Small Amplifier Board Repair Guidance

Discussion in 'General Electronics Chat' started by cwackduck, Jul 30, 2014.

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  1. cwackduck

    Thread Starter New Member

    Jul 29, 2014
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    Hello everyone! I found this site by looking for some tips on repairing the amplifier for a set of speakers I'm pretty fond of. A while back, someone posted here about a Klipsch Promedia 2.1 system Previous topic that lost audio on the left channel. I have a similar issue, but not quite the same as his, and that thread was never resolved. His actual speakers appeared to be damaged while mine don't seem to be affected.

    I've narrowed the source of my problems down to the Left-Channel High Frequency amplifier board. In this system there are two identical boards for the left and right speaker.
    A full schematic of the speaker system can be found here:
    Klipsch ProMedia 2.1 Schematic

    Upon my initial inspection I found a resistor burned out on the board. Further testing found continuity on some of the transistors and a somewhat-puffy capacitor. Long story short, I was in a hurry to get the speakers working again for an event and was careless/frantic in ordering new parts. New parts went it, new parts failed/exploded. Pretty sure that was alllll my fault. SO, I've ordered new parts that I'm sure are the exact replacements this time and not some "close enough" parts.

    So here's the terrible…
    The Left HF board as it sits now:
    [​IMG]
    [​IMG]

    and after testing, here's the current failures:
    [​IMG]

    Fortunately all of the other components on the board test properly.
    I've currently ordered new R15, R17, Q7, Q8, Q9, and Q10.
    Also, since R14 was so close to R17 when it went "poof", I'm replacing it even though it tests good.

    I replaced all of these same parts (including C3) originally. The first time R17 was burned up, but no other components showed damage. This time Q9 popped but I'm pretty sure it's because I got the wrong components for Q7 and Q8.

    All of the parts should be arriving in the next two weeks (Slow boat from China) and I'm hoping that replacing them properly should fix the issue, but my concern here, and what I'm writing all of this for, is what might have caused this in the first place? I just don't want to wire this all up with good, working parts and have them blow again right away because I didn't look far enough up the chain. For the record, I did buy duplicates in case that exact thing happens…

    I don't remember any one event that would have caused this. I was using the speakers for a party where many people were hooking their phones up to play music. When I moved the speakers back to my computer a few days later the left channel was out, the right channel sounded weak and terrible, and the subwoofer worked as expected.

    For those that know what they're looking at in the schematics, what other components should I be testing that are sending a signal (or too strong of a signal) to these boards? I'm hoping the failure is contained to this one HF board but I don't know enough to trace the line back and find exactly where I should be looking for a surge or something.
    Any help or guidance is extremely appreciated! Thanks!
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    one fortunate thing is that with a stereo you have two identical amplifiers, you can compare measurments between the two, and get the numbers off of good parts that are on the good one where the parts may be burnt on the bad one.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Before you begin testing with installed components, there are a number of voltage checks you can perform.

    Don't connect a loud speaker.
    Don't install Q9 and Q10.
    Replace all other suspect components.
    R15 looks like 6200Ω in the schematic.

    Measure the DC voltage at both sides of R17.

    Do you have access to an oscilloscope?
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Mr Chips is right, there are some preliminaries before reconstruction.

    Firstly are you sure of the DC voltage supplies?

    I would be suspicious of C8, C9, C10 and C11 given the events.

    Was your measurement of R15 in circuit?
    If so the 2N3904 (Q6) might have a collector base (partial) short.
    Change it, it will only cost pence.

    Check the board itself for damage or shorting bridges with a magnifying glass.
     
    Last edited: Jul 30, 2014
  5. AlphaDesign888

    New Member

    Jul 27, 2014
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    Use an analouge meter. Never measure parts in circuit. The results will be wrong.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Also, measure the voltages +V-HF and -V-HF. If they are correct I would expect them to be roughly equal in magnitude (but opposite polarity, obviously) and ~ 25 to 30V.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Strange that R18 and R19 are shown as 0-5W.
    I would have preferred to see something like a 0.22Ω resistor in there.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think those 0,5W are 0.5Ω resistors.
    In some printings the Ω-sign will show-up as W.

    Bertus
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    The photo of the board shows R318 and R319 as wire jumpers.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    With a DC coupled amp; you pretty much want to test every semiconductor out of circuit - any leakage anywhere is likely to incinerate the new power transistors.

    Don't forget any diodes - any resistor showing any sign of distress should be replaced, any checked in situ that read high - are high, any that read low are shunted by other parts, you could always lift one end and get the actual value.
     
  11. to3metalcan

    Member

    Jul 20, 2014
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    Once you've followed the above excellent advice, a longer-term potential problem here is that Q6 (the amplified diode that sets the quiescent current for the output devices) is being biased by fixed resistors. I have fixed many an audio amp where this was fine coming out of the factory, where all the units were fitted with a single batch of similar devices, but when I replaced the output transistors later the bias was suddenly totally amok. R15 looks like it's 6200 (not 8200) ohms on the schematic...this makes sense because together with the 4700 ohm resistor, they would add up to almost exactly 10K, which is a standard bias trimmer value (they probably determined the value with a trim and replaced it with fixed resistors.)

