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Tektronix 2235 CRO SMPS problem.

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Crash

New Member
Have a Tektronix 2235 CRO Power supply died. If I bypass the regulator and feed 45V into the SMPS section it seems to work but runs at about 100KHz instead of the correct 20KHz. Cannot fault any of the active components. Could it be one of the transformers? Has anyone any ideas?
 

mvs sarma

Well-Known Member
Have a Tektronix 2235 CRO Power supply died. If I bypass the regulator and feed 45V into the SMPS section it seems to work but runs at about 100KHz instead of the correct 20KHz. Cannot fault any of the active components. Could it be one of the transformers? Has anyone any ideas?
as you may have the manual with schematic, please upload the smps section of the schematic. It would help some one to advise you.
check whether contents of this link would be of some help
http://sci.tech-archive.net/Archive/sci.electronics.repair/2006-08/msg00558.html
 
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Externet

Active Member
Hi.
9½ of 10 Tektronix oscilloscopes fail on the power supply.:mad:

That model schematic has an error !! I will point the correction if posted.

Highly probable, there is an open electrolytic capacitor.

Miguel
 

Crash

New Member
Sarma/Miguel,
Thanks for the replies.
I have attached a copy of the smps circuit
I have checked all electro capacitors, both for C and ESR and have replaced any that seemed to be marginal. I have also checked most resistors in the switching circuit as well as diodes and semiconductors.
When I supply 45V to the switching circuit (this simulates the pre-regulator output and also gets rid of a mains connection) the current starts at about 1.4A and rises to 2A over a period of about 1 minute. The two TIP31 switches get hot. The waveforms are generally what you would expect as per the handbook but the switch is switching at approximately 125KHz instead of 20KHz. Output volts from the output transformer are about 30% low.
 

Attachments

mvs sarma

Well-Known Member
Sarma/Miguel,
Thanks for the replies.
I have attached a copy of the smps circuit
I have checked all electro capacitors, both for C and ESR and have replaced any that seemed to be marginal. I have also checked most resistors in the switching circuit as well as diodes and semiconductors.
When I supply 45V to the switching circuit (this simulates the pre-regulator output and also gets rid of a mains connection) the current starts at about 1.4A and rises to 2A over a period of about 1 minute. The two TIP31 switches get hot. The waveforms are generally what you would expect as per the handbook but the switch is switching at approximately 125KHz instead of 20KHz. Output volts from the output transformer are about 30% low.
thanks for your effort
but the haziness of the images is really troublesome. Practically unable to make up. can we improve the readability. other wise if it is a link, please indicate we will take a download..
 

Externet

Active Member
Hi.
That schematic revision has the error already corrected.
The typical causes of failure I have mostly experienced are C906, C940, C941, C942, C943, C925, R940. on the primary side.

For the secondary side, the notes I have for that schematic show
TP940 = 50Ω to TP950
+100V point = 12KΩ to TP900
+30V point = 963Ω to TP900
+8.6V point = 160Ω to TP900
-8.6V point = 80Ω to TP900
+5.2V point = 290Ω to TP900

Unsure now, it has been a long time, if they are the expected loads for each supply branch. Readings other than those, clues the circuit to follow.
Problem in the secondary can be isolated by lifting L960, L961, R971, and some branching "W" wires, until Q946, Q947 overheating goes away.
I have never, ever seen a bad switching transformer. From your observations, it may not be the power supply but a failing secondary circuit.
Hope it is of some help,
Miguel
 
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Crash

New Member
Hello Miguel/Sarma
My last attachment was poor quality. New attachments are better.
Sarma, the 45V is introduced across TP940 & TP950
Miguel, I have checked or replaced the components you listed and also checked the loads per your info. all are within the values.
I have removed all CRO circuit loads from the main transformer,T948, and replaced them with resistors across the output windings to simulate the loads but the switch still runs at approx. 120KHz. I seem to only have the transformers left to replace.

regards..Peter
 

Attachments

mvs sarma

Well-Known Member
Hello Miguel/Sarma
My last attachment was poor quality. New attachments are better.
Sarma, the 45V is introduced across TP940 & TP950
Miguel, I have checked or replaced the components you listed and also checked the loads per your info. all are within the values.
I have removed all CRO circuit loads from the main transformer,T948, and replaced them with resistors across the output windings to simulate the loads but the switch still runs at approx. 120KHz. I seem to only have the transformers left to replace.

regards..Peter
unfortunate that i cant still resolve the clarity, though slightly improved.

don't reduce to 90 kb
take it at 4 or 5 mpix as a portrait or close up ( leaf symbol comes) the post straight let itt be 400 to 700kb
so that people like me can see them.( 64+ age)
 
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Externet

Active Member
Hello Peter. I know the feeling. From several dozen of those I repaired, some gave me the nastiest frustration feel.
Please check C945, C944, R919, C919, R945, R921, R916.

Sarma... am not a magician able to read the posted schematics ;) ... I have it printed.:rolleyes:

Miguel
 

mvs sarma

Well-Known Member
Hello Peter. I know the feeling. From several dozen of those I repaired, some gave me the nastiest frustration feel.
Please check C945, C944, R919, C919, R945, R921, R916.

Sarma... am not a magician able to read the posted schematics ;) ... I have it printed.:rolleyes:

Miguel
That is fine
After all we need to help Crash restore the scope. i hope to see that happen.

perhaps soon.
 

mvs sarma

Well-Known Member
Hi Sarma,

I understand the problem. I too am 64+. See the new attachments.

