Bad Tube in my "new" Hammond C2 Organ preamp? HELP w/ diagnosis!!

jeffstev

Member
2008-01-27 10:42 pm
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
 

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Re: Bad Tube in my "new" Hammond C2 Organ preamp? HELP w/ diagnosis!!

jeffstev said:
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.

Jeff

This paragraph is the key to your problem. The sound fading away is the signature of a tube filament dying out. The high plate voltage clinches it. Without current flow through the tube, the plate voltage will be high. Basically the same as the B+ side of R24. Because there is no current draw, there is no voltage drop across it. Chech the tube and/or replace it.

Victor
 

jeffstev

Member
2008-01-27 10:42 pm
Re: Re: Bad Tube in my "new" Hammond C2 Organ preamp? HELP w/ diagnosis!!

HollowState said:


This paragraph is the key to your problem. The sound fading away is the signature of a tube filament dying out. The high plate voltage clinches it. Without current flow through the tube, the plate voltage will be high. Basically the same as the B+ side of R24. Because there is no current draw, there is no voltage drop across it. Chech the tube and/or replace it.

Victor

OK, THANK YOU!! As I've been rolling this around in my head for the last 2 days, it's only logical and it's why I keep coming back to that section of that particular tube. Transistor circuits operate in a similar fashion so this should be somewhat analogous. Open collector/emmiter & no current, B+ voltage appears.

Thanks for all the thoughts on this. I'm the kind of guy who really wants to understand the problem so I can learn from it, not just throw stuff against the wall and see what sticks (although I've done that WAY too many times in the past!).

I've really been through hell and back to get to this point with this Organ - for a fun read:
http://organforum.com/forums/thread/47333.aspx

Jeff
 

m6tt

Member
2007-08-08 12:12 am
Some of the old hammonds are unfused...a short from B+ to ground or some other problem may be overtaxing the rectifier? Many Hammond organs use a couple identical tubes in the preamp. Have you tried moving them around and seeing if the problem changes? (i.e. my m2 has 2 6sj7s, and one was bad, I found it by swapping the two and noting what changed)
 

jeffstev

Member
2008-01-27 10:42 pm
ThSpeakerDude88 said:
See if one of the sections does not glow in a dark room. That will tell you if the tube has half a filiment burned out. ;) If you havent done that already....

Now there is a simple, elegant troubleshooting technique that I knew would bubble up to the top from one of you guys. It reminds me of when I was a 13yr old kid and I started fixing stereos and TV's. look for the burned out fuse, resistor, diode or transistor that's cracked in half. It's just that my "tube experience" is limited, so I didn't even think of that.

Guess what? The filament on the side of the 6SC7 tube that shows the high voltage and isn't working is DARK. Imagine that? :smash:

Thanks guys for all the tips. I'm finding a new tube today!

Jeff
 
Re: Re: Re: Bad Tube in my "new" Hammond C2 Organ preamp? HELP w/ diagnosis!!

jeffstev said:

I've really been through hell and back to get to this point with this Organ - for a fun read:
http://organforum.com/forums/thread/47333.aspx

Jeff
Hi :D
If You like church organs, this is a link of Italian historical builder
that have worked Also for vatican city.
http://mascioni-organs.com/

Sorry for this OT but I can't reach Your email.

bye :)
 
Noisy Hammond C2

I have a C2 also and am experiencing a static type sound through two different speaker systems. What I mean to say is that with a leslie 122 and a PR-40 amplified speaker systems hooked up at different times that static exists. I am suspecting the AO10 amplifier system in the C2. The sound is there when playing or not. and it gets louder with volume increase and on the lower notes. Any ideas what I might be looking at??

Thank you!
Rowland