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  1. #106
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    Quote Originally Posted by BaronJ View Post
    Good Morning Steve,

    That makes more sense ! A transistor version of a thyristor.

    The three capacitors combine to provide the delay. If D1 is OK and I think it is, then the two transistors are suspect. I think one has gone short. I would replace both with modern silicon ones. BCxxx series will work OK. You may also find that the timing constant changes as well. I'll do more analysis later. Honey Do time !

    Attachment 385640
    A little animated picture should help !
    Thanks John.
    Sorry, missed this post for a bit there as it came through while I was creating my next one!!

    Just refreshed my dim memory on thyristors.
    That would make sense - in that a thyristor 'latches' once its triggered, and you need to remove the supply to turn it off.
    Fits with the behaviour that I've observed.

    Time to refit those caps and diode and make sure I haven't buggered it completely before moving onto replacing the transistors.
    Hopefully I've got sufficient transistors in my stash...

    Steve

  2. #107
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    Soldered the original caps and diode back in and tested. All good - still ticking so I haven't broken it any worse.
    Replaced Q2 with a PN100. Ticks a bit quicker.

    Finding a PNP transistor to replace Q1 might be more of a challenge....

    Edit: Managed to rat one out of my youngest daughter's electronics experimenter kit from primary school days. Must be close to 20 years since that box has been opened to actually get anything out!
    Replaced Q1 - unfortunately no change.
    Still ticking....

    Steve

  3. #108
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    Hi Steve,

    That yellow cap is 0.2uf. For the time being those three transistors are favourite for being faulty / low gain.

    I'll redraw the circuit the way I think it should be. The relay driver transistor might be duff, Emitter base short, which means that the other two transistors are doing the switching of the relay. I can't make out where R5 goes. I can see that one end goes to a transistor pin, from your diagram it goes to ground.

    Can you confirm that I've got the transistors marked correctly please ?

    TX100 board.png


    Edited to add: Those transistors cannot be replaced they appear to be germanium types. You would get away with replacing all of them with silicon ones. Types BC413/4C for NPN and BC415/6C for PNP. If you have a choice, go for highest Hfe and voltage rating. There are dozens of equivalents.

    This is the data sheet for them.
    bc413_bc414_bc415_bc416.pdf

    TX100.png

    As promised a new circuit diagram this should be correct now. Please check it. Thanks.
    Best Regards:
    Baron J.

  4. #109
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    Diagram looks perfect John. Really appreciate your help with this - thank you.
    I've double checked - R4 and R5 connect together, and that junction goes to Q2 emitter like you've drawn.
    The other end of R5 goes to C4 and Q3 emitter.

    EDIT:
    On looking again, I think there's an issue (but likely I'm just not reading the diagram correctly).

    This is how I was referring to the 3 transistors:
    CB_front Qs.JPG

    Q1 is KC307 PNP
    Q2 is KC507 NPN
    Q3 is KC509 NPN

    I think the LH transistor in your diagram should be Q1 (PNP) but you've got it as NPN.

    I'll see if I can chase up some BC413/415's today.
    I used a BC558B to replace Q1, and a PN100 to replace Q2 last night (which resulted in no behaviour change).

    Out of interest I played with the trim pot (as compared to the main delay adjustment pot). It gives a very coarse adjustment on timing.

    Steve

  5. #110
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    I thought my local Jaycar might have those BC413/415's but no luck.

    Only really PN100 and PN200 general transistors, and some other varieties of BCxxx.
    Would the BC547 and BC557 be suitable? Otherwise I'll order some 413/415's online.

    Jaycar_transistors_PN100-200.JPG

    Jaycar_transistors.JPG


    I was reading this thread https://electronics.stackexchange.co...wo-transistors earlier about it being impractical to just grab a pair of NPN and PNP transistors and make a psueudo thyristor out of them as needed to be hand picked to match or wouldn't work. The practical solution was to add a couple of resistors or a resistor + diode.

    Just theorising (and probably proving I have NFI what I'm on about) - with the behaviour that I'm seeing where the circuit is working but not latching it indicates that all 3 transistors are turning on OK (since the relay is turned on), but there's not enough current flowing in the second one of that "thyristor" pair to keep the first turned on once the trigger voltage on the base of the first is removed?

    If that's the case then there's likely some component value tuning required around Q1-Q2

    Steve

  6. #111
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    Good Morning Steve,

    Any of the BC547 and BC557 complementary pairs with similar gains should work just fine. Just watch the collector voltage. Some are rated quite low, only 25 volts.

    Just one thing I noticed this morning, actually I should have picked up on before, the diode feeding the base will probably be a germanium device as well, so its voltage drop will be only 0.3 volts. It might not matter as its only triggering the transistor pair.

    If you have a scope or an analog VVM you should be able to see the rising voltage on the timing capacitors C2, C3, C4. A digital meter probably won't respond fast enough for you to see the voltage rise. Looking at Q3 base should show the trigger voltage suddenly appear.

    Now if Q3 has a base to emitter short there could be enough current to operate the relay. The same may be true of a collector to base short.

    Thanks for the picture showing the item ID's.
    Best Regards:
    Baron J.

  7. #112
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    Thanks John. Jaycar has a mixed "prototype" transistor pack which has a selection including the BC547/557 so I'll grab one of those packs tomorrow.

