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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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    Why would R7 burn up?

    Just wondering why you tested R7 as defective .... that is .... what happened here?
    Maybe a current pulse jumped back from the motor coil, and blew out something else .... CR2 .... R5 .....Q1 ....Q2
     
  2. ZTatZAU

    ZTatZAU Member

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    88- Ooops! It's me again. My DMM was on a 2K scale and I had to go to 20K to get a reading.

    It must be a 10K (mid point). About 3K per turn. Started at 10.7 Had it down to 3.7 and up to 14.7 then both times back to starting point with the same turns and had 10.7 again. R-7 is fine! I'm gonna put this down for awhile and put it all together with the 2N5192 when the new bulb arrives.

    I'll let ya'll know what happens. ZT
     
  3. ZTatZAU

    ZTatZAU Member

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    Gyro Coil Circuit. My progress...

    Well I replaced the 14v lamp and the 2N5191 (2N5192) power transistor with a cross referenced NTE184. Put it all together and still only get a half a turn on the rotor when power is applied. Sequential adjustments of R7 din't help either. When power was applied Voltage across L1 started at 10-12 volts and then quickly dropped "steadily in 2 or 3 DMM updates" to zero. Removing the board, applying power and measuring across Q3 C-E Voltage varied between 6-12 volts with a flashlight sweep across the photo sensor. So it seems that Q3 is doing something??? Also, the mounting depth to which the lamp can be inserted is adjustable to some extent so I plan to try sliding the lamp in a bit farther to see if that might help.

    I've also resigned myself to better understanding this circuit. To that end and armed with a crash course in LTSpiceIV and Duffy's help with the marking of the bottom of the board, I set out to draw the schematic of the board I have. I've drawn the transistors as Duffy depicted them, but Duffy, "How did you know which transistors were PNP and NPN when you marked the bottom of the CB photo?"
    Anyway, here's what I came up with...

    [​IMG]

    I know it can probably be untwisted some and laid out more logically, but does my schematic look anywhere close to depicting the labelled circuit board below?

    [​IMG]

    No word back yet from the old timer with the paperwork but (knock on wood) I've got covers coming off these devices all over America to determine if there's a missing component on my board (where Duffy shows the ? and arrow) or if this space was not used on my version of the board. I'll let you know what I find out.

    Thanks! ZT
     
  4. dave

    Dave New Member

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  5. user_88

    user_88 Member

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    From your observation .... only half a turn on the rotor ... it looks like you are only getting current flow in one direction, through Q5.

    ... Trying to find the main current path when Q5 is abruptly turned off ....
    I am looking at the three diodes, D1,D2,D3, and the following series resistor, R1.

    The sum of the turn-on voltages of these three individual diodes equals the turn on voltage of the part marked CR1 in the parts list page 1.

    Can you check the integrity of D1, D2, D3, and R1?
    Are these parts good? Do the three diodes conduct in one direction only?
     
    Last edited: Apr 21, 2009
  6. duffy

    duffy New Member

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    I design circuits like that for a living. Are you giving it the full +14V? When you hit the sensor with the flashlight, did the rotor twitch at all? Also, check the black paint on the rotor, see if it's falking off or anything - that opto needs the highest possible contrast between the black and reflective parts. Matte finish is better than a gloss finish for this.
     
  7. ZTatZAU

    ZTatZAU Member

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    Hi 88 - Hi Duffy! Thanks for your replies!

    Does this mean my new schematic looks to be pretty accurate? If so, all component IDs referred to now are from the new schematic.

    Can you explain what is supposed to abrubtly turn off Q5 and why? Is Q5 ultimately controlled by what Q3 sees in terms of the amount of reflected light from the bright and dark rotor?

    Could there be a start up mode? Perhaps where L1 is intially controlled by R7 and Q1 and then, once the rotor is up to speed, control of L1 switches to the optics of Q3?

    And can you tell if Q5 simply turns off and on? Or, as you say "...only getting current flow in one direction...) actually reverses the polarity of L1 in this new schematic?

    88 - Diodes 1, 2, and 3, all check out good individually. Without removing them from the circuit, end to end they are open (low resistance) in both directions (through R1) the other way around. R1 checks good but reads 1.002 on the 2K scale. On any smaller scale it reads open. Does this sound like a 1K resistor? From the color bands I thought this was a 22 ohm- 5% (red-red-blk-gold) resistor. Can someone clear this up for me?

