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SWR Strawberry Blonde II; Permanent Loud Hum w/Cyclical Popping

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  • #16
    Same here. It comes the hard way when you're a "this" guy or a "that" guy rather than an overall electronics guy. I cut my teeth on SS a few years ago and these guys were CRITICAL to the projects success.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    • #17
      The Fender phone guys are not technicians, they have no way to look at a schematic and know if it is what you want or not. All they can do is look up in their data base, and see what it returns. Unless a note is made in the data base, they have no way to know that the circuit needed for model XYZ power amp is the same as the parts of model ABC power amp. And to be candid, there is a limit to what they will call the actual repair techs off their work to research schematics to hand out. So all we can do is make specific questions, and they can answer them or not.

      It was said the Rumble 100 was the same power amp circuit. So then, does that schematic seem to match your amp?


      Call me crazy, but when you have 41v power rails and only 25v on the output, I tend not to suspect a shorted output transistor. On the other hand, if +41 is missing, then -25 sounds reasonable to find.
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        Originally posted by Enzo View Post
        Call me crazy...
        Okay, you're crazy.

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        • #19
          Just got a quick look at the Fender Rumble 100 Power amp, and it looks very close. I hope to check the rail voltages this weekend.
          I'll post my findings when I have more info.

          Thank you for your help.
          Nick

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          • #20
            Thanks a lot! This appears to be the correct schematic for my SWR Strawberry Blonde II Power amp PCB!

            Nick

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            • #21
              The Fender Rumble 100 amp schematic is the right power amp schematic for this SWR Strawberry Blonde II. Using that schematic, it appears that Q15, Q17 & Q19 all have shorted Base-Collector junctions. All three are PNP devices. They are the last 3 output transistors on one side of the Power Amp output stage. I have direct part# replacements on order, as well as the two big power supply caps, C53 & C54. Though there did not appear to be any measurable trouble with these, I figured as long as I had the Power Amp PCB pulled out anyway, I'd go ahead and replace them also. Only a few $$$. I should have the replacement components later this week. The 41V and 15V power supplies appeared to be OK.
              I'll report back after I've replaced the components.

              Nick

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              • #22
                I'm not generally one for "shotgun" repairs. That is, just replacing a bunch of stuff because you haven't isolated the actual problem. I do however fully agree with doing maintenance in a convenient manor rather than wait until something breaks. It's a double edged sword
                "Take two placebos, works twice as well." Enzo

                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                • #23
                  SUCCESS!!! After checking components while using the Fender Rumble 100 schematic, I found the 41V and 16V supplies to be OK. The output driver stage on the PNP side was bad. Q15, Q17 & Q19 all had B-C junctions shorted. I replaced the Q15 Driver Stage transistor and the 2 Power Output transistors Q17 & Q19. It sounds great now.
                  But alas, I'm now concerned about the heat coming from the + & -16V Zener Diodes, D34 & D35. The cement 5W Power Resistors R101 & R102, between the 41V supplies and the 16V supplies are even hotter. I put some fairly massive heat sinks on them to see if that might help. But after 45 minutes of constant use, I could have fried some bacon & eggs on them. This circuit had been modded before I got the amp. Both R101 & R102 had another 5W cement power resistor added in parallel to cut their resistance pretty much in half. R101 is now 200 ohms, changed from 470 ohms in the schematic. R102 is now about 235 ohms, changed from 470 ohms in the schematic. I can't be certain, but it may have shipped from the factory like that. The PCB below the Zener Diodes was scorched, but not burnt. When I replaced the power transistors I mounted the Zener Diodes almost 1/2" above the board for better cooling. I seems hard to believe the PCB below the resistors was not scorched, also. Are these current limiting resistors for the 16V supplies?
                  I'm considering buying some aluminum 25W Power Resistors (200 ohm & 250 ohm) to replace the cement ones, and mounting them to the power output transistor heat sink. The transistor heat sink was still just a little above room temp.
                  Anyone have any other suggestions?

                  Nick

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                  • #24
                    I don't know why the added resistors were stuck in there, I would suggest going back to the original 470 ohm value. If you want to move them off of the board and chassis mount them, you can do that, but I think that you can just go back to the original style cement resistors with no real problems.

                    By lowering the value of the resistors, the resistors need to dissapate more heat than if the ohm value was higher. Remember that the voltage drop will be the same regardless of the resistance value.

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                    • #25
                      Thanks 52 Bill, I will try changing the resistor values back to 470 ohms and try that.

                      Nick

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                      • #26
                        I have finished working on this amp and have it all buttoned up and it's working great. I went ahead and removed the additional power resistors so that both the +_ supplies have a 5W, 470 ohm resistor. Far less heat is being generated now, and with a nominal heat sink across the two resistors, the temp stay very warm, but constant. The two +_ 16V Zeners don't even get hot enough to have to remove my finger from them.
                        Thanks to everyone who contributed suggestions & advice. Your help was invaluable!

                        Nick

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                        • #27
                          Hi, anyone here have a schematic for the preamp of this SWR SB II.
                          Mine is cutting out occasionally.

                          Thanks

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