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Need help with Circuit

Discussion in 'General Electronics Chat' started by ZTatZAU, Apr 11, 2009.

  1. user_88

    user_88 Member

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    .... I'm not seeing any transistor testers at RS online .... I guess they are no longer for sale there.
    Professional versions ... like BK ..... would probably be prohibitively expensive.
     
  2. ZTatZAU

    ZTatZAU Member

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    Thanks 88!

    I have a DMM so I can check them with the diode mode as described in a previous post. The DMM actually has a ebc transistor testing connection labeled Hfe but I don't have the dedicated lead. If you think that would be a better way of testing I could probably make up a three wire test lead.
    ZT
    BTW Might you know where I can download an operating manual for a BK Dual Trace Triggered Sweep 1470? I'd kinda' like to see that sawtooth waveform you described in a previous post.
     
  3. duffy

    duffy New Member

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    Yes - and that's a very good analogy.


    Oops, you're right - not a Darlington.
     
  4. dave

    Dave New Member

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  5. ZTatZAU

    ZTatZAU Member

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    88-
    I've asked my friend to see if that old timer can find page two of the parts list; if not the correct drawing and parts list that might only be a page or two away from what he sent).
    ZT
     
  6. ZTatZAU

    ZTatZAU Member

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    Hey 88-
    I took a break and traced the green board circuit from the end of the long gray pot to the base of what would be Q1 in the schematic. Like the schematic, my "Q1" collector attaches to the base of "Q2" and "R8" which looks to be 68K-5% (BluGrayOrgGld Hope that's right.)

    I plan to continue the physical tracing from there, on my board, comparing each connection and component with the schematic we have, and see how and where they differ. If I don't get lost and keep good notes, I will try drawing a schematic of the green board and post the results.
    ZT
     
  7. duffy

    duffy New Member

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    No, those are right, they even have a little "+" and "-" by them. Do you have something wired differently?
     
  8. ZTatZAU

    ZTatZAU Member

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    No you're correct. Everything is wired good too! I have a hard time keeping this straight when the board is flipped over. The tracings and component symbols you put on the bottom board picture is really helping me go through the circuit.
    I will post again with more answers or questions.
     
  9. user_88

    user_88 Member

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  10. user_88

    user_88 Member

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    I'm not sure I understand the Q1 and Q2 transistor arrangement, yet.
    Is Q1 normally on? ... And then pulsed off when the Q5 voltage spike occurs?

    Maybe page 2 of the parts list will help out.

    At any rate, you ought to be able to start the rotor motion by tweaking R7, if the other components are okay .
     
    Last edited: Apr 15, 2009
  11. duffy

    duffy New Member

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    Looks like Q1 is suppose to turn off when the drive is off - I'm assuming the motor winding resistance + R10 + R4 + R5 is much less than R6 + R3, so that R7 can be used used to set that "off" state within .7V from the positive rail. Q2 is a little tricky, it looks like it's switching Q4, but it is actually switching Q5. Either Q4 or Q2 can operate the driver, the former is obviously coming off that phototransistor, and Q2 appears to be part of a pulse generator using C1,3,&4 as the reactive components.

    Thing I can't figure is how this little ******* is suppose to actually spin. I mean, there's nothing in there to reverse that polarity on that coil, far as I can see. Suppose the wheel was stationary, but its magnet was already aligned up and down with the coil when it turned on? Seems like it would just sit there. If the wheel started out the other way, then yeah, it would try to right itself and maybe start spinning - but with this thing it looks like we need some copper ring "shades" or a small permanent biasing magnet in the pole pieces to rotate that field a little to get it started off a single-ended drive like that.
     
    Last edited: Apr 15, 2009
  12. ZTatZAU

    ZTatZAU Member

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    Hi again! Duffy's right!
    The gyro always stops in a particular parked position ("...only spins a 1/2 a turn and then stops abruptly".) Stops abruptly being the key words here. There IS a small bar magnet tucked into the mounting bracket of the gyro housing. I thought that was "normal' and figured it was to position the rotor in a "parking place" so as to allow the first pulse to get the rotor going and then the pulsing or reversing of the coil would begin. So that parts works repeatedly!

    Sounds to me like Q1 and Q2 do the pulsing or reversing but if you guys can't decide how they work, how do think I feel?

