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Damn near started a fire in my Hot Rod Deluxe

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  • #16
    No, sorry, not sure how to check the zeners. I just received replacements by mail and will install new ones as soon as the 470 resisters I ordered arrive.

    How do I check the zener diodes? Sorry for what must be a beginner question.

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    • #17
      https://www.allaboutcircuits.com/tex...ck-of-a-diode/
      "Yeah, well, you know, that's just, like, your opinion, man."

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      • #18
        Thanks. And I carefully prodded
        the IC’s with a finger and didn’t feel any heat. I know the 470r resistors are hot but that is likely normal.

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        • #19
          The resistors are gonna get real hot if they're dropping 45 volts. You shouldn't run it more than long enough for testing that way. You need to find the shorts/near shorts on that supply first. My guess is that the zeners are shorted, but it could be any number of things.
          "Yeah, well, you know, that's just, like, your opinion, man."

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          • #20
            Thanks. If I have time today I will install the new Zeners and see if that improves it.

            The Zeners I got from Digikey have the specs/part number but are 2/3 the physical size. Is that a concern?

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            • #21
              Was just about to install new Zeners when a heard a Loud boom and the power went out. Went outside, neighbors tree feel across the power line even took out the telephone tree- found parts of the transformer up near my house in pieces.

              Gonna take the rest of the day off.....

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              • #22
                Wow.
                Click image for larger version

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                As far as the zener size, the specs I found for new one is approx. 3.5mm diameter x 9mm length, so yes, quite smaller than older type, but still spec at 5W so no problem.
                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                • #23
                  Make sure to install them the proper direction. Zeners "zene" in reverse direction. Forward they are just diodes. So in correctly, a 15v zener has 15v across it. If in backwards, it is just a diode and you have about 0.7v across it.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #24
                    Reality check: the Diodes have a silver stripe (collar) on one end. The PSB has an outline that has an arrow on one end. Safe to assume the collar goes in the direction of the arrow? I guess I should use the correct terminology- cathode side or anode side.

                    Btw. I tested the old Diodes with my DMM and the readings were way off, tested the new ones and both were something like 580 ea. Not at home so that may not be an accurate number.

                    Prolly get the 5w 470k resistors in the mail tomorrow so I will put this all in on Monday if we have power.

                    Thanks to all for the patient guidance. I’m gaining a least a small bit of knowledge.

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                    • #25
                      The line end is the cathode, the arrow is the anode. The arrow points at the line
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #26
                        Originally posted by Oldmactech View Post
                        Reality check: the Diodes have a silver stripe (collar) on one end. The PSB has an outline that has an arrow on one end. Safe to assume the collar goes in the direction of the arrow? I guess I should use the correct terminology- cathode side or anode side.

                        Btw. I tested the old Diodes with my DMM and the readings were way off, tested the new ones and both were something like 580 ea. Not at home so that may not be an accurate number.

                        Prolly get the 5w 470k resistors in the mail tomorrow so I will put this all in on Monday if we have power.

                        Thanks to all for the patient guidance. I’m gaining a least a small bit of knowledge.

                        470K ??? 470 ohm, right?
                        Experience is something you get, just after you really needed it.

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                        • #27
                          Originally posted by nickb View Post
                          470K ??? 470 ohm, right?

                          Yes, 470R is what they are. Poor typing skills.

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                          • #28
                            Installed the two 470r resistors (had to jump one connection on each), installed two Zeners (had to jumper them also.)

                            Fired it up and the yellow channel light was still lit,no reverb, no channel switching. Got late so I gave up for the night.

                            Readings at the resistors is troubling. R79 -49.6 -17.9. R78 55.8 55.8. I obviously missed something, maybe jumper failed although I checked continuity. Possible the resistor failed out of the box? No smoke, actually played GTR through it for a minute-not bad.

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                            • #29
                              Look for a broken connection coming out of R78 (zener side).
                              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                              • #30
                                Rebuilt the traces on the 470r resistors and the two Zener Diodes.

                                R78 and R79 are now +/-48 and +/-16 in spec.

                                Some symptoms remain, sone changed.

                                Yellow overdrive light still lights even before standby switched.
                                Stuck on drive channel, master and drive pots now function not previously. No channel switching.
                                Normal volume now no longer functions. .
                                No reverb.

                                Guitar sounds good if you like the drive channel. Input one is louder than input two.

                                Going back to the schematic to look for clues.

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