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CRATE BV amp Ch1 prob- "Crunchy" Noises.

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  • #31
    Well the fact that the Clean Volume has absolutely no affect on the noise points to the noise being 'after' the volume control.
    So from the entrance to the OC to the beginning of the power amp you have a problem.
    To be honest, I cannot help all that much as I would use my Oscope to find the source of the noise.
    Thinking beyond that thought, you may be able to 'see' the noise with a volt meter.
    It may show itself as an erratic DC voltage , where it should be steady.
    Or it could be seen as an erratic AC voltage riding on the signal.
    There are a number of Jfets that are used to switch in & out certain parts of circuits whether the amp is set on clean or gain.
    Truthfully, I do not believe that you are setup for the level of troubleshooting that is called for on this problem.
    You know the problem with the amp.
    That is the information that a tech needs.
    "What's wrong".
    So, my kindly advise is to take it to a competent tech & let them have at it.

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    • #32
      Great! that makes sense and altho Im not an expert (!) its the area i thought surely must have some logic to therefore concentrate on given the symptoms I have soley in ch1.

      The noise as I jusy previously mentioned remains whether vol is then turned up or not. IE when Ch2 is switched to and vols down, its quiet.. but when I switch across to ch1 (vol down) a persistant intractable noise hits me as Ive described. Upping the ch1 vol does not affect this noise.

      Great- I think Im finally getting s'where!! Ok so herefore Im inclined to replace each C and R in this little "ch1 section we agree on area.. as I have no idea how to isolate/ test the problem further (ie to a single component). What do you think of this idea? does it make any sense to you?

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      • #33
        Originally posted by g-one View Post
        Yes, I would look at the branch around the Ch.1 tone controls. But what does the clean volume control do to the noise?
        If you can get rid of the noise by turning down the clean volume, then the problem is BEFORE the clean volume pot.
        If turning the clean vol. down does not affect the noise, then the problem is the vol. pot or something after it.

        Edit: Sorry, I missed the part where you said the volume control has no affect on the noise. So that rules out anything before the pot.
        Great! that makes sense and altho Im not an expert (!) its the area i thought surely must have some logic to therefore concentrate on given the symptoms I have soley in ch1.

        The noise as I jusy previously mentioned remains whether vol is then turned up or not. IE when Ch2 is switched to and vols down, its quiet.. but when I switch across to ch1 (vol down) a persistant intractable noise hits me as Ive described. Upping the ch1 vol does not affect this noise.

        Great- I think Im finally getting s'where!! Ok so herefore Im inclined to replace each C and R in this little "ch1 section we agree on area.. as I have no idea how to isolate/ test the problem further (ie to a single component). What do you think of this idea? does it make any sense to you?

        Comment


        • #34
          Originally posted by Jazz P Bass View Post
          Well the fact that the Clean Volume has absolutely no affect on the noise points to the noise being 'after' the volume control.
          So from the entrance to the OC to the beginning of the power amp you have a problem.
          To be honest, I cannot help all that much as I would use my Oscope to find the source of the noise.
          Thinking beyond that thought, you may be able to 'see' the noise with a volt meter.
          It may show itself as an erratic DC voltage , where it should be steady.
          Or it could be seen as an erratic AC voltage riding on the signal.
          There are a number of Jfets that are used to switch in & out certain parts of circuits whether the amp is set on clean or gain.
          Truthfully, I do not believe that you are setup for the level of troubleshooting that is called for on this problem.
          You know the problem with the amp.
          That is the information that a tech needs.
          "What's wrong".
          So, my kindly advise is to take it to a competent tech & let them have at it.
          Absolutely right JazzP. All that makes sense. But just before I do take to a tech (I dont have one anywhere nrby/ know of any for even 50m) I will try at least to do the blind-but-with-some-logic swapping a few Cs and Rs etc --IF-- I can even if at the very least distinguish a possible/ probable problem area. Thats all I wanted to do. Thats all I expected to do with knowledge from this thread. I just needed to find the likely area in Q, by way of the sytmptoms I gave.

          I can swap all the Rs and Cs now in Q. If that doesnt do it I will do the 3 pots. THEN/ after that: it is a total write-off.

          Comment


          • #35
            Originally posted by Sea Chief View Post
            Great- I think Im finally getting s'where!! Ok so herefore Im inclined to replace each C and R in this little "ch1 section we agree on area.. as I have no idea how to isolate/ test the problem further (ie to a single component). What do you think of this idea? does it make any sense to you?
            No it does not. We just proved it could not be anything before the volume pot, so that rules out all those C's and R's in the Ch.1 section.
            Now I will throw you another curve ball. Put the amp on Ch.2 and crank up the master (with gain down). Is the same noise present? If so, the noise problem is not isolated to Ch.1. A noise problem that is common to both channels would not be heard on Ch.2 if the master was turned down.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #36
              Christ. I thought the 'after' was this ch1 area we agree on.

