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  • #16
    Originally posted by Helmholtz View Post
    I see low D, E preamp supply voltages.
    If the preamp has a low +D and +E voltages check 400VAC transformer voltage.
    If transformer voltage 400VAC, voltage at (first) resistor R5.6K-1 should be + 560VDC.
    It's All Over Now

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    • #17
      I don’t see a R 5.6k-1 anywhere...can you please tell me what R it is?
      Last edited by Perkinsman; 11-06-2020, 10:42 PM.

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      • #18
        resistor R5.6K-1 on your schematics page 2 top left
        1)
        R5.6K-1 is a resistor in the filter for preamp voltages
        Last edited by vintagekiki; 11-06-2020, 10:50 PM. Reason: 1)
        It's All Over Now

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        • #19
          Ok, thanks. I found it, 436vdc on one side, 369vdc on the other. 406VAC across blue to yellow transformer wires.
          Last edited by Perkinsman; 11-07-2020, 12:24 AM.

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          • #20
            The amp still doesn’t have any volume when switch is on “hi”, very low volume on “lo” & a decent volume on “clean”.

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            • #21
              Originally posted by Perkinsman View Post
              Ok, thanks. I found it, 436vdc on one side, 369vdc on the other. 406VAC across blue to yellow transformer wires.
              Are you sure you measured DCV at the first of the two 5.6k resistors, the one which connects to the 1M resistor and the rectifier diodes?

              If so voltages are much too low.
              - Own Opinions Only -

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              • #22
                Originally posted by Perkinsman View Post
                The amp still doesn’t have any volume when switch is on “hi”, very low volume on “lo” & a decent volume on “clean”.
                Change the V1 (12AX7) with a new tube (not like new). Through V1 pass "hi" signal.
                If with changing V1 get a "hi" signal, but "hi" and "lo" signals have low volume, check (replace) V2 or (and) V3 (12AX7) tubes for processing "hi" and "lo" signals.
                It's All Over Now

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                • #23
                  Originally posted by Helmholtz View Post
                  Are you sure you measured DCV at the first of the two 5.6k resistors, the one which connects to the 1M resistor and the rectifier diodes?

                  If so voltages are much too low.
                  With 406VAC voltage at blue / yellow transformer wires, voltage at the first of the two 5.6k resistors should be the order 560-570DCV.
                  Maybe the voltages 436vdc / 369vdc values for + D and + E.
                  It's All Over Now

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                  • #24
                    I replaced V1, 2,3&5. Remeasured Yello/blue=409vac
                    One side of 5.6k-1=455vdc, other=406vdc
                    Do preamp tubes impact those voltages? If so, I’m wondering if all the tubes should be replaced.
                    Attached Files

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                    • #25
                      Originally posted by Perkinsman View Post
                      I replaced V1, 2,3&5. Remeasured Yello/blue=409vac1)
                      One side of 5.6k-1=455vdc, other=406vdc 2)
                      Do preamp tubes impact those voltages? If so, I’m wondering if all the tubes should be replaced.3)
                      1)
                      What did you get with the replaced V1, 2,3 & 5. Is the amp working properly now.
                      2)
                      Voltage drop on 5.6k-1 is 49VDC and it is made by current 0.00875 which can be fine for 9 triodes (0.98mA per triode).

                      Voltages on schematics should be observe with reserve because something wrong with the values. Probably is error in writing value for voltage between page2 and page3.
                      page2 D+ = 366V page3 D+ = 348V
                      page2 E+ = 365V page3 E+ = 345V
                      Tube V3B on the anode has 367VDC, no anode resistance, that same anode connected has D+ = 348VDC.
                      3)
                      A voltage drop is created through resistors R5.6K-1, R5.6K-2 and R470-1 by flowing the current of all preamp tubes. The bigger the current, the bigger the voltage drop.
                      If not a problem look that between preamp tubes by mistake did not "stray" 12AT7 which has about 10 times the anode current from 12AX7.

                      Basically if you want the amp to be in mint condition, the best option is to replace the preamp tubes V1 to V5, because you don't know how long they worked before.
                      It's a expensive option and thus won't learn anything. One new tube (not like new) 12AX7 is enough to fix the amp, and the procedure as described # 22
                      1)
                      Resolder point on 1N4007 (red arrow) because seems as cold solder.
                      Click image for larger version  Name:	image_52346.jpg Views:	0 Size:	682.3 KB ID:	917550
                      Last edited by vintagekiki; 11-08-2020, 04:41 AM. Reason: 1)
                      It's All Over Now

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                      • #26
                        Originally posted by vintagekiki View Post
                        Resolder point on 1N4007 (red arrow) because seems as cold solder.
                        Click image for larger version Name:	image_52346.jpg Views:	0 Size:	682.3 KB ID:	917550
                        That ^^^.
                        The low DC at the 5.6k-1 looks like a bad rectifier dode connection.

                        - Own Opinions Only -

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                        • #27
                          Originally posted by vintagekiki View Post
                          1)
                          What did you get with the replaced V1, 2,3 & 5. Is the amp working properly now.
                          2)
                          Voltage drop on 5.6k-1 is 49VDC and it is made by current 0.00875 which can be fine for 9 triodes (0.98mA per triode).

                          Voltages on schematics should be observe with reserve because something wrong withe values. Probably is error in writing value for voltage between page2 and page3.
                          page2 D+ = 366V page3 D+ = 348V
                          page2 E+ = 365V page3 E+ = 345V
                          Tube V3B on the anode has 367VDC, no anode resistance, that same anode connected has D+ = 348VDC.
                          3)
                          A voltage drop is created through resistors R5.6K-1, R5.6K-2 and R470-1 by flowing the current of all preamp tubes. The bigger the current, the bigger the voltage drop.
                          If not a problem look that between preamp tubes by mistake did not "stray" 12AT7 which has about 10 times the anode current from 12AX7.

                          Basically if you want the amp to be in mint condition, the best option is to replace the preamp tubes V1 to V5, because you don't know how long they worked before.
                          It's a expensive option and thus won't learn anything. One new tube (not like new) 12AX7 is enough to fix the amp, and the procedure as described # 22
                          1)
                          Resolder point on 1N4007 (red arrow) because seems as cold solder.
                          Click image for larger version Name:	image_52346.jpg Views:	0 Size:	682.3 KB ID:	917550
                          1. All channels work now
                          2.Good eye on the D+/E+ difference, I’ll call Mesa & inform them so they can revise
                          3.I decided to replace all V1-V5 since they’re all old original. I’m also replacing the power tubes with a new matched quartet of 6L6gc’s
                          4. I reflowed that diode, thanks

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                          • #28
                            I had several Mesas on my bench with low volume issues and in most of them the culprit was a blown mute FET so make sure to check those as well.

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                            • #29
                              That’s a very good idea, because there is something else going on regarding the volume beside the tubes. Which Fet is typically the problem?

                              Can they be tested while in circuit? Are they proprietary or can they be had from Mouser?
                              Last edited by Perkinsman; 11-09-2020, 04:18 PM.

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                              • #30
                                There are 2 mute FETs on page 4 of the schematic. Those are Q3 and Q6. Q3 is the "main" mute FET and if blown would affect the overall volume. Q6 is the reverb mute FET and is not expected to affect the volume. Both FETs are J175 type which still can be found from many sources including Mouser.
                                They can be checked in circuit by measuring the gate (mid pin) voltage which in this case is expected to be around 11-12V. When blown usually it's much lower than that. You can also measure the resistance from source to drain (the two outer pins) which if dead will be very low.

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