    Duffy - Yep! A nicely charged 12V with a 1.5a trickle charger attached. Wen flashing the opto sensor with the flashlight the board was dismounted from the coil. I was watching the DMM and didn't think to watch the rotor.

    I do have another gimble/rotor assembly with slightly nicer (smoother) bearings. I was going to swap it out after polishing up the brass section and then masking and painting a new coat of flat black paint on the other half. On the next go around, it sounds as though I should be pushing the little bulb in as far (as bright) as it can go. Eh Duffy?
     
  8. user_88

    user_88 Member

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    With power applied .... 14 V .... What is the voltage across each of the D1, D2, and D3 diodes?
    If the diodes are good, they should read approximately 0.7 v DC across each one.
    ... Also, what is the voltage across R1?

    Resistance check of diodes is not a valid test...

    red red black = 22Ω
     
    Last edited: Apr 21, 2009
  9. duffy

    duffy New Member

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    The schematic looks accurate, thanks for putting those values and callouts on there. R12 turns off Q5. There's nothing in the circuit that reverses the current flow to the coil. (I'm guessing the permanent magnet is the thing that does the trick for flipping the rotor back) Yes, there's a start-up circuit, then control switches to the optics of Q3.

    Yes, bright is probaby better. Turn the rotor slow and watch the voltage across R4. When it's on the "dark" side, Q3 should be mostly off and you would see a lower voltage across R4. When it's bright, Q3 is mostly on and you should see a higher voltage across R4.

    Also, remember this thing may be only suppose to work in the dark. It's really the sharpness of the transition from black to reflective that triggers it, not the absolute level. So try it in a dark room, see if that gets it going.
     
    Last edited: Apr 22, 2009
  10. user_88

    user_88 Member

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    The reverse current path may not be there .... I don't see any components with a sufficient amp rating to indicate that there is supposed to be a reverse current through the coil. .... It looked like a good idea though.

    Maybe if you have a sufficiently strong initial half turn rotation, there will be a flywheel effect to carry the rotor around for the second half.

    The function of the permanent magnet could be simply bring the rotor to rest at a particular point, or it could have something to do with completing the rotation.... not sure about this.

    As far as things to try out, maybe get a different power supply... something that has 14 or 15 v DC, and at least a 4 or 5 amp capability.

    Are you reasonably certain that the circuit board that you are using is built to operate on 14 V? I seem to recall that a popular aircraft power supply was specified to generate 28 V.
    If the board was designed to be run on 28 V, it might not spin up on 14 V.
    Without a verifiable circuit diagram, one that matches your circuit board, it is difficult to say for sure whether it is 14 V DC or 28 V DC.
    Some circumstantial evidence ...
    Aircraft electrical systems
     
    Last edited: Apr 22, 2009
  11. duffy

    duffy New Member

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    No, I thought about that for the same reasons you did - not getting enough of a kick, maybe the voltage is too low.

    But look at R2. How big would you say that is? Looks like a 1/2W to me. At 28V it would be have to burn off nearly a watt. Less than 1/4W at 14V. Gotta be 14V.
     
    Last edited: Apr 23, 2009
  12. user_88

    user_88 Member

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    What does resistor R1 in the photo above look like?
    I think it is a half watt 22Ω ceramic resistor... but could possibly be a 1 watt rating.... It is a different body color than the rest of the carbon resistors.

    In the original parts list, the diode which is in series with R1 is listed as an SR 1741.
    An SR 1741 is a 2.1 V forward voltage, 2 amp axial schottkey rectifier.... In the current photo above, the CR1 diode appears to be replaced by the D1, D2, and D3 series connected diodes.

    These parts might define a path to ground for a large current flux ... as the Q5 resistor shuts off. In effect, this would allow the coil current to change directions on the second half of the rotor spin, attracting or repelling the rotor magnet as required, and completing a full revolution of travel.

    With all the confusion ... different circuit boards and diagrams .... it is difficult to say for sure what is going on though.
     
