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    Matching output transistors on an audio amp?

    I got a crown dc300a amp that had one distort channel and one good one. In my "testing", I managed to cause "issues" to the good channel which now causes the main fuse to blow... :-( There is a thread in the audio section for that effort. I thought this thread would be more "general"... maybe about components / theory.

    So after a long "sigh" and many days of contemplating and reading, I started back. Isolating the two banks of 5 output transistors on the 1 channel (4 2n5631 and one 2n3859a) that span +-60 v rails. I found a 2n5631 on each "side" shorted.

    Reading various places, I found comments about matching "pairs" and matching in general, but I am not sure I understand what it means. Do the transistors (ie. 2n5631's) all have to match? vintage number? only a "pair" match? what is a "pair"? Another question is that the crown 3990 isn't readily available but some "mexico 2n5631" ones are... are these an OK replacement? or are they like some other "fake" units / semi-conductors that people find in the tv repair world?

    Thanks for any guidance on this.... I hope I can eventually resurrect this poor unit.

    #2
    Re: Matching output transistors on an audio amp?

    most amps it doesn't matter about matching . just use same batch numbers if possible .
    pairs are complimentary so sub with complimentary pairs .

    Comment


      #3
      Re: Matching output transistors on an audio amp?

      The output transistors do not need to be closely matched because the emitter resistors provide local feedback for each transistor, it looks after any imbalance in the bank. A bank is the entire group for the top or bottom, two banks per ch. So mixing different part numbers is OK. Matching is important with a discrete front-end diff. amp. (Crown here is op-amp IC), and some Crown pre-driver parts are hand selected for match or high hFE.

      2N5631 140V 16A 200W fT=1MHz is an old, slow power transistor. You can't really find old slow parts. It is OK to use faster output transistors but some power amp designs were really tight and compensated just for the original transistors.
      For replacements, I would consider:
      MJ15024 250V 16A 250W fT=4MHz
      MJ21194 250V 16A 250W fT=4MHz are modern perforated-emitter technology.

      If you blow an output/driver transistor, always go back one stage and replace the driver/pre-driver even if it checks good. They can test OK yet are damaged from the overcurrent when the output tranny failed and end up blowing the new O/P trannies and cause grief. I usually get suspect if the diode-test voltages E-B and C-B are quite different or way off compared to a part on the other channel.

      Check closely diode-test E-B and C-B, and hFE to ensure the remaining output transistors are not weak or damaged.

      Comment


        #4
        Re: Matching output transistors on an audio amp?

        thanks for the extensive reply. You give me more than I can chew...:-) :-(

        So to do those additional checks (I was only checking collector to emitter resistances as a "gross check" and comparing across all same devices), I have to demount all the transistors, right? Further when you refer to "back one stage", that stage is not on the output board but on the main control board... sorry I don't really know much about amp design / layout. Lastly, should I be able to totally disconnect the output board from the main without causing an issue to power up to see if things are "working" on the main board?

        Comment


          #5
          Re: Matching output transistors on an audio amp?

          I swapped a transistor on the output of a 25WPC amp with a random transistor I had on hand that I could not find specs of. After adjusting the bias point purely for idle power dissipation, my ears couldn't really discern the difference between the repaired channel and the other channel...

          Comment


            #6
            Re: Matching output transistors on an audio amp?

            oh... that's another "lesson point"... if / when I replace any of the output transistors, do I have to do some "board adjust" (ie. bias...?)? Will the thing function without any adjustment or will it "runaway"?

            Comment


              #7
              Re: Matching output transistors on an audio amp?

              I'm also interested in this topic.

              I have a Crown Amp with Output transistors that are in groups of 4 and they are marked with a little factory stamp 1,2,3 or 4 on each device, along with the part number.

              Apparently they were selected to have the same gain in each batch and when I find the same original devices on ebay (they are obsolete) the listings say I can't just stick any transistor in, and if the 'gain' number does not match the others in the Amp they they will fail, probably with a bang knowing my luck!

              Some of them in the amp are short circuit but I've never tried to fix it as I couldn't find the same gain transistors as replacements.

              This contradicts the advice given here. Why is that?
              Last edited by dicky96; 09-16-2019, 02:17 AM.
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              Comment


                #8
                Re: Matching output transistors on an audio amp?

                Originally posted by budwich View Post
                thanks for the extensive reply. Lastly, should I be able to totally disconnect the output board from the main without causing an issue to power up to see if things are "working" on the main board?
                Yes I do this without a problem. Same as you can happilly run a blown amp in mono with only one channel output stage fitted. It's a cheap way to drive a Sub for instance.

