as you may have the manual with schematic, please upload the smps section of the schematic. It would help some one to advise you.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?
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.
unfortunate that i cant still resolve the clarity, though slightly improved.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.
That is fineHello 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.
I understand the problem. I too am 64+. See the new attachments.
quite possible , your thoughts are highly appreciated Externet !! but we need to check the method used for knowing 20KHz or 120KHz!!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.
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.