              Right this does leave ONE possibility. To me. That is the only thing remaining after the vol pot in the ch1-area-we-agree-on is the OC1. Now, I cant see anything at all it could be (as there isnt physically anything else) other than the OC1.. and more specifically, if its 'switching' diode 1/2 is obviously ok (it switches) and its other 1/2 looks like a resistor under its shell.. then the problem cannot be anything/ anywhere other than this resistor section blown- can it?

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              • #37
                You are assuming too many things. Sure OC1 could be noisy. Easy trick is to short across R12 with a wire, or otherwise ground pin 7 of the tube. If that stops the noise, then the noise is before that point, and apparently after the volume control. Only thing between those two places is OC1. NO Rs and Cs involved. If in doubt, you could swap OC1 and OC2, trade places. The noise goes to the other channel or not, and there you go.

                There is only one PATH after the OCs, but that path is not unchanging.

                But what if that doesn;t kill the noise? I thought I pointed out JFETs Q2,3,4,5,6. All of those are in the circuit, they switch other parts in and out to set gain and signal level. Look at Q5,6. They ground the bottom end of the tone stack and post control for the other channel. But when they are supposed to be off for the CH1, they are still there, and could potentially inject noise. For that matter D4,5 could be noisy. The other JFETs could be guilty in similar fashion.

                SO the key is always to isolate the problem. You turned down the volume and the noise remained, so it is after the volume control. You have two tubes, so pull V1, it is the first two stages. If the noise goes with the tube, then we know the noise is from those first two stages somewhere. If it remains it is in the V2 half of the preamp.

                You want to hunt for it? I had you ground the grid pin 7 of V1.

                Note: I arbitrarily called the two sections of V1: V1a and v1b The schematic doesn't name them. I needed to call them something, so sorry if I caused confusion. For me and in this thread only, the A section is pins 1,2,3 and the B section 4,5,6.

                If grounding pin 7 of V1 kills the noise, we know it was before that point, but if it doesn;t, move along to V2a, short across R16 or otherwise ground pin 2 of V2. Kills noise? No? Short across R28 or otherwise ground the grid pin 7 of V2. Grounding each of those points tells us if the noise is before or after each point.

                I can ground those grids because they all sit at zero DC volts. If I add a cap in series with my grounding test wire, I can ground signal path nodes that do have DC on them. But we don;t need to.

                There are many, many techniques for hunting down a noise in the signal path.
                Education is what you're left with after you have forgotten what you have learned.

                Comment


                • #38
                  Enzo- thanks for that reply. Ive already learnt what a triode is from this thread, the first triode then pins 1,2,3 (a) and second triode pins 4.5.6 (b) and the other three pins two of which are heaters and erm last pin erm s'thing or other.

                  Ok I still have my beady eye on the OC1. Call it a numpty's hunch-fine, but s'times numptys can be right. I did think of swapping the two OCs around, but they are pigs to remove & tracks are not terrific at all/ easily ruined. So plan of attack:

                  1) short across R12. If noise goes, does that mean I have found cause to be OC1 or is that a dead R12 (or both maybe?).
                  2) put a ground jumper wire from pin 7 of V1 to ground. If noise goes I -have- found OC1 to be the culprit.

                  I will do these first- then I will consider the pesky transistors: these take me time to figure out on the schematic, which may be easy for you guys, but are hugely difficult for me to follow; bar the symbol for * is * they're like another language.

                  Comment


                  • #39
                    g-1 pointed out to me my typo. The B section is of course pins 6,7,8, NOT 4,5,6. Sorry.

                    Think of the amp as a process, a route from one end to the other, a path. Like a highway. The whole idea is to determine where on that route the problem exists. We then look at the point for the cause. If there is only one part it could be, then great. If it is three or four possible parts, OK, we determine which one. My grid grounding just pinches off the path at each grid.

                    Look at the circuit. R12 is there for either channel, your symptom is only on CH1. SO I doubt R12 is the problem, but if shorting it kills the noise, it means the noise is from before it.


                    Lets try it the other way. You think OC1 is noisy? Short across it then with a wire. Then its LED is lit, its resistor element goes lo resistance. Well a dead short with a wire is about as low resistance as it gets.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #40
                      (Yes I noticed your/ my typo. 123/a 678/b triodes understood. Thats good even if I throw this **** in the river!)

