Tektronix 2235 transformer - Page 1
Forum section: electronics / testgear

author: oldwhitelodge
author_rank: Newbie
timestamp: July 27, 2020, 12:37:27 pm
content: I've read the previous post in regard to this. Can anytell me what the expected inductances I should see on the 120-1594-00 inverter transformer? AS I've eliminated everything else and I've taken it off the board and all windings read resistive rather than inductive so I guess it is gone.Therefore I'm looking for a replacement if anyone can help? I've tried the obvious sources such as Qservice. I'm in the UK.BestChris

author: bd139
author_rank: Super Contributor
timestamp: July 27, 2020, 05:48:38 pm
content: Good luck with that. Your best bet is to find another 22xx to pull the transformer from or get rid of the thing quick. The entire power supply is nasty in them. I’ve done two so far (check my threads on here) with marginal success. I managed to get a transformer and multiplier from the US but it set me back £60. Hardly worth it.I’m inclined to not bother with these scopes any more.

author: tkamiya
author_rank: Super Contributor
timestamp: July 27, 2020, 10:09:23 pm
content: I have a Tek 2236 for parts. I wonder if it's the same in power supply??

author: Satbeginner
author_rank: Regular Contributor
timestamp: September 21, 2020, 11:29:45 am
content: Hi all,I am fighting with a 2235 PS too.I previously repaired several 2213 and 2215A's, and it "always" was the primary switcher, but this time....I removed the Power FET to apply a external 43VDC to see if the scope would work, but no, it looked like a full short, the secondary switcher would not work at all.I disconnected all secondary diodes from transformer 120-1601-01 , still the same.I disconnected the 2kV wire and disconnected the two filament wires, still full short, no switching in the secondary switcher.I checked all components in the secondary switcher circuit, all OK.I removed the transformer 120-1601-01 (PITA!) and temporarily replaced the two primary windings in the secondary switcher circuit with two 24 Ohm resistors, and behold, the secondary switcher is working!So, is my transformer dead?The weird thing is, I applied a 400mV 50kHz sine to one of the primary windings, and all secondary windings give a voltage which looks like OK with respect to what is to be expected.My thoughts were: if the input is roughly 40V switched, I know roughly what to expect, so if I use a similar, but lower voltage from a generator, I could check the individual outputs?I still do not know if the transformer is dead or not, but when I connect the two primary windings it will not switch at all.I will start to measure the inductance of all the individual windings and report back.Saludos from Spain,Leo

author: Satbeginner
author_rank: Regular Contributor
timestamp: September 21, 2020, 12:35:36 pm
content: Hi all,back again.Before measuring all inductances, I did the following first: I did a "ringing test" on the transformer.Basically you apply a fast rise and fall square wave to (one of) the primary winding(s) and measure using a scope the output of one of the secondary windings at the moment of the Rising flank.More info can be found here:https://www.flippers.com/fbt-main.htmlAttached to this message you see the result of the TekTronix transformer and the result of an other transformer I got from a switched mode power supply.When there is (hardly) any "ringing" that means there is a shorted winding, or at least a heavily loaded winding.But since these transformers are now out-of-circuit, there should be no load.However, the TekTronix transformer is showing hardly any ringing, so somewhere in there is a short....Now I need to find another transformer, 120-1601-01 or similar (2213, 2215, 2215A, ...)Saludos,Leo

author: bd139
author_rank: Super Contributor
timestamp: September 21, 2020, 12:55:45 pm
content: If you do find a replacement make sure you find out why it failed otherwise it'll eat the next one as well instantly.

author: beanhauler
author_rank: Contributor
timestamp: September 23, 2020, 10:40:34 pm
content: Hmm i also have this problemThank you for the suggestion on fast pulse checking , i might have to build the box and try itInteresting thing is i have lifted the resistor in the emitter of the two driver transistors and i get a rock solid 42.x V across the inverter driver circuit electrolytic.This is between the two test points. I have not hung a bulb on it to see if it holds up but i expect it will. i also need to check the waveforms on the TDA chip but i assume alright. I assume that something is loading the driver transistors of the inverter from starting up or one of the inverter transistors is just dumping full load current to the 0.2 ohm resistor which shuts down the pre-reg.So i removed all secondary loads one at a time including filament wires and the sneaky 5 volt bleed ...still no go.Checked all diodes across the secondary and appear all ok. I have now removed the transformer completely and of course the 43 is still there.I have put an inductance meter on the primary and it measures 36mH across the two outer windings with the windings joined for the centre tap.Interestingly if i measure the windings separately i get 6.58uH per winding on the primary which is strange.Next up i will put a signal genny on it and see what the secondarys measure although i suppose i should measuer at 20khz and not a few k's.Like the 24ohm resistor check as a substitute for the secondary but how does this oscillate with no base drive from the primary feedback circuit?this all points to the rise time ringing check but if it fails i guess in the bin it goes. The transformer seems vacuum potted and impossible to rewind.Forgot to add. This all started with an excessive amount of jitter on the display and then the familar cycle of inverter tring to start with the "squeak" at 12hzUpdate.This is alittle bit of a mysteryChecked the inductances on the transformer and this is what i get:primary 3 and 4 shorted out with a clip ..normally connected via pcbbetween 2 and 3 6.85uHbetween 4 and 5 6.85uHbetween 2 and 5 36.5mHresistance between 2 and 5 0.3 ohmssecondary 12-14 41.2 mH14-16 41.2mH13-15 36.5mH12-16 46mH13-14 25.2mH14-15 25.2mHtried to measure the hv to what i think is pin 10 and shows 500k resistance and zero inductanceMaybe there is a cap in the way?worrying is the inductance measurement on the primary between the centre tap and a collector of a transistor would be 6.85uHThis seems too low but if anyone out there can tell me otherwise i would be grateful.If i join 3 to 4 as it would be normally on the pcb then the outer 2 to 5 read mH but that is not what would be seen by a transistor collector.7uH would saturate very quickly i think.Is this broken?Think i have found my problem.Did a frequency test of the transformer putting 1 v at 600 ohms between pins 2 and 5 with 3 and 4 shorted.Ran it at one k and teh outputs were all low but there and in proportion but a little low. Increased frequency to 10k and level doubled. Increased to 20 K and not much change. On the face of it it looks fine, however on deeper looking i notice a hairline crack through the ferrite core. Now it makes sense. Inductance drops and primary current increases to maintain same volt secs across core . Current increases beyond limit on current sensing resistor and trips main inverter supply pre reg.I suppose if i cou;d find an identical core i could replace but is probably a long shot.Need to look for a spare transformer now.