                I also often do this the other way too - remove the output board, and power it up with my dual bench supply to +/- 30V. The current limiter will save you if something is still badly wrong and it's often easier to work on/diagnose the output board when on the bench and not in the amp. in some cases you can also easily inject a signal such as sine wave or audio into the output board.
                Last edited by dicky96; 09-16-2019, 02:24 AM.
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                  #9
                  Re: Matching output transistors on an audio amp?

                  Added some pics.
                  Output devices C8188-2 and C8187-4, with batch numbers stamped on them
                  Crown Power-tech 3 amplifier

                  Why did they go to the effort of doing that?
                  Attached Files
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                    #10
                    Re: Matching output transistors on an audio amp?

                    if running parallel transistors they should be matched if no balancing resistors in the circuit.
                    even so they are better to be gain matched
                    always check idle current and dc offset after working on amps
                    Last edited by petehall347; 09-16-2019, 04:33 AM.

                    Comment


                      #11
                      Re: Matching output transistors on an audio amp?

                      Originally posted by redwire View Post
                      The output transistors do not need to be closely matched because the emitter resistors provide local feedback for each transistor, it looks after any imbalance in the bank. A bank is the entire group for the top or bottom, two banks per ch. So mixing different part numbers is OK. Matching is important with a discrete front-end diff. amp. (Crown here is op-amp IC), and some Crown pre-driver parts are hand selected for match or high hFE.

                      2N5631 140V 16A 200W fT=1MHz is an old, slow power transistor. You can't really find old slow parts. It is OK to use faster output transistors but some power amp designs were really tight and compensated just for the original transistors.
                      For replacements, I would consider:
                      MJ15024 250V 16A 250W fT=4MHz
                      MJ21194 250V 16A 250W fT=4MHz are modern perforated-emitter technology.

                      ... stuff deleted...

                      Check closely diode-test E-B and C-B, and hFE to ensure the remaining output transistors are not weak or damaged.
                      one more question about the possible replacements, are you saying that these would be a "direct drop in replacement" without any modification to the existing circuit "design" which I don't think I want to attempt.

                      Comment


                        #12
                        Re: Matching output transistors on an audio amp?

                        Yes, I would use MJ21194 or MJ15024 as a drop-in replacement for 2N5631. The odds of vintage parts being counterfeit or just old and crusty, is much worse than a new part upsetting a finicky design - which is really rare.

                        In the 70-80's power transistors could have very low gain hFE ~15 which was terrible, and about 4:1 spread for the same part number. So you could get 15 or 60. Crown would have not wanted lazy transistors. Many manufacturers got semi's house-numbered and sorted according to hFE, to keep their products working best.

                        example 2N5631 $10 eBay feedback:
                        "For the price, it is not bad. I have few different transistor testers. I bought 25 of these 2N5631 transistors for my testings. They had DC gain from 8 to 165 hFE.. 4 of them had 8 hFE under their spec. 10 of them in the range of 20-30 hFE, and 8 of them between 40-50 hFE. 3 of them were between 90-165 hFE."

                        Modern power transistors have at least twice the hFE of old parts with less variance- so today it's not such a big deal, you don't need selected parts.


                        Most amplifiers don't work with the output board disconnected. They need feedback from the output back to the input stage and this can get lost when you disconnect things. I would check the circuit and wiring to see if it will work OK.

                        P.S. I use Q-tips to wipe up extra silicone grease and contain the mess.

                        Comment


                          #13
                          Re: Matching output transistors on an audio amp?

                          thanks for this explanation. As usually, another question arise. So a "drop in" works (hopefully), the question is do I need to replace all 2n5361 transistors on the blown channel or can I just replace the blown transistors. Will there be issues in levels between the right / left channels if only one side is replaced?

                          These question arise because I am somewhat "afraid" to disturb too many things both in terms of "finger problems" but also soldering skills.

                          I am continuing to measure / compare areas of the channels to check things out further while I wait / decide on the best mode for me to get the amp going again.
                          Last edited by budwich; 09-16-2019, 01:54 PM.

                          Comment


                            #14
                            Re: Matching output transistors on an audio amp?

                            It depends on how good you are at testing transistors. Miss one bad transistor, a power amp could kill the new transistors and the boss would chew me out because we'd have to buy a second batch and eat the cost of them.

                            Testing audio transistors - they have three leads, so six possible combinations to do with your multimeter on diode-test (three of both polarities). You should only get readings E-B and C-B, around 0.5-0.8V for two of the six combinations.
                            If I get an oddball reading, I then use multimeter on ohms, which should always read open circuit or maybe a megohm or higher, unless a junction is damaged which reads much lower.

