phase linear 400

VSAT88

VSAT88

Veteran and General Yakker
#446
Joined
Sep 7, 2017
Messages
1,819
Location
Phenix City Alabama
Tagline
Like my BlueTooth ?
#446
I am going to message that guy with the bolts and see if he will trade me for a 400 4 fin with new caps a protect board and all stock output transistors.
 
ABebar

ABebar

Journeyman
#453
Joined
May 28, 2021
Messages
165
Location
Warrenville, IL
#453
Hmmm... It appears that I may have spoke too soon on getting her up and running this weekend. That was back when I wasn't anticipating any problems.

I brought it up with the control board wired in and the bottom row of output transistors installed. It came up fine. I was able to adjust the bias on both channels to 0.35v at around 2:00 (60%) on the pots. I left it on for 5-10 minutes and it held stable there. Offset and output were both basically 0v. Satisfied, I slowly powered back down, discharged the bulk caps, and installed the next row of transistors - 2nd from the bottom. I started to power back up and as soon as I apply voltage through the Variac, my bias skyrocketed. I didn't let it get beyond a volt or two, but it was quickly past mV range and my input voltage was still single digits at this point. I'm sure I wasn't past 10vac (I was looking at the DVM and not the Variac meter). No illumination on DBT at all. I quickly powered down, discharged the bulk caps again (they barely had anything) and did some checks. No shorts to ground anywhere I look. Cases to chassis ground show no shorts. I checked my bias transistor wiring, and they are in firm and measure a short between the molex screw and the pad on the backplane board - good conductivity for all six wires.

What gives? Any suggestions on what to check first? Does it possibly jump up to a certain level until the circuit stabilizes and I just didn't notice it last time?
 
Gepetto

Gepetto

Administrator
Staff member
#454
Joined
May 15, 2011
Messages
13,590
Location
Sterling, MA
Tagline
Old 'Arn Enthusiast
#454
ABebar said:
Hmmm... It appears that I may have spoke too soon on getting her up and running this weekend. That was back when I wasn't anticipating any problems.

I brought it up with the control board wired in and the bottom row of output transistors installed. It came up fine. I was able to adjust the bias on both channels to 0.35v at around 2:00 (60%) on the pots. I left it on for 5-10 minutes and it held stable there. Offset and output were both basically 0v. Satisfied, I slowly powered back down, discharged the bulk caps, and installed the next row of transistors - 2nd from the bottom. I started to power back up and as soon as I apply voltage through the Variac, my bias skyrocketed. I didn't let it get beyond a volt or two, but it was quickly past mV range and my input voltage was still single digits at this point. I'm sure I wasn't past 10vac (I was looking at the DVM and not the Variac meter). No illumination on DBT at all. I quickly powered down, discharged the bulk caps again (they barely had anything) and did some checks. No shorts to ground anywhere I look. Cases to chassis ground show no shorts. I checked my bias transistor wiring, and they are in firm and measure a short between the molex screw and the pad on the backplane board - good conductivity for all six wires.

What gives? Any suggestions on what to check first? Does it possibly jump up to a certain level until the circuit stabilizes and I just didn't notice it last time?
Click to expand...
Go back to your first row is the starting suggestion, ensure all is OK.
 
ABebar

ABebar

Journeyman
#459
Joined
May 28, 2021
Messages
165
Location
Warrenville, IL
#459
One more thing I did was tighten down the thermal straps on the bias transistors. I was expecting to never lift the control board again. Just checked for shorts across any of those leads or to ground. Looks okay.
 
You must log in or register to reply here.
Top