    I'm still trying to figure out what turns Q5 on and off. Which I can now see as a direct switch to the coil. I'm guessing it's Q4's collector when Q4 is turned on by Q3 the phototransistor's emitter. Am I close here?

    I understand that it's the reflection of the light coming off the bulb (in various amounts depending on the reflective surface of the rotor being black or brass.
    If that's true we know Q3 and Q4 both work 'cause they both turn on at least once and then turn off. We know Q5 works (if that's the only direct connection to the coil L1) cause the rotor will spin the 1st half turn.

    So it looks to me that the problem has to be in the "magic" of Q1 and Q3. Or R7, I haven't messed with that yet?

    Could the 6V bulb (instead of the specified 14V bulb) be too bright or too dim to operate Q3 correctly?

    Do the transistors have to be removed from the board to accurately check them with my DMM as I described in a PP? Or can I test them where they're soldered into the board and know if there's a short or an open through the transistor where it shouldn't be?

    The resistance of L1 is 7.5 ohm +/- 0.1 ohms on three coils I've checked. R-7 and R-10, I don't know yet. They're working on page 2 of the parts list
     
    Last edited: Apr 15, 2009
  13. user_88

    user_88 Member

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    At this point, it looks like there is nothing to indicate that any of the components are 'obviously' defective.

    I would suggest trying an iterative adjustment of R7.
    Turn the adjuster 1/4 turn one way, and turn on the power.
    If you don't see rotor rotation, turn it back a quarter turn, and reverse a quarter turn.
    If still no result, try for two quarter turns, or a half a turn variation.
    .... Before you start changing R7, take your DMM and measure between two of the R7 leads to get a benchmark or reference reading, so that you can have some assurance or confidence that you have returned to the original 'state' .... if necessary.... Maybe record the measurement and which particular leads they are between, before you change anything.

    Components, particularly electrolytic capacitors, can change value with age, heat, humidity. .... Maybe there is some oxidation that has built up internally on the R7 potentiometer.

    As additional insurance, I usually make a pair of small alignment marks on the adjuster and the component case ... a dot of paint, a marker pen, or something similar ought to work.

    If Q5 isn't turning off at the correct instant, you won't achieve rotation of the rotor.
     
  14. duffy

    duffy New Member

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    I recommend the same thing, but R7 is a 10, 15 or 20 turn pot, so turn it all the way one way (carefully, you can strip these if you force it), see how it spins, turn it all the way the other way (counting turns), check again, turn it back half way, check again, turn 1/4 of the way in the most favorable direction, check again, etc.

    User88 is right, it's an inverting pair, not a Darlington pair. It does not reverse, just amplifies and inverts the coil pulse.
     
  15. ZTatZAU

    ZTatZAU Member

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    Thanks guys!

    I tested the power transistor (Q5 in the drawing: 2N5191) in the green board with my DMM. (The transistor was still in the circuit, so my test may not be valid.) Each connection E-B & B-E, B-C & C-B, E-C & C-E, read the same (low resistance) in each connection. Does it sound like this transistor is definitely shorted?

    The corresponding transistor in the little gray board is a 2N5192. I removed that transistor and it tested good! All Specs on the two are the same except for the following...
    Vcbo Vceo Vceo (sus) hfe DC current drain

    2n5191 60 60 60 @Ic=1.5A Vce=2V: 25-100 @IC=4A Vce=2V: 10
    2n5192 80 80 80 @Ic=1.5A Vce=2V: 20-80 @IC=4A Vce=2V: 7
    Can these two be swapped out; one for the other here? Or should I be trying to find a new 2N5191?

    I also have the correct miniature bulb and a new tip for the solder sucker on the way. I'll put it all together add some tweaking on R-7 and see if it makes cake. ZT
     
  16. user_88

    user_88 Member

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    In-circuit testing of transistors can be difficult ....
    Usually, the procedure is to apply power to the circuit, and then look for the characteristic 0.7 V drop at the emitter-base junction, and also test voltage with respect to ground at the collector, and then the emitter, and maybe also the collector to emitter voltage.

    Also, you have to know if the transistor under test is sequenced to be on or off .... difficult to know unless you are familiar with the circuit operation.

    Testing an in-circuit transistor without applying power .... just with resistance and conductance is not something that I would try. Maybe it can be done, but usually, the resistivity readings are affected by adjacent circuit components.

