What Is Probably Wrong With This Circuit?

Discussion in 'Technical Repair' started by abrogard, Dec 20, 2018.

  1. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    I got TR7 out and tested on the diode setting and got these results:

    Neg Pos Reading
    B E 1
    E B 378
    B C 1
    C B 365
    C E 1
    E C 1

    which I think is okay? So I'll move onto the next. It's quite tricky isn't it? Burnt some insulation on adjoining wires already. :)

    p.s. so i can't troubleshoot the thing by stabbing around the live board with my probe and reading voltages off around the transistors, etc. ? That doesn't work? Need a CRO for that maybe? Or that's just not the way it's done?

    TR6

    Neg Pos
    B E 1
    E B 367

    B C 1
    C B 368

    C E 1
    E C 1
     
    Last edited: Jan 7, 2019
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    You can troubleshoot by probing. If the series lamp energizes,there is still a problem.

    You can do voltage tests referenced to G or Ct, means the black probe is connected there and the red probe can check each of the previous points mentioned.

    Before you do any voltage tests, did you do the diode test on the two diodes?
     
  3. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Hello. I didn't realise you'd answered. I've made things worse.

    In my struggles to get the transistors out I've scratched through the green track and scratched through the black track in places.

    Fine hairline scratches. I didn't realise scratching through the black would reveal green beneath. I still don't understand that.

    I wasn't careful enough. I wasn't careful at all. I was concentrating on getting the things out. The bent over bits wouldn't come free of the board. No matter how much I 'wicked' them. So I was scratching in there trying to get a fine screwdrive blade under them to lever them up.

    I suppose/hope there's a chance it can be fixed? Drop of solder on a green track fixes that. But how to fix a black track. A drop of some non-conductor? What?

    I've got TR4 and TR5 out now. I was going to test them but I'm not sure which leg is which. Their cans have little lugs on them and I thought I'd google and find some explanation somewhere but though I found the datasheet for the 2swhatever it didn't explain that.

    I did the diode tests, yes. The results are up there somewhere in my posts, reporting results.

    So I might as well finish up this route now and get them out and test them. Then fix the board if fixable. Then reassemble and probe around the live board if that's a good way to find what's wrong.

    Unless it is magically fixed the lamp is going to light alright. It has always lit up from the beginning. As soon as I switch the organ on. I've reported that a couple of times and observed that I'd expect that in my ignorance because wouldn't there be current running if it's standing by for a keypress? But maybe you mean the lamp indicates more current than a 'standby' current. Or I'm totally wrong and there should be nothing....

    edit:
    Yes. the diode was post #54

    I said "

    Okay. Measured the diodes.

    The Black one on the 20K range shows 1 from the centre to either leg with the black lead on the centre.
    It shows -3 from centre to either leg with the red lead on the centre.
    It shows zero either way from outside leg to outside leg.

    The Red diode is the same except it shows about -6 with the red lead in the centre out to either leg."

    [​IMG]
     
    Last edited: Jan 8, 2019
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    I remember the diode test from that and it was a resistance check and not a forward/reverse check using the diode function.

    As far as the PCB runs being "accidentally cut" yes, they can be repaired by bridging the crack with a small piece of wire, or a simple solder bridge.
     
  5. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Sorry, Joe, I didn't see this post of yours until just now.

    Okay, I'll do the proper diode test. :)

    p.s. how to do a 'proper' diode test with a three leg diode?
     
    Last edited: Jan 10, 2019
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    One of the diodes is a common anode. In that case, with the positive lead on the anode, the cathodes when touched with the negative lead will read 0.6 to 0.8 on the other two legs when you touch the leg with the negative lead.

    The other diode is a common cathode. In that case the negative lead connected to the common point, the positive will read those voltages on each leg when the anode is touched with the positive lead.

    I suspect the middle would be the common point.

    In each case when you do the reversal of the leads, the voltage should read the same as if the leads were not connected to anything.
     
  7. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Well the diodes check out like this:
    Red diode: With Negative in the middle '1' to each leg.
    With Positive in the middle '400' to each leg.

    Black diode: With Negative in the middle '400' to each leg.
    With Positive in the middle '1' to each leg.

    which looks good to me except you say it should be 600 to 800. Could 400 be right for this brand of diode?

