Screw loose!

I have been fighting a drifting bias problem on my VTA-70 for a couple of days. The problem was related to a single socket, and was not tube related. The bias on that socket would drift from 35-45ma, and change as the temp increased. I checked all the bias resistors, and bias circuit. Everything was spot on. While making another measurement on the 10 ohm bias resistor, I noticed that the resistance was jumping from 10 to 16 ohms! Closer examination uncovered a loose socket screw, which the bias resistor uses to make its ground connection. Tightened the screw, and all is well. Seems like a good idea to periodically check all screws/nuts related to the ground distribution!

Mike

mazeeff wrote:I been fighting a drifting bias problem on my VTA-70 for a couple of days. The problem was related to a single socket, and was not tube related. The bias on that socket would drift from 35-45ma, and change as the temp increased. I checked all the bias resistors, and bias circuit. Everything was spot on. While making another measurement on the 10 ohm bias resistor, I noticed that the resistance was jumping from 10 to 16 ohms! Closer examination uncovered a loose socket screw, which the bias resistor uses to make its ground connection. Tightened the screw, and all is well. Seems like a good idea to periodically check all screws/nuts related to the ground distribution!

Mike

Screwing in stainless steel is not a perfect way of grounding. Myself used a
coarse copper wire as grounding point, this is itself grounded at the PSU minus post.

peterh wrote:
Screwing in stainless steel is not a perfect way of grounding. Myself used a
coarse copper wire as grounding point, this is itself grounded at the PSU minus post.

Good idea. Did you just run the copper ground bus, across the length of the chassis? How did you mount it?

Mike

Only screw loose here is sitting in the listening chair.

Good point though. Another fairly common area of concern is a loose multi-cap. That can cause substantial hum even if all the actual electrical connections are good.

Honorable mention also to the RCA jacks. Those can loosen up and rotate, shorting out a channel.

mazeeff wrote:

peterh wrote:
Screwing in stainless steel is not a perfect way of grounding. Myself used a
coarse copper wire as grounding point, this is itself grounded at the PSU minus post.

Good idea. Did you just run the copper ground bus, across the length of the chassis? How did you mount it?

Mike

I user solder tags : see http://n.manet.nu/vta70/

( i think i made a thread about this build, but here is one picture again)

peterh wrote:

mazeeff wrote:

peterh wrote:
Screwing in stainless steel is not a perfect way of grounding. Myself used a
coarse copper wire as grounding point, this is itself grounded at the PSU minus post.

Good idea. Did you just run the copper ground bus, across the length of the chassis? How did you mount it?

Mike

I user solder tags : see http://n.manet.nu/vta70/

( i think i made a thread about this build, but here is one picture again)

Thanks. Simple enough, and I have the parts on hand!

Mike

sKiZo wrote:Only screw loose here is sitting in the listening chair.

Good point though. Another fairly common area of concern is a loose multi-cap. That can cause substantial hum even if all the actual electrical connections are good.

Honorable mention also to the RCA jacks. Those can loosen up and rotate, shorting out a channel.

My VPI turntable came that way, loose RCA jacks and you could spin 'em into a short.. But VPI seemed to have used a pneumatic drill to install the plate that held them in. It was a bitch to fix.

well I have been known to have some loose screws........but yes, I too have been caught out with those loose blighters! It is something that could be easily overlooked!