Chasing down the problem

Couple of nights ago, I was listening to my mono block tube amps and after about ten minutes the right channel goes silent! What the heck!! So begins the search for the it upstream of the amps, or is it the right side mono block that has an issue...and what kind of issue.
Luckily, I own another stereo ss amp, so the first thing to do was to hook that up and see if the problem continued. This would eliminate a few pieces of gear as the culprit...basically everything in front of the tube mono blocks...except the connection from the preamp to the mono blocks...the ic’s.
The system played fine with the stereo ss amp in the chain...therefore leading me to believe that the following components were not at fault...1) the front end digital player and the front end turntable set up ( I did play both to be certain) 2) the tube preamp, 3) the connections between these components...speaker cable and ic’s ( I use different ic’s and sc’s to connect the preamp to the ss amp than with the tube amps).
The next day, I hooked up the tube mono blocks again...and this time after making sure that the connections at the amps were tight and that the tubes were all re-seated. Music played again in both channels ---for about twenty minutes! Then the same issue reared its head again, the right channel was silent. Hmm.

Therefore, unless I am wrong, the only two possibilities left are the right tube mono block...or the right ic feeding that mono block from the preamp. My next thing to do was to change the ic’s for another pair ( could have swapped left for right, but since I have other pairs of ic’s that i know are good, i figured why not try this next) Plan being to check the ic’s and then finally start to move tubes from one mono block to the other and vice versa to see if the issue changed channel.
Last night I listened to the same set up but with just the ic’s changed out between the amps and the preamp....and no problem. ( this after a two hour session).
Hopefully, I have identified the issue..would seem that the original right side ic has a iffy connection somewhere ( a lot easier to address than to have to get the mono amp worked on).
Anyone else have a problem like this that took a little ’work’ to identify?

749cdfb3 0814 490e b189 a364ad773263daveyf
Yes. Had a bad solder joint inside the RCA of one interconnect. Very hard to find as it worked fine for so many years, then gradually progressively acting up. Seemed from the beginning to be a connection issue, but exactly where? Intermittent faults are the worst. You get there but slowly. Eventually narrowed it down to inside the plug, and still wasn't sure (just seemed so unlikely!) until taking it apart the wire just pulled right off! Re-soldered, and no problems since.

Yours sounded from the start like another contact problem. In your case parts expand as they heat up, eventually reaching a point where they lose contact. It could be like mine where its inside the IC, where the wires solder to the pins. Or it could be the RCA connection itself. Even a tiny amount of play could be enough for the IC to wiggle just enough for the signal pin, the part that sticks out, to lose contact with the clip, which is inside where you can't see it. 

Not saying its either one, only saying now its narrowed down to a smaller area you have to really zoom in on every little thing going on in that area.
Great detective work. 
@gdnrbob, Thanks, who knew when we got into this hobby that one had to learn to be a detective as well as everything else, LOL.

@millercarbon  The plug in question is a WBT locking RCA. I suspect that there is indeed a bad solder joint. The cables in question are Nordost Tyr's...which have been very reliable up to now. I am probably going to have to send it back to Nordost for servicing. 
I had a very similar problem recently but in my case the left channel amp would kick out a very nasty hum to my speaker. I opened the amp up and everything looked fine. I ran another pair of  rca cables from my surround sound system which is separate from my stereo system and had the same problem.I was going to take it out for repair but decided to take one more look and again everything looked fine. When I touched the connection to the rca input it moved away from the connector. A little soldering fixed the problem.
@lwin  Your problem was actually at the RCA amp that correct?
I think mine is the RCA ic's...
@daveyf  Initially I thought it was a poor or problem connection on the cable end and it ended up being the rca connection inside the amp.
@lwin Thanks. That's interesting, although I have never had a hum....just channel drop off. 
Well, the problem is back...and this with different ic's. So, I am guessing that at this point that the ic is not the issue. Next step is to swap the left side driver tube on the left side mono block to the right side mono block and vice versa to see if the issue goes to the other channel. Stay tuned.
BTW, could a driver tube failure cause a cessation of sound? Seems a little odd, but onwards I go.
Have you tried pushing up from up under the RCA connector at the input of the amp or output of the preamp? I had this problem long ago on a BAT amp. I had to keep constant upward pressure on the RCA which meant I had to place a wood block under the RCA. Maybe not the best solution but it worked.

