HP 5065A lamp oven repair details! - Page 1
Forum section: electronics / general

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: May 27, 2019, 04:43:02 pm
content: Hi,This is the first post of the details on how to repair a 5065A that has a defective lamp oven winding.The insulation of the original winding fails and shorts out to the oven cylinder or to itself. This usually causes excessive temperatures and can in some cases completely cook the lamp oscillator board. I have repaired quite a few over the years and have learned a few major lessons. First do a continuity check on the Db9 to check the windings.1-2 Thermistor >2K at room temp3-4 cell oven should be 50 Ohms4-5 lamp oven should be 50 Ohms (likely shorted, open, or shorted to case)6-7 C-field coil around 11 Ohms8-9 Thermistor >2K at room tempIf all Ok except the lamp oven then remove the lamp assy. To get to the lamp assy. you remove the outer nut and fiber washer on the threaded rod that exits the lamp end of the optical unit. Then remove the outer shield the inner nut and the two inner shields. Remove the 3 small screws around the outside edge of the lamp assy. and pull out the lamp assy. If its stuck wiggle it around and work it loose. Don't worry about the lamp assy. yet just put it aside. Now stuff a rag or paper towels down in to cover the glass filter cell to catch debris and protect it. On the picture you will see two black marks I made on the oven cylinder. Note that they bracket the location of the slot where the big cable enters the optical unit. This is IMPORTANT!!! The area between those marks is what I call the exclusion zone. You MUST NOT remove the foamed in insulation between these marks! If you do you WILL damage the large cable inside the foam and be out of business! Now to getting the oven cylinder free!In the picture you will see a Pittsburgh 5/64" screwdriver from a 6 piece set. Note that I have removed the plastic coating on its shaft to correspond with the depth of the oven cylinder. When the screwdrivers insulation just butts up against the oven cylinder the end of the screwdriver should be just shy of the bottom of the cylinder. (maybe 1/16") Then tape up the outside cables so they don't flop about. Taking the screwdriver rotate and pry to break into the foam right up against the cylinder and working (patience!) around and down slowly drill down until the screwdrivers insulation hits the top of the cylinder. Stay TIGHT against the oven cylinder and pick out debris and pieces of oven winding as you go. DO NOT dig into the exclusion zone! Once you have reached bottom all around except the exclusion zone you can insert a large round item into the cylinder and pry laterally away from the exclusion zone. The cylinder should pop free. In the pictures you will see the removed cylinder and the surface of the exclusion zone. Pry out any remaining oven winding stuck in the exclusion zone. You should also carefully scrape off a SMALL layer of the exclusion zone just deep enough so that the original winding grooves are gone. At this point it's a good idea to recheck the readings on the Db9. Note the before and after picture of the oven cylinder. I had 9 grooves machined into it 1/16" X 1/16" to accept the new windings so that they don't protrude. Also extended the lateral groove to go all the way end to end. More to follow! Cheers, Corby

