Announcement

Collapse
No announcement yet.

Marshall VS65R signal path help?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #31
    ok so Im not getting any AC reading on either T9 base or T9 emitter (well, 2 mV if that counts). I cant see a Q4 so just in case you have a different diagram Ill try post the other two pages of the ones I have.


    Click image for larger version

Name:	65 preamp2.JPG
Views:	1
Size:	175.7 KB
ID:	847551
    Attached Files

    Comment


    • #32
      Sorry, I got my Q's and T's mixed up.
      So you had 1.78Vac at T4 base, I wanted to know T4 collector. Since you said zero at T9 base, it looks like there will be nothing at T4 collector, but check it anyway.
      While you're at it, check that R7 (at T4 emitter) and it's connections are good.
      Originally posted by Enzo
      I have a sign in my shop that says, "Never think up reasons not to check something."


      Comment


      • #33
        No, nothing at T4 collector. R7 measures 22R as it should (measured in circuit) and have checked connections and continuity to other components etc.

        Comment


        • #34
          Well, at least the problem area now seems to be isolated.
          How about DC voltages (no signal applied) at E,B, and C of T4.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #35
            E= -40vdc
            B= -39vdc
            C= -1vdc

            Measured in respect to signal ground, no signal applied

            Comment


            • #36
              Ok. How about DC, no signal, E,B,C voltages for T6, T9, T11.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #37
                yep, I got:-

                T6 E= -1 B= 0 C= +1

                T9 E= 0 B= +1 C= +40

                T11 E= 0 B= -1 C= -40

                I know we are concentrating around the power amp but you are aware that I did try swapping this board with a good one and the good one gave the same results? Also, I really do appreciate your help with this, its a good learning curve for me and when you ask for certain measurements, if they are not what you expect them to be would you mind just telling me what you expect and why?

                Comment


                • #38
                  There may be more than one fault, right now there appears to be one on this power amp board, so I don't want to assume it's good just because the amp didn't work with another good power amp board. There could be a preamp problem as well.
                  In post #33 you found no signal at T9 base, even though you had earlier found signal at T4 base (post #29). This indicates a problem.
                  Now I'm just trying to get basic DC operating conditions before continuing chasing the signal.
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #39
                    Have we documented all the DC power voltages in the amp? The Power amp and preamp DC voltages are important to check and post to the thread. Excuse me in advance if all of that has been posted already but I can't seem to see any of them posted. I have seen faults that occur on the preamp, i.e. a shorted small voltage capacitor, where it had a major effect on the power amp. So at first my trouble shooting lead me down the rabbit hole to rule out a fault in the power amp board voltage supply. In the end the fault was in the preamp which made it seem like the power amp had the short. Either way it would be a good idea to post all the working voltages at the start of the power supply. Not saying that is what is wrong but they should be checked.

                    Edit: Looking again I do see that there is confirmation of the -/+15v preamp voltages. And it is probably fine. Still I guess my main point is that perhaps the preamp is causing a related issue on the power amp board too. Is there a way to disconnect the preamp from the power amp to test the power amp independently? Probably not in this case but that has helped me in some instances in the past.
                    When the going gets weird... The weird turn pro!

                    Comment


                    • #40
                      g1 No I fully understand that I just wanted to make sure you had seen the post where I mentioned swapping it.

                      DrGonz78 yes I checked the voltages for both DC value and AC ripple. The reason I went towards IC7 at the very beginning is because although its on the preamp board I thought it was more concerned with the power amp? I can split the preamp and power amp by putting a signal into the return jack as g1 had me do but as I said i get a nice clean tone through it from my signal generator but nothing from the lower strength signal of a guitar. Again I didnt know if that was somethingn to do with IC7?

                      g1, I assume by your comment regarding T4 and T9 that your reasoning is that T4 doesnt appear to be working otherwise it would pass the base signal out the collector and onto the bases of T9 and T11 yes? Where to next?

                      Comment


                      • #41
                        The DC voltages look basically ok. Really, I expect T4 to be working.
                        Try again with signal and AC voltage readings, 100mV signal input to FX return jack, mix pot set full.
                        AC voltage readings at E,B,C for T4, T9, and T11.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #42
                          ok, set up as abov except its 120mV signal input ('cos thats the only setting it has!) I get:-

                          T4
                          E 112mV
                          B 112mV
                          C 13mV

                          T9
                          E 11mV
                          B 13mV
                          C 111mV

                          T11
                          E 11mV
                          B 13mV
                          C 111mV


                          All ac measurements, mix pot full up.

                          Comment


                          • #43
                            So look back at post #29 (and post #25), how do you explain such a difference?
                            You had almost 2VAC at T4 base.
                            Maybe double check.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #44
                              good question!! I have no idea where I got that from! Ive just double checked the voltages from post #42 and got the same again so I can only assume i measured the wrong bit back in post 29, Sorry.

                              Comment


                              • #45
                                Then redo post #25 and post results.
                                Originally posted by Enzo
                                I have a sign in my shop that says, "Never think up reasons not to check something."


                                Comment

                                Working...
                                X