Tektronix 2235 issue! - Page 1
Forum section: electronics / testgear

author: angelothewolf
author_rank: Contributor
timestamp: December 13, 2013, 07:13:04 pm
content: Hello, and thank you for reading my post.I've recently acquired a used Tek 2235 at a local auction. Cosmetically, theunit looked brand new like it was hardly ever used. The reason why may bethat there is a slight problem with the unit, possibly somewhere in thepower supply section. When power is applied, the unit exhibits somethingsimilar to the 12kHz "chirp" symptom, in that once every second or two, thepower LEDs flicker and an audible chirp noise can be heard coming from thetransformer.I've tried some basic troubleshooting steps as follows:- Visually inspected the circuit boards for obvious component failures- Tested all diodes and capacitors of the power supply out-of-circuit tomake sure they were still OK- Checked the power supply outputs as best I could for any unusualload/short conditionsInitially, I thought that there was a short tapping off of the +8.6V supplysomewhere on the board. I checked the resistance between the +8.6V line atW960 to ground and measured it to be around 66 ohms. I tested the circuitvery briefly (less than a couple seconds) with W960 open, and the chirpingnoise went away, the power LEDs remained lit, and the HV supply seemed toprovide the proper voltage to turn on the CRT. The waveforms from severalpoints in the power supply circuit (specifically, waveforms 37 through 44,with appropriate ground references to TP940 and TP950 as directed in theunit's service manual) all appeared to be within specifications with nodeformations or voltage abnormalities. Reconnecting W960 returned the unitto its initial state - 12 kHz "chirp" with no operation.I opened up just about every test point related to the +8.6V power supply asshown in the power distribution diagrams, as well as many others that weretotally unrelated. Of course, none of this pointed out any obvious defectsin any particular section of the oscilloscope, and like a person with a limpwandering aimlessly in a forest, I kept coming full circle back to theswitching power supply, and assumed there was something that just wasn'tright about it. But what?Since the switching power supply kept going into a fault condition, I wantedto double-check the +8.6V, -8.6V, and +5V power supply lines to see just howmuch current was being pulled. Using my HP E3631A, I applied these voltagesdirectly to the main circuit board at points W960, W961, and W968respectively. I noted that the +/-8.6V supply crossed over from CC to CV ataround 440mA, and the 5V supply crossed over from CC to CV at around 365mA.Does this load current sound about right for the outputs of these supplies?Or is it indicative of a short circuit somewhere on the board that I haven'tbeen able to track down yet?If there is in fact no problem with the +/- 8.6V and 5V supplies, thenperhaps removing the load from the +8.6V supply temporarily relieved anoverload condition somewhere in the HV supply which allowed it to operatefor a brief moment - I just don't know and at this point I am stumped.This is a nice piece of test equipment in beautiful condition, and it wouldbe a shame to toss it in the dumpster if the problem is something reallysimple that my lack of experience caused me to overlook. Any helpful tips oradvice regarding where to look next in repairing this unit would be greatlyappreciated!Thanks!

author: tautech
author_rank: Super Contributor
timestamp: December 13, 2013, 07:25:37 pm
content: Sorry i am not familiar with 2235 however symptoms sound like PSU overload.Dried out electrolytic caps or shorting Tantalum beads are my first suspected culprits.

author: angelothewolf
author_rank: Contributor
timestamp: December 13, 2013, 07:55:54 pm
content: Quote from: tautech on December 13, 2013, 07:25:37 pmalready replace all electrolitics in the unit, and nothing! i was suspecting on the HV caps breaking down,perhaps in the -2KVcathode supply, but i couldnt find replacements for that ones ( .01uf 3Kv). i test that caps out of the board and they apear to be ok.Now you mentioned, just find a tiny tantalum cap around the TL594 IC. gonna check it out!Thanks!