Blackstar series one 50 help

[malukaluis]

So this friend leaves her blackstar series one 50 in the back of her car after a gig, throws her back out so can't lift anything so the amp ends up in the car all winter, it's been in there for about 9 months. So feeling better starts gigging again finally pulls the amp out and surprise surprise the amp does not work properly.

It fires up fine and works normally for about 2 to 3 mins then it seems as though the supply to the preamp is cut off, it presents just like when a mixing desk is turned off before the power amps, it makes that pop sound.
However if the amp is powered down, left for about 5 mins, it will come back to life but for the same short time frame.

It also makes a quiet high pitch oscillation sound when the fault occurs, but lasts 15 or so seconds like a cap discharging.
I first thought dry joins but quickly ruled it out as all joints looked in good order (only one and a half years old, bought brand new) and i ran the iron over much of them.
As it was working before winter and all it was exposed to was cold for about 9 months i'm at a loss as to the cause.
Would the cold affect capacitors? i'm not sure.
Valve amps are not one of my strong points so any help or schematic would be grateful.

[Randall]

Sounds heat related. If it "works fine for 2 - 3 mins" something is probably heating up and expanding, and breaking a connection. I would look for a mechanical failure by probing around with a chopstick to see if I could either make it fail when it is working, or make it work when it has died. It has been subjected to temperature (expand/contract), as well as vibration related stresses, so I'd look for a mechanical before a component failure.

Also, if it was lying on it's back, look at the bigger components like filter caps or even tubes that would have been vibrated on a different plane than they were intended if it was standing in the normal vertical position. If it has caps glued in place, the glue can get brittle in the cold and allow the caps to vibrate free and cause a fracture in the solder joint that could certainly be a problem when warming up,

[malukaluis]

Sounds heat related. If it "works fine for 2 - 3 mins" something is probably heating up and expanding, and breaking a connection. I would look for a mechanical failure by probing around with a chopstick to see if I could either make it fail when it is working, or make it work when it has died. It has been subjected to temperature (expand/contract), as well as vibration related stresses, so I'd look for a mechanical before a component failure.

Also, if it was lying on it's back, look at the bigger components like filter caps or even tubes that would have been vibrated on a different plane than they were intended if it was standing in the normal vertical position. If it has caps glued in place, the glue can get brittle in the cold and allow the caps to vibrate free and cause a fracture in the solder joint that could certainly be a problem when warming up,

I think you are right.. i've now gone over the entire amp...... every joint the problem's still there. I now suspect the EL34's... I replaced the ECC82 and 83's, that did not resolve it so it leaves the 34's.
One observation, when it does work is that the sound is very tinny (running music from my Ipod). I didn't notice it at first but as we become so acquainted now i'm noticing other things.....

[malukaluis]

A quick update.....dodgy ECC82. I discovered that when the amp was upside down it work well until i tapped the ECC82, replaced it hey presto working fine.

[Chuck H]

Don't toss the old tube. Trust me, it's innards (which are in a vacuum) didn't go bad because of cold weather. Try cleaning the pins. And since that is the likely culprit, you should clean and retention the sockets as well to avoid future problems.

JM2C

[malukaluis]

Chuck H the pins were fine, I think something went amiss with the valve, remember it was in the boot of a car for 9 months being driven back and forth. Any how it caught me out. Just when you think you've seen it all, someone comes around and pulls your pants down.

[malukaluis]

I've been quite busy lately but wanted to share how I biased this amp, as you can imagine Blackstar were not forth coming with any information, so i trawled the net and discovered the following.

There is a 2 pin connector (con 1) on the EL34's board, connect your meter to that.......


The bias is adjusted with the preset to the left of the large heatsink (pr 1). I set this amp to 33ma, anode voltage was 454v...