[bluenoteJML]

Hi - I've got a cornet2 (about 2 years old) and the sound on one output (left) has gotten very low.  The other channel is fine.  I've tested everything else with a pro-ject tube box II and the sound is fine on both channels using it.  Does anyone have any ideas as to what might have gone bad?  Thanks.

[decal]

bad tube?

[Bill Epstein]

Before you look inside, switch the 12AX7s to see if the lower output follows them. If not, then replace the 12AU7.

Next, switch the interconnects, but they're usually intermittent or dead if bad.

Finally, if you're able, carefully take the measurements shown on the board and post the results. If you can't, then you'll need a technician to have a look. 280 volts DC with 60 milliamps of current is potentially lethal.

[bluenoteJML]

Thanks for the replies.  I, of course, don't have a 12AU7 on hand to swap out but do have several 12AT7's from an tube roll on my amplifier.  Will a 12AT7 work temporarily while I await the order I placed last night for new tubes?  On another topic - where do you all buy your tubes from?  Thanks. 

Jon

[tubesforever]

Tubesandmore.com is a decent source.  JJ's and Tung Sols are highly recommended for the 12AU and 12AX7 tubes.   Get them matched for your gear.  Its worth the extra little bit of investment.

Partsconnexion sells tubes as does PartsExpress, TubeDepot.com and others.  Sorry if I left out your favorite spot.  What's Kevin Deal's company called?  He is a great tube sources. 

Cheers!

[amandarae]

Hello,

First off, you have to have replacement tubes.  It is more simplier to troubleshoot the Cornet circuit if you do not want to go to "component level" if you have known tubes that work.

Switching the 12AX7 will not do anything, except moving the tube that is not working from stage 1 to stage 2 or vice versa  of the amplification circuit.  If you look at the schematic, the 12AX7's are being use "one triode" each per channel.  That is L channel, in your case, uses one triode of the first 12AX7, then the other triode of the next 12AX7.  The 12AU7 which is a dual triode also is being use as a cathode follower(CF).  I doubt it if the problem is on the 12AU7 because they are configured unity gain(gain=1).  If they are faulty, IMO, you will not hear any sound(audio) on the output.

If you do not have tubes for replacement and you already open the Cornet, and had tried different interconnect pair, inspect visually regarding RCA's connection to the board and ground.  Maybe, you were playing with interconnects earlier and the terminal got "pulled" for what ever reason.  You can also measure voltages test point indicated on the board to verify for the channel that's having a problem.

With the new tubes in hand (you can use 5751 if you have some in the interim replacing the 12AX7's), you can easily verify which tube is the culprit in no time and if it is really just a tube issue.

Goodluck!

Abe

[Bill Epstein]

Switching the 12AX7 will not do anything, except moving the tube that is not working from stage 1 to stage 2 or vice versa 

Good point. I recommended it from force of habit.

[amandarae]

Hello Bill,

Yeah!  I am guilty of doing that too.  I know that you un-intentionally thought that each 12AX7 contains both the gain stages for a particular channel.  I had that impression when I read the post, until I checked the schematic.  Just want to clarify it to the OP for easier troubleshooting of his problem.

Happy new year!

Abe

[bluenoteJML]

Hi All,
Thanks one and all for the replies.  I agree, I should have extra tubes on hand and do for all but the 12AU7 which I have rectified by ordering several different ones from The Tube Store.  I'll post the results tomorrow when I get the new tubes.

Jon

[bluenoteJML]

Okay.  I've gotten the new 12au7 tubes and tried one then the other.  No sound still on left channel.  Then I switched out both 12ax7 tubes and again no sound.  I'm still unpacking my stuff and have not found my extra rectifier tubes to try.  I'll take the power measurements tonight or tomorrow and post those.  Thanks.   

I also have a pro-ject tube box II phono-preamp that I had before the cornet2 and I tried it to be sure that the no sound issue was not another piece of equipment.  Perfect sound etc from both channels. 

[bluenoteJML]

I've taken (or tried would be more appropriate) volt measures on my cornet2.  No voltages were measurable at except at the R101 and R102 resistors.  Voltages there were 288.4 and 288.9 respectively.  No other places noted on the PCB with a volt measure showed any volts at all.  I've about reached my limit of expertise on this.  Thanks. 

[Bill Epstein]

Check your filament voltages with the neg probe on pin 9 and pos on 4 or 5 for each of the 3 12Axx. Let us know what they are. Then with the neg probe clipped to ground (ground terminal of an RCA jack will do) and your left hand in your pocket (no kidding), measure voltage at pins 1,2,3,6,7,8, and post those results. Also test point marked on board (150V) at R212 R&L.

Post those numbers. We'll figure something out.

[bluenoteJML]

Here are the measurements (had to keep right hand in pocket as i'm left-handed    Pin measurements are in order.  1,2,3, etc. 

12AU7 - 315.8, 141.2, 147.7, 5.8, 5.8, 315.9, 142, 148.7
12AX7 (closest to 12AU7) - 141.3, 0.0, 0.0, 5.7, 5.8, 142, 0.0, 0.8
12AX7 - 140.4, 0.0, 0.8, 5.8, 5.8, 143.3, 0.0, 0.8

R212L - 141.3
R212R - 142.1

Hopefully this will tell you something.  Thanks for helping.

Jon

[Bill Epstein]

The 12AU7 has what it needs and neither AX7 do, so the problem is getting B+ to the AX7.

See on the schematic how the B+ goes first to the 12AU7 and then in series to the 12AX7? Between the R216 point that should read 330 Volts and the 12AX7 there is R214, C204, R212, R211, R209, etc.  Re-solder all the thru hole solders, test the parts and re-solder all the tube solder pins.

There's probably a solder blob with an open joint lurking inside.

It's even possible a tube pin socket is broken, likely candidates would be 1&6, 2&7, 3&8, 9. Or you could say all but 4&5

If that doesn't work, take a couple of hits of something and call Jim in the morning.

[GRD]

As I look at the schematic, 150v is indicated for the plates of both AX7's.  The AX7's have a 150k Ohm resistor between the B+ and the plates, and the tube draws 1 milliamp.  So R202 and R209 drop 150 volts when the tube is conducting.  So it looks to me like the AX7's are working (drawing 1 milliamp and bringing the voltage down form B+ to around 150 volts).  The measurement of 0 volts on V201, pin 3 doesn't seem right - that should also read 0.8 volts.

I think the suggestion to reflow the solder joints along the signal path seems like a good one. 

[bluenoteJML]

Thanks.  Will do on the solder.  I just moved and had the tubes out of the cornet2 during the move, is it possible I broke a solder joint during the re-insertion of the tubes?  Regardless, I'll re-solder the joints and try again.  Thanks again.

[bluenoteJML]

Good Morning!  I'd like to thank Bill, Amandarae, decal and GRD for their suggestions and help.  It turned out to be a broken solder joint on one of the 12ax7 tubes.  All is now right with the world (I just hope that my neighbors downstairs like Little Ed at 8:30 in the morning) Thanks again.

Jon

[GRD]

Excellent!!  Good to hear you're back in business. 

 

[Bill Epstein]

It turned out to be a broken solder joint

What? I can't hear you! Turn that darn record player down!