Normal operation. Monitor from the 'antennas>6-meter>receivers multiple>cryo' rtd page.

Troubleshooting.&nbs= p;

Cooldown from ambient temperature.

·   Normally it is not necessary to repump the d= ewar with a vauum pump, unless the dewar was opened.

·   Check helium pressures on the gauges on the right-hand wall of the receiver cabin.&nbs= p; With neither compressor nor cold head running, the static pressure should be approximately 250 psi.  Add helium (only ultra high purity, grade 5.0) if necessary.

·   On the platform, open the lid to the compres= sor hut, set the speed controller switch to 'fast-cooldown= ,' turn on the helium compressor.  In the receiver cabin, open the large black valves on the right hand cabin wall, turn on the cold head power switch.

·   If the compressor trips off in a few minutes (most likely in cold weather), it may be necessary to go through the 'primi= ng' procedure: remove the helium lines from the back of the compressor, install U-shaped priming tube, operate compressor for about half an hour to warm up= the oil, then reinstall normal helium lines and try again.

·   The dewar requires about 5 hours to cool.  After the temperatures bottom out – typically stage1 is at 40-50 K, stage 2 at 11-14 K, stage3 at 5 K – flip the speed controller switch to 'slow-no= rmal.'  This slows down the refrigerator c= ycle from 60 Hz/72 rpm to 25 Hz/30 rpm. &n= bsp; Within minutes the stage3 temperature should drop below 3.5 K.

Adding helium.<= /p>

·   Use only ultra high purity, grade 5.0 helium.

·   In the receiver cabin, remove the brass cap = from the charging fitting on either the supply or return side of the manifold on= the right-hand cabin wall.

·   Crack open the valves on the gas cylinder and pressure regulator; flush helium through the yellow charging hose for 10 or= 20 seconds, then attach the hose to the charging fitting.

·   Open the small black valve to allow helium i= nto the manifold. 

·   Target pressures: 250 psi supply and return = if the compressor is not running; 280-300 psi supply if the compressor is runn= ing and the dewar is warm; 250-270 psi if the compressor is running and  stage3 is at 4 K.

·   Close valves when finished, replace brass ca= p on charging fitting.  Be certain = to close the main valve on the helium cylinder tightly, since the pressure regulator tends to leak.

Simple defrost procedure.

After several months of operation the stage3 temperatu= re typically creeps up because contaminants freeze out at the cold end of the refrigerator.  If the stage3 temperature is 4-5 K, the following 'defrost' procedure can sometimes help.  It takes about 10 minut= es and can be done remotely.

·   ssh control@bimaX    // log onto antenna com= puter as control.

·   bimaRx heater3=3D12    //=  turn on mixer block heaters.=

·   Wait 5 to 10 minutes.  The refrigerator continues to run,= but stage3 temperature increases to 7K, stages 4 and 5 to 12-17 K.

·   bimaRx heater3=3D0      // turn off heaters.

·   Ideally, stage3 will cool to lower temperatu= re than before (doesn't always work).

Partial warmup decontamination procedure.

If the simple defrost doesn't lower the stage3 tempera= ture, a more thorough defrost may be in order.&n= bsp; This will take several hours.

·   Close the large black valves on the helium manifold, sealing off the cold head from the compressor – this will k= eep contaminants from migrating back to the compressor.

·   Turn off the cold head with the switch in the receiver cabin; allow the helium compressor to keep running.

·   (Optional) Connect rapid warmup heater suppl= y to the dewar (4-pin Bendix connector).  Turn on the stage2 heater. This greatly speeds up the defrost proces= s.

·   Turn on the stage3 heaters using the simple defrost commands.

·   Wait until stage3 is 35-50 K; if the rapid warmup heater is used, this will take only a few minutes, and stage2 will b= e at 100 K or so.

·   Turn off all heaters.

·   Attach helium bottle to the charging fitting= on the supply side of the manifold, after flushing out the yellow charging hos= e.

·   Turn on the cold head, open the small black valve on the supply side of the manifold, and crack open the small black va= lve on the return side of the manifold.  Helium will flow from the gas cylinder through the cold head, and es= cape from the return fitting.  Do t= his for 20-30 seconds.  This will = flush contaminants out of the cold head.

·   Close black valve on return side, pressurize= the cold head to 250 psi; then close supply valve.  Open the large black valves to the compressor and begin cooling down again.

Background information – basic description.

The dewar on each 6-m telescopes is cooled by a 3-stage cryocooler.  This is a CTI 102= 0 cold head modified by the addition of a 3rd stage in Berkeley.  SIS mixers and feed horns are conn= ected to the 3rd stage, at about 3.5 K.  IF amplifiers and an inner radiation shield are connected to the 2nd stage, at about 12 K.  An outer radiation shield is cooled by the 1st stage, at about 40-80 K.

The cold head in the dewar expands helium from about 2= 80 to about 50 psi.  A helium compre= ssor (CTI 1020R) on the azimuth platform supplies the compressed gas stream.  The compressor is mounted inside an insulated 'hut' with a variable speed ventilation fan.  The fan sucks = outdoor air through louvers at one end of the hut and exhaust the air out the other side.  When the inlet air temperature drops below about 50 F, a fan controller (Phase-On) reduces the= fan speed to idle, and the louvers swing shut (unless, alas, the wind blows them open again).  Heater tapes, en= abled by a switch and an inline thermoswitch, are wrapped on the compressor and o= il sump to keep the compressor warm in extremely cold weather.  Another thermoswitch prevents the compressor from starting if the oil sump temperature is below 15 C.

Flexible helium lines carry the supply and return heli= um lines over the elevation wrap into the receiver cabin.    Self-sealing Aeroquip fittings are used on all helium lines.

A speed controller is used to slow down the cold head = from its normal cycle frequency of 72 rpm to 30 rpm.  At 72 rpm the stage3 temperature b= ottoms out at about 5 K.  Slowing the= cycle frequency reduces the stage3 temperature to about 3.5 K.