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    #41
    Re: Crest CPX1500

    Have you checked all other semiconductors for shorts? You don't need to remove them, a shorted device will show as shorted in-circuit anyway.
    "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
    -David VanHorn

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      #42
      Re: Crest CPX1500

      yes.. I have checked every diode, every transistor, and every once and again I hear a beep, but after checking the scheme it's allways because of some small resistor in the circuit.

      I have also checked bridge rectifiers.. and unless something can fail while live and show a good condition when switched off... The only unusual thing I see is resistance through some micro ceramic caps.

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        #43
        Re: Crest CPX1500

        I don't think I ever had this amp running at low voltage(with the bulb in series) in a good condition, unlike other amps I had repaired and finally lit propperly.

        It coulnd't possibly be trying to power up but but not having enough voltage... couldn't it?

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          #44
          Re: Crest CPX1500

          If you think the ceramic capacitors are suspicious, replace them, sure. Why not?

          I think it is indeed possible that with the lightbulb it may not be getting enough current to run properly. What does the lightbulb do when the amplifier tries to start up?
          "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
          -David VanHorn

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            #45
            Re: Crest CPX1500

            I'll fetch some when I go down to the city.

            About the semiconductors, when they fail, do they short with a 0ohm value? or can that value stay at a random number like say, 30 or 80ohm? That is something I have allways wandered.

            And about the light bulb, it blinks the same as the leds, intermitently.

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              #46
              Re: Crest CPX1500

              I was asking about semiconductors because I did have a pair of diodes(CR100,101) that were suspicious, they all read the same both ways, so I took one out to see (after a long thought) but it read fine out of circuit. Those were the last semiconductors that read something similar to a short. Appart from IC's, that I am not 100% sure how they are tested.

              But still, have any of you encountered bad semiconductors that read something different to 0?? I am very curious about this... It is something to take into account..

              So following the trace, I finally figured out it arrives to R194,195. Whitch are all toast(also on the other channel). they read around 1-2k. I have some unused biggish 4.7 resistors, I guess around 2-5w, but I think they are too big to fit propperly, so I'll have to wait till shop time.

              Thanks again

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                #47
                Re: Crest CPX1500

                Hi guys,

                I just came back from the shops, and there is not a single 4.7ohm SMD resistor in the city. I couldn't take it so what I did was buy some 1/4w resistors and fitted a pair with some heatshrink to insulate from other connections.

                Put everything in it's place, and after checking everything looked "safe", plugged it into the bulb, and hit the switch. The two first seconds were fine, fan turned, DDTleds turned on and the bulb lit dimly.

                Then, the bulb flashed once and nothing else. Fuse401 passed away... farewell.


                So after dinner I might get into checking everything again and tell you what I see

                Comment


                  #48
                  Re: Crest CPX1500

                  Yes, you're right. Shorted ICs don't necessarily have to read 0 Ohms. Often they read close to it, likely under 300 Ohms or so. Some ICs could read a low value even if they were OK, but it's less likely.

                  Another thing to remember is that if an IC shorts, it can often draw a high current, burn up (there are sometimes no outward physical signs of this) and then go high-resistance or open-circuit afterwards. So just because an IC doesn't read a low resistance or zero ohms, doesn't mean it's OK.

                  Take those resistors for example. They have seen overcurrent and burnt up to the point that their resistance has increased substantially from what it was.

                  Now, for both R195 and R194 to burn up, it would indicate a short-circuit in something they supply power to. This is basically U100, U101, and C102\C107. Since C102\C107 are in series, both would have to short at the same time, so this is unlikely. It's more likely that U100 or U101 have failed.

                  If R295 and R294 also failed, U200 and U201 are also suspects.

                  It's telling that F401 blew
                  Last edited by Agent24; 05-16-2017, 02:05 AM.
                  "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                  -David VanHorn

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                    #49
                    Re: Crest CPX1500

                    Thank you, that is very usefull to know. Both the fact that they can be shorted showing a resistance different to 0, and also that they can be bad even without showing low resistance.

                    In fact, U100 and U101 show around 24-230ohms between some pins. Also U200 and U201, in different pins.
                    So I guess they are all toast.

                    I checked to see if anything else burnt up but everything looks fine

                    I went to the city to try and find them and all I got told was good luck, so I ordered some online along with a bunch of other stuff, and it should get here around july... so stay comfy.
                    I have had this sitting around for almost 2 years, so I can wait for another month or two

                    Comment


                      #50
                      Re: Crest CPX1500

                      Hi guys, I recieved the parts yesterday, and after installing the 4,7resistors and U100/U101
                      the amp now starts without blinking.
                      The only thing it does now is the fan and channel B red DDT led and turn on, also U400 starts heating up. Diodes around read fine so my guess is U400 is bad.

                      Comment


                        #51
                        Re: Crest CPX1500

                        I went to get stuff at the shop and got a pair of 7815 and 7915 of the same make, the ones I had were different brands, and I thought, what the hell. But no luck.

                        What made U400 heat up was CR115, a shorted 15v zener that goes to ground, will fetch some tomorrow.

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                          #52
                          Re: Crest CPX1500

                          Did you replace U200 and U201 also? They are likely bad too, if U100 and U101 were.
                          "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                          -David VanHorn

                          Comment


                            #53
                            Re: Crest CPX1500

                            Hi people.
                            Yes Agent, I had replaced U200/201 (allthough I'm trying to center on getting at least channel A going first)

                            I have replaced CR115 and with that, channel A power led CR300 came on after the usual delay startup time. But now I feel heat around C113 and also Q105. I replaced both with no change. Everything else stays quite cool.

