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JVC RX-6042 5 secs then shuts off

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    JVC RX-6042 5 secs then shuts off

    i have gone through this amp and i can see anything visiably wrong. i have checked all the caps and replaced the ones that measure infinite ESR. i have pulled all of the heatsinked transistors out and tested them. im not sure what else to check for i cant seem to find any schematics online and im not 100% what i am looking at. i am much more used to looking at car amps. this appears to have 5 seperate audio amps they all look very similar. i have drop in boards for digital audio,analoug audio, radio tuner, and some analoug tv stuff. there is a main power board coming off of the mains transformer and from there it goes to what i think is a bunch of heatsinked rectifiers for DC.

    im not sure where to start looking so i took just a few pictures if anyone can help me i will gladly post more, im just not sure where to start.

    all caps have been checked and all TO-220 transistors have been checked

    but now i am only left with T0-92 and im not familiar enough with this stuff to know all of thiese pieces and part numbers. and now because they are so small its time consuming looking all of them up, and it doesnt stay on long eough to start stabbing away with a meter
    Attached Files

    #2
    Re: JVC RX-6042 5 secs then shuts off

    I do think a schematic will be necessary - I see a number of sites have one. Are we talking about a speaker relay shutting off, or the mains relay on the power board? Lights on the front panel? Usually there will be a standby 5 volt supply to the micro on these units. When you press "on", the micro sends a signal to turn on the mains relay. At that point, protection networks on the speaker outputs may shut the unit down.

    You do not have to remove any of the transistors for an adequate test. Just put your meter in diode check mode and check the 2 transistor junctions according to the number on the part. That method will work 99% of the time. However, this problem does not sound like a failed transistor, but of course anything is possible. Try to obtain the schematic and I'm sure you will get some good help to guide your repair.
    Is it plugged in?

    Comment


      #3
      Re: JVC RX-6042 5 secs then shuts off

      because i am so new to this i have a hard time making heads or tails of the engineering design side of schematics but they help me out determining collector vs emitter etc..


      you are right i have some standby voltage present to light up a led light on the power button and enough to engage relays when its depressed. it is a mains relay shutting off everything after a few mins. the front display and everything and forcing it back into standby

      i am trying desperatly to post the schematic but this site wont let me post it even though its only around 3MB .pdf file.

      which cirucuit should i be looking at or which failed components should i be looking at? or even which board?

      Comment


        #4
        Re: JVC RX-6042 5 secs then shuts off

        i went back and tried tesing some of the TO-92 cases and im not sure if i am doing something wrong but on diode check i get around 1-3v on all 3 of the legs on ohms i get even weirder and more inconsistant readings of 8k-100k on all 3 combinations of the legs and on beeping i cant get it to beep at all.

        i would love love love to know how to check these things in the board or if there is a tool i can get to test them in place. i was always taught that the only way to test these is to yank them out

        Comment


          #5
          Re: JVC RX-6042 5 secs then shuts off

          Originally posted by scampo77 View Post
          on ohms i get even weirder and more inconsistant readings of 8k-100k on all 3 combinations of the legs and on beeping i cant get it to beep at all.

          i would love love love to know how to check these things in the board
          For transistors "Q", I usually test them "in circuit" first for shorts. I test all 3 pin combinations, 1-2, 1-3, and 2-3. Any reading under 30 ohms suggests a short. However, just because it is not shorted doesn't mean it is 100% working.

          PS. Using continuity or beep is not a very accurate test. Some of the really cheap multimeters will beep "continuous" for any reading under 1500 ohms.
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          We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

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          Comment


            #6
            Re: JVC RX-6042 5 secs then shuts off

            does anybody else have any suggestions?

            Comment


              #7
              Re: JVC RX-6042 5 secs then shuts off

              i have now double checked all of the transistors and all of the caps i really dont want to throw this thing away if i dont have to can anyone else help me?

              Comment


                #8
                Re: JVC RX-6042 5 secs then shuts off

                Post the schematic on a filesharing site, or export relevant portions to an appropriate size (pixels and filesize) image you can post or link.

                First random thought is, if main PSU relay is de-energizing, trace back that circuit to see what the protection or source is and what inputs or values it is using to determine on/off.