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: June 01, 2019, 10:05:29 pm
content: Hi again,Here is episode 2 of the lamp oven repair! At this point if you look into the opening you willsee the original thermistor sticking out of the bottom where the cylinder seated. Just cut it off flush.Now that the oven cylinder is in hand and machined you will need the following.11 feet of pelican wire company P2332ADVFEP.009BL wire, a Digi-Key 723-11184-ND 100 Deg.C thermostat, a Digi-Key 615-1007-ND thermistor, and some high temp RTV. The thermostatwill be added to provide overtemp protection for the future!You find the middle of the wire, bend it over to double it. Place an end in the chuckof a portable drill and the other into a vise. Keep tension on it and run the drillto give a tight twist. (about 5 twists per inch) Two people are handy for this. one to run the drill andone to run a hand along the wire. Once done keep tension on the wire andrelease the vise end. It will curl up a bit but is easily straightened. Take thethermistor and place some tiny tubing over the leads. Then attach two leads cut off a resistor(I use silver solder) and insulate with heat shrink. Locate the longitudinal slot in theoven cylinder and place a strip of teflon tape centered on it. place the thermistor into theslot with the bead centered halfway down. The bead MUST be surrounded by a good amount ofheat sink grease! I use a grease that is fairly liquid as I don't get good results withthe pastes. leaving around 2 1/2" of the two oven wire ends protruding from the top edge ofthe slot start winding, using the slot to make the jog to the next groove. Thewire should hold the thermistor tight once all is wound. Then fill the grooves flushwith high temp RTV. Don't RTV the top bit of the lateral groove with the wiresprotruding yet. I have also had good results just using a good (thin) tape.Once the RTV is set (24 hours), position the cylinder in its location in the optical unit.Make sure you can center it and it's level. Pop it back out and run a bead of the RTV aroundthe bottom where the cylinder will seat, also a small bead around the bottom egde of the cylinderitself. Reinsert the cylinder and make sure its bottomed out and level. I use a few wood shims oround the top edgeto hold it while it sets (another 24 Hours!). Once set you bend outward the new wiresso that the lamp mounting plate won't hit them and RTV around the top edge of the cylinder.I stuff a small bit of foam or silicone rubber in first that sits about 3/16" below the topof the cylinder to give the RTV something to sit on. Another 24 hour wait and thenlooking down into the cylinder use a sharp razor knife to trim the RTV atthe bottom of the cylinder flush with the cylinder. The lamp assy. wontgo in if its not trimmed! Open up the backshell of the Db9 connector and removethe two wire connected to pins 8 and 9 (thermistor) and 4 and 5 (oven). You will have removed twowires from pin 4. Ohm from pin 3 to these two removed wires. You will be reinstalling the wirethat shows 50 Ohms to pin 3 note its color. The other 4 wire can be cut off or bent 180 and stuffed up into the bundle. Make two twisted pair wires using small wiring that are long enough to wrap aroundthe outside of the big cable and run from the Db9 pins to the inside of the Lamp chamber towire to the new oven and thermistor leads. Solder the ends of the thermistor wires to pins8 and 9 of the Db9. Solder the two ends of oven wires to pins 4 and 5 of the Db9 and alsosolder the original wire back onto pin 4. then Cut the leads of the thermostat shortand solder one end to one of the new oven wires. Leave enough slack so that you can mountit later to the closest big phillips screw on top of the lamp assy. Find the two ends ofthe oven wires from the DB9 and solder one to thermostat lead and one to the remainingnew oven wire. Now take the two ends of the thermistor and solder to the new thermistor wires.(Insulate all with heat shrink!) Now recheck the DB9 values and make sureAll look good. Remove any rags you stuffed in and blow out any debris, also take an alcohol wipeand clean the top surface of the filter cell. Whew! Now lets assume your lamp assy. is good.Reinstall it with its tiny phillips screws. Remove the big phillips nearest the thermostat andreinstall it with the thermostat under it. There is a small hole on the top of the lamp assy.This hole lets you run a thermocouple or other probe down to the front surface of the lamp assy.Go 1 5/8" down fro the top of the lamp plate. Position such a probe, install the middle shield only. Hook the db9 and the lamp power connectors to the mainframe. Power up the mainframe and you should see max readings on the lamp and cell ovens. In 25-35 minutes they should drop dwn off of max. Wait another 15 minutes and note the lamp assy. temperature. You should be between 90 and 92 Degrees C. If not you will need to select another R6 resistor on the A11 board. This resistor sets the lamp temperature. Once you are happy with the temperature remove the probe and install all 3 shields. The nut just inside the outer shield should be positioned so that the outer shield will be clamped between it and the outer nut and fiber washer. You can connect the other Optical unit wires (RF from A3, Signal to A7, and TED across the diode between a8 and A4). Red TED wire goes the end of the diode at A4-1, and the black to the end of the diode at A8-1. All these connections can be extended so that the optical unit can sit on top of the unit as shown EXCEPT the RF from A3. Extending it will ruin the RF match but it is long enough to reach by itself. Now you are done and can align/checkout the rest of the unit! After alignment you can reinstall the optical unit in the mainframe.Good Luck!

author: uncle_bob
author_rank: Unknown
timestamp: June 08, 2019, 03:28:32 pm
content: HiVery interesting !!!!Bob

author: Theboel
author_rank: Frequent Contributor
timestamp: January 24, 2020, 03:46:09 am
content: Hi Corby,is that the brown thing around the lamp is heat insulator ?Do You think keep the lamp at more stable temperature made difference especially for telecom Rb ?

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: January 24, 2020, 05:15:54 am
content: Theboel,The aluminum lamp oven cylinder is held in place by insulating foam. I think that's what you are asking about.The main reasons the 5065A performance beats the telecom units are:-triple magnetic shields.-actively regulated C-field current.-classic 3 element optics (lamp, filter, resonance cell) however a few telecom units have classic optics-resonance cell Rubidium vapor density control via a tiny thermoelectric cooler.-and the main reason a large diameter resonance cell that is at a much lower temperature than the telecoms.Cheers,Corby

author: Theboel
author_rank: Frequent Contributor
timestamp: January 24, 2020, 08:45:28 am
content: Quote from: 5065AGuru on January 24, 2020, 05:15:54 amThank You Corby, thats all I need to know