                            I tried hooking up my testing speaker and a source but as soon as I fire up the device it sends the speaker this nasty hiss, that gets louder past the delay startup time. I hear nothing from the source even when I turn volume up.

                            I know I have said this before, but I can not find any more shorts. The difference, is this time once the amp starts the light bulb goes off completely.

                            But if some components still heat up.... does it have to be because of a short, in the one in question, or in further ones?


                            Thanks for your time

                            Comment


                              #54
                              Re: Crest CPX1500

                              there is more stuff that heats up, U304, U400, U401, R124, R135.

                              Comment


                                #55
                                Re: Crest CPX1500

                                Maybe you have found all the shorts now. It's good that the light-bulb now goes off after startup, that indicates there is no major problem. You should still use it at this point though, just in case.


                                Heat around C113 could just be from CR115, or R124.
                                R124 is rated as a 5 Watt part, so I would expect it to run at least warm. The same goes for R135.

                                U304 seems to be just for powering the fan, and assuming that's working normally, then U304 is probably OK. Power regulators often run warm anyway, but if it's significantly hot, it may have had damage from the overvoltage incident.

                                Again, U400, U401, they are regulators, they supply power to a significant portion of the circuitry, so running warm for them would be expected.


                                Q105 getting hot may be a problem, though, especially if its complement, Q104 does not. If a replacement did not solve the problem, then you need to look at the surrounding components.

                                Is R134 also getting hot? Are C117 and R133 OK?

                                Also, I would be suspicious of any of the following diodes being leaky or open-circuit: CR123, CR122, CR121, CR110, CR111, CR112.


                                You may also like to start doing signal tracing and find where the signal disappears and where the hiss begins.

                                If you have freezer spray, you could try spraying things and see what diminishes the hiss. Likely it gets louder as something heats up, and you may find the faulty part or area by freezing it.
                                Last edited by Agent24; 06-26-2017, 04:28 PM. Reason: typos
                                "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                -David VanHorn

                                Comment


                                  #56
                                  Re: Crest CPX1500

                                  I think the major heat source, one leg of r135 and r124

                                  Then about the fan I have doubts, I have an 8ohm testing speaker I don't love too much, then a 4ohm and a 10ohm big resistors, what should I use preferably as dummy load?
                                  I tested with the speaker and the 10ohm, the first made the fan change speed about 3 times and settled, but with the second the fan went crazy at random speeds.

                                  I don't know much about ceramic disk caps, so C117 is a mistery to me yet, will fetch some ASAP.
                                  R133 reads 4.57k

                                  I have just swapped CR110, CR111, CR112, CR121 and CR122 but don't have an ISS244 at hand for CR123 so that'll have to wait. Now both red leds light up during delay startup time and channel B's goes off after, while channel A's green led lights up. sound is still the same.

                                  I'll start tracing tomorrow or so, I just started and I don't know how a fuse went, I guess i'm too tired.

                                  It was a good day though, thanks

                                  Comment


                                    #57
                                    Re: Crest CPX1500

                                    I obviously shorted something while trying to trace the other day, making both fuses go. Is it possible that the opamps suffered enough to go bad by shorting two legs together for short periods? I mean, if fuses went...


                                    I havn't gone to the shops yet to get caps and diodes, but appart from that I just got my tracer up'n rolling again and decided to start from the beggining, so I started poking on P101 (assuming either BDFED/DDTS1A/DDTS2A would be carrying signal) to find only a nasty noise on DDTS2A, so I guess it comes like that from the input board. But that won't appear in the schematics, and I'm having trouble finding it online, does anybody have it or know where to find it?

                                    I also made a few laser thermometer checks from different angles and I am now pretty certain Q105 and Q104 aren't getting all that hot, more likely it's the legs from R124/R135.

                                    Comment


                                      #58
                                      Re: Crest CPX1500

                                      ...And I begun from the end didn't I? hurray!

                                      no, now seriously, I just realized P101 aint the beggining as I assumed but the end, the beggining comes along p100, whitch doesn't carry any line either so im in the same spot.

                                      I took the board out and checked diodes, cross-checked resistors (only one missmatch) and the capacitors.. I used a capacitance check on my DMM whitch I didn't really believe was real(it is a cheap unit) and managed to read some values that matched with their pairs. :O

                                      Signal traced and found traces on few places but the signal dies after some resistors, allthough without schematic don't have a clue..will need a better magnifying glass... looks like this will be a slow one!
                                      Last edited by 5inc; 07-04-2017, 02:32 PM.

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                                        #59
                                        Re: Crest CPX1500

                                        If you bridged the opamp power pins you could have shorted the supply and blew the fuses. Or perhaps a ground clip in the wrong place?

                                        Anyway, can you post some good photos of this input board? And point out where these resistors are, where the signal disappears.

                                        Might help also to show pictures of which cables connect to which connectors on each board.
                                        "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                                        -David VanHorn

                                        Comment


                                          #60
                                          Re: Crest CPX1500

                                          okay my mistake, the input board is just fine.
                                          While trying to find where the signal dies to show you, I swapped channel to find the path went all the way through on this one, and compared all the way from there untill I found there was a switch turned off. Set both buttons like the channel that worked and got away with it without having to heat anything up yay!

                                          Now, signal arrives through P100 in the B channel board, and after raising the volume knob signal arrives to the main board through P103, and then in between R100 and R101 is the last point there I hear the signal propperly, after R101 it diminishes, and after R100 it dies. Just after that come Q100 and Q101, where I reckon trouble is waiting. Will fetch some tomorrow.

                                          Do you still want me to post the pics about the connectors? I think it's pretty straight-forward from now, I'm starting to get confident with the tracer And yes, I probably bridged the opamps, I make all tests inside the box, so no ground clips yet.

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