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

[jeffstev]

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/hammon...anual/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

[astouffer]

Measure R24 and see what resistance it is. Anything of this vintage should have coupling capacitors replaced and resistors checked for tolerance. As to why all the voltages seem higher than normal is because AC line voltages have crept up over the years. The schematic says 115 volts but its odd to see anything below 120 now.

[jeffstev]

R24 is 339k so it's right in the ballpark. C6, C15, C16 are all ok (ie not shorted). If the voltage at pin 5 is 300v, that's where I've been focusing my attention....

Jeff

[astouffer]

What voltage do you get at pin 4 of V4? It should not read positive. If a cap is bad you probably won't be able to tell on a digital meter. The wax seals go bad and moisture starts to collect and DC will leak through at higher voltages.

[HollowState]

Originally posted by jeffstev
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]

Originally posted by HollowState


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

[ThSpeakerDude88]

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....

[m6tt]

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]

Originally posted by ThSpeakerDude88
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?

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

Jeff

[Gold_xyz]

Originally posted by jeffstev

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
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