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Tektronix 2235 Oscilloscope Troubles

T

Tony

Jan 1, 1970
0
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
 
J

Jim Yanik

Jan 1, 1970
0
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

did you check the PS filter caps for good ESR? Switchers (and 2200 series
scopes)are prone to bad electrolytics WRT ESR. Don't forget the electolytic
cap for the housekeeping supply for the TL594 IC. (100uf,IIRC)

(I don't have 2235 schematics or manual,so I can't go into specific part
numbers,my memory just isn't that good.;-) )

You could have a problem with a HV cap breaking down,perhaps in the -2KV
cathode supply.
 
T

Tony

Jan 1, 1970
0
Jim,

If you're ever in the Chicagoland area - I owe you a cold one. I pulled the
caps from the power supply and replaced them with brand new NTE
replacements, and some NOS HV caps I had left over from a laser power supply
project. I also replaced all of the 510K 1/2W resistors near the HV output
stage since they looked like they were overheating and the epoxy coating was
flaking off.

I turned the unit on, and after a suspenseful second or two of silence, the
unit powered up, the LEDs stayed lit, and I got a trace on the screen!

I'm going to leave the unit on for a few hours to burn it in and see if it
remains stable, but it appears you hit the bull's eye on this unit. Sometime
this week, my Sencore LC75 analyzer should arrive in the mail from Florida,
and I'll be able to determine exactly which part or parts were faulty.

Thanks again!!

-Tony
 
J

Jim Yanik

Jan 1, 1970
0
Jim,

If you're ever in the Chicagoland area - I owe you a cold one. I
pulled the caps from the power supply and replaced them with brand new
NTE replacements,

I hope you used low-ESR caps intended for switchers.(105 degF,low ESR)
Otherwise,they will not last long.

and some NOS HV caps I had left over from a laser
power supply project. I also replaced all of the 510K 1/2W resistors
near the HV output stage since they looked like they were overheating
and the epoxy coating was flaking off.

Good,they were a common problem for loss of focus;the carbon film resistors
would increase in value until they opened.They should be replaced with 0.5W
carbon composition types.
I turned the unit on, and after a suspenseful second or two of
silence, the unit powered up, the LEDs stayed lit, and I got a trace
on the screen!

I'm going to leave the unit on for a few hours to burn it in and see
if it remains stable, but it appears you hit the bull's eye on this
unit. Sometime this week, my Sencore LC75 analyzer should arrive in
the mail from Florida, and I'll be able to determine exactly which
part or parts were faulty.

Thanks again!!

You're welcome,glad to be of help.

BTW,I use(and love) the Dick Smith ESR meter.
Hats off to it's inventor.
 
T

Tony

Jan 1, 1970
0
I just got my Sencore LC75 in today, and found the culprit to be a 10uF 100V
cap, which was reading a value of a few picofarads, and an ESR of about 436
ohms. All of the other caps tested ok. I think I am going to love this new
addition to my bench!

- Tony
 
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