Tweet
I had received a Fender Twin Amp from a steady client about a week and a half ago, which had been red-plating. I didn't find that happening while I had it on the bench, though there was evidence of it having cooked two of the four tubes, and only two of the four were still workable tubes, from a reasonable plate current match standpoint. Not having a matched quad set of 6L6GC tubes on hand, I ordered that set, which came in last Friday. When I installed them Monday before the client came to pick it and an Ampeg SVT-AV up, it was suddenly pulling loads of AC mains current in the Low Power mode, so I wasn't able to let it go until I got to the bottom of that.
Before I opened it up yesterday, I had pulled up the Service manual for it, and in their Tech Notes in the manual, Tech Note TN96-3, one of the known issues with it is from the failure of that Output Power Switch arcing internally as a result of continual switching power levels when the amp is in Operate mode (not in Standby). DPST toggle switch. One set of switch contacts changes the HT supply to the O/T Center Tap, while the other set changes the bias voltage from -52V to -20V (approximately). The fun begins when the HT contacts arc, and no longer reduce the HT voltage from 455VDC to 230VDC in the Low Power mode.
That seemed to be what I was getting, while it was fine in the High Power mode. After cutting off the fixed leads of the Cutler-Hammer DPST switch, and re-terminating the leads for the replacement DPST switch, I ground off the two rivets holding the switch together, having already left enough lead length to strip and test with the ohmmeter. My Fluke 8060A DMM has a Conductance Mode, that being the reciprocal of ohms, allowing VERY HIGH resistances to be measured (though not as high as an Electrometer). My 'low current' measurements indicated it was reading in excess of 10,000 Mohms, though measuring between the two sets of contacts, there was a reading, which there wasn't in the normal Ohms Mode (which will read up to 300Mohms). After I opened the switch for inspection, sure enough there was evidence of arcing between the terminals used on the HT side.
I didn't find any of the parts in the power supply circuit that appeared damaged, and all sounded/measured fine again with the new switch. I will confess having switched the Twin Amp and other amps having an Output Power switch while in Operate mode, though normally, I'd only change it while in Standby mode. Odd there's no warning posted around the switch regarding that distinction.
Tech Note TN96-3.pdf
Twin-Amp-1995-2001-Schematic-PCB Layout.pdf
Before I opened it up yesterday, I had pulled up the Service manual for it, and in their Tech Notes in the manual, Tech Note TN96-3, one of the known issues with it is from the failure of that Output Power Switch arcing internally as a result of continual switching power levels when the amp is in Operate mode (not in Standby). DPST toggle switch. One set of switch contacts changes the HT supply to the O/T Center Tap, while the other set changes the bias voltage from -52V to -20V (approximately). The fun begins when the HT contacts arc, and no longer reduce the HT voltage from 455VDC to 230VDC in the Low Power mode.
That seemed to be what I was getting, while it was fine in the High Power mode. After cutting off the fixed leads of the Cutler-Hammer DPST switch, and re-terminating the leads for the replacement DPST switch, I ground off the two rivets holding the switch together, having already left enough lead length to strip and test with the ohmmeter. My Fluke 8060A DMM has a Conductance Mode, that being the reciprocal of ohms, allowing VERY HIGH resistances to be measured (though not as high as an Electrometer). My 'low current' measurements indicated it was reading in excess of 10,000 Mohms, though measuring between the two sets of contacts, there was a reading, which there wasn't in the normal Ohms Mode (which will read up to 300Mohms). After I opened the switch for inspection, sure enough there was evidence of arcing between the terminals used on the HT side.
I didn't find any of the parts in the power supply circuit that appeared damaged, and all sounded/measured fine again with the new switch. I will confess having switched the Twin Amp and other amps having an Output Power switch while in Operate mode, though normally, I'd only change it while in Standby mode. Odd there's no warning posted around the switch regarding that distinction.
Tech Note TN96-3.pdf
Twin-Amp-1995-2001-Schematic-PCB Layout.pdf
)
Comment