Author Topic: Tektronix 2235 transformer  (Read 454 times)

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Offline oldwhitelodge

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Tektronix 2235 transformer
« on: July 27, 2020, 12:37:27 pm »
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.

Best

Chris
 

Online bd139

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Re: Tektronix 2235 transformer
« Reply #1 on: July 27, 2020, 05:48:38 pm »
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.
 

Offline tkamiya

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Re: Tektronix 2235 transformer
« Reply #2 on: July 27, 2020, 10:09:23 pm »
I have a Tek 2236 for parts.  I wonder if it's the same in power supply??
 

Offline Satbeginner

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Re: Tektronix 2235 transformer
« Reply #3 on: September 21, 2020, 11:29:45 am »
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



You need a scope to repair a scope, and you need many multimeters to repair another multimeter!
*Tek 2467B, Tek 2465B, Tek 2465B, Tek 485, Tek 475A,  Keithley 175A, Keithley 2000, HP 3468B, HP 3457A, HP 34401A, PM 6671, PM 5716, Fluke 45, Fluke 75, Fluke 77, Fluke 79, AFX 9660BL, KPS 605D, etc. *
 

Offline Satbeginner

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Re: Tektronix 2235 transformer
« Reply #4 on: September 21, 2020, 12:35:36 pm »
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.html

Attached 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



You need a scope to repair a scope, and you need many multimeters to repair another multimeter!
*Tek 2467B, Tek 2465B, Tek 2465B, Tek 485, Tek 475A,  Keithley 175A, Keithley 2000, HP 3468B, HP 3457A, HP 34401A, PM 6671, PM 5716, Fluke 45, Fluke 75, Fluke 77, Fluke 79, AFX 9660BL, KPS 605D, etc. *
 

Online bd139

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Re: Tektronix 2235 transformer
« Reply #5 on: September 21, 2020, 12:55:45 pm »
If you do find a replacement make sure you find out why it failed otherwise it'll eat the next one as well instantly.
 
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Offline beanhauler

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Re: Tektronix 2235 transformer
« Reply #6 on: September 23, 2020, 10:40:34 pm »
Hmm i also have this problem
Thank you for the suggestion on fast pulse checking , i might have to build the box and try it
Interesting 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 12hz

Update.
This is alittle bit of a mystery
Checked the inductances on the transformer and this is what i get:

primary 3 and 4 shorted out with a clip ..normally connected via pcb
between 2 and 3 6.85uH
between 4 and 5 6.85uH :horse:
between 2 and 5 36.5mH

resistance between 2 and 5 0.3 ohms

secondary 12-14 41.2 mH
14-16 41.2mH
13-15 36.5mH
12-16 46mH
13-14 25.2mH
14-15 25.2mH

tried to measure the hv to what i think is pin 10 and shows 500k resistance and zero inductance
Maybe 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 6.85uH
This 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?




« Last Edit: September 24, 2020, 11:42:30 pm by beanhauler »
 
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Offline bdunham7

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Re: Tektronix 2235 transformer
« Reply #7 on: September 23, 2020, 10:54:15 pm »
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.

Best

Chris

I 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? 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit DMM is good enough for most people.
 
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Offline Satbeginner

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Re: Tektronix 2235 transformer
« Reply #8 on: September 24, 2020, 06:56:13 am »
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
« Last Edit: September 24, 2020, 07:01:20 am by Satbeginner »
You need a scope to repair a scope, and you need many multimeters to repair another multimeter!
*Tek 2467B, Tek 2465B, Tek 2465B, Tek 485, Tek 475A,  Keithley 175A, Keithley 2000, HP 3468B, HP 3457A, HP 34401A, PM 6671, PM 5716, Fluke 45, Fluke 75, Fluke 77, Fluke 79, AFX 9660BL, KPS 605D, etc. *
 

Offline tppc

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Re: Tektronix 2235 transformer
« Reply #9 on: September 24, 2020, 10:04:28 pm »
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!  :-DD

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.
« Last Edit: September 25, 2020, 05:25:43 pm by tppc »
 
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