    It's a lot more effort, but if you could de-solder Q5, and test it free ... out of the board, you might have better results determining if it is defective. If you have a known good substitute transistor you can compare the out of circuit resistive/conductive meter readings. Another possibility ... if you have an hfe test port on your DMM, make a set of jumper wires, with clips on one end, and use the bare wire ends to insert in the hfe section of the meter.
     
    Last edited: Apr 16, 2009
  17. ZTatZAU

    ZTatZAU Member

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    Thanks for the info 88!
    I'll pull it out while the board's detached. I do have an hfe port and will make up some leads. What readings will be displayed for a good or bad transistor?

    And how about the 2n5191 and the 2n5192... Swappable?
     
    Last edited: Apr 16, 2009
  18. user_88

    user_88 Member

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    The test with the hfe port will give you a nominal signal amplification gain ....
    If the number is anything reasonable, compared to the transistor data sheet specification, then the transistor is good. If the hfe is out of range, or zero, then the part is bad.
    The only thing to be careful about with the hfe test is to be sure that the transistor leads are connected to the right holes in the hfe port. Usually, you have to go to the data sheet... there should be a sketch or diagram of some sort ... make sure that you are looking at the transistor from the correct orientation .... either from the top or from the bottom, and then identify the c,b, and e leads.

    The 2N5192 should swap just fine with the 2N5191 .... It even has a higher C-E max voltage ... 80 vs. 60. The nominal data sheet DC gain is 20 for the 2N5192 ... so anything in that general range will be a good reading for the hfe test. ... Probably not much test current on the hfe test port, but it should show some sort of number, even 5 or 10.
     
    Last edited: Apr 16, 2009
  19. user_88

    user_88 Member

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  20. ZTatZAU

    ZTatZAU Member

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    Hi 88-

    I pulled out the 2N5191 and it too tested OK as follows...

    RESISTANCE READINGS OF A GOOD TRANSISTOR
    TEST LEAD CONNECTION DESIRED RESISTANCE
    (+ / -) NPN PNP
    ----------------------------------------------------------------------------------
    BASE - EMMITER LOW HIGH
    EMITTER - BASE HIGH LOW
    BASE - COLLECTOR LOW HIGH
    COLLECTOR - BASE HIGH LOW
    EMITTER - COLLECTOR HIGH HIGH
    COLLECTOR - EMITTER HIGH HIGH

    Can you tell me if this test any good?

    I hooked up some test leads and paperclips inserted in the ebc sockets of the DMM and did the hfe test.

    Gain was 2 on the 2N5191 I took out of the green board
    Gain was 10 on the 2N5192 from the old gray board. (USE THIS?)
    Is this the same as front-to-back ratio?

    The bigger bummer is the long gray pot seems to be open. Unless I'm measuring it wrong? Looks like three connects to the board but two of them are connected together. So from either of those two across to the third should measure the resistance? Open! And there seems to be almost no resistance to turning the adjusting screw. Should I feel a tiny little drag like a wiper? I tried the pot from the other old gray board but it's adjusting screw was frozen.

    It's marked "Helitrim 79PR50V". Do you know where I can get one of those? I'll have to try and pick one up on the way home ???

    ZT
     
  21. user_88

    user_88 Member

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    I would definitely go with the 2N5192 .... gain of 10.

    The 2N5191 sounds marginal .... if not defective.

    Re: the R7 trim pot .....
    There should be the full resistance value between the two outer or extreme terminals, which are not adjustable ... if I recall ..... and the resistance between one outer lead and the center lead should have a variable resistance .....
    If you are not getting a reasonable resistance measurement between any two leads, then something is wrong internally ... maybe it is burned out ... shorted.

    ...And that also opens up the question of whether something else in that current path is defective ..... Q1 maybe .....

    Not absolutely sure about the 79PR50V .... I will have to look it up someplace.

    I found a Helitrim 75PR10 which measures 10.0 ohms between outer terminals, with center terminal being variable pick-off.

    Guess that 79PR50 is a 50.0 ohm potentiometer.
    ... not sure about V letter... maybe V means made of ceramic ..
    If you can find any sort of trim pot ... similar in shape ....50 ohms .... it would be worth a try.
     
    Last edited: Apr 16, 2009

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