trying to repair tektronix 2245a

[undone.evil]

hello i have been lurking around for awhile and decided to make an account.

i have a tektronix 2245a that when it is turned on hums, clicks, and resets over and over. the first thing i tried was removing components untill the power supply would stay on.

i started with the processor board and with it removed the power supply resets a couple of times and then starts and stays running. at this point i checked the output voltages and the 43v and they are all good. i then removed the power supply from the main board and powered it up and it starts the first time.

i then started suspecting that maybe the caps in the power supply were weak and it was not able to provide the proper volts/amps on start up but i have removed most of the electrolytic caps and they were within spec. i also checked for shorts across all of the diodes and found nothing.

im not sure if im even on the right track here and was looking for some outside opinions/ideas. i do HVACR so i have an understanding of how electricity works and how it acts in simple circuits but this is way out my league. i am a hobbyist so i play around with crt tv's, plasma tv's, basic electronics, and old junk from the 80's-90's but i am no electrical engineer.

anyways any help or advise would be greatly appreciated, thank you.     

[bob91343]

I would put aside the power supply and try to run it from a separate power supply.  These scopes can be very flaky at times.  I repaired one by replacing the power FET in the power supply.

[undone.evil]

i dont really have the equipment to run it on a bench power supply, i would probably need 4 of them with all the different voltages also a used as is power supply for this scope cost as much as a whole working scope.

[bob91343]

That isn't true.  The scope runs on a single supply, 43 Volts or something like that.  The source of that is a separate board that is easily disconnected.

[aqibi2000]

Or it could be powered from 1 PSU with various voltage regulators.

[WattsThat]

That isn't true.  The scope runs on a single supply, 43 Volts or something like that.  The source of that is a separate board that is easily disconnected.

Yup, 44 volts to be exact with eight output voltages derived from it. But the 44 volt supply is not separate on that scope, it’s one rather busy board.

If the unit comes out of chirp mode with loads disconnected, it’s not the 44 volt supply.

You can remove the supply from the unit and load the 5 volt supply to test it out of the box. Just beware the 130 volt supply caps stay charged for a long time. Put a 10k bleeder resistor across it otherwise the caps will bite you bad for a good five minutes or more.

FWIW, the Tek service manual can be found online or purchased on eBay from artekmedia for $12.95. There is a detailed section on troubleshooting contains it the manual. Start there.

[undone.evil]

so i removed the W2201 jumper wire on the power supply which from my understanding isolates the 43v to the rest of the board and loaded it with a 40w incandescent bulb and it fired up instantly, i also tried a 150w bulb and it took a few tries to start the supply but i think that is just because the 150w is to high of a load for the supply but i could be wrong.

@ Watts That - how much of a load would you recommend i put on the 5v line to test it. and good advise on the bleed resistor i got bit already.

i have the service manual for this scope but it seems to jump around and alot of this is beyond my knowledge so its like greek to me.

i attached some images of the power supply

[WattsThat]

I remember using a 100 watt incandescent lamp on the 44V supply and mine fired up fine once I fixed the problem which was a bad transformer, T2204. Nothing was open but it simply wouldn’t allow the oscillator to start. I loaded 5 volts to 2 amps and the +/-7.5V to about 500ma. Fortunately I had a connector that fit the  pins that made it much easier to test.

I do have the remains from a good supply board that I harvested T2204 from, so if you need any Tek specific parts, PM me, happy to send them to a good home.

[WattsThat]

The jumper W2201 does not disconnect the 44 volt supply, it’s the pre-regulator current source and the easy place to see waveform 12I. Don’t attempt to connect a scope to this power supply without a high voltage differential probe since the 44 volt side is not isolated from mains, you’ll blow up your scope and maybe yourself.

There is a very good detailed description of the power supply beginning on page 3-55.

Edit: also buried somewhere in the service manual, and I forget exactly where, is a table of resistance readings for each of the power supply voltage rails. That can be an easy place to start looking for shorted tantalum caps and other things that would load the supply down such that it will not start, known as “chirp” mode.

[undone.evil]

heres what i have with a 150 watt bulb = 10ohm load

5v=250ma
7.5v=300ma
15v=400ma

44v=750ma after several restart attempts

i have also been wondering about the T2204, i noticed that pins 12-22 all have a direct connection to ground and that doesnt seem right seeing as those pins are outputting through the diodes to the header pins to the main board.

yes i seen the table your talking about, from what i can tell those readings have to be taken with the power supply in the scope but the manual doesnt say. when i checked the resistances nothing seemed way off.

im used to sony stuff so this seems really vague or maybe im just not looking at it right.

also heres the table you were referring to

[undone.evil]

ok, so i have been messing around with this power supply (i had to give it break for a bit) and i remembered that the 130vac was giving me weird readings and that all of the pins on the output side of the transformer T2204 seemed to have low resistance to ground.

i lifted all of the diodes on the output side of the transformer and pins 13-22 all have between 1.2-0.2 ohms to ground and each other also pin 12 is open to all other pins on the transformer and ground, now im pretty positive its the T2204 transformer which sucks because they dont exist anymore. the only places that i can find them are websites that want me to send information for quotes and im pretty sure these places are scams. i thought about using a couple of transformers to replace it but im not sure thats doable. 

now im wondering how this thing has been making the proper voltages with a short to ground, maybe im crazy but that doesnt seem possible. as far as i can tell rest of the circuit to the supply pins for the scope seems fine (no shorted capacitors or bad diodes).

maybe someone can chime in on how i had correct voltages at the test points with shorted windings.

lastly i would post on the buy/sell to see if someone has the part but i cant make a post and i read somewhere that its only for businesses and somewhere else that i have to make X amount of posts but i cant seem to find the true answer so if someone could enlighten me i would appreciate it.

thank you   

[techneut]

That you measure low resistance on the transformer to ground is because pin 19 and 14 are connected to ground so you are measuring the secundairy coils. That you can not measure pin 12 is a problem because you wil be missing 130V ac and dc. Only you said before that you were bitten by the cap?

[undone.evil]

i meant that i was bitten by something and that a bleeder resistor is generally a good idea.

i understand that pins 19 & 14 are ground but the highest reading i get across any pins to each other or ground is 1.2 ohms, i would think that 22 to 16 would give me more than that but i usually work on larger step down transformers than this so im not sure if this is correct for the voltages here.

so i wonder if i can get a small transformer and feed 130v into the board at pin 12 and it will work.