  13. duffy

    duffy New Member

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    You're confused, I'm not. D1-3 are in there to match the voltage drops of the Q1-Q2 Darlington plus the base-emitter drop on Q4. Nothing reverses the current to the coil, as I've said before.
     
  14. ZTatZAU

    ZTatZAU Member

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    Today, I measured the Voltage across the diodes 1-3 and across R1 with 12 v battery power via 5A breaker to the circuit. R1 - 0.0v, R2 - 0.14v, R3 - 0.30 V. Across R1 - 0.34v. At one point, while validating these readings the power was "left on too long" and something heated up. I could smell it but I saw no smoke. (Remember this board once heated up another L1 coil and melted a gyro housing!) A visual inspection found the long circuit board trace about midway from the L1 connection to D1 on the labeled photo above, had cracked apart and lifted somewhat from the board. So I'm dead in the water till this is repaired.

    I looked online and found a few methods for printed circuit repairs... (conductor pens, foil tape, epoxy, lap joints, drilling, wire & solder bridges, etc) what the best fix here?

    88 - Here is a picture of R1 (outboard - just to the near side of the long gray R7). From the bottom Red-Red-Blk-Gld).

    [​IMG]

    The three diodes are at the near corner of the board numbered, from the corner - in, D3, D2, D1. I'm not sure they're all the same but D1 is marked 52 over 7220. Can you ID that diode?

    Also can you ID the transistors described on the parts list as:
    "SPS 3816, Motorola (selected) 48S48" ???
    The components are marked "M224 over 48S48" ???
    If so, how about "48S64 & 48S69" ???

    In the meantime I'll reread and try to understand your discussion on the circuit above.
    ZT
     
  15. user_88

    user_88 Member

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    One option to fixing a gap in a circuit trace is to buy a roll of de-soldering tape ... It comes in several widths, and roll lengths.
    This product is a sort of wire mesh .... usually on a tape-like roll, that is normally used with a soldering iron to absorb solder when de-soldering pins and other parts.

    The advantage of using a splice type link of the de-soldering tape is that is can carry quite a bit of current. The only preparation necessary is to clean a reasonable section of the circuit board trace, on each side of the break.

    Also, when soldering on a circuit board, or even when de-soldering anything, it is advisable to purchase a small bottle of liquid solder flux. Usually a brush is included in the bottle cap ....It is a resinous, liquid. Somehow, this liquid flux helps the solder to flow ... surface tension, or capillary action maybe. The only negative aspect, is that when you have completed the repair, it is sometimes necessary to use a flux cleaner spray to remove any excess solder flux ....

    The only caution here is to be meticulous when cleaning the solder spot areas .... often there is a protective coating of lacquer or something.
    Maybe get some fine sand paper or other abrasive material ... also maybe a spray or two or electrical contact cleaner to de-grease the area. Solder will not stick if there is any foreign matter .... dirt, dust .....

    If you think that the circuit trace will not be carrying any significant current, maybe just a signal type voltage, then you might try a liquid product ...
    not sure of the exact name ... liquid copper ... liquid nickel .... a metallic type liquid in a small bottle, which dries to a conductive ... plastic like consistency.

    How is your oscilloscope working?
    After you have repaired the L1-D1 break in the board trace, it might be interesting to see what kind of voltage is present across R1. However, in order for current to flow through R1, the three series diodes, D1, D2, and D3 have to be in good order.
    Can you read the part numbers of any of these diodes?
     
    Last edited: Apr 23, 2009
  16. ZTatZAU

    ZTatZAU Member

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    Thanks 88!

    I'll check out the de-soldering tape. I cannot see the ID on all three diodes. So I don't know if they are all the same. Diode D1 is marked "52 over 7220" (See my last post for component ID questions)

    ZT
     
  17. arunb

    arunb Member

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    where's my bear ??
     
  18. ZTatZAU

    ZTatZAU Member

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    Thanks again to all who've helped me and provided their advice, suggestions, and observations on this project. I have learned a lot since posting here.

    My last schematic was close but not quite right. Some more digging led to an old manual for another dated device by the same manufacturer that happened to use the exact same gyro and circuit as the unit I have. I've double checked each component and their values with my board and they all match perfectly. The missing component that Duffy spotted is only used on the 28v boards. So I now have THE CORRECT SCHEMATIC, a component diagram and parts list, and some pretty good info on theory of operation and troubleshooting the gyro. I've included copies below and ask that future references to ndividual components be as per the new correct schematic, diagram, and parts list. I apologize again for all the confusion but I am making progress.

