Audio Research PH7

Hello everyone. Going to post this in this forum as well as the amp/preamp forum. Just want to run this buy you guys to see what comes out of it. I recently purchased gently used Audio Research PH7 phono preamp. The Preamp had 3 hours on the tubes according to the display. I have had it for a week now and till now it has worked flawlessly. Typically I will mute the preamp to change records, etc. Today I muted changed a record and when I unmated the right channel did not play. turned the unit off and waited for about 10 mins and powered it back up, and the right channel came back. muted/unmuted right channel goes out. According to the seller the PH7 lay dormat for a few years prior to selling it. I am wondering if it could be one of the tubes could simply be failing, but it seems more like a curcuit problem than a tube problem. Any thoughts or experience with this particular issue with the ARC PH7?
My personal experience with a weird/intermittent tubed equipment 
its rarely/never the tubes. It is however, easy enough to pop in a known good tube if you have spares or just switching like tubes.

A dead channel in my tubed phono, had tech support stumped.Eventually found  a cap wasn't seated correctly on the board.Apparently wasn't easy to see the problem.

Thanks Tablejockey. I agree about the tubes. I figured I would at least try and reseat them just to see what if any impact it would have. Further investigation I have determined that it has something to do with Temp as well. When I let the tubes cool and reseated them the unit performed as expected until it got to full operating temperature. Once hot the problem returned. It plays fine until I either mute it or put it in mono.Other than that it sounds great. As long as I don't mute it lol.  The PH7 originally was made with 3 power supply tubes which ARC came out with a upgrade to the unit to convert it to 1 power supply tube. This unit does not have that upgrade and I was considering sending it in anyway to have that done. I thought I would take my time doing that since I just got it and enjoy it for a month or two but I think I am going to just send it in with this new development. The buttons on the front panel connect to a small circuit board in the front of the unit that is not part of the main board so maybe its just simply replacing that board. 
Randy-11 I would assume so, but not sure. When I purchased this unit I was a little concerned about if the caps could have discharged while sitting so long, but I would think that would take 15+ years for that to happen. I am not a electrical engineer but based on what I have read it would take longer than a few years. Could be wrong about that. 

Randy-11 according to the shipping labels on the box it was shipped from ARC in 08.. it has been used since then but obviously with only 3 hours on the tubes not much since then 
Ok just took a look and yes there are several large electrolytic Caps on the main board. A couple on the piggy back board that the front panel buttons connect to. Visually they do not look bad i.e not deformed, or none are leaning or discolored. 
@tablejockey , +1. I doubt it is the tube, but it won't hurt to try.

Sounds like it could be a cold solder joint, as temperature seems to have some affect, or a capacitor. It could have been a weak cap to start with, however, two enemies for caps are high temperatures, and dormancy. 
Lack of cycling (on/off cycles) and/or high heat can shorten a capacitors lifespan significantly.
Trivia - There is a (square?) button on the main board to reset the tube hours counter to zero. Two 6H30 power tubes were replaced with one 6L6G/5881 tube because of short life and high failure rate. The 6922 signal tubes can be rolled with outstanding results. I have a used PH7 for 4 years now and very reliable. Great pre! It works best with .5 mv carts although my DV XX2 MKII is .28 mv and works great. The PH7 is very quiet.
@wlutke Good to know on reseting the tube hours. I figured there had to be a way to do it since tubes can be replaced. I agree that the PH7 is an outstanding pre. Great sound staging and very quiet even though I have not had the power supply upgrade. I have a call into ARC to see if that upgrade is still available. After more testing this appears to definitely be a temp problem and most likely with a bad resistor or maybe a cap. If I remove the cover to allow more ventilation the problem does not occur. Once I put the cover back on 15 mins later after heat collects in the cabinent the problem is back. Sonically there is no problem. it sounds fantastic, but don't want this to lead to a larger problem down the road. I found a guy here in the Atlanta area that specializes in tube gear and I am going to drop it off today. If I send it to ARC Im sure that i can count on being without it for 4-8 weeks. I will post back to let you know what the problem turned out to be. Thanks for all that replied. 
  Hi guys well we found the problem with the PH7. The mute relay was bad. The tech found the problem in about an hour including the time it took to fail. Right under $200 to fix the problem. Hopefully we have had our glitch for this mission.
Thanks for all the help. Back to the music.
I have been using a PH7 since 2009, having had a PH5 before that.  Mine developed an issue with either a cold solder joint or a weekend solder joint and so on occasion I get a loud hum when first turned on. I know where I have to push on the board to get rid of it. I sent mine in for the upgrade. I thought would've been repaired BUT it went to the upgrade w/o going through the proper procedure for repair. Hence my issue wasn't cured. But the upgrade is worth it. The (3) 6H30's were all aging faster than they should have and all at different rates. After the upgrade you're left with 1-6H30 and a 6L6G/5881 plus the 4-6922. I have rolled many a tube to get the sound I need. I really like the PH7 and wouldn't go with an all 6H30 ver. like the newer units. I have switched to the EH 6H30 gold pin versions and they are much better, warmer with a more even top to bottom sound. The 6L6G/5881 also rolls well. 
I also found that the unit sounded better with the top off, so I have mine velcro'd on and pop it off for serious listening sessions.  I wouldn't trade the RC cart. loading for anything.  Every once and a while it won't turn on either by RC or front panel. So I have to unplug-plug-in the cord from the back and all is fine.  ARC didn't seem concerned about it and told me what I was doing was what they recommend. I'm not sure why it does this and ARC never said why either. 

i agree that ARC should have found the problem with the unit when In for the upgrade based on the checklist that they say the do whenever the unit comes in for service. However I can see them missing it if it was not exhibiting the symptoms at the time. I would think if it's not a weak solder joint but rather a weak resistor or other component sometimes those things are hard to see with a visual inspection.they say the check for cold/weak joints.  I am however happy you found a work around that you are ok with. 

In regards to the power supply tube upgrade I did finally get a call back from ARC a few days after the unit had been taken to the local tech and the upgrade is still available for $300 and I do plan to send it in, but I am going to enjoy it for a few weeks first. I was told it would be a 3-4 week turn around and with round trip shipping add another 2 weeks  plus another $100 lol. 

I hear you on rolling the output tubes and i have have made a note of the ones your using and I have planned to do just that at some point. The unit sounds so darn good As it is and it is hard to imagine it sounding any better but that is the beauty of the hobby right. 

Regarding it soundibg better with the the top off I wondered about that and may operate it in that way as well. It kinda makes sense when you think about it and I'm sure that is the reason why most of their newer gear has the tubes exposed on the cabinet design. 

Thanks for the info. Good stuff.