                Pull the daughter boards off, at least those that aren't soldered on, then see if power stays on. If so, it tends to indicate a daughter board fault. It also looks like there is a lot of darkened PCB area aroud some transistors, check that the solder joints and traces are intact.

                Comment


                  #9
                  Re: JVC RX-6042 5 secs then shuts off

                  http://www.4shared.com/office/xOxKnDUd/mb167sch.html
                  or

                  or
                  <a href="http://www.4shared.com/office/xOxKnDUd/mb167sch.html" target=_blank><img src="http://dc468.4shared.com/img/xOxKnDUd/mb167sch.pdf" border="0"></a>

                  this is my first time doing doing a file share like this and im not sure what to post a link of, so i posted all of them.

                  im not sure which is the board i need to start at? from the pictures i posted on the first post #4 seems to be where the power starts and there are 2 relays on this board.

                  Comment


                    #10
                    Re: JVC RX-6042 5 secs then shuts off

                    Here are a couple of suggestions. First, when testing diodes or...the 2 "diodes" which make up a bipolar transistor, you must use the diode check position of your multimeter. This position is different than the ohms position. In diode check mode, the meter will read in d.c. volts. A typical silicon diode or a working junction in a transistor will read .6 to .7 volts forward and 0 volts reverse. That is the definition of a diode.

                    It is a good sign that the unit powers up into the standby mode and fires the mains relay. The most likely reason for the unit to shut down after a few seconds will be a problem with one of the main amplifier output stages. Here is a simple test. Hook up your multimeter to each speaker output - one at a time. Have it wired up before turning the unit on - use the d.c. volts or the millivolt scale if you have one.

                    Power up the amp and look at the meter reading. The reading should be very near to 0 volts d.c. If you find one or more channels that are not 0 volts, then you have confirmed a problem in the power amplifier stage(s). Then, you can get to work on that area of the circuit. What could be simpler than that!
                    Is it plugged in?

                    Comment


                      #11
                      Re: JVC RX-6042 5 secs then shuts off

                      ^ while I agree with that, some amps do have a few mA DC on their outputs when functioning "as designed". Although opinions may vary what an acceptable level is, some say <= 20mV DC, maybe a little bit higher like up to 100mV for power amps. Regardless if the output stage(s) have failed causing DC offset, it'll be a lot more than that although it is up to the designer to determine at what DC output level a protection circuit kicks in.
                      Last edited by 999999999; 01-12-2012, 06:38 PM.

                      Comment


                        #12
                        Re: JVC RX-6042 5 secs then shuts off

                        I made a mistake in typing: a good silicon junction should show about .6 volts forward and infinite or "OL" on your multimeter in the reverse direction, indicating no current flow. Great! Now we have a schematic to look at. Thanks. Notice that your output devices are power darlingtons - a package containing 2 transistors tied together. Darlingtons test a little different from single transistors and the fastest way to check these, and other output transistors is simply use your ohms scale and measure from emitter to collector, looking for anything near a short (low ohms reading). That would be a bad part.

                        As far as output offset voltage, no need to get technical at this point. Lots of equipment will operate up to 600 mv offset which is where some protection transistors start to work. So a 5 second shutoff situation should show an obvious dc voltage on the speaker terminals, if that is the cause of the trouble.

                        This unit has both a mains relay, and individual speaker output relays. So first, determine if your speaker relays are actually operating, or else the measurement at the speaker output terminals will be a waste of time. If you don't hear the speaker relays click, that is a clue. In that case, measure the d.c. voltage on the other side of the relay, or the "hot" output for each channel. That point will be easy to find, since there are only 2 terminals on the relay (ignore the relay coil windings).

                        So, as soon as we have the results on the amplifier output voltages, we can move on to the next exciting step.
                        Last edited by Longbow; 01-14-2012, 12:03 PM.
                        Is it plugged in?

                        Comment


                          #13
                          Re: JVC RX-6042 5 secs then shuts off

                          i pulled all 5 amp boads out of the unit and still the same symptoms. i thought it was worth a shot.

                          with the amp boards back in i measured the 3 speaker output relays and all 3 are energizing

                          with amp boards in i measured the speaker outputs and have 1.8-2v ac and dc on all channels well beyond 600mv

                          should i try with no amp boards in?

                          thanks for being so fired up for this project!