author: bdunham7
author_rank: Super Contributor
timestamp: September 23, 2020, 10:54:15 pm
content: Quote from: oldwhitelodge on July 27, 2020, 12:37:27 pmI may have that part or an equivalent, I'll have to check. Can you tell us what exactly is wrong with the scope and why you are looking at the transformer?

author: Satbeginner
author_rank: Regular Contributor
timestamp: September 24, 2020, 06:56:13 am
content: Hi,The main problem was: no power at none of the output voltages of the power supply.The primary switcher circuit is trying to power the secondary switcher (very briefly) but is switched off because of an overcurrent situation.I need the transformer T948, because when I took it out I could test it.Also, the secondary switcher circuit that drives T948 does work when I replace the two primary windings in the switcher circuit (pins 3-4-5-6) with two 24 Ohm 5W resistors.So, transformer out of the circuit, and two resistors in the circuit.For this test I removed the power primary switching FET, and powered the secondary switcher using 43V at TP940 and TP950.I did not build that "ringing test" box, I just connected a 1kHz square wave output from a function generator (in my case a HP3314A) and triggered my scope on the leading flank.I used the scope to look at one of the secondary windings, when you see that nice, slow decaying ringing, the transformer is OK, in the example of the broken T948 after two (distorted) periods the signal is gone.That indicates there is a shorted winding somewhere inside that T948 transformer.I found that several different part numbers were used during the lifespan of this 22xx series scopes.120-1348-01, 120-1348-03, but also 120-1601-01 was used.Please let me know if you have one available??Regards,Leo

author: tppc
author_rank: Regular Contributor
timestamp: September 24, 2020, 10:04:28 pm
content: Hi there,I had a similar issue with a Tek2211.As I could not source a replacement transformer, I decided to unsolder the copper shilding and open the transformer.In my case, the problem was on the HV winding (many turns burned and shorted).I uncoiled it and counted the turns, even though this was back in sept. 2006, I can still remember there were 916 turns!Then I bought a roll of 0.1mm wire and rewond the 916 turns!(1 layer joint winding, electrofuge spray, 1 turn of plastic film, 1 layer joint winding, electrofuge spray, 1 turn of plastic film etc...)can't remember how many layers, maybe 10 or so.This scope still works wonderfully.That said, not sure it is the same cause, but it would be worth checking.Regards.

author: Satbeginner
author_rank: Regular Contributor
timestamp: October 02, 2020, 03:24:19 pm
content: Quote from: tppc on September 24, 2020, 10:04:28 pmHi,Since it was broken anyway, I opened it up to see if a rewind of (most likely) the HV winding would be possible.First of all, it was completely potted, so already quite difficult to open up.And, on top of that, the HV winding in this version of the transformer turned out to be physically in between the pins 12,13,14,15,16 and pins 17,18,19 winding.So no "easy" access to the HV winding without destroying another winding first.So, still looking for a TekTronix 22xx T948 transformer... :-)Saludos from Spain,Leo

author: Satbeginner
author_rank: Regular Contributor
timestamp: November 14, 2020, 03:18:42 pm
content: Follow up on this repair:I managed to find a replacement transformer, so from that point on and a full recap of the PS the Power Supply was working again, but....I also added a 12V 60mm fan to help this scope a little it. The components for that were already on the A1 PC.Only to find the CRT was damaged....In the top half of the screen the traces were showing distorsions and also a fuzzy spot, caused by internally broken glass support beams. :-(I could confirm this by removing the CRT and I could see the loose glass particles moving around inside the CRT.Although it did not make sense at all economically, I decided to give this scope a new life.I managed to find a replacement CRT (not the exact same type, but newer), and after adding the three components R897, CR 897 and C897 providing a DC voltage to pin 12 of the CRT, it was back in business.After a thorough cleaning of the input attenuators and a full calibration it is as good as new :-)Thanks and hope you are all safe,Leo

author: markce
author_rank: Regular Contributor
timestamp: November 14, 2020, 06:17:41 pm
content: Ah, you have a 2235A model. Now that makes a bit more sense to invest in.In general, if you buy a 2235/36 first thing is to recap the power supply elco's if this is not already done.This will prevent selfdamage of the PWS.Also these elco's are critical for the scope's performance so only take take the best (low ESR).