Bad Tube in my "new" Hammond C2 Organ preamp? HELP w/ diagnosis!!
So there I was, ripping out a solo during a practice on my "new" Hammond C2 organ and the sound falls away and gets soft to the point of no volume in the span of about 15 seconds. Down to nothing, nada.
I'm good at electronics (BSEE) and can fix anything, but I grew up with semiconductors, not tubes!! Using the schematic from the C2 AO-10 Preamplifier, pic/schematic attached and also located here:
http://www.keyboardpartner.de/hammond/diagrams/B-3_A100/service_manual/3-47.jpg
(it's the bottom circuit diagram)
I think I traced the problem to V4, a 6SC7 tube. The voltages are:
Pin 2 = 137v (stage 2, input from the exprssion pedal)
Pin 5 = 300v (stage 1, after the input from the vib scanner) - shouldn't this also be 137v????
Pin 6 = 1.6v
Therefore, I'm concluding that the first stage of that tube does not appear to be working. For my own edification, does this indicate a short? Open? I have good signal at the SCAN terminal if I send that directly to the amp, and touching pin 3 of this tube causes hum to be sent to my amp so that section appears to be working. The measurements on the power supply also seem WAY off, although ~ +10% range.
My +290v supply measures 316v, the +280 measures 309v. The heater voltage is 6.75v. Pin 8 of the 6X5 tube (rectifier tube? must be) is 346V. The coils on the power transformer measure 590v (297/297 to ground). Any problems with these numbers being this high?
Given all this info, I'm SURE that one of you is going to give me the magical answer.
I ordered a new cap kit a few days ago but I don't think there's a cap problem... this organ hadn't been played in years before I brought it home and I've been playing it for 4-5 hours at a time for the last several days/week.
Comments? Ideas? Of course, I don't have a spare tube but know where I can get one. It would just be nice to know what is going on before I just start shotgun replacing things...
Jeff
So there I was, ripping out a solo during a practice on my "new" Hammond C2 organ and the sound falls away and gets soft to the point of no volume in the span of about 15 seconds. Down to nothing, nada.
I'm good at electronics (BSEE) and can fix anything, but I grew up with semiconductors, not tubes!! Using the schematic from the C2 AO-10 Preamplifier, pic/schematic attached and also located here:
http://www.keyboardpartner.de/hammond/diagrams/B-3_A100/service_manual/3-47.jpg
(it's the bottom circuit diagram)
I think I traced the problem to V4, a 6SC7 tube. The voltages are:
Pin 2 = 137v (stage 2, input from the exprssion pedal)
Pin 5 = 300v (stage 1, after the input from the vib scanner) - shouldn't this also be 137v????
Pin 6 = 1.6v
Therefore, I'm concluding that the first stage of that tube does not appear to be working. For my own edification, does this indicate a short? Open? I have good signal at the SCAN terminal if I send that directly to the amp, and touching pin 3 of this tube causes hum to be sent to my amp so that section appears to be working. The measurements on the power supply also seem WAY off, although ~ +10% range.
My +290v supply measures 316v, the +280 measures 309v. The heater voltage is 6.75v. Pin 8 of the 6X5 tube (rectifier tube? must be) is 346V. The coils on the power transformer measure 590v (297/297 to ground). Any problems with these numbers being this high?
Given all this info, I'm SURE that one of you is going to give me the magical answer.
I ordered a new cap kit a few days ago but I don't think there's a cap problem... this organ hadn't been played in years before I brought it home and I've been playing it for 4-5 hours at a time for the last several days/week.
Comments? Ideas? Of course, I don't have a spare tube but know where I can get one. It would just be nice to know what is going on before I just start shotgun replacing things...
Jeff