                      Ok looing at it again I am now confused as to why you suggest test R12 as it seemed to affect both channels (and pin 7 too) so i couldnt apply any logic to these two tests as surely there woud be symptoms on the other ch2 if either was iffy-? Can you say if I am right, or wrong here. If Im wrong there is no point me continuing with this as its complete double-dutch now/ I cant see anything that makes any sense.

                      Again the noise is ONLY on ch1.

                      I think I have to presume, and test accordingly. I will to start with, presume OC1. Ok now when you say short across it, it has 4 legs so not so simple. Now, afaict (from the schematic) it has two parts, a resistor side and a diode side. So each has two legs that need to be ID correctly before shorting. Am I thinking on the right lines here??

                      A) If the led works, then can I assume the diode side is good.. and therefore IF it is indeed the OC1 then that must have narrowed it down to the resisitor side. Am I thinking along the right lines here in order to put a short across two of the four legs? (or do you suggest shorting both 'sides' or do you not consider there to be 'sides' at all?)

                      I have been trying and trying, as Ive said many times, to try and get help to 1. at least narrow-down the area in which I can concentrate. Then I can hopefully 2. narrow it down to a few components. And then hopefully 3. to one component. All

                      I need is help with process 1. to start with. Then I will ask further for help to try and get to 2. etc.

                      Comment


                      • #41
                        The Vactrol VTL5C6 is marked on it's upper surface as to which legs are what.
                        You will find 'Cell' on one end & 'LED' on the other.

                        What Enzo is asking you to do is to short across the 'Cell' end.
                        That in itself (the short) will eliminate any OC issues from the equation.
                        If the noise is gone, then that OC is bad.
                        Another thing, Enzo is trying to hammer home is troubleshooting technique.
                        Often times a tech will utilize a 'divide to conquer' approach.
                        You narrow down where the problem is by starting at the beginning & the end of the circuit.
                        If the problem is at the end, what you would then do is 'divide, the circuit in half.
                        Again, is the problem at the middle.
                        If 'yes', then the problem is between that point & the beginning.
                        If 'no', then you divide the circuit in half again, towards the end.
                        This is a very effective approach to narrowing down where the problem is.

                        Note: as an aside I feel for you as there is not any documentation on the physical location of the components on the circuit board as referred to the schematic.
                        It is called a 'Layout Diagram'.
                        In your case it may help you if you actually draw a crude diagram of the circuits in question.
                        (yes it will take time)
                        It doesn't have to be pretty, but something is better than nothing.
                        Last edited by Jazz P Bass; 09-16-2013, 06:17 PM. Reason: Spelling

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                        • #42
                          Originally posted by Sea Chief View Post
                          I am now confused as to why you suggest test R12 as it seemed to affect both channels (and pin 7 too)
                          It may be common to both, but this is simply a diagnostic procedure to isolate the problem. You have already proven the problem is after the vol. control. You are trying to figure out if OC1 is bad. While set on CH.1 you short out R12 with a wire. This will kill ANY noise coming from before R12 (like from OC1). If the noise is still there, then it is NOT coming from OC1.

                          Originally posted by Sea Chief View Post
                          presume OC1. Ok now when you say short across it, it has 4 legs so not so simple. Now, afaict (from the schematic) it has two parts, a resistor side and a diode side. So each has two legs that need to be ID correctly before shorting. Am I thinking on the right lines here??
                          Identify and short the 2 legs that are the resistor side, while set on Ch.1. Is the noise still there or not?

                          Edit: Sorry JazzP, simulpost.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #43
                            Ok but JazzP the word 'cell' means nothing to me. (Apart from a battery).

                            Is 'cell' another word then for a Resistor??? or have I seen the 1/2 of the OC as a resistor.. when in fact it is (a cell) something entirely different?

                            Pls help me on this:
                            1) Is the OC a two-section, two-halved component?
                            2) If so is one a diode, and one a resistor?

                            Comment


                            • #44
                              These are all 'static' tests by the way.
                              No signal is to applied at this point in time.

                              Comment


                              • #45
                                Man you have got to learn to look things up.
                                Google VTL5C6, images.
                                Here is the datasheet: http://www.excelitas.com/downloads/DTS_vtl5c6c7.pdf
                                (I am on an XP computer at the moment. For some reason, starting in the not too distant past, I can not upload images.)
                                You will see, as I have already stated, that one end of the unit is marked 'Cell".
                                There are two leads coming out of THAT END of the OC.
                                Put a jumper wire across them.

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