[orthobiz]

...before sending it back to Frank.

Short story: A few years ago my college roommate sent me his highly original Dynaco ST-70 as a surprise (actually he had threatened to give it to me for many years). I did not even plug it in, I did the only rational thing and sent it to AVA immediately for upgrade/refreshing. The AVA team did a great job. Mary worked on the chassis and it cleaned up pretty well despite years of neglect. Frank said it looked like it was stored in a garage on the East Coast and indeed, it was in my buddy's garage in Marblehead, MA!!

Anyway, I gave it to my daughter, it's been in service probably for 4 or 5 years. Now when it's turned on it has some static and makes a loud screeching noise out of both speakers after about 3 or 4 seconds. The preamp is disconnected, nothing plugged into the RCA jacks.

Of course it may need to go back to Frank for eval, but any ideas of what to try first? Unplugging some tubes or something??

Thanks for your help,

Paul

[WGH]

As Jim McShane, tube seller extroidair always tells me when I have noise, clean the pins. He is always right. I used electronic contact cleaner from Ace Hardware. Those copper tube pins on NOS tubes get oxidized.

Pins only, not the sockets

[danielgk]

Plug the preamp back in, turn it on, set the volume to zero.

Some (most?) power amps do not do all that well with open inputs as far as noise goes.  There is really no reason for them to be dead quiet with open inputs.  They are designed to be quiet with an input that's turned all the way down, which is more like a low impedance (short) to ground than an open.  This may not fix your problem, but it is a better way to test it.  That, or use shorting plugs which actually short the inputs to ground.


Dan

[orthobiz]

First line of attack will involve contact cleaner that is on its way. Then will try plugging the preamp back in. The noise is the same with or without the preamp hookup, whether the preamp is turned on or not.

Updates soon, thanks for the suggestions!

Paul

[orthobiz]

So most of the tubes have been cleaned with Deoxit BUT the two tubes on the main board don't budge. The ones with white ceramic skirts...Do we just pull upon the tube glass? Seems like something gonna break, or is there a twisting motion that's necessary?

Thanks again.

Paul

[JackD]

Yes gently pull the tubes straight up by the glass.  The "white skirts" are the tube sockets and are soldered through the board.

[danielgk]

gently rock the tube back and forth while pulling up.  Without the rocking motion it will be extremely hard to get the tube out.  A rocking motion also helps when putting the tube back in.

Dan

[mick wolfe]

gently rock the tube back and forth while pulling up.  Without the rocking motion it will be extremely hard to get the tube out.  A rocking motion also helps when putting the tube back in.

Dan

Agreed. More often than not, the rocking motion is needed. In the end it's likely you'll still want to amp checked out regardless. You're issue could be a faulty component or even a cracked or cold solder joint. This especially where the tube socket legs are soldered. Just reflowing the solder in these areas is all that is necessary in most cases. This also provided the tube sockets themselves are still in good shape.

[mick wolfe]

Oops, just looked at my post again.  Missing a word or two plus a misspelling. I definitely need another cup of coffee, maybe even a Red Bull.

[orthobiz]

Deoxit on the posts delayed the onset of the feedback but did not eliminate. I'm gonna have her call Frank!

Paul

[avahifi]

Most likely a bad tube. If problem is the same on both channels, then probably the 5AR4 power supply rectifier.
Replace this one first.

Frank

[orthobiz]

That tube is replaced, same problem. My daughter should be calling to send it in...thanks for input, Frank.

Paul