    I'm currently trying to test tr4 and tr5. I have TR4 out and I'm all hung up with it. It has a leg from which I read 400 to either of the other two legs. Doesn't matter which. So is that broke?

    And I can't figure out which is which leg. I can't be sure how it goes back in the board even if I knew which hole was which in the board.

    But I'm not even sure about that. You'd think it would be easy enough to follow a track, see where it goes, identify it on the schematic and there you are. It would be easy for you I guess. :)

    But it all goes wrong for me. Things don't seem to check out. I'll keep at it.

    I'm going to get one of those el cheapo dsm's is that what they call them? new fangled cro's. Someone told me they'd be better at reliably testing transistors.
     
  8. dendad

    Well-Known Member

    Feb 20, 2016
    2,209
    575
    Connections...
    TO3.png
    1 = Base
    2 = Emitter
    3 = Collector (case)
     
  9. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Yep, thanks Dendad. It's not the 1030 I'm hassling with but the 485. I couldn't find a diagram anywhere saying where the lug was supposed to be.

    But I've got photos of the board and I should be able to match up its orientation from that.

    And I've finally got organised and figured out how to use the dmm to find things on the board instead of trying just to eyeball it all.

    You find somewhere there should be a direct nil resistance connection and then stick one probe into the transistor hole (or whatever device) and prod around until you find the zero ohm connection, isn't it?

    So from that I've identified TR4 Emitter - goes straight to the Base of TR6 and the Collector goes straight to B+.

    Now I can put the old one in or a new one when they turn up.

    Testing TR4 on the diode testing setting I found one leg with about .4 V to either of the other two legs with Positive on that leg.

    When I go round it with Neg and prodding the other two with the Pos terminal I find only 1 leg with .4V

    I don't know what to make of that. Right or wrong?.

    That's not right, is it?
     
    Last edited: Jan 12, 2019
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    The 400 seems like a reasonable number, especially with the diodes being higher current and you are feeding it a 1 mA current from your multimeter.

    You can do a hfe test on the four transistors. That test is described in your meter's user manual.

    You may have to construct some wires to complete that task.

    Is this similar to your 2SC485?
    2SC485.jpg
     
    Last edited: Jan 12, 2019
  11. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Yes, it's like that. I'll do the hfe as soon as I can. got a few chores to do first... :)
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    Do you remember which transistor measured the same (0.4 V) in both directions?
     
  13. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0

    It was TR4. I just measured it again. Using diode measurement facility.
    From Negative to Positive: e-b 1
    e-c 1
    c-b 400
    c-e 1
    b-e 1
    b-c 1
    From Positive to Negative: e-b 1
    e-c 1
    b-e 400
    b-c 400
    c-b 1
    c-e 1

    Haven't tried the dmm transistor hfe testing yet. I'll do it now..

    edit: well I just did it and it gave me 0. zero.
     
  14. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    Ok....Let's put that one aside. Just so you know do the hfe test on another transistor, like the output ones, the 2SC1030.

    The smaller transistors, you can plug right into the hfe test points. The driver and the power amplifiers you can't.

    Here is a tutorial for testing using the diode functions, just remember when they say OL (overload) your meter will read 1.

    here is the datasheet for that transistor.
     
    Last edited: Jan 12, 2019
  15. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Okay.

    For TR7:
    Hfe: zero
    Neg to Pos
    BE 377
    BC 1
    CE 1
    Pos to Neg
    BE 1
    BC 1
    CE 1

    For TR6
    Hfe: zero
    Neg to Pos
    BE 315
    BC 1
    CE 1
    Pos to Neg
    BE 1
    BC 1
    CE 1

    I found another transistor in my toolbox. A new one. I used it to test if the DMM was working okay.
    It was a 2N3055 and I got these results:
    Hfe 001
    Neg to Pos
    BE 445
    BC 1
    CE 443
    Pos to Neg
    BE 1
    BC 1
    CE 1

    What does all that mean, I wonder, if it means anything at all?

    p.s. that's a good job you do on your images, Joe. What do you use - Photoshop?
     
  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    Those are not my images on that website.

