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TDS540C Power Supply Problems

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neil.johnson
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TDS540C Power Supply Problems

Post by neil.johnson » December 15th, 2018, 2:45 pm

Hi all,

First time poster, long time listener.

My favourite TDS540C is finally struggling to keep going now. First things first I checked the electrolytics. Nothing leaking, they'll all small black rectangular things not shiny round things.

Then pulled the power supply out and on the bench. The service manual has a list of expected unloaded output voltages so I am checking those out.
First thing to note is the sound: in the standby mode (no main power) there is a nasty zizzing sound coming from somewhere near the large bridge rectifier (can't make out the component refdes). It could be something else in the vicinity so I am loathe to just replace without evidence (and the HV probe is at work at the moment).

Anyway, that doesn't appear to stop the power supply doing something, and bringing it out of standby brings up the main regulators (smoother high-frequency hissing from the transformers). Now some of the output voltages are wrong :(
The +25V output on J6p17 should be in the range +23.5 to +27.5 volts. I read +4.90V
The +5V output on J5p39 should be in the range +0.59 to +0.81 volts. I read +0.05V
The -5V output on J5p17 should be in the range -0.39 to -0.61 volts. I read -0.69V

All the other rails agree with the service manual. Other than replacing the PSU board with a known-good one I wonder if anyone here has the schematic for this board? And/or any experiences in getting this board back up and running?

Many thanks,
Neil

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » December 15th, 2018, 3:58 pm

Note: I did find the schematics for the TDS540 on the w140.com website, but that doesn't quite line up with my 540C - for example, the DC power connector J5 is actually 50 pins compared to 40 pins on the schematic. If anyone has a link to the 540C schematics that would be a great help!

Neil

strick
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Re: TDS540C Power Supply Problems

Post by strick » December 16th, 2018, 6:56 am

Neil,
The 520B Component Service manual has the schematic for the 400 watt power supply. You can download it from K04bb's site. If you have the tds520 service manual #2 (from Artekmedia) you can generally follow the procedures for checking the power supply out. the earlier manual is for the 350 watt power supply and there are differences in how the standby supply works.

When you say that the 540C is struggling to keep going, what do you mean? Power supply problems generally manifest themselves with:
no power at all, ever.
only powers on if unplugged for awhile
suddenly turns off on it's own

They are very well regulated and strange voltages will generally cause the supply to simply not start.

BTW. you should be using an isolation transformer to look at anything on the high voltage section as lots of things are above 400 volts.

Strick

hexley ball
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Re: TDS540C Power Supply Problems

Post by hexley ball » December 16th, 2018, 12:54 pm

neil.johnson wrote:
December 15th, 2018, 2:45 pm
The +25V output on J6p17 should be in the range +23.5 to +27.5 volts. I read +4.90V
FWIW, the power supply in my TDS644B produced odd voltages on the +25 output until that rail was loaded with >500 mA.

For testing, I had built a load board that put 500mA loads on each of the 5 volt rails, 100mA on each of the 15 volt rails, but only 50 mA on the +25 rail; and that made the +25 rail unhappy.

BTW, the problem with my supply was two bad TVS suppressor diodes. After replacing them with new parts, I did a post mortem. The two removed suppression diodes were tested using an adjustable 0-400 volt HV supply, a safety series resistor, and a 1 mA VOM to ground. Voltage across the diode was measured with a bench DMM. A couple of the new replacement suppressor diodes were also tested. Results:

The bad 250 volt suppressor passed 1 mA with 10.6 volts in one direction, and 23 volts in the other direction. Yikes!
The good 250 volt suppressors passed 600 uA at 246 volts in either direction. This is as rated.

The bad 220 volt suppressor passed 750 uA at 220 volts in one direction, which is as rated. In the reverse direction, however it passed 1 mA at only 50 volts. Yikes again!
The good 220 volt suppressor passed 800 uA at 216 volts in one direction, and 213 volts in the other. Seems to be as rated.

After replacing the suppressor diodes, I also swapped in a new BU508A switching transistor for good measure. The scope has worked fine since.

strick
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Re: TDS540C Power Supply Problems

Post by strick » December 16th, 2018, 2:08 pm

Hexley,
I think your methodology is correct to place a load on all the rails to get the proper response. The diodes you speak of are documented in the 350 watt supply (TDS520 manual) and serve to regulate the voltage for the standby supply. it appears to be a free-running oscillator with the transistor (Q9) you spoke of as the switch. They are VR4 (1.5K220CA) and VR5 (...250CA). This design also occurs in some 400 watt supplies.

Many of the newer 400 watt supplies have a pulse width modulator (pwm) instead of the free running circuit. It's easy to tell which you have by looking for two large semiconductors on the main board just underneath the heat sink surrounding the large 470 uf (450V) cap. The diodes are connected and at right angles to each other. If you have the newer pwm, it is a single non-heatsinked 3-pin (TO-220 type) device about an inch in front of the same heat sink toward the 40 pin connector.