@paulcreed Thanks. Yes, I tried that and the problem still persists.
I swapped one of the driver tubes to the other mono block and vice versa...still problem persists. Tomorrow, after the amp has cooled down again, I will swap the other driver tube and try again. ( The amps employ one 12au7 and one 12ax7 per channel as driver tubes. The 12ax7 swap is next.) If that still doesn't expose the issue, then it will be replacing the power my case four KT 150's...and if that still isn't the issue...well then she's off to the repair shop..:0(
daveyf, a couple of questions. Why don't you just swap all the mono block tubes at once so you can confirm or eliminate a tube problem in one shot?  Are you using a SS or tube preamp, and if a tube pre amp when you hooked up your SS amp did you give it enough time that if it is a preamp problem it would show itself?
@jetter  Thanks for the suggestions. I did play the preamp with the ss amp for a few hours.Nothing showed up, so I am assuming that the preamp is ok. As a last resort, I will hook up the ss amp again and give it a very long test to see if the problem rears up again. As to replacing all of the tubes at the same time in the amps...I thought about this, but it wouldn't tell me which tube is the culprit if I did that...just that it is a tube problem in the amp. So, I am doing one set at a time, from left to right and vice versa. This is more time consuming, but should zero in on the particular tube if it is the issue. Lastly, i was thinking to swap my speakers over...left to right, as it is maybe possible that a x-over part is overheating, although I doubt this as the issue was not present when I used the ss amp. Nonetheless, it is easy to check...and it will be the last thing on my list before sending the amp for repair. Onwards--
Once again, good sleuthing techniques.
One question-
Can you open up the amp and check to see if the wiring to the connectors looks solid? Or, if anything might be shorting it out?
@gdnrbob  I could do that, but i am hesitant to do so as the amps are "theoretically' under warranty. I say "theoretically" as you never know with this stuff until you request service!
So tonight I swapped out the 12ax7 driver tube...and after a few minutes of idle power...a loud and nasty hum and buzz through my right speaker! ( the same channel that was acting up before!) I quickly powered down...and decided to change out all of the power tubes. ( I figured this might be a classic power tube failure). Upon replacement of all KT150's, I powered up again. This time, no loud hum and apparent issue at idle. I switched on the high voltage...and sure enough..same channel is still down!..:0(  
I'm pretty flummoxed at this point, so I think next is off to the dealer.

We need a tube wizard.
Hey, @Atmasphere!

Here's why tubes can be a major PITA! 

Tonight, I figured one of my tube mono blocks was I swapped in my ss Rowland stereo amp ( in place of the tube mono blocks) for a night of music and lo and behold...the same darn issue persists!! No sound out of the right channel! 
This is so very odd as my first thought, see my OP, was to do this test and the sound was fine before when the ss amp was in the chain. This would seemingly now mean one of two things..a) the preamp has a problem or b) the right speaker is blown. 
I swapped over speakers...put the left speaker on the right side and vice versa ( easy to do with stand mounts, luckily) Nope--the right channel is still silent. Hmmm. Seems to be a preamp issue ( which could still mean a tube has gone away, as I use a CAT tube preamp...) 
So, onwards we go...
@jetter looks like you you’re right in regards to the preamp... although I am so surprised that all was fine when I hooked up the ss amp and played it for a few hours previously. Any idea how a preamp tube can cause one to lose sound in one channel?
daveyf, I don’t have much knowledge about the inner workings of our gear like many here posses. I do know that preamp tubes last a very long time, but I would hope that you just need a new tube.

This is what I would do:

If you have not done so already, switch the interconnects from one preamp channel to the other and see if the problem follows the interconnect to the other channel.

If no luck, with the preamp off switch the tubes to the opposite channel and if the dead channel follows the tube switch you hopefully just need a new tube.

My preamp has two tubes for the line stage and two for the phono stage, and they are mounted horizontally, see this I have lost a channel two times over the years. Both times were because somehow a perfectly good tube or the vertical board the socket is mounted into had lost connection with the preamp. The first time I lost a channel I removed and reseated the two line stage tubes in the same socket and all was well. The second time I lost a channel, with the system off I just gently nudged the tube a little and the channel came back. Not sure why this happens but only twice in many years of use.

If that doesn’t work, while the lid is still off, as a poster mentioned above explore if by chance the RCA connecter in your preamp is damaged.

From here, if you haven’t already, I guess I would switch speaker cables left to right to make sure I don’t have a bad solder joint on a speaker connecter and if that doesn’t help with my limited knowledge of electronics its off to the shop for repair.
@jetter   Thanks. I will swap tubes in the preamp...left to right. But I can not see how this could be the issue as the preamp utilizes separate/different tubes for the phono stage and the line stage ( left and right channels)...and in my case, both the phono and line stage exhibit the same problem. I checked the cables, all are fine. I think it likely that this is going to be a repair for the tech.
To conclude this story...I finally had to take the preamp ( yes, not the amps at all!) to the tech. It turned out it was a switch on the preamp that was not functioning well and was intermittently cutting off the right channel!! My tech diagnosed this is minutes!
While I was playing with the amps, I inadvertently destroyed one of the Tele 12Au7 driver tubes and one of the KT150 power tubes went bad...but this didn't cause the problem in the OP! So, now that the preamp is up and running again, all is well. 
Motto of this story seems to be that for those who are less technically inclined--like myself--diagnostics isn't our strong point. 
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@daveyf ,
I, for one, think you did a pretty thorough job of trying to trace the problem.
Unfortunately, it was something you didn't consider, nor would have I.
A dodgy switch in the preamp would be one of the last places I would think to check.
Though, if it were to happen to me, I would contact Ralph Karsten (atmasphere). Though I own his equipment, I do know he will help anyone in distress, as his presence on Audiogon shows.- A very nice and generous person.
@gdnrbob. Thank you for the vote of confidence. Luckily, the issue has seemingly been resolved with the preamp. I recently rolled in NOS Telefunken 12AX7 smooth plates and Amperex 6922 PQ's into the phono stage, a very nice addition. The CAT really responds well to NOS tubes..
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@millidas   As I stated above, I did indeed go to my local tech, who solved the issue. He wasn't located in a shopping center however...:0)
daveyf, you can easily take the RCA apart, look for shorts and re heat the solder joints. Take 10 minutes. 
@mijostyn  If i do that on my Nordost cable, i lose the warranty. No thanks.
Then send it back. But even if it was on warranty it would take me less time to fix it then dragging it to a UPS store and I would have it to use immediately. 
@mijostyn  If you read the thread, you would see that the Nordost cable was not at fault! Good thing I did not get my soldering iron out......
BTW, could a driver tube failure cause a cessation of sound? Seems a little odd, but onwards I go.
It certainly can!

