Tek 2230 external psu, help needed!


Pitpat
 

Hi guys! Few monts ago I've got this nice scope that was struck by a lighting, I've managed to get it going but it seems that T906 is affected with a shorted turn on one of the secondaries that lead to a way to hot q9070 that eventualy with the excessive heat give up busting itself, the fuse and, depending on how lucky I am, other components.
It also make a really funny noise when operated, note that all the voltages after the inverter section are spot on!!
The idea is: since is really hard to find a replacement for the T906 and a rewind is way out of my competence can I replace all the preregulator with a new smps? I'll try to attach a file that should make clear what ive got in mind.
The red cross represent the components that i would remove for this job.
Thanks
Let me now what you guys think!!


Pitpat
 


Pitpat
 

My bad, all the 48vAC should be DC


alastair.knights@...
 

Yes you can replace the Pre-Regulator stage with a 42V 3A SMPS or even a laptop-style external power brick. One snag to watch for is that on start-up, any momentary SMPS over-voltage may trigger SCR Q935. So I would first test the SMPS for overvoltage on another scope. I'd also check that the R934 (3ohm) is in place in case Tek Mod 050-2239-03 has been implemented, which replaces R934 with a short. Also you must ensure that your SMPS has a proper current limit. When I tried this on my own 2239, Q935 triggered, shorted the SMPS, which delivered 7A, enough to blow up Q935. So yes it does work, but be careful about over-current protection.


Pitpat
 

Thanks for the tips! I tink I'll use a voltage regulator that also has a current limiter after the smps.
Federico


Pitpat
 

I'm not quite sure how to properly set the current limit, any help would be appreciated :)
Also from my understanding the boost converter works manipulatating a pwm like signal that could lead to a noisy output, I guess that I'll have to probe it when I'll get the module.


alastair.knights@...
 

Some form of current limit for the external supply is highly recommended as another fault could burn out the main inverter transformer T948, which is un-obtainable. The short-circuit current from a SMPS power supplies can be very high - my 48V 3A SMPS module reaches 7A when shorted before it cuts out. A fuse would not provide adequate protection either. (The Tektronix pre-regulator has built-in current sense/limit circuitry.)

If I was attempting this modification, I would use an external 48V 2A laptop style power brick with mains and power lead/plug (UK price about £16 from eBay/Banggood/etc). Inside the scope, I'd fit a DC-DC buck converter to drop the voltage down to 42V and also provide an electronic current limit to protect the rest of the scope. The normal current draw at 42V is around 1.5A, so a 2A current limit would be sensible.

I'd also want some form of over-temperature cut-out to protect the DC-DC converter from a short in the scope power supply as it might try to dissipate 96W inside the scope case! You might also want to buy a 22ohm 100W resistor so you can test the SMPS and Buck converter combination before connecting it to your scope. You can also use it to test the Pre-Regulator if you want to attempt any further repairs.

Finding a DC-DC Buck converter that can output 42V at 2A with current limiting and which fits inside the scope chassis might be a challenge. You may need to use adapt/modify a cheap basic buck converter module to suit, so a 22ohm 100W test resistor would be useful to test the final converter. A quick search on eBay shows a 50V 5A Adjustable DC-DC Step Down Buck Module with current and power limiting, but you'd need to remove the PCB from the case and possibly remove some parts from inside the scope chassis to make room - eg; the mains inlet connector, the line input filter and C906.

Hope this points you in the right direction,

Alastair Knights


Pitpat
 

Hi thanks! Your suggestions really pointed me in the right direction!, I'm tinking of using a boost converter (that has current limiter and a voltage regulator builtin) because as you said finding a good buck converter that support 50vdc as an input is a tough job.
I'll try experiment with that using my variable bench psu as an input at first and then choose a power brick.