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » December 19th, 2018, 3:13 pm

strick wrote:
December 16th, 2018, 6:56 am
Neil,
The 520B Component Service manual has the schematic for the 400 watt power supply. You can download it from K04bb's site. If you have the tds520 service manual #2 (from Artekmedia) you can generally follow the procedures for checking the power supply out. the earlier manual is for the 350 watt power supply and there are differences in how the standby supply works.
Thanks for the lead, I managed to download a schematic and PCB layout that looks much more like the board I have in front of me.
strick wrote:
December 16th, 2018, 6:56 am
When you say that the 540C is struggling to keep going, what do you mean? Power supply problems generally manifest themselves with:
no power at all, ever.
only powers on if unplugged for awhile
suddenly turns off on it's own
Mostly symptoms #2 and #3 are shown by the scope, although once it starts turning off by itself it then won't turn on at all.
strick wrote:
December 16th, 2018, 6:56 am
They are very well regulated and strange voltages will generally cause the supply to simply not start.

BTW. you should be using an isolation transformer to look at anything on the high voltage section as lots of things are above 400 volts.

Strick
Yes I have a 240-to-110V isolated site transformer to power this while I am working on it.

Thanks for your help!

Neil

strick
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Re: TDS540C Power Supply Problems

Post by strick » December 19th, 2018, 5:47 pm

Neil,
if it turns on occasionally and/or turns off on it's own, I would strongly suspect the standby supply in the earlier models (VR4 and VR5 usually) or in the later models, the bias supply (U12, the pwm usually). I did a post earlier to be able to tell which version you have.

in both cases, these parts are driven directly by the main input bridge rectifier and filtered by C12, the large 470uF/450 volt capacitor. in each case, they provide the 12-15 volt bias voltage that is necessary for everything else to function. That also means that they run - ALL THE TIM that the scope is plugged in and the mains power switch is on. Cycling the on/standby switch simply applies power to the rest of the circuits to provide the rest of the rails. Sometimes the PWM can be revived by cycling the mains power switch or simply unplugging for a period.

Usually, C12 is fine, but I just got a scope that was supposedly "dead" that worked fine - for a few minutes. Then stopped turning on. C12 was cracked on the top and showed no capacitance at all!

Good luck. Strick

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » December 20th, 2018, 9:39 am

Strick,
Yes I believe I have the newer 400W model as I have found the PWM device just where you described.
What I have also recently found is that this power board has seen some repair work before (previous owner). Not a clean professional job :roll:
So I need to assess that work as well.

Cheers
Neil

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » February 6th, 2019, 3:32 pm

So, after a bit of a hiatus on other projects I'm back on this beastie. Confirmed that this board has the PWM chip U12, but also interestingly the large diodes by the 470uF cap (CR14 and CR16) are not at right-angles to each other, but run parallel. A small change, but a difference nonetheless.

Neil

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » February 6th, 2019, 3:53 pm

Hmmm, so here's the list of parts that look like they have been replaced or touched up:
CR34 (-5.1V output crowbar zener)
RT1 (input inrush protection)
C12 (main reservoir cap)
Q2 and Q4 (FET drive)
R2 (FET drive)
Q5 and Q6 (the FETs being driven)

Also note that CR46 and CR47 have had their anode ends touched up (flux on the joints).
As has the anode joint of CR6 (more flux).

Pretty much all on the input side, which makes me think this might have been hit by some nasty surge spike at some time in its life?

Neil

strick
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Re: TDS540C Power Supply Problems

Post by strick » February 6th, 2019, 6:10 pm

neil,
if you have a 3-pin U-12, then you have the later supply and the 520B schematic applies. The parts that I was referring to are VR4 and VR5 in the older supplies (300 and early 400 watt) and they are special 2-way diodes with a pass voltage of 220 and 250, respectively. Actually, I just came upon a supply that had VR4 and VR5 in parallel, so I need to update my last

the CR14 and CR16 you speak of are diodes after the main power mosfets. These are present in old and new supplies.

thus, first check out the standby (called bias in the schematic) supply. - if it's putting out the voltage sufficient to power the 15 volt bias, then the problem is beyond that point. Otherwise, simply replace U12. Since it has to work all the time that the scope is plugged in (standby mode), it's not surprising that it fails often.

Strick

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » February 9th, 2019, 4:13 pm

It looks like U12 is ok - I get bias voltages on both sides of T7, although not quite what the schematic would suggest: "+12V BIAS" reads 16.3V, and "+15V BIAS" reads 12.4V. So I think it is running, so the issue lies further down the road.

I also notice that when the supply is up I hear a gentle hissing from the big T6, which is kind of expected. But when the failure occurs it stops hissing. Time to check the gate drive.

Neil

neil.johnson
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Re: TDS540C Power Supply Problems

Post by neil.johnson » February 20th, 2019, 3:11 pm

Yes, it looks like the gate drive from the auxiliary board stops when the supply shuts down. Further probing shows that something is telling the SMPS controller chip to shutdown, but there are many sources all diode-ORed together. *sigh*

Neil

BurntChips
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Re: TDS540C Power Supply Problems

Post by BurntChips » February 21st, 2019, 8:29 am

Visually check the polarity of all the power supply electrolytic capacitors and ensure they are installed correctly. I am working on a '540C right now where the electrolytic on the -5V rail was installed backwards at the factory. I doubt that it's the only one that was built this way.

Ken

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