Hello, and thank you for reading my post.
I've recently acquired a used Tek 2235 at a local auction. Cosmetically, the unit looked brand new like it was hardly ever used. The reason why may be that there is a slight problem with the unit, possibly somewhere in the power supply section. When power is applied, the unit exhibits something similar to the 12kHz "chirp" symptom, in that once every second or two, the power LEDs flicker and an audible chirp noise can be heard coming from the transformer.
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 to make sure they were still OK
- Checked the power supply outputs as best I could for any unusual load/short conditions
Initially, I thought that there was a short tapping off of the +8.6V supply somewhere on the board. I checked the resistance between the +8.6V line at W960 to ground and measured it to be around 66 ohms. I tested the circuit very briefly (less than a couple seconds) with W960 open, and the chirping noise went away, the power LEDs remained lit, and the HV supply seemed to provide the proper voltage to turn on the CRT. The waveforms from several points in the power supply circuit (specifically, waveforms 37 through 44, with appropriate ground references to TP940 and TP950 as directed in the unit's service manual) all appeared to be within specifications with no deformations or voltage abnormalities. Reconnecting W960 returned the unit to its initial state - 12 kHz "chirp" with no operation.
I opened up just about every test point related to the +8.6V power supply as shown in the power distribution diagrams, as well as many others that were totally unrelated. Of course, none of this pointed out any obvious defects in any particular section of the oscilloscope, and like a person with a limp wandering aimlessly in a forest, I kept coming full circle back to the switching power supply, and assumed there was something that just wasn't right about it. But what?
Since the switching power supply kept going into a fault condition, I wanted to double-check the +8.6V, -8.6V, and +5V power supply lines to see just how much current was being pulled. Using my HP E3631A, I applied these voltages directly to the main circuit board at points W960, W961, and W968 respectively. I noted that the +/-8.6V supply crossed over from CC to CV at around 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't been able to track down yet?
If there is in fact no problem with the +/- 8.6V and 5V supplies, then perhaps removing the load from the +8.6V supply temporarily relieved an overload condition somewhere in the HV supply which allowed it to operate for 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 would be a shame to toss it in the dumpster if the problem is something really simple that my lack of experience caused me to overlook. Any helpful tips or advice regarding where to look next in repairing this unit would be greatly appreciated!
Thanks in advance,
- Tony