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Amplifier Repair - electrolytic capacitors expose metal top?

Discussion in 'Troubleshooting and Repair' started by looxuser, Jan 21, 2016.

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  1. looxuser

    looxuser

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    Dec 28, 2015
    Hi I am new to electronics fault finding, why when i touch dvm probs to ground and the top exposed metal part of an electrolytic can i see a voltage please?
     
  2. Gryd3

    Gryd3

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    875
    Jun 25, 2014
    What voltage do you see? The case should not be tied to either pin, the voltage you are seeing could be caused simply by the average charge residing in the capacitor. There should be no current flow.



    http://www.teapo.com/WebSiteFile/Download/Specification.pdf
     
  3. HellasTechn

    HellasTechn

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    Apr 14, 2013
    That depends on the circuit. I think that normally you should not
     
  4. looxuser

    looxuser

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    Dec 28, 2015
    Interesting, views thanks, the capacitors are 6600uf 50v i think and they are smoothing capacitors I suspect just after a full wave rectifier on an ac supply to an audio amp. There are two one reads about 44v and the other 0.04v?
     
  5. looxuser

    looxuser

    77
    2
    Dec 28, 2015
    I should have said both are the same, never have even thought to test to ground in that way as my knowledge said that was not tied to ground, I was just curious and surprised to see it. One interesting thing i read is that the material used to bond these to the pcb can become conductive and i thought as the stuff is plastered around the base that was what was happening but the bonding material doesn't seem conductive when i tested it with a DVM.
     
  6. davenn

    davenn Moderator

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    1,986
    Sep 5, 2009
    then it is possible that if they are large smoothing caps like that, that the metal can is connected... some styles have that option
    we would need to see the cap, and how it is soldered to the board

    photos would help
     
    Gryd3 likes this.
  7. looxuser

    looxuser

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    Dec 28, 2015
    i'll remove them from the pcb and post some photos. On testing elsewhere earlier oddly quite a number of other smaller caps read voltages too
     
  8. Gryd3

    Gryd3

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    875
    Jun 25, 2014
    I would not call that odd when you consider how they work or what they are doing.
    Remember that measuring yourself from a ground potential may result in a voltage.
    If the capacitor's can is (by design) tied to one of the terminals, than this is to be expected. If the can is left floating, I would expect this as well, however, there should be no current from the can to ground in this case.
    If the Capcitor's negative terminal is at 3V, and the positive terminal is at 12V, I would most certainly expect to read something on the can.

    Correct me if I'm wrong in this case, but if the entire capacitor is at an elevated potential... it would make sense to see a voltage on it. If one of the legs is ground, then it's much less likely, but still quite possible.

    (Would this relate to leakage in the capacitor?)
     
  9. looxuser

    looxuser

    77
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    Dec 28, 2015
    Guys, So here is a snapshot of an amp circuit board i'm trying to repair. Started by testing various voltages around the board, I'm supposed to have -31volts (-VFL whatever that stands for) feeding another control board pcb but measure 28v on my dvm. Other board voltages seem ok as per diagram.
    Theres 33.3v AC on the legs of fusible resistor FR501 (82ohm not 56 shown) direct from the power supply transformer. I assume diode D511 is a half wave rectifier and C509 smoothing capacitor. My assumption then is that I should have 33.3v DC at the collector of Q506 & Q507 and that these are being used as switches. This is all concerned with power regulation I understand. I also assume that resistors 510 to 512 must be ok as i would not get any reading -VFL through these PNP transistors. Not sure of the function of C510 and the Zener. If somebody could kindly help explain the function of this part of the circuit that might help a novice like me learn too. Many thanks
    Also a snapshot of the corresponding schematic.

    upload_2016-1-21_23-45-33.png upload_2016-1-21_23-46-45.png
     

    Attached Files:

  10. looxuser

    looxuser

    77
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    Dec 28, 2015
    HI Gryd3, yes i thought leakage too but does that mean the cap is necessarily bad. The amplifier circuit board is well over 20 years old by the way so leakage could well be a factor. Thanks for the link by the way very informative.
     
