal at all. On opening the unit I found 2 resistors badly scorched and one of the output transistors was also running very, very hot.

    I have been in correspondence with the manufacturer. They agree with my belief that the 2 main transistors in the main output circuit were blown (which the say causes the resistors to run hot). They also think several other general purpose npn transistors are also likely to be damaged and are currently in the process of securing a total of 9 components which they will supply free of charge (very good of them) and asked if I will take a photo of one of the npn transisters to make sure they send the correct items (5 of them the same)

    New concern - the point of this thread

    I had the unit open today to take the requested photo and thought I would check a few of the diodes in a small cluster in the output path. Using my DMM in the diode mode. Alternating the pos & neg probes on anode & cathode I got a reading in both directions (on a total of 7 diodes - see pic). Typically the diodes read 0.612V (with pos lead on anode and neg lead on cathode) and 2.30V with probes reversed. I had expected to get NO reading when testing with pos probe on the cathode. All diodes have a very similar reading but diodes elsewhere, outside the output path gave me the expected reading (i.e NIL reading when reversing probes). Could I be missing something basic when testing diodes in-circuit like this or is it likely that all 7 diodes could have shorted simultaneously? I haven't even examined the 2 zener's (not sure how to test them) and if all these components (total of 16 to date have gone south, I wonder if zeners and opamps are likely to have blown too :eek:

    I've attached a photo of the out portion of the pcb . Sorry for the long post. diodes4.jpg
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,965
    889
    It used to be easy with through hole parts - just snip one of the diode leads and you can test it unencumbered by other circuit elements.

    You should ignore any reverse leakage because its probably not the diode - and some diode types normally have some leakage.

    You could probably reduce the task by using a DMM diode check function to read the Vf of each diode, I'd certainly remove any that read shorted and have a closer look - but some diodes have a lower Vf, and other circuit elements can sabotage the readings.
     
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  3. Standisher

    Thread Starter Member

    Jan 16, 2015
    80
    7
    Thanks for the reply @ian field . When you say:

    I was in diode check mode on the DMM and I thought the 0.612V I quoted WAS the Vf...was it not? I was more concerned that I was getting 2.3V reading when I reversed the probes on the diodes.
     
  4. LesJones

    Member

    Jan 8, 2017
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    89
    The reverse voltage reading is probably due to the components that the diode is connected to. You would need to trace out the schematic of the commponents connected to the diode.

    Les.
     
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  5. ian field

    Distinguished Member

    Oct 27, 2012
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    Out of circuit; it should read "out of range" for most diode types in reverse.

    You can't trust any kind of reverse leakage reading with the diode in circuit.
     
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  6. Standisher

    Thread Starter Member

    Jan 16, 2015
    80
    7
    OK guys, I think the best thing I can do is desolder one of the diodes with strange readings and see what that produces out of circuit. Trouble is with these miniature, surface mount components, I'll have to be careful not to damage it in case it was OK in the first place :)
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Those round SMD diodes are a PITA. I'd order the service manual and find out what they are - and then just replace any suspects.

    As I'm a cheapskate - I'm just as likely to form the leads on regular diodes to match the pads - but its still worth finding out what they are.

    Predictably - its easier to solder the new part in than get the old one out. My method is to tin the ends with fresh solder and flux, then keep moving the iron from end to end till it heats up and comes adrift. You can get a special ultra low melting point solder that dilutes what's there and makes unsoldering SMD parts much easier - maybe someone can remind me the brand name
     
  8. Standisher

    Thread Starter Member

    Jan 16, 2015
    80
    7
    Thanks Ian. I think the low melting stuff you're thinking of is Chip-quik (or something like that)? Would love to be able to get a service manual but can't find one out there despite giving it the heavy google fingers. Even the manufacturer doesn't seem very knowledgeable about what components are in there...hence asking me to take a photo even though I had given them the part # stamped on the transistor (1AM) !

    If anyone knows a good source for service manuals it would be great.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,965
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    When I did TVs for a living - most manuals places charged too much for practically unreadable lousy copies - any time they were readable; they turned out to be for a different version of the chassis.

    There are online archives of test equipment manuals and schematics that just take a bit of finding. There was the BAMA boat anchor archive - but I haven't looked in for a while and can't confirm they're still there.
     
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