                            Testing in-circuit can work but only if there are no low value resistors or other transistors to get in the way. It's a hassle to pull/disconnect two leads each, but a solder sucker or wick makes it easier.
                            You can also do (multimeter ohms) comparison readings between the same spots on the two channels to look for trouble, if you have one channel working.

                            So I take my time and test all of them, many junctions can be examined in-circuit. I look at resistors for damage or going high in value.

                            Comment


                              #15
                              Re: Matching output transistors on an audio amp?

                              :-) ... tough boss.... :-) but I understand, your effort is money and customer success. Mine is trying to get to the end without blowing a restricted budget and my "boss" who says "why don't you just get rid of it, you don't use it" ... :-) she is probably right... but I can't bring myself to give up that easy.

                              thanks for your input... its good for pointing me in the right direction.

                              Comment


                                #16
                                Re: Matching output transistors on an audio amp?

                                Yeah I suspect some manufacturers are very anal about their circuits and require specific gains and other specifics to make their circuit work for the aforementioned reasons and thus requiring exact replacements. I just so happened to have an amp that didn't require specific gains.

                                The balance offset wasn't absolutely necessary to tweak - it worked more like class A, but it did bring down the heat generation and back to the class AB operation zone.

                                Comment


                                  #17
                                  Re: Matching output transistors on an audio amp?

                                  I put an order in for mj15024's on the "slow boat"... my fingers are crossed. At this point, while I wait, I will give an attempt at removing one of the transistors that I "suspect" (based on the tests so far) to see how much difficulty I will have.... my fingers will be crossed, but not on the soldering iron.. ouch! :-)

                                  One thing that I just noticed in the schematic, there appears to be some "circuit changes" for different transistors... in this case, there are some different resistors / caps (minor differences in resistances) listed in places IF you have 2n5631's versus RCA 1B05.... hopefully this won't come back to bite me.
                                  Last edited by budwich; 09-17-2019, 08:31 AM.

                                  Comment


                                    #18
                                    Re: Matching output transistors on an audio amp?

                                    Originally posted by redwire View Post
                                    It depends on how good you are at testing transistors. Miss one bad transistor, a power amp could kill the new transistors and the boss would chew me out because we'd have to buy a second batch and eat the cost of them.

                                    .... stuff deleted....

                                    Testing in-circuit can work but only if there are no low value resistors or other transistors to get in the way. It's a hassle to pull/disconnect two leads each, but a solder sucker or wick makes it easier.
                                    You can also do (multimeter ohms) comparison readings between the same spots on the two channels to look for trouble, if you have one channel working.

                                    So I take my time and test all of them, many junctions can be examined in-circuit. I look at resistors for damage or going high in value.
                                    You were right about using a solder sucker... a large tip soldering iron and a couple slurps on each contact, and the thing fell out (had previously removed the screws). It tested basically a short in all direction and combination.... this was one that tested "poorly" on the CE resistance along with BE junction. I will probably take out one that is testing good to do an out of circuit to confirm some of my results... along more de-soldering practice.

                                    So new question is about heat compound, the "original" was corning dc-340 which is still made but can any type of heat conducting compound be used... that doesn't "run" (ie. high temperature)?

                                    one more question: as I removed the transistor, I see there is a "plastic ring / insert" on the screw holes of the transistors which is some form of insulator for the heat sink. Do I have remove these and swap them over to any new transistors or will the new transistors come with same inserts?
                                    Last edited by budwich; 09-18-2019, 01:25 PM.

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                                      #19
                                      Re: Matching output transistors on an audio amp?

                                      Alright! It's good to find the bad parts, getting closer to a fix. The output transistors are in parallel so in-circuit readings will not tell you which one is shorted or open.

                                      Silicone heat-sink compound is still around and pretty messy, Dow DOWSIL 340. But a huge expensive tube to buy. They use zinc oxide in the grease base.
                                      The industry moved to non-silicone based thermal grease and inferior thermal pads (not good enough here).
                                      For smaller tubes, a computer store will sell all kinds of thermal grease like Arctic Silver or Noctua etc. I'm not sure what stores are in your locale.

                                      Comment


                                        #20
                                        Re: Matching output transistors on an audio amp?

                                        thanks for the info on the grease. Do you know anything about the plastic sleeves / inserts for heat sink isolation? Do I need to buy them as a standalone item, or are included in the transistor design?

                                        further, I have a "cheap" transistor / capacitance checker.... first time ever used it for transistors... anyway, the good transistor gives all the "vitals" well the bad transistor shows a basically a pair of resistors (because of the three pin connection to the test block). So that is good.

                                        We have "active electronics" store here in ottawa so I have access to some stuff although a lot gets sourced out of toronto so its not always a "walk in and buy".

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