    Last edited: Jan 22, 2016
  11. 73's de Edd

    73's de Edd

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    Aug 21, 2015
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    Sir looxuser . . . . . .

    Are you having display problems on that unit . . . . . .as
    -VFL is a negative supply voltage for florescent display power for your unit.
    Referring back to the mentioned 33VAC, it feeds into D511 to create a NEGATIVE supply that then passes into a series pass regulator composed of Q506 and Q507 and their reference voltage to be regulated to, is being determined by the presence of 30 volt zener diode D512.
    There is an earlier input filter capacitor of C509 and DO note its polarity, as well as an output filter as C510
    Check that output voltage over at pin 10 of the connector, and if it is approx minus 30, all is well in that circuitry.
    BTW use the D512 zeners cathode connection and / or its associated buss, as negative meter probes ground reference for metering throughout your tests.

    Pass us the make and model of this unit, if further help is needed. Since, a lot of what you might be needing is not being shown or is being too small to make out.

    73s de Edd




    .
     
    Last edited: Jan 22, 2016
  12. looxuser

    looxuser

    77
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    Dec 28, 2015
    Hi 73's de Edd, no the main display is fine. given what you shared that makes sense to me. I'll post full schematic of the unit later as the fault is very low volume on the amp and I've nearly exhausted all checks within my capacity. Thanks for your help
     
  13. looxuser

    looxuser

    77
    2
    Dec 28, 2015
    Hi 73's de Edd, I was going to share what I'd done and tested so far but on reviewing the circuit diagram I think i may be onto something. Before i explain i cannot upload the service manual it to large can i send it to you somehow? This evening I tested the board further and found that at IC 801 the electronic volume control I had 12.2V at pin 16 (supposed to be 11v and a fraction of a volt at pin 1 (supposed to be -7v). Given this -7v also supplies the buffer amp is suspect those components coming off the -8.5v feed namely R810, D805 (switching diode 1ss133) and Cap 826. I couldn't get to the leads of the cap but tested to GND either side of R810 and got -8.4v and on the output of the resistor -7.5v but nothing at pin 1 of IC801. What are your thoughts on this please? Given the IC801 has seen 12.2v instead of 11v do you think this could have damaged the IC and also if Cap 826 or D805 are not performing as they should could this also cause the lack of -7v at pin 1 IC801 and also at the buffer amp resulting in my low volume issue. I also found a faulty transistor Q510 in the over current detect circuit and replaced that with no improvement. Kind Regards, Andy
     
  14. davenn

    davenn Moderator

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    1,986
    Sep 5, 2009
    upload it to a filesharing site and give a link to it
    make sure it's a site that we do not have to log into
     
    Last edited: Jan 26, 2016
  15. looxuser

    looxuser

    77
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    Dec 28, 2015
    Hello 73's de Edd & other forum members.

    Guys please ignore my posting from Monday this is incorrect. I have now reached the limits of my ability on this so would really appreciate some help with troubleshooting this amp. I have uploaded the circuit diagram to http://www.filedropper.com/fxwz95smaiwaen1. So lets start at the beginning with the faults and my thought process from there, so the volume is very low and whilst the volume seems to control up and down louder and quieter via rotary volume control it never reaches anything higher than a low sound output. Also when the source of the sound is selected on the front panel buttons its hit and miss whether they work and in most cases you can hit say the CD source and you hear the tuner, play a cd and you can hear the cd over the tuner and various combinations of the same, phone, video etc, never seems to be the same twice. So my logic told me that because these were not switching cleanly that the power supply to the ics which control this may also be supplied to the amplification sections which would effect the volume namely the input attenuator IC603 & buffer amp IC803 & IC804 and Power Amp IC851 (I replaced this as a matter of course so I know it to be good)

