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 8th May 2011, 06:44 PM #1 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH Phase Splitter Help http://support.fender.com/schematics..._schematic.pdf The link above is the schematic. Anyway I am having problems with the amp so I took some voltage readings. I got to the Phase inverter and at test point 17 (at the cathode before resistor 39) I am getting 75vdc when it should be 17vdc. I had some resistors laying around and replaced the two plate loaders and that didn't help. What should be my next step in diagnosis?
 8th May 2011, 07:17 PM #2 djoffe   diyAudio Member   Join Date: Oct 2008 Leaky C18? Check C18 for leakage...easiest way...there should be very little voltage drop, a few volts, at most, across R38. If there's more than that, then C18 is leaky. Another way...lift one side of C18 and remeasure the DC volts. If it drops close to normal, then C18 was your culprit.
 8th May 2011, 08:03 PM #3 llwhtt   diyAudio Member   Join Date: Jun 2008 Location: SoCal The schematic is wrong, if you calculate the current thru the cathode resistor, using 17VDC, it not even close to the sum of the plate currents. Check your plate resistor voltage drops, calculate the currents, add them together and that will be close to the combined cathode currents, multiply that by your combined cathode resistors and you have the voltage reading you should see, I guessing the 75 volts is close. What is your initial problem? Craig
 8th May 2011, 09:01 PM #4 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH The initial problem is when playing guitar through the amp there is a crackle/rattle sound that seems to follow certain notes that are sustained. It is present in both channels, I tried different speakers, tubes, and looked for bad connections and bad solder joints ( I basically reflowed all solder connections ). I then started to take voltage readings and stopped on test point 17 because it wasn't close to what fender states. I will take voltage readings and post ASAP
 8th May 2011, 09:12 PM #5 llwhtt   diyAudio Member   Join Date: Jun 2008 Location: SoCal For whatever reason Fender neglected to put grid stoppers on pin 5 of the output tubes, on the schematic anyway. Try installing some 1.5KOhm 1/2 Watt resistors, exact value is NOT important, right on pin 5 and see what you get. This always seems to fix weird noise problems after you've pulled all of your hair out. Many old Marshalls never had them and with today's tubes they always act up. Craig
 8th May 2011, 09:17 PM #6 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH voltage drops are 77 and 84 77/80000=.0009625 84/100000=.00084 .0009625+.00084=.0018025 .0018025*39820=71.77
 8th May 2011, 09:18 PM #7 llwhtt   diyAudio Member   Join Date: Jun 2008 Location: SoCal The schematic is WRONG just like I said. Put those grid stoppers in! Craig
 8th May 2011, 09:22 PM #8 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH So I place them in series after C21 and C22 to pin 5? I don't have 1.5k can I use 1k? Last edited by famousmockingbird; 8th May 2011 at 09:31 PM.
 8th May 2011, 09:39 PM #9 llwhtt   diyAudio Member   Join Date: Jun 2008 Location: SoCal Try the 1Ks, there are NO resistors there now are there? Pull the wire off pin 5 of each output socket, clip the leads of the resistors to 1/4" or so, install the resistors on pin 5, then connect the wire to the resistors. If this doesn't fix your problem they should be there anyway. Craig
 8th May 2011, 09:41 PM #10 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH Yup no resistors, firing solder gun up will check back shortly.

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