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Tektronix TSG-271 PSU

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Tektronix TSG-271 PSU

Post by Cathovisor » Thu Mar 24, 2016 9:18 pm

Some time ago my company bought a Tektronix TSG-271 PAL test signal generator. Unfortunately, it blew fuses when I got it... and after a lot of investigation, there was a considerable amount of silicon carnage on the mains side of the SMPSU - to wit, the chopper transistor itself, its drivers, an active snubber circuit, the 0.47µF drive coupling cap (wrong type - should have been polypropylene) and a 0.75Ω current limit resistor amongst others!

I've now got it to the point where bringing it up on a variac will allow it to run nicely, but any attempt at a cold start results in the destruction of the 1A fuse and the chopper transistor :aai

Any thoughts from any SMPSU gurus? A snippet is reproduced below.
TSG-271 PSU.GIF

 
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Re: Tektronix TSG-271 PSU

Post by Ed Dinning » Thu Mar 24, 2016 9:46 pm

Hi Catho, look at the surge protection circuit; the soft start circuit and current limit area. This may show you if it is in the AC to DC front end or the regulation circuits. I can't see a power factor corrected front end, which is good as it can give further complications.

In a lot of these units the switching transistor is driven on for the first few pulses and turns off on either a timer or the current limit circuit. This is to overcome the difficulty of the voltage control loop not being stabilised (caps to charge) until after the first few pulses.

Ed

 
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Re: Tektronix TSG-271 PSU

Post by Cathovisor » Thu Mar 24, 2016 10:18 pm

Thanks, Ed: I'll have a look in the PSU and see if I changed the LM393 in the current limit circuit - so much silicon got smoked in that PSU that I've lost track of what I changed! The chopper control chip certainly did.

I had been fitting what I found in the PSU - BUW12As - as chopper transistors, but I have ordered some MJH16002As from the States for it.

 
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Re: Tektronix TSG-271 PSU

Post by Cathovisor » Fri Mar 25, 2016 12:56 pm

A little cold measuring around the PSU last night revealed that R647 (270kΩ) has gone o/c - I wonder if this is the problem? The diode and the 680pF cap are okay.
TSG-271 PSU detail.GIF
TSG-271 PSU detail.GIF (9 KiB) Viewed 1033 times

 
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Re: Tektronix TSG-271 PSU

Post by Refugee » Fri Mar 25, 2016 4:55 pm

Have you checked all the secondary diodes. The current limiters on those things often just fail to work.
The damage on the mains side just acts as a distraction.

 
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Re: Tektronix TSG-271 PSU

Post by Cathovisor » Fri Mar 25, 2016 4:58 pm

Refugee wrote:Have you checked all the secondary diodes. The current limiters on those things often just fail to work.
The damage on the mains side just acts as a distraction.

Well, as mentioned Ref - when wound up on a variac, it works perfectly; it's a 'cold start' that kills it. And yes, I have checked the diodes but will do so again, just in case.

 
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Re: Tektronix TSG-271 PSU

Post by Ed Dinning » Fri Mar 25, 2016 8:31 pm

Hi, snubber resistor (R647) could give problems as currents in main device will be higher at start up and with the resistor OC it could cause problems.

Ed

 
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Re: Tektronix TSG-271 PSU

Post by Cathovisor » Sat Mar 26, 2016 9:06 pm

Say it quietly, but... I think we have a win. :aan

In between visiting scrapyards and the like this morning, I visited one of my stores to grab some 120kΩ resistors to calibrate my Leader CRT tester (next on the bench - I didn't like the burnt resistors in the PSU, nor the split sleeve on one electrolytic) and also some 270kΩ to try in the PSU. This also gave me an excuse to exercise the 'scope and its 100x probe.

So... connect the 'scope to the collector of the chopper transistor and wind up the variac. These traces are taken with the o/c 270kΩ resistor still in place. This is with the PSU running at about 200V or so AC.
WA000004.png
Yes, that's really 100V/div!

WA000005.png
Note p-p amplitude is nearly 800V.

Having disconnected, and allowed the two smoothing caps to discharge, I replaced the o/c 270kΩ resistor with a new one and again, wound the variac up to the same voltage. In all this time, the 'scope has not been touched.
WA000010.png
p-p amplitude has dropped by some 200V.

WA000009.png

Finally, the moment of truth: will it take a cold start?

The answer is.... yes! There is a heart-stopping moment though because the longest second or so ever passes between switching on and the PSU bursting into life! So finally, I've ordered some 350V 1W 270kΩ resistors to go in the position previously occupied by the o/c one as I'm unsure of the rating of the temporarily fitted one.

I'm left wondering if the cold start produced a spike that exceeded the SOA of the switching transistor when that 270kΩ was open circuit and punched a hole in the junction...

 
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Re: Tektronix TSG-271 PSU

Post by Refugee » Sat Mar 26, 2016 11:45 pm

That is a lovely demo of what the snubber does on one of those SMPSUs.
It sure cuts the spike in half.


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