July 8, 2005

430 tx problems - began about June 15. Tx had worked well on a 2 or 3 day run for Frank Djuth. Modulator pulse on console scope was ideal: no hum, absolutely rectangular.

Next run was to be a long world day campaign. Ran first night ok. By morning, there was strong RFI showing on the A-scope. Quickly diagnosed as coming from the transmitter. Didn't seem to be through wave guide, as the strength on both CH and Greg rcvrs remained the same when the power splitter was adjusted from side to side. And the interference remained with the RF drive turned off.

Threshold voltage was about 60kV. Below 60, the impulse spike noise disappeared. We disconnected the hvps from the mod tank - both hv and ½ hv lines - and when the cap bank was brought up as far as 100kV, no noise was observed. Problem seemed to be something arcing in the oil tank. Made attempts to isolate the three elements in the tank: klystrons, floating deck, and buffer deck. Noise seemed go away, except when in the operating configuration.

July 7, Thursday

Mod pulse on the console scope looks fair - though spikes appear when the hv is brought up. There is a strong though variable spike during the turn-on of the floating deck.

As this spike occurs when the one-shot should operate in the buffer deck, we removed the fiber optic drive signal to the buffer deck. Expecting to see the mod pulse rise quickly retain its flat top through the pulse period, and then exponentially decay (4Meg x 900pF = 3.6msec) we observed a fast rise, downward sloping top, initial fast decay for about ½ msec, and then the expected slow decay.

We also observed the high voltage using a scope connected to the lowest resistor in the bleeder chain.

Here saw a sharp decrease in voltage (positive going, since the supply is negative), followed by sloping decay, a sloping increase, and an eponential tail recovery.

7-11-05, Monday

Victor Iguina and Joe noticed heat cmint through the oil from the 120 VAC connector on FD. FOund burned bakelite - bad connection. Repaired it with some G10 board. (Probably not the cause of the arcing problem.)

Plan is to remove plate connector on BD DP11

But first: noitced that the cathode follower plates are glowing on the BD. Turns out to be normal: Each 4-400 draws 120mA quiescent, so power is .12(2300-400) = 240 W so we would expect the (fairly small) plate structure to incandesce orange.

Alont the way, found a 10 ohm resistor, R428, was actually 10k. (Orange band was almost brown).

Grid parasitic suppressor. Didn't really matter, as 4-400s dont draw grid current in this circuit. (Both were glowing, indicating full quiescent current).

Noon. Went to disconnect plate of V401 (tail clipper. Found plate connection loose. Also one screen resistor on a 4-400 follower had a cold solder joint.

Aha! This must be the root cause of the problems. [fat chance]. Go for the home run: fix connections, put everything back into the full operating configuration. .(DIDN'T CARRY OUT THE TEST OF DISCONNECTING THE PLATE OF V401).

7-15-05 Friday. Things back together, run a test. Seems to be working. No arcing noise observed on receiver.

7-18-05 Monday. Schedule test at 4pm. Bring up HV. All traces on the console scope now steady.

Mike Sulzer turns on receiver. Arcing is there, just as it had been. Sure enough, looking at the beam current of Kly1 or Kly2 on the console scope with the gain and intensity way up, tiny "grass" can be seen - the arcing. So we are back to Square One.

This time the arcing tended to heal itself, i.e. after raising the hv to produce arcing, the rate of arcing would slowly decrease, even to zero. Further raising of the hv would again start the arcing, which would once again improve itself, suggesting a sharp point that is being 'conditioned' (smoothed).

The arcing, as before, behaves as a relaxation oscillator; raising the hv increases the rate of arcs.

Something charges through a high resistance path, then breaks down producing a quick discharge. The charging process then begins anew. The time constant for the FD+mod anode capacitance and the 4Meg bleeder would be 900uF x 4 Meg = 3.6 mS. (Try to measure the arc rate - did this, arc spikes can be as closely spaced as 1mS).

7-19-05 Tuesday

Follow up on the 4Meg, 900pF relaxation osc theory. This would be an arc from the floating deck. Theory 1: The arc is from one of the feedthrough insulators to the klystron mod anodes.

Experiment: remove the wire from the FD that feeds the two feedthroughs. Bring up the hv carefully, watching the klystron currents. (We are making the assumption that the mod anodes, while floating, will charge negatively and keep the klystrons turned off.

If the voltage can be brought up and no arcing occurs, we wil then connect one mod anode at a time to locate the arc.

But if the voltage is brought up and arcing begins, we will disconnect the klystron cathodes, totally removing the klystrons from the circuit. If arcing still occurs, we then find which deck is responsible.

Problem with Theory 1 is that we observe the floating deck voltage on the console scope. We should see a sawtooth waveform if the arcing is the relaxation oscillator described above. Let's go ahead with the test as planned, however. If the arcing persists (and I suppose it will), then we can investigate whether it might be inside the DP11 of the FD. (though the internal capacity of this tube would

predict that the arcs would be more closely space in time).

Experiment: 7-20-05

1. With the mod anodes floating (removed cable), high voltage was raised and the arcing was still prominant on the receiver A-scope.

2. Leaving the mod anodes disconnected, the 50kv line was removed from the modulator bushing (outside the tank). High voltage raised - arcing still present.

3. Removed HV connection to tank (plastic rod instead of alum. tube above hv bushing).

High voltage raised

This confirms again that the arcing is inside the tank, but that it is not at the mod anode bushings (or mod anodes themselves). Test 2 shows that the DP11 in the floating deck is not arcing.

Next test: remove cable to klystron A cathode, leaving the 1/2 voltage disconnected. This leaves the hv

connected to the buffer deck, via the 5kv supply and to Klystron A. If arcing persists, it must be from Klystron A or the buffer deck to ground, or from the floating deck to ground. The floating deck will only be getting high voltage via the 4 megohm resistor from the buffer deck. Since the arcing seems to be a relaxation oscillator, the 4 meg resistor is a prime suspect. The next test would be to remove the resistor and see if the arcing occurs with high voltage on the buffer deck alone.

Result, Still arced. Next test: remove the cable to klystron B. Both klystrons are now totally out of the circuits (their mod anodes and their cathodes are disconnected). If arcing persists, it must be from the buffer deck to ground, or from the floating deck to ground. The floating deck will only be getting high voltage via the 4 megohm resistor from the buffer deck. Since the arcing seems to be a relaxation oscillator, the 4 meg resistor is a prime suspect. The next test would be to remove the resistor and see if the arcing occurs with high voltage on the buffer deck alone.