vvr help

[andrew]

Hi, I can't get my VVR for cathode biased amps working. I replaced the old mosfet with a new 2973 but I get no voltage coming out of the unit. All connections are correct. Could a bad Zener diode be the problem? Thanks for any help guys.

[rock_mumbles]

Are you sure the MOSFET is insulated from the chassis?

You should make sure there is no continuity between the drain (middle lead) and the chassis before powering up.

Dana recommends that you DO NOT use a heat sink compound as a lot of varieties of heatsink compound are electrically conductive which will short out the MOSFET.

[andrew]

Yes, it is insulated with the piece supplied with the nte 2973. I took the diode out and with neg on the cathode and pos on the anode it read 10 using the diode test function on the meter. I took care in handling the mosfet ($15!) I'm stumped.

[rock_mumbles]

OK, what kind of amp ... where have you put the VVR in circuit? Pictures?

[andrew]

I am regulating the whole amp and it is a Rocket type with a ss rectifier. I had this unit on a previous build and it quit working so I took it out thinking it was a bad mosfet. I wanted to use it on this Rocket so I bought a new mosfet. 480 volts in but no volts out.

[John_P_WI]

Does your new mosfet have a metal tab? If so the screw may need to be insulated with a shoulder washer and plastic washer to electrically isolate it from the chassis.

[andrew]

yes the nte mosfet has a built in washer insulating it from the bolt. I also unbolted the mosfet and dangled an inch free of the chassis just to be sure.

[John_P_WI]

Just throwing this out, the new mosfet has the same pin out of the source, gate and drain?

[andrew]

yes the NTE brand is used with Dana's VVR. The new one is also an NTE.

[andrew]

I found a problem. I disconnected the vvr OUT from the B+1 / 1K 25w resistor junction and the OUT wire reads full voltage now. When I attach it back to that junction I get 120 volts. Then if I only disconnect the B+2 from the 1K 25w resistor I get full voltage at B+1 and high voltage after the 1K. These tests were done without the power tubes. Is this a bad B+2 filter cap? I've never had this happen before.

[andrew]

I bypassed the VVR and the amp works fine with correct voltage on all the nodes. I don't understand why with the VVR installed the voltage would drop so much when the B+2 filter cap is attached.

[Tillydog]

I bypassed the VVR and the amp works fine with correct voltage on all the nodes. I don't understand why with the VVR installed the voltage would drop so much when the B+2 filter cap is attached.

Going by the schematic posted here, there is a current limiting aspect in the VVR. When the drop across R3 (10R) exceeds the zener diode voltage (12V), the output voltage is pegged at whatever it takes to supply 1.2A for those values.

Since nothing went up in smoke when you bypassed the VVR, I don't think you're drawing 1.2A.

Either the resistor is too big (damaged, cracked, dry joints??) or the Zener voltage is too small (wrong way round, or failed).

You say that the zener measured "10" on your tester. Is that 10V? I suppose Dana could have tweaked the value from that schematic ??

What does a normal rectifier diode measure on your meter? (0.6?? 600??). The zener should measure ~0.6V / 600mV one way and 12V the other way - It would probably just show 0.6V one way and open circuit the other way on a meter. If it measures the same both ways on your meter, then it is definitely shot.

HTH

Andy

[andrew]

Tillydog, the zener reads the same in both direction = bad zener. Thanks for the help Andy.