    Thinking about the operation logic here, the inputs come in via IC601 & IC602 and are then passed through an amplifier (INPUT ATTENUATOR) IC603. These amplified signals then going to the electronic Graphic Equalizer on the front board (I know this front board to be good without faults) At the GEQ you can calibrate the input source so that on switching it is the same volume as the other sources. Interestingly even though theres low volume there are no GEQ bars moving on the display. I'd have thought they should even at low volume. Also theres a very loud speaker pop when i press the calibration button. From the GEQ these then route to the surround IC701 and from that go to the electronic volume control IC driven from and encoder. This volume IC then drives to the Buffer Amp (which i read somewhere typically delivers 30db amplification) and to IC851 the power amp. Theres also the over current detect circuit which I suspected instructed the power amp to reduce power in an overcurrent situation. I found a duff transistor at Q510 and replaced it without any improvement. So common to both the source switch IC's and the amplifiers is the -8.5v (-Vcc) and +9v (+Vcc) supplies which i've tested at -8.4v and +8.9v respectively. A fresh set of more experienced eyes hopefully will give me the breakthrough on this as i've now hit a blank after checking these voltages around the board and on some components & IC pins. Kind Regards, Andy
     
  16. looxuser

    looxuser

    77
    2
    Dec 28, 2015
    Guys, in addition to the above I see that IC604 (Functional attenuator) seems to take digital signals from the front panel to drive the actions of functional switching IC's 601 & 602. Also as the Microphone socket has its own amp separate to the rest so in theory you would expect when using a MIC some response on the GEQ digital display and indeed correct volume level. If then that was the case would it be fair then to direct attention to the INPUT & FUNCTIONAL INPUT Attenuators or do you think the problem is something obvious. Worth giving a try do you think as i'd have to buy a MIC?
     
  17. looxuser

    looxuser

    77
    2
    Dec 28, 2015
    Evening all, well i tried a mic and guess what a response shown on the GEQ display which suggests to me this could be input attenuator related. Problem is though there's no sound at all from the amp now as if what was going wrong has finally given up completely. I could use some advice please on how to measure around the circuitry for music playing through various stages with a DVM, if that is even possible?
     
  18. 73's de Edd

    73's de Edd

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    1,364
    Aug 21, 2015
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    Sir looxuser . . . . .


    Looks like your 13 Meggle-hurtz of compressed data has just now, finally made it down the pipeline .
    I now see that you must have a $ Kilobuck $ system if being totally complete.
    The separation of all of the different stacked units would make it hard to work on, IF they just have small short length custom flat cabling interconneting between the different units.
    BUT . . . the main power unit seems to have all of the control that we will initially need.
    And that unit will be the primary one of interest, since it has the Audio Output Amp circuitry in it.

    The backs of the units seem to have ~ 10 wire separate interconnecting custom flat cable daisy chained between them and some

    of them are depending upon +12VDC and some power from a lower level AC voltage supplied by two of the connections from the

    AC supply, incoming from the PT1 power transformer in the main unit.

    The first questiion now, is if this is your unit /system from day and that you are completely familiar with it and have been using it for years and it just stopped working now ? . . . . . Answer.
    OR is this something new to you /used and possibly not even working to begin with, and you COULD be fooled by a simple switch or function being in a wrong position, like tape position, or in monitor position or even a control mis setting related to all of the complexity added by the Dolby, and Surrouind Sound aspects of the unit versus direct (bypassed) ?. . . . Answer.

    You mentioned hearing a febble FM/AM tuner sound, as well as the mike input lighting up the scales of the Equalizer display.
    But as I said, in order to troubleshoot the main unit and therewith, have its top and bottom covers off for access

    /and/readings, any use of the other Tuner and Eq units seems unlikely* . . .am I right ? ( * Due to the short 10 connection custom

    flat cable being used . . . . . Answer

    You say:
    " I see that IC604 (Functional attenuator) seems to take digital signals from the front panel to drive the actions of functional switching IC's 601 & 602."