Regards....Peter
Thanks Peter, I could see them much better now.
Let me see, for a while and I shall be back to continue discussion. Nice to know that you too are at the same age group and with hobby interests common. Really comforting me.

While we need to see reason for low voltages, perhaps the way we measure the frequency may not suite, as the counter, if used shows harmonics. the best appears to measure from one pulse start to next pulse start and then evaluate the frequency by help of another oscilloscope, if readily accessible. hope i am able to express myself.
 
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Externet

Active Member
Any chance that the timing components in post #9 were previously replaced with a wrong value by mistake? :(

Open trace on one of those after desoldering several times...?

That would be interesting to explore. Changing value (or paralleling something) on which one shifts the switcher oscillation frequency to something lower... I really do not know what frequency is supposed to have in healthy conditions. :eek:

Miguel
 

mvs sarma

Well-Known Member
Any chance that the timing components in post #9 were previously replaced with a wrong value by mistake? :(

Open trace on one of those after desoldering several times...?

That would be interesting to explore. Changing value (or paralleling something) on which one shifts the switcher oscillation frequency to something lower... I really do not know what frequency is supposed to have in healthy conditions. :eek:

Miguel
quite possible , your thoughts are highly appreciated Externet !! but we need to check the method used for knowing 20KHz or 120KHz!!
 

mvs sarma

Well-Known Member
Peter, Can you (CAREFULLY) measure the voltage on the filter capacitor after the bridge. In case your voltage is 230V, it should read approximately 390 to 400V. If it is 110 supply, you need check for the 110/240 selection.
Next doubt, --whether all the 4 diodes of bridge are OK?
 
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Top Cap

New Member
I would have gone for R919, C919 and maybe the decoupler C922. I last repaired one of these nearly 20 years ago and though the following notes taken at the time may not be directly related it may be of assistance.

"Investigation showed two diodes in bridge circuit, CR904 and CR902 both short circuit. Why? I removed U930 and disconnected the FET. I then did a test I used before on these scopes by winding up an external PSU from 0V to 40V, connected across C940. The scope controls were set to find beams and display traces. Having found scope OK, I transferred positive of the external PSU, set to 20V, to CR920 cathode. The slow start circuit can now be checked using a multimeter that should rise slowly until 14V is reached when there is a quick upward swing. (Better seen on an analogue multimeter). This scope however did not swing and I found Q930 very ill (TEK 151-0164), it is a 2N2907A PNP.
After replacement the slow start operates. Re-fitted U930 and FET and for caution, I powered up scope on a variac. Scope worked OK with 40V rail settling at 42.5V.
Note I replaced the diodes with 1N4007 which have a higher piv rating."

Transcribed from a scribbled note in my old engineers reference book, it may be useful to someone with a 2235/2215 fuse blowing problem.
Les
 

mvs sarma

Well-Known Member
I would have gone for R919, C919 and maybe the decoupler C922. I last repaired one of these nearly 20 years ago and though the following notes taken at the time may not be directly related it may be of assistance.

"Investigation showed two diodes in bridge circuit, CR904 and CR902 both short circuit. Why? I removed U930 and disconnected the FET. I then did a test I used before on these scopes by winding up an external PSU from 0V to 40V, connected across C940. The scope controls were set to find beams and display traces. Having found scope OK, I transferred positive of the external PSU, set to 20V, to CR920 cathode. The slow start circuit can now be checked using a multimeter that should rise slowly until 14V is reached when there is a quick upward swing. (Better seen on an analogue multimeter). This scope however did not swing and I found Q930 very ill (TEK 151-0164), it is a 2N2907A PNP.
After replacement the slow start operates. Re-fitted U930 and FET and for caution, I powered up scope on a variac. Scope worked OK with 40V rail settling at 42.5V.
Note I replaced the diodes with 1N4007 which have a higher piv rating."

Transcribed from a scribbled note in my old engineers reference book, it may be useful to someone with a 2235/2215 fuse blowing problem.
Les
The Note is really useful

while i suspected broken (open) Diode- may be generally rare to happen.
 

Crash

New Member
Hi Les, Sarma, Miguel,

Les, I note your comments and will use your approach when I look at the pre-regulator. My problem is in the switching circuit which covers Q938, 939, 944, 947 & 948.
I connect 45V across C940 (TP940, TP950) and although the switcher does oscillate it is operating at about 120KHz instead of 20KHz and does not give required volts out of T948. All active devices have been replaced, all capacitors have been replaced, all resistors have been checked. The loads on all secondaries of the transformer T948 have been disconnected and checked against the figures provided by Miguel. I have removed all diodes from the transformer secondary and loaded the windings with suitable resistors. Switch still runs at 120KHZ. the circuit draws 2A from the 45v supply and Q946/947 overheat. I seem to be left with the transformers T944 and T948. T944 would seem to be a possibility because it is supposed to control the switch frequency. I removed this transformer, checked it for shorted turns etc but cannot fault it. Have also checked T948 as best I can (very difficult to remove from board so did not try) and could not fault it either. Help!!!!

Best regards....Peter
 

mvs sarma

Well-Known Member
In case you have access to the parts of the sub module U 975 ( supposed to be multiplier) can you check the capacitors for leakage and replace them.
All said, I am suspecting things around the winding (10,23) perhaps a high voltage winding. A partial short could be possible on this winding, or a leaky cap on the voltage multiplier, thus loading the whole drive.
YOU NEED TO BE CAUTIOUS -
- DISCHARGE EVERY CAP FIRST, BEFORE HANDLING THIS SUBSECTION.

In the section, that you suspect, please check the Zener diodes for the voltage indicated. You may simply replace electrolytic caps, C940, C942 and C944.
 
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