                          Comment


                            #14
                            Re: JVC RX-6042 5 secs then shuts off

                            also, when i hook up a speaker to the outputs i am getting very loud mechanical noise, im not sure if this is signifigant or not

                            Comment


                              #15
                              Re: JVC RX-6042 5 secs then shuts off

                              ^ don't hook speakers up to the outputs after having measured > 600mV DC, it can damage them.

                              As already implied in the topic, odds are either your output transistor(s) are shorted or an upstream bias transistor is shorted or leaky, though you could have a failed power supply smoothing cap (measure both power rails for excessive AC ripple).
                              Last edited by 999999999; 01-15-2012, 04:59 PM.

                              Comment


                                #16
                                Re: JVC RX-6042 5 secs then shuts off

                                if it were a output transistor wouldnt it be this voltage on only 1 channel?

                                Comment


                                  #17
                                  Re: JVC RX-6042 5 secs then shuts off

                                  i went through all the transistors inside this thing except these 2 big monster ones. im not sure what they do but they areright at the begining and there are 10x larger then the largest caps on the board. one has 2.5 ESR and one is open.

                                  is this my problem?

                                  ill go to the store tmrw and get new ones
                                  Attached Files

                                  Comment


                                    #18
                                    Re: JVC RX-6042 5 secs then shuts off

                                    If by the two big monster ones you mean those mounted on the big heatsink, yes those are in a position likely to fail in an amp.

                                    It's a good question why it's not only on one channel. You might trace back the audio path to see if it's DC prior to those transistors and keep tracing till you find where the DC starts.

                                    Comment


                                      #19
                                      Re: JVC RX-6042 5 secs then shuts off

                                      Originally posted by scampo77 View Post
                                      i pulled all 5 amp boads out of the unit and still the same symptoms. i thought it was worth a shot.
                                      I have no idea what the unit should do when boards are pulled out. That is not a useful troubleshooting approach.

                                      with the amp boards back in i measured the 3 speaker output relays and all 3 are energizing
                                      This symtom suggests that something on the output side of the relays may be faulty, otherwise the speaker relays would probably not energize at all. Have a look at the speaker relay board. This board contains some protection networks that look at both sides of the relay contacts. I'm not familiar with this design, but the problem may be on this board. FW303 pin 1 should have 24 volts dc on it.

                                      with amp boards in i measured the speaker outputs and have 1.8-2v ac and dc on all channels well beyond 600mv
                                      1 to 2 volts dc? or more than that - maybe 30 volts dc? the AC component is probably hum - ignore this for now. What d.c. do you measure at the speaker terminals?

                                      should i try with no amp boards in?
                                      you said you already did that. Forget that method.

                                      Set your meter to ohms scale and measure emitter-collector resistance on all the power darlington outputs, looking for shorts. Make sure that your +-50 volt rails are ok. See if only one channel or all channels are showing dc voltage on the output. Check the parts on your speaker relay board, looking for open resistors, bad caps and bad transistors.
                                      Is it plugged in?

                                      Comment


                                        #20
                                        Re: JVC RX-6042 5 secs then shuts off

                                        I've had a better look at the speaker relay board and my thought is that all the protection transistors are checking that the speakers are connected correctly and not cross wired. That's the only explanation that I can come up with, since some lines are monitored on the amp side of the relay, and some are monitored on the speaker side of the relay.

                                        Ultimately, all the "cross wiring" protection on the speaker relay board gets mixed into one signal which appears on plug FW301, pin 5. Monitor this pin. Normal operation once the speaker relays fire, is for pin 5 to be +5 volts (it is labeled incorrectly as 0 volts). If this pin does not stay at 5 volts, the unit will shut down. Note that D1356, a 5 volt zener, supplies 5 volts to all the protection transistors. Make sure that R1355 is not open, and actually reads 3.9 Kohms.

                                        On the same board, another protection circuit monitors all 5 amplifiers. This circuit has 3 transistors (right in the middle) and 5, 100K resistors - one connected to each of the 5 amplifiers. This is a convoluted design, but necessary to prevent smoke. First step is to measure each amplifier output d.c. level and report the findings. The problem might be a blown amp, or it might be a problem with the protection circuit. You can do it!
                                        Is it plugged in?

                                        Comment

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