OCXO repair help! - Page 1
Forum section: electronics / repair

author: TheSteve
author_rank: Unknown
timestamp: October 01, 2015, 07:18:22 am
content: I have a 100 MHz OCXO made by Wenzel that has absolutely no output. The oven is still working fine.I opened it up(not a fun task) and have poked around but nothing obvious is jumping out at me.The corner pins are ground, the second pin on the top left is power in @ 24 volts DC. The third pin is the control pin. The forth pin is a logic level output that goes low when the oven is up to temp - it works just fine. Note the vintage red LED just above the heater transistor - it goes out when at temp. The 5th pin is the 100 MHz output. The voltages across all of the tantalum caps looks fine to me. Probing around I can see there is oscillation - I do see a steady 1 volt @ ~97 MHz across either side of the capacitor marked A4 that is connected to the Dale inductor on the smaller hybrid board. I see no signal anywhere else. I did remove the transistor marked 7K(2N7002) and the oscillation stopped so I am pretty sure it is fine. The SOT23 diode with the blue mark tests fine.Happy to perform any tests or provide any voltages/measurements people think might help troubleshoot this.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 01, 2015, 07:38:01 am
content: Check the transistor (?) on the top right of the daughterboard and the one on the main pcb above that (signal+DC).

author: TheSteve
author_rank: Unknown
timestamp: October 01, 2015, 07:55:28 am
content: Quote from: PA0PBZ on October 01, 2015, 07:38:01 amThe parts you have referenced are both the same and are marked "C2NIB" or "C2N1B" - I assume it is the first one but can't be 100% certain. They both measure very strange to me. Between the two pins on the same side of the package they both measure 32 ohms. Measuring across the package from the side with the single contact they measure 77 ohms and 79 ohms. Both of the devices have the same resistance readings within an ohm. I find the readings pretty strange but kind of ruled them out because both measure the same. Unless they both experienced the exact same failure.EDIT:OK, if I only use "C2N" for the part I find it could be a CMPT404A PNP transistor. I removed the first one and when out of circuit I can see the base to the collector looks ok(diode), but the base to the emitter is 32 ohms. With it removed I also see some weak 100 MHz signal floating around on the board. So I think there is a pretty good chance we have a winner here. So is the part really a CMPT404A (in stock at Mouser!).I removed the second one and it measures exactly the same.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 01, 2015, 08:51:14 am
content: The transistor seems to be "designed for chopper applications"... ? When I search for C2N I see the same result so who knows, 32 ohms between B and E is not really what you expect. Replace them and start wondering why they both failed the same way.

author: TheSteve
author_rank: Unknown
timestamp: October 01, 2015, 08:57:36 am
content: I will order a few tomorrow, should have them Friday. If all goes well my Anritsu 10 MHz to 8.4 GHz sweep generator should come back to life. I picked it up locally for a great deal so my fingers will be crossed this does the trick.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: October 13, 2015, 03:35:27 pm
content: Quote from: TheSteve on October 01, 2015, 07:18:22 amOpening an OCXO can is not real fun to do. Did you torch it open? Wenzel makes some nice OCXOs, got a few flavors of 10mHz varieties. hope you get this going. Could be an upgrade for your Anritsu. Keep us posted.

author: TheSteve
author_rank: Unknown
timestamp: October 13, 2015, 07:54:45 pm
content: Quote from: SoundTech-LG on October 13, 2015, 03:35:27 pmStill working on it - made several foolish assumptions early on, all were incorrect, lol. I will update this thread with more details soon, including how I opened the OCXO.