Close call with a Cornet 2

Biff · « **on:** 4 May 2010, 10:51 pm »

Hey all - I'm new around here, so I apologize if something like this has been brought up recently.

I had a black plate RCA 5814A in my Cornet 2 that "misbehaved." I don't know if it was shorting or what, but I got some bad distortion in the right channel (though not very loud) and the tube smelled pretty bad. I quickly shut everything off.

The Cornet is working fine but this event wiped out the tape input on my receiver. The right channel is now very low in volume. Other inputs are fine.

I'm not too concerned about that - I'm just wondering if the Cornet may have sustained any hidden damage. It seems to work fine (with a different tube obviously). Should I just leave it alone or take a peek inside?

Thanks -
Biff

GRD · « **Reply #1 on:** 4 May 2010, 11:05 pm »

Better take a look inside. Since tubes don't smell, something in the Cornet got hot. Good time to check things out.

Biff · « **Reply #2 on:** 4 May 2010, 11:37 pm »

Hey there, thanks for the reply. I swear the tube smelled when I held it up to my nose, but I guess it would smell anyway!

Everything looks okay inside. At first I thought the transformer might have taken a hit because of the look of it (see photo), but I think that's just the paint they glop on the outside. It looked that way before the "event" (I took a photo when I first got it).

I think we're cool. No more screwing around with old tubes unless they're thoroughly tested.

Thanks again -
Biff

hagtech · « **Reply #3 on:** 6 May 2010, 08:19 am »

Look for char. Usually there will be some black residue or some visual indication of a failure via heat or smoke. Check the vents on the electrolytics. And then try cheap regular tubes. Make sure circuit is fine before you swap in something rare.

jh

Biff · « **Reply #4 on:** 6 May 2010, 01:57 pm »

Thanks, Jim. I think everything is okay.

Edit: it occurs to me that the Auricaps don't have vents as do typical electrolytics, correct? I guess they would be bulging if they had a problem.

B.

Yoda · « **Reply #5 on:** 6 May 2010, 07:04 pm »

I would double check voltages to make sure they're still in spec.

Biff · « **Reply #6 on:** 6 May 2010, 07:56 pm »

I didn't build it so I'm not sure where to test for voltage. I see 150V marked on the schematic in a couple of places.

Edit: I found all but the 365v on the schematic - I guess I'll find that one printed on the board. Negative is the chassis I assume.

Left hand in pocket...

Biff · « **Reply #7 on:** 7 May 2010, 12:51 am »

They were all right on except the two 155 points were closer to 165. Is that okay?

Thanks.
Biff

Yoda · « **Reply #8 on:** 7 May 2010, 02:39 am »

Yes - the manual says there could be +/- 10v on those voltage points depending on tubes, etc. Also make sure that heater H+ is correct - around 6.2v. Too high could lead to early tube failure, too low could mean poor performance.

Biff · « **Reply #9 on:** 7 May 2010, 03:12 am »

I looked all over for that one but couldn't find it. Where on the board is it?

Thanks for your help.

Yoda · « **Reply #10 on:** 9 May 2010, 03:12 pm »

http://en.wikipedia.org/wiki/12AX7

Bill measure across pins 4 and 9, and 5 and 9 - see the diagram above. The 12au7 pin layout is similar

Biff · « **Reply #11 on:** 10 May 2010, 01:23 am »

Thank you again.

bean_counter · « **Reply #12 on:** 12 May 2010, 09:12 pm »

I have been wondering... if the cathode follower tube was bad, and the end result was that the receiver tape input took the hit - that would imply that whatever it was that took out the receiver must have passed through the output coupling capacitor. I think that would be C8, looking at the original Cornet schematic (C208 in the Cornet 2?).

Would the cap be suspect after such an event?

Biff · « **Reply #13 on:** 12 May 2010, 09:24 pm »

Good question. How would I check that? I should add that I don't have a capacitor testing device, and it obviously doesn't have a short because the unit is working fine. All the caps look okay, no blown out vents or anything.

Biff

poty · « **Reply #14 on:** 13 May 2010, 11:47 am »

Re: "Would the cap be suspect after such an event?"
I think - no. But in each case it may be different. Your thoughts about why the bad tube affected the next input? - It could be sparking inside the tube (periodic shortages) which led to very high voltage AC noise which can easily go through the cap.

Re: "How would I check that?"
The main reason for the capacitor - to block DC. Then the simplest way to check if the capacitor meets that duty - to measure DC from the output.

Biff · « **Reply #15 on:** 13 May 2010, 01:49 pm »

That works for me, I wouldn't have to take the bottom plate off again...

What level of DC voltage am I looking for? Or should I just check the other channel and see if they're about the same?

Thanks -
Biff

poty · « **Reply #16 on:** 13 May 2010, 01:51 pm »

There shouldn't be any DC. Disconnect the output from the preamp before measuring.

Biff · « **Reply #17 on:** 13 May 2010, 02:12 pm »

I'll give that a try. Thank you all, I appreciate your help.

bean_counter · « **Reply #18 on:** 13 May 2010, 03:43 pm »

Quote from: poty on 13 May 2010, 11:47 am

It could be sparking inside the tube (periodic shortages) which led to very high voltage AC noise which can easily go through the cap.

I had initially thought that too, but where would the high voltage AC come from? The only AC present is signal level, from the amplifier stage.

Unless all bets are off when the tube shorts/sparks, and all kinds of unexpected things happen, like AC high voltage??

Biff · « **Reply #19 on:** 13 May 2010, 03:52 pm »

Nothing sparked inside the tube. The right channel was sort of rumbling but not loudly. The left was fine. Whatever it was, the receiver didn't like it.

When I verify that there is no DC on the output, do I touch the probe to the outside or inside of the RCA jack? Or both?