Tektronix 2235 issue!

[angelothewolf]

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!

[tautech]

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.

[angelothewolf]

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.

already replace all electrolitics in the unit, and nothing! i was suspecting on the HV caps breaking down,perhaps in the -2KV
cathode 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!