    It turns out that... When power is first applied, the rotor element is positioned (via a parking magnet) so the non-reflective side is facing the photo transistor and light source. In this position Q1 is off. With Q1 off, Q2 is off and Q3, 6, and 7 are switched on. With Q7 conducting, initial motor torque is achieved by current flow through L1.

    As the rotor turns and the reflective side appears, Q1 turns on. With Q1 on, Q2 will be on and Q3, 6, and 7 will be off. This removes current from L1 but sufficient torque has been applied to bring the non-reflective side of the rotor back to the light source. At this point the cycle repeats and the rotor will continue to spin.

    (I understand how the switching is supposed to work but could use some help in understanding how Q1 and Q2 switching on and off turn Q3, 6 and 7 off and on! Any help on this would be appreciated!)

    And... As the rotor gains speed, a feedback voltage is devleoped across L1. (I don't understand much of this at all. Again any help appreciated!) This feedback voltage is rectified by CR4 and C3 and applied to the base of Q4 through trimmer R12. This voltage is mixed with a reference voltage at the base of Q4. Without a feed back voltage, Q4 is on causing Q5 to be off. With an increase in feedback, Q4 will switch off and Q5 will switch on. The switching of these two transistors will occur at a rate proportional to the rotor speed. When Q5 is on, it will tend to regulate the pulse width of the signal driving Q6. Thus the speed of the rotor (21000 rpm) is regulated by trimmer R12.

    The other good news is that I've also now had some success running the gyro! First thing that really helped was to ditch all the cables, battery, charger, clamps and clips, etc., and rig up a sturdy stable test platform with an on/off switch, a good regulated power supply and a 5A breaker. With the help of a friend, the board's damaged circuit trace was repaired nicely with a lead from a new resistor bridging the broken trace as well as another similar repair to another section of trace that showed evidence of heating/burning.

    I had already replaced Q7 (the old Q5) and now have also replaced CR3 (the old D1) and swapped out the 14v lamp with the specified 6v bulb (generously inserted for more light). We put it all back together and then turned it on...

    The gyro started running and steadily gained speed. Pretty high speed! I don't know if it went to 20K rpm or not but it really got going good. It ran a total of about 30 - 40 seconds. My ears aren't that great and my friend's are worse but I thought it sounded great! At one point I could hear a slight buzzing in the circuit (maybe when control of Q6 - Q7 passed to Q4 and Q5) as it seemed to try to self regulate the rotor speed. My friend was fixated on the cresendo of the lamp pulsing and getting brighter as the speed came up. Shortly thereafter it started to slow and eventually stopped. The bulb by then had become steady and was at full brightness. Nothing seemed to get hot, it just stopped. Subsequent attempts at powering up did nothing. Not even a 1/2 a turn on the rotor.

    Next morning I tried it again and voila! It ran again for 30 seconds or so. And now I find repeatedly that after waiting 15 - 20 minutes or longer between power ons, the gyro will run for various times (10 - 40 seconds) and at various speeds (i.e. slow - midspeed - high speed) before shutting down and stopping. Sometimes it seems that speed control actually switches to Q4and Q5 but other times it never seems to get to that point.

    So I am making progress and feel I've come a long way. Other than my two questions above (in bold and in parenthesese), I'm wondering if the above sounds like a capacitor is failing after it warms up-charges up or whatever, or more like a transistor or diode that warm up and fail??? Any suggestions on specific components to look at based on this new behavior and the new drawings?

    I'd also like some help in identifying the two transistors Q4 and Q5, marked with a (?) question mark. The two components are the same and marked M224 48S48 (mfg p/n). I've seen this "48S48" described in various parts lists and drawings as: Motorola SPS 3816 (Selected); Transistor, PNP, Small Signal; and Transistor MPS 6519. I'm guessing the MPS and SPS numbers refer to a Manufacturer's Purchase Specification" or "Special Purchase Specification" so maybe hard to find. Any help on finding equivalent components would be appreciated! ZT

    The component diagram, parts list, re-labelled photo of board, and the true schematic follow...