    Yes, your units design uses a continuous and updating series digital stream for control functions, or in a few cases of MC4051 or MC 4052 CMOS being used in switching cases , they just use a logic lo/hi from the u/p to toggle their states.


    Now, surely I can't expect you to pull out a HP combined audio osc and dist analyzer, along with a companion Tek or Hoooolet

    Plastered triggered scope . . . . . so this testing technique will be utilizing KISS and Mac Guyver technology, for your convenience / test equipment deficiencies..

    So the game plan is to troubleshoot by signal injection in either using some of the multiplexed "whine" generated by the IC151

    microprocessor , or solder tack together a voltage divider and impedance matching network to use the VAC feeding between the

    units, or the use of a VCR or CD player that we can tap off its output audio with use of a shielded patch cord with RCA connectors and use anywhere from a .01 thru 1 ufd poly or ceraamic capacitor for DC blocking and AC coupling into the amp.
    Now answers pls . . . . as well as your possibly having the simplest good audio source, as was just mentioned in the very last option.

    I placed the primary info that we will be consulting in the attached mark up.

    The STK power amp is on the far right since the schematic was erroneously designating pin 8 as one of its inputs . . . . which is
    supposed to be 18 and now is properly marked on the block diagram.
    Also, I "flipped" some of the switches to the proper positions that we will be working with.

    Starting at the left GREEN arrow, it is showing one channels tuner audio path taken all the way to the input of the STK AF power module.

    Talk to me and fill me in on the ?'s.

    Tech Ref : (Clickee-Clickee)

    [​IMG]


    Dat unit . . . . itsalookalikeadis . . . . . on its frontal knobs and buttons markings:

    [​IMG]

    73's de Edd



    .
     
  19. looxuser

    looxuser

    77
    2
    Dec 28, 2015
    Hi 73's de Edd, thanks for responding, to answer your questions in turn then...

    Yes its my hifi from the 90's from new. Stopped using it years ago and pulled it out of the loft in say 2007 to find it had this fault and put it away again. Never got round to sorting it until now. The system can function with just the amp and GEQ (one unit) and the tuner attached, it doesn't need any of the other separates such as cd etc. The unit must be over twenty years old but it has sentimental value to me. I read somewhere that loft storage (excessive temps and no power for extended periods) can dry out the electrolytic caps and give rise to problems.

    With just the tuner connected by the ribbon is how ive been using and testing and like i say until yesterday it was producing the low volume and now nothing. There is some crackling on the speakers when you tamper with the speaker wire terminals so theres some power at them it would seem.

    So to confirm all sources were a feeble very low volume, not just the tuner. And non of them were able to light the GEQ bars, except my test yesterday with a MIC which of course has its own Pre Amp.

    I am able to see both the top (component) side & bottom (solder track) sides of the board for testing. The flat cable of the tuner can be used with the board in this case as a sound generator if thats needed, equally the cd could perform the same function. the ribbon cables are long enough to facilitate this.

    My father has a scope I can borrow and a simple signal generator he made many years ago.

    Yes it appears there were a number of STK amp IC's used in construction, the idents on the pinouts show the 22pin version but mine is the 18 pin version, the pin numbering shown in brackets alongside the 22 pin idents.

    Great work on the logic diagram and showing me the Tuner LCH path btw. This clearly took some time and I'm greatly appreciative thanks.

    So i understand we'll be testing left to right on that diagram? Ive generally found a ground to ground the DVM probe and then moved the other probe around the board to measure voltages at various components. I'm concerned about making sure I don't damage anything so if in your guidance you can tell me where to put probes etc that would also be a great assistance.

    Kind Regards, Andy
     
    Arouse1973 likes this.
  20. looxuser

    looxuser

    77
    2
    Dec 28, 2015
    Ok now have a scope and signal generator if we need that.
     
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