phase linear 400

VSAT88

VSAT88

Veteran and General Yakker
#446
Joined
Sep 7, 2017
Messages
1,865
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Phenix City Alabama
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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
177
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
14,045
Location
Sterling, MA
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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
177
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.
 
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