    How are you connecting TR6 and TR7 to the hfe connector? Take a picture of either of those connected to the NPN sockets on the HFE connector. You would need to make up some wires that fit and attached them to the proper pins on the transistor. Put the 2SC485 (TR4) into the hfe test connector in the NPN pins. Then do the same for the 2SA485 (TR5) using the PNP pins. I suspect there would be two or more digits in the HFE readings.

    When you say negative to positive, you also state which two pins your testing. For example, you have BE and then what I assume is the forward voltage on that emitter to base connection.

    you have only to do six tests .... on each transistor. I read that Neg to Pos and then B to E as NEG B, POS E. NPN's the E(-) B(+) should show the diode forward voltage. On PNPs the meter leads are reversed.

    E (-) B (+)
    B (+) C (-)
    E (+) B (-)
    B (-) C (+)
    E(-) C (+)
    E(+) C (-)

    I asked for the resistance tests (2k scale) because the reverse on those transistors could be higher than 1V, and less than the 2V on that diode test scale.

    That 2n3055 has a problem ...
     
    Last edited: Jan 14, 2019
  17. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    Yep, I've been finding my way with this testing and recording of it.

    You'll see I finished up with just six tests - plus the hfe.
    I don't understand the phrase 'forward voltage'. I'd guess it means from positive to negative.

    The way I've listed it works like this:

    Neg to Pos Means the negative lead will be on the leg listed on the left of the page.

    So Neg to Pos
    BE means negative lead on Base, positive on emitter.

    And
    Neg to Pos
    BE
    BC
    CE
    means negative lead on Base, pos on emitter followed by negative on Base positive on collector followed by Neg on Collector positive on enitter.

    I think that's pretty clear but I'm happy to list them any way you like.

    I made up leads to connect the 1030's. I'll hook them back up and photo them for you. It's because I wondered how good my lead make up was that I measured that 2n3055.

    The main worry for me was that the wires into the dmm are loose and I think just make a contact at the bottom of the hole, not clamped to metal sides at all, I think.

    But it seemed to be working alright.

    I'll take a pic.

    Here it is on imgur: [​IMG]

    well it wouldn't do that. just gave me a crossed out image icon. Perhaps I've used my quota. Here's a link:
    https://imgur.com/QMgrzN3
     
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    I cant tell if your base connection is in the NPN or PNP connection. It looks like your using solid bare copper wire. The connection when you inserted the wire was firm, not loose?

    You can upload your images right here at this site using the upload a file button and choosing the file. QMgrzN3.jpg
     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,958
    1,759
    My concern with the hfe test is you getting zeros.

    Both TR6 and TR7 are bad, based on your clarification of the connections and yes, that could cause the fuse to blow. You will need to check TR4 and TR5 as well.

    Your 2n3055 is also bad, primarily because of the CE reading.

    When you test a good transistor, the hfe could be 35 and up on your meter, depending on how it tests hfe. It certainly will not be zero or 1.

    On edit, I seen where you tested TR4 and it too looks bad. You can still confirm that with the hfe test (it should read 000 as well). TR5 I didn't see any test information about that one.
     
    Last edited: Jan 14, 2019
  20. abrogard

    Thread Starter Member

    Dec 20, 2018
    60
    0
    I'm not careful enough, eh? Should have made a better pic. It is in the NPN, in fact. I'm actually using MIG wire.
    I didn't test TR5 yet. Sort of getting all this information in about exactly how to test and whatnot.

    The 2n3055 is completely new. Never been used.

    Straight out of the packet onto the bench for that test. I'll have to find where I got it from and not do business with them again.

    It harks back to one of my aborted attempts to get into electronics by building something from a web thing. Bought all the bits and never built it and now I don't know what it was supposed to be. :)

    TR4 should read Hfe 000 for 'bad'? Or for 'good' ?

    So could well be that four transistors are bad?. I doubt that happened all at once in the normal course of events so would it have happened by me switching the thing on after it got broke and made the noise? Have I probably zapped transistors?

    And should I be safer now in that respect with this series lamp?

    I can't do anything tonight. Tomorrow. Your help is good, Joe. Thanks. :)
     
Loading...