Topics

2235 Repair having no power


satbeginner
 

Hi all,

I am fighting with a 2235 PS.
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 5W 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.

So to test further, 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 on this "ringing test" can be found here: https://www.flippers.com/fbt-main.html

Here are some pictures I took: https://groups.io/g/TekScopes/album?id=254000

In the mentioned album 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 both 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 for my 2235, preferably in the EU, 120-1601-01 or similar (2213, 2215, 2215A, ...)

Saludos, stay safe,

Leo


 

On Mon, Sep 21, 2020 at 02:47 PM, satbeginner wrote:


Now I need to find another transformer for my 2235, preferably in the EU,
120-1601-01 or similar (2213, 2215, 2215A, ...)
Hi Leo,
I have a 2215 wreck (very incomplete) with a transformer, will have to check.
I also have a 2235 with a defective primary switcher. Never tried to repair.

Contact me if interested.

Raymond


tekscopegroup@...
 

I don't have a 2235 manual on hand but this sounds very similar to my case with the T948 main inverter transformer (120-1348-03) used in the 2213A that had failed, in my case on a scope I fixed about 10 years ago. Once I replaced that transformer with one taken from a scrap board, everything came back to life. So these sometimes do indeed fail, although apparently not very often. I never did any testing to the failed one, so no idea what really went wrong. Wish I could offer more insight but that's it for now.


satbeginner
 

Raymond,

Contacted you through MP.

Leo


satbeginner
 

Hi,

Since it (the transformer) 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.

Here are some pictures I took: https://groups.io/g/TekScopes/album?id=254000

So, still looking for a TekTronix 22xx T948 transformer... :-)

Saludos from Spain,

Leo


tekscopegroup@...
 

Leo, best bet to setup an ebay search, although you probably already did that.

About a month ago 3 transformers where sold (item 283967868607), and another in July (item 323980491533). So there might be a good chance that the next one will come up at some point. Notice that I used both your transformer PN and the PN from the one I replaced to get these search results. Both PNs seem to be fully compatible to your 22xx needs, so don't limit yourself to just that one PN. Perhaps there might also be other compatible PNs out there for the exact same transformer.

If it where me, I would even contact that guy that sold the 3 transformers, just in case he might still have another one he found afterwards, or one of the purchases fell through, etc. One never knows. I once was able to get the very last quite rare display unit from a seller that had sold all his stock some months earlier, but luckily had missed one and was too lazy to relist it. A nice worded friendly inquiry sometimes yields a reward in that the seller might volunteer to sell you that last piece he still might have around.

Another idea is to find a "for parts does power up" 2213A, you can find those under $100, but obviously the shipping to Spain is most probably going to be a deal breaker.

Good luck hunting,
Alex


satbeginner
 

Hi all,

it's been a while but the 2235A is back in the land of the living.
This repair made no sense whatsoever economically, but it had to be done. (My mind works in mysterious ways..)

I found a transformer on eBay, tested it and put it in, and yes, I had a trace again.... but:
It was crooked, only in the upper halve of the screen, I tested it in X-Y mode and I could find the very same place where the trace would distort when applying DC voltages to X and Y inputs..

So.... the CRT was shot as well.
After taking it out, I saw the little glass particles inside the CRT.

Thanks to Raymond I was able to get another broken 2235A and a spare chassis with a 465 CRT in it.
I was able to transplant the 465 CRT to the 2235A.
To do that I needed to add a few parts and a wire to Pin 12 of the CRT to provide a 100VDC to it, and now it is all back up and running.
I replaced the PS caps and did a full calibration on it, so one Tek back to operation !!

Actually, two Tek's back into operation, because the other broken one I got, I also repaired, with that scope it was "just" the secondary switcher components.

One extra (temporary) trick I performed was with U930, that one had it's output on Pin-10 shot as well.
However, when I tested U930 by applying an external voltage, Pin10 did nothing, but Pin9 did!
In the datasheet Pin10 and Pin9 are the same pulse, but shifted in phase, so I temporarily removed Pin10 from the IC, and connected Pin9 to Q908 instead.

I ordered replacement IC's for U930, but for now it is running perfectly. :-)
Also caps replaced and calibrated, so now to the next project.
BTW, that one is the scope where I used the not original knobs.

Here are some pictures I took: https://groups.io/g/TekScopes/album?id=254000

Saludos, stay safe,

Leo


Bert Haskins
 

On 1/21/2021 11:20 AM, satbeginner wrote:
Hi all,

it's been a while but the 2235A is back in the land of the living.
This repair made no sense whatsoever economically, but it had to be done. (My mind works in mysterious ways..)

I found a transformer on eBay, tested it and put it in, and yes, I had a trace again.... but:
It was crooked, only in the upper halve of the screen, I tested it in X-Y mode and I could find the very same place where the trace would distort when applying DC voltages to X and Y inputs..

So.... the CRT was shot as well.
After taking it out, I saw the little glass particles inside the CRT.

Thanks to Raymond I was able to get another broken 2235A and a spare chassis with a 465 CRT in it.
I was able to transplant the 465 CRT to the 2235A.
To do that I needed to add a few parts and a wire to Pin 12 of the CRT to provide a 100VDC to it, and now it is all back up and running.
I replaced the PS caps and did a full calibration on it, so one Tek back to operation !!

Actually, two Tek's back into operation, because the other broken one I got, I also repaired, with that scope it was "just" the secondary switcher components.

One extra (temporary) trick I performed was with U930, that one had it's output on Pin-10 shot as well.
However, when I tested U930 by applying an external voltage, Pin10 did nothing, but Pin9 did!
In the datasheet Pin10 and Pin9 are the same pulse, but shifted in phase, so I temporarily removed Pin10 from the IC, and connected Pin9 to Q908 instead.

I ordered replacement IC's for U930, but for now it is running perfectly. :-)
Also caps replaced and calibrated, so now to the next project.
BTW, that one is the scope where I used the not original knobs.

Here are some pictures I took: https://groups.io/g/TekScopes/album?id=254000

Saludos, stay safe,

Leo
Great job Leo!

I used the "ringing test" a few years back to find a bad transformer in a 2215A.

I bought a 2235 as a parts scope and plan to swap the CRT over to a 2221A if I can find the time.


Bert






Roy Thistle
 

On Thu, Jan 21, 2021 at 08:20 AM, satbeginner wrote:


After taking it out, I saw the little glass particles inside the CRT.
Did you see... or do you know what they are from. ( I'm always curious about how things fail... and about CRTs in general.) Regards.