As you've found out though, that wasn't it. Here's a bit of troubleshooting advice, should you run into a problem that occurs in one channel and not the other:

The first thing to do **always** is simply swap the interconnect cables left for right at the inputs of the amplifiers. Then run the system. Did the problem move? If yes, the amps and speakers are off the hook. If no, its in the amps or speakers.

If yes, the problem moved, then you swap the interconnect cables left for right at the output of the preamp. Did the problem move? If yes, the cables are off the hook- its in the preamp or a source. If no, its a bad cable.

In this manner you can test every component in your system and its a lot lot easier than swapping out for another amp which could potentially have its own problems, creating confusion. Intermittents like the switch in your preamp make finding the problem harder; if the system seems to work fine after a change just let it play- sooner or later the bug will raise its ugly head.

If you had followed this procedure, you would have known much sooner (probably by a day) that the preamp was the problem, and no ancillary casualties. So keep this in mind. Electronics are very hard to make completely failure proof, so knowing how to go about this can make for far less frustration- something no-one needs!
@atmasphere Thanks Ralph. I did exactly as you recommended and followed this protocol. This is what ultimately lead me to the preamp, but in my case, since the issue was intermittent, and for some reason did not happen to show up when I changed amps, the true culprit was further hidden. To add to that, i changed the left and right interconnects at the amp and the issue followed the left to right change! This lead me ( falsely) to suspect the cables. I now believe that the way Ken Stevens has these toggle switches connected can lead to a intermittent issue with either channel, as the signal passes through all of his switches before going to the input or the output connectors. Not sure if this is typical in tube preamps, but there we have it.
If you are referring to those toggle switches on the front panel, yes, they are very capable of doing that. It is possible to clean them a bit, by putting the preamp on its rear panel and spraying a bit of Deoxit in the green and black can directly into the crescent moon that appears under the toggle handle, then operate the switch and do it to the crescent moon on the top. Then work the switch back and forth about 20-30 times. For this operation you don't even have to remove a cover.
Thanks again, Ralph. This is pretty much what my tech did when he received the preamp, except he did remove the top and sprayed from behind onto the offending switch. Seems to have solved the issue. When i told Ken that is what was the issue, he was not happy that Deoxit was spayed onto his gear, feels it is like salad dressing, LOL. 
I use break cleaning fluid for that, the old chlorofluorocarbon stuff. It is pretty much the same stuff they can as electronics cleaner. 
In my experience and I do not have a lot, once a switch starts doing this it will do it again sooner and eventually fail. Atmasphere do you think this is correct? I also think it depends on the quality of the switch. There are some pretty nice switches out there now. 
So, daveyf you still might have to pull out that soldering iron:)
@mijostyn  I was wondering about long the repair will last. So far, so good. These toggle switches are still available and I have bought several replacements if needed. It is no easy task to replace them and my tech feels it would be a pain to do. So here's hoping the fix continues to last.
The deal is a lot of toggles and switches in general, get a build up of worn material INSIDE the switch housing.. Different aerosoles, with different blends, do different things. BE very careful spraying brake fluid or starting fluid on ANYTHIG... Much less MEK.. and some solvents..

I've drilled hundreds of switches, sprayed, cleaned, and resealed with a dab of silicone... A LOT of crap usually comes out.. Then they work, half the time again, and half again... You can prolong the life a LONG time with maintenance.. :-)

3M (WD40), Deoxit, Liquid wrench, ALL make electronic products. I would use the correct one. One will wash and dry, one will lubricate with polymers, or petroleum. ALL should dry though... Some lubricates the contacts some lubricates the actual switch. It all wears, and leaves a mess..

@oldhvymec, +1,
I would use DeOxit- they have a number of different formulations for differing applications, but they are for electronics.