    [​IMG]

    [​IMG]

    [​IMG]

    PS. Just in case anyone would like to see it... Another friend who's helping with this project sent me the following picture of the simple little board in the schematic I originally posted.

    [​IMG]
     
  19. duffy

    duffy New Member

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    I came up with a parametric cross to a 2N3906 for Q4 and Q5. These are readily available.

    Put your finger on the transistors (especially Q7) and see if one of them is getting hot after the 30 seconds. Could be a cap or something, but a good guess is that it's a transistor...

    ...or that incandescent lamp.

    Transistors increase their beta (gain) with temperature. There are several ways this could cause problems.

    But my first guess would be the lamp itself. Incandescent lamps are notoriously non-linear. If the one you have is a substitute, it may glow so bright you lose the pulse transition off Q1, or settle at a different current and change the voltage on the emitter of Q6. A clue to this is that CR7 and its resistor in the dashed line circuit off to the side there are referenced to a "table". I suspect the table values depend on the characteristics of the lamp, and you have a mismatch which is causing the problem. Do you have that table?
     
  20. ZTatZAU

    ZTatZAU Member

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    Hi Duffy,

    What's a parametric cross? How do you come up with the 2N3906's? DId you ever hear of those MPS and SPS type "spec" numbers before? Was this "special marking" intended to keep some proprietary secret and/or parts source?

    The 6v lamp is new and on spec. Even the 24v version of the board uses the 6v lamp. Current to the lamp is kept at a nominal 5.2v by a zener diode. Not sure of the number, CR7 I think, but it's one of the components in the dashed circuit. Yep! I do have the table. It simply states these components were added after a certain "-2" build using the 6v lamp. My board's build does indeed include the components so I went back to the 6v bulb. I might play with moving the lamp in and out of the grommet mount but I think your hunch that the more light the better will be correct. I've been real paranoid about heat! (And this board did once melt down a gyro housing!) I've been watching for it but there is no apparant heat in either Q7 or L1 when the rotor shuts down. Power to the board is still good and the lamp is still on but nothing on L1; till everything "simmers down". Could Q6 or Q7 turn on at first and then go open when warmed up? Or more likely a diode or cap upstream of Q6 and 7 that is failing to turn them on after they're warmed up?

    And can you explain about how Q1 and Q2 being on or off does the opposite to Q3, 6 and 7? And how about the feedback thing with Q4 and 5? Can you shed some light on that?

    I'm not sure but I think I read that LTspice program can "simulate" a circuit drawn with the program. Does this mean that if I reproduce the circuit in LTspice with all the component values the program can display current flow for different states of say... Q1? Or show the voltage across individual components or the current in a branch of the circuit?
     
  21. user_88

    user_88 Member

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    For preliminary troubleshooting, you might try a spray or two of a cooling vapor such as the following product:
    http://www.electro-tech-online.com/custompdfs/2009/05/6403321_MSDS.pdf

    This might assist in the location of a defective component that is caused by excessive heat build up .... excess current flow.

    There are one or two other possibly defective components that would not become apparent by merely using a component cooling spray.

    Sometimes electrolytic capacitors fail due to aging, drying up of the internal electrolyte paste, or possibly voltage spikes which exceed their limits.

    One possible candidate that might be defective is capacitor C2, which is a 22 µF tantalum capacitor.

    Tantalum capacitors can be easily damaged, more so than other types of electrolytic capacitors.

    Capacitor C2 returns a feedback voltage signal, from the action of the Q1 photo-transistor, and consequently affects the switching speed of the Q1 transistor.

    If your oscilloscope is working, it might be instructive to observe the voltage signal at the positive terminal of C2, adjacent to R13, as you apply power to the circuit, and the rotor spins up, and then decelerates, as a result of something that is defective.

    If C2 is replaced with a new part, care should be taken to orient the + and - leads correctly when soldering the part into place, and to not allow excess voltage or sparks to contact the new capacitor prior to installation.

    I see that C1 is a 1 µF tantalum capacitor. You might check this one also.

    C5 ... 0.47 µF tantalum cap?
    C3 ... 0.1 µF tantalum?
     
    Last edited: May 3, 2009

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