    To find out if the bias is set ok the way it is, check the voltage across R18 (it will be millivolts) in both the repaired circuit AND the circuit that's still good. Do the same with R19. If you're seeing a big difference from one circuit to the other, you may need to replace the fixed resistors with a trimpot.
     
  12. MrChips

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    Oct 2, 2009
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    Your idea has merit except according to my arithmetic 6200 plus 4700 is 10900.

    Also note that R18 and R19 are solid jumper wires, not resistors.

    However, I agree. You want to adjust the DC bias so that the voltage reading at R18, R19 is zero volts.
     
  13. to3metalcan

    Member

    Jul 20, 2014
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    Hey, now. I said ALMOST. ;)
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    No. You said "almost exactly". That's an oxymoron.:rolleyes:
     
  15. to3metalcan

    Member

    Jul 20, 2014
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    Here I thought that's what you got if you left a moron's contacts exposed to air and humidity. :)

    While we're nitpicking, a zero DC offset at the output (which the circuit is pretty likely to have if the bias is even close, given the emitter current source on the input diff. pair) doesn't actually guarantee that the transistors aren't running too hot, especially at the crossover point.

    I sort of thought R18 and R19 were maybe on the other side of the board, or something...without 'em, I guess you're gonna have to interrupt the circuit with an ammeter, somewhere?
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Quite possibly, I also wondered if the wire links in the photo were there for topological reasons.
    We could really do with a copper side photo as well.

    I second that observation about the fixed bias on the amplifed diode connected transistor.
     
  17. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Just global replace all the transistors and diodes, and any damaged resistors. It's a quick and easy job compared to trying to faultfind it and guess which transistors might still be ok...

    And put a polyfuse on the output, to stop it happening again.
     
  18. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I hope the OP has not gone the same way as his circuit board.

    :eek:

    I'm wondering why we have not heard more from him.
     
  19. cwackduck

    Thread Starter New Member

    Jul 29, 2014
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    Yes, I'm still here! :D
    I didn't expect this thread to blow up like this! Usually I'll post and go away for a while and get maybe one response, so thanks everyone!

    I'll go through and try to hit all the relevant info:
    It is fortunate that I have the other working board. I've been using it to test components against the other and compare it to the schematics. The schematics I've linked were made based on observation as opposed to coming from the factory. I've found a few small discrepancies on my actual boards, one being R15, which is an 8200 on both boards, and measures within that tolerance on the good one.

    All measurements I've done so far have been with the components still in-circuit.

    I've looked the board over pretty well with a jeweler's loop and have found a few places where the pads have started to lift but all traces are complete and I haven't seen any shorts. I'll go over everything again more closely when I've finished soldering the new parts in to double check.

    R18 and R19 are, in fact, wire jumpers. I'm not sure where he got the values in his schematic other than I believe I may have a later revision of the speaker system. I know there's a newer version out there with an entirely different set up inside and I think I've heard of there being two versions of my set up with very small changes made.

    There were two diodes on the board that I tested in-circuit that tested fine. I can retest them out of circuit to be certain.

    I haven't really measured much else because I didn't know where to start. I do have access to an oscilloscope at work but I don't really know how to use it. I could figure it out with instruction or I can pick one of the engineer's brains. (I work at a TV station)
    Can I measure the output of the transformer with this board removed? I didn't know if I could plug the base unit back in with these two amplifiers removed and not cause damage somewhere else. I have avoided plugging any speakers or the sub back into it until I see that it's running stable and not smoking.

    I've still got a few more days until a majority of my parts show up, and probably 2 weeks until Q7 and Q8 arrive. In the mean time I'll remove the components and test them out of circuit. I'll make sure to look at C8 - C11 as suggested as well. I'll do that as soon as I get a chance and report back. Is there any way to test the input signal to the board without it being in place?

    For reference, here's a pic of the working HF board:
    [​IMG]
    Thanks again, everyone!
     
  20. MrChips

    Moderator

    Oct 2, 2009
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    AAC is a very active forum and there are a lot of keeners here. So don't be so surprised to see so many quick responses.

    It is not unusual to see some variations in component values in final production units. Sometimes design engineers like to tweak values for better DC balance.

    As stated before, I'm a bit surprised to see 0Ω at R18 and R19.

    With a DC voltmeter, you should be looking for 0V at the output (also at R18 and R19) if the circuit is properly biased. Hence with Q9 and Q10 out of circuit, the voltage across R17 should be equally balance about 0V, perhaps +0.6V at the base connection to Q9 and -0.6V at the base connection to Q10.

    If you had an oscilloscope, you could input a sound source, e.g. music from an mp3 player, and then observe the waveforms at R17 and then at the output when you have installed Q9 and Q10. You should see a signal equally balanced about 0V with no clipping.
     
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