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Hotrod Deluxe channel switching

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  • #1

    Hotrod Deluxe channel switching

    Yep. Another thread haha.

    Well. I've tirelessly been at this thing for 2 days now. It wouldn't switch channels at all at first. Replaced U3 and it started switching. But no MORE DRIVE. I've gone thru the voltage test points at least 5 times. Everything looks good EXCEPT:

    TP34 pin3
    -12v
    -11.7
    -11.7

    TP36 pin1
    -15v
    -15v
    -15v

    I replaced Q4 because it seemed that it should be turning OFF when pin7 goes low. Replacing it didn't help. Same readings. Replaced U3 again thinking maybe I blew it probing around. Then I replaced Q4 again because still same readings.

    I cannot for the life of me figure out why Q4 has the same -12v on C and E. I replaced it with mps6531 NPN 40v 600ma just like the 2n4401. Same leads too.

    Again. All other test points are correct.

    Baffled. Circuit board says 2002 fyi. So I think this is the right one. http://www.thetubestore.com/lib/thet...-Schematic.pdf
    Last edited by lowell; 01-25-2018, 11:02 PM.
    Tags: None
  • #2
    Have you checked the 'indicator' LED?

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    • #3
      What is the voltage on U3 pin 7 for clean / drive?
      Experience is something you get, just after you really needed it.

      Comment

      • #4
        Ok I also meant to mention that the MORE drive works if I lift D17. The red LED works too.

        U3 pin7
        +15.5 clean
        -10.5 drive
        -10.5 more

        However. Even in Clean with +15.5 on pin7... Q4 base is -11.5v?? This is why I've replaced it... very odd.

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        • #5
          Originally posted by lowell View Post
          Ok I also meant to mention that the MORE drive works if I lift D17. The red LED works too.

          U3 pin7
          +15.5 clean
          -10.5 drive
          -10.5 more

          However. Even in Clean with +15.5 on pin7... Q4 base is -11.5v?? This is why I've replaced it... very odd.

          I expect U3 pin 7 to go lower than -11.5V else Q4 will never turn off. Take a look at the data sheet:


          Click image for larger version Name: 4560 swing.JPG Views: 1 Size: 43.5 KB ID: 848400
          Is this a known reputable good 4560? If so lift R84 and remeasure U3-7. That 4560 has to sink a lot of current in 'drive'. Margiinal design IMHO.
          Experience is something you get, just after you really needed it.

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          • #6
            Again my bad. I replaced it with a 4558. Then replaced it with a TLO72. So yeah not sure those work in there as designed.

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            • #7
              Originally posted by lowell View Post
              Again my bad. I replaced it with a 4558. Then replaced it with a TLO72. So yeah not sure those work in there as designed.
              Naw, neither of those will work. Still the engineer that came up with this gem - using an opamp to drive a relay - should have been taken to the cleaners at the first design review. You could add a resistor say 47k from the base of Q4 to -16V. I think that will do it.
              Experience is something you get, just after you really needed it.

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              • #8
                Duh. 4558 definitely can't keep up when looking at the data
                Attached Files

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                • #9
                  I have to admit after all these years the opamp continues to be a fly in the ointment.

                  Nickb you rock thank you. Devil was in the details on this one. I'll try the 47k mod.

                  So I'm assuming pin7 will still be able to pull up Q4 base in Clean mode. To about 63% between +/- 16v. 47/(47+27) So about +4v. Hows the math? Probably off LOL.

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                  • #10
                    Originally posted by lowell View Post
                    Probably off LOL.
                    Well...that bit was right

                    It'll go to about -10.2 = -16+5.1+0.7 max , I would say.
                    Experience is something you get, just after you really needed it.

                    Comment

                    • #11
                      What about the 47k to -16v though? Doesn't that change Vb of Q4? I'm confused. How did you arrive at 47k from -16v to Q4b?

                      So now I am only getting -7v on Pin7. (Without 47k) Don't know what changed...other than this opamp possibly going bad.

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                      • #12
                        When pin 7 is low then the 27k and the 47k will form a potential divider. When pin 7 is high the 5.1V zener and the Vbe of Q4 will limit the voltage. 47K was a bit of a guess we just need to make the base a bit lower than it was.

                        If you are now getting only -7V at pin 7 then something has gone South. U3 maybe hot and into short circuit protect? Fitting the correct part is the proper solution.
                        Experience is something you get, just after you really needed it.

                        Comment

                        • #13
                          Right on. Well I tried 47k. Didn't work. So I kept lowering it till 27k and boom.. MORE drive. Not elegant... But it's working.

                          On another note. I couldn't for the life of me figure out why this drive channel sounded better than any HRDL I'd ever heard. Found that V2 pin7 grid had +2v on it. Wasn't the coupling cap leaking... bad 12ax7. My god this thing sounded killer. But alas... now it's "fixed." That was one HOT biased 12ax7 haha.

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                          • #14
                            So it only switches to MORE drive SOMETIMES with the footswitch. I think I'll wait till I get the 4560 in and install it.

                            Comment

                            • #15
                              You might as well try the stock type transistor in there too?
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              ​

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