    Just pulled Q3 and put the meter over it.
    Q3 emitter to base resistance is only 0.9k
    In comparison Q2 emitter to base is 36M

    If I put the meter on diode test mode,
    Q3 emitter to base is 0.37v - I get the same regardless of polarity which doesn't seem right
    Q2 emitter to base is 0.6v one way, and 0.9 (same as open circuit) the other.

    Does that suggest Q3 has seen better days?

    I do actually have an old scope - 20MHz Hitachi V-212. I pulled it out last night to try and remember how to use it, and I'm not convinced its working properly. I got it many years back when I built a few guitar valve amps, but I'm so rusty I was searching online for "how to drive an analog scope" tutorials!!
    My perceived issues with it could be 100% operator..

    Steve

  8. #113
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    Hi Steve,

    I think that you have it cracked

    Those figures are typical of failed germanium junction transistors. The scope should have a probe test reference connection on the front panel. Usually a 1 volt square wave, used for setting the probe loading. That will show if the scope is capable of doing what you want.

    If you connect the scope probe between ground and the top of C3, C4 you should see the voltage rise when it is switched on.
    Best Regards:
    Baron J.

  9. #114
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    Default The joy of having a TOS.... ( FNGJ32 mill )

    Quote Originally Posted by BaronJ View Post
    Hi Steve,

    I think that you have it cracked

    Those figures are typical of failed germanium junction transistors. The scope should have a probe test reference connection on the front panel. Usually a 1 volt square wave, used for setting the probe loading. That will show if the scope is capable of doing what you want.

    If you connect the scope probe between ground and the top of C3, C4 you should see the voltage rise when it is switched on.
    Its a 0.5v reference on this one. Finally managed to pull and twist the right knobs to get it displaying that OK. Doesn't help that one channel has the 5x gain knob in the middle of the volts/div broken off....

    Put it on C4 and ground, and can definitely see the voltage rise, then sharply drop, then rise again. Adjusting the time pot varies the period

    If I put the probe on Q2 I get this weird form displaying - looks like 3 helical wires rotating slowly...

    Had to look out the window for a bit after that - thankfully it was a beautiful sunset...



    Steve

  10. #115
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    Hi Steve,

    Lovely picture, you don't often get to see a sunset like that.

    The waveform that you describe is exactly what you should see. If you have the hang of the voltage reading, between the bottom of the trace and the highest point, that will tell you how high the voltage gets before it resets.

    You will need to tell me which pin ! You should see the voltage rise on the output (cathode) of D1. That point should be the base to collector between Q1 and Q2. The base of Q3 should suddenly jump up and the relay should operate.
    Best Regards:
    Baron J.

  11. #116
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    Default The joy of having a TOS.... ( FNGJ32 mill )

    Quote Originally Posted by BaronJ View Post
    Hi Steve,

    Lovely picture, you don't often get to see a sunset like that.

    ...
    We're very lucky where we are. Edge of a semi-rural town, and about 200m vacant land between us and the nearest westerly neighbour.
    We call it our "sunset window".
    Here's tonight's one:


    Back to this damn timer though...

    Replaced the transistors today with BC547/BC557. Would you believe it, no change. Its still ticking....

    I pulled the good one out of the mill to compare. I probed the same point on C3 where I could see the slow voltage rise on the dodgy one - and all I got on the scope was the wiggly rope instead of a nice trace.
    Put the multimeter on the good one - approx 30v across C1
    Check the dodgy one - only 21v

    Low input voltage is never a good sign.

    Next I checked the AC output from the transformer. Roughly the same on both - just under 25v
    Does that mean there's a diode thats gone open circuit?

    Steve

  12. #117
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    Across C1...same scope settings for both readings

    Faulty timer


    Good one


    Steve

  13. #118
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    Default The joy of having a TOS.... ( FNGJ32 mill )

    Next I fed the faulty timer some nice 30vdc from my benchtop supply.
    Connected pins 3 + 4, and click. It latched.

    Replaced the rectifier diodes - no change.

    Only one thing left, C1.
    Replaced that with a 47uF electrolytic I had in my box (one removed had 20/35 on it), and its fixed!!!!!!

    Here's the culprit:



    Steve
    Last edited by OxxAndBert; 10th Apr 2020 at 12:24 AM. Reason: Had incorrect replacement cap size

  14. #119
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    Hi Steve,

    20 uf @ 35 volts. Unbelievable ! The capacitor breaking down at a fixed voltage.

    They usually just explode or don't work at all. Did you replace all the other components ?

    Anyway congratulations are in order
    Best Regards:
    Baron J.

  15. #120
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    Default The joy of having a TOS.... ( FNGJ32 mill )

    Thanks John - I honestly couldn't (and wouldn't) have done it without your input so are in order for you too!!
    With setting up the VFD control the way I have using the original contactors I actually need this timer - so its not just a spare.

    Components that I've ended up replacing are: the relay, 3 transistors, the 4 rectifier diodes and C1.

    My better half has been very tolerant of the electronics workbench that has mushroomed on her bench, but this morning's job is to finish up and pack it all away and clean up before I outlive my welcome!!



    Then I can get back to finishing the VFD install.....

    Steve

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