Blown PSU in a Tek T935A scope - diyAudio
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Old 5th August 2009, 08:50 PM   #1
00940 is offline 00940  Belgium
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Default Blown PSU in a Tek T935A scope

I'm pretty sure the transformer is toast but I'll still ask

I've got a T935A which blew the HV fuse.

Before changing the fuse, I've tested the following components:
- R458 and R457 are reading ok.
- C455 and C458 are ok.
- Q458 (a tip150) is also ok.

Testing with power on but no fuse on the HV line reveals that all the low voltages supplies in the scope are ok (+/-8v, 100v, 33v).

So I changed the fuse and started the scope again, monitoring the current through r458. It went up to 320ma so I quickly powered it down. According to this website: http://www.logwell.com/tech/oscillos...ice_notes.html , " If R458 gets very hot or burns out, check C458 for a short, check T460 for a shorted primary, and/or check Q458 for proper operation." .

C458 and Q458 seeming ok, I removed the cover over the HV section and tested the transformer (after checking that the 10kV was down, I'm not crazy ). There is indeed a dead short in between the pins 4 and 5 of the xformer (less than 1R and continuity tester "beeps").

Things seeming toast anyway, I powered the thing two seconds again to check the voltage in between the pins 4 and 5. I got less than 3V...

Two things:
- The xformer didn't get hot... but with a 3W resistor dissipating 16W, I didn't let the power on for more than 3sec.
- There is a diode soldered from pin 1 (ground) to pin 5 of the xformer. It's not referenced in the service manual. It's soldered directly on the leads.

I attach the schematic of that section. The full service manual can be found here : ftp://bama.sbc.edu/downloads/tek/t932/

Thanks in advance for any tips
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Old 5th August 2009, 09:17 PM   #2
star882 is offline star882  United States
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It's a switching power supply, so the transformer would read as a short on a meter.

Check for a short on the secondary side.
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Old 6th August 2009, 07:38 PM   #3
00940 is offline 00940  Belgium
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I did my homework. I'm also attaching the schematic of the whole oscillator section.

In between :
- 1-9: 183r (9 leads to the 3x multiplier to get 10KV)
- 1-8 : 106r
- 7-10: 110r
- 3-6: 1.3meg
- heaters: 0.3R (continuity meters beep)

I've also found additionnal parts air-wired : there's an additionnal RC filter in between CR465 and R465.

How can I properly test a xformer for a switching supply ? Feed 1V of AC to 4-5 and see what happens ?
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Old 6th August 2009, 08:20 PM   #4
star882 is offline star882  United States
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Connect a light bulb in series with the input voltage for current limiting. Then measure voltages to find the problem.
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Old 7th August 2009, 06:24 AM   #5
Eva is offline Eva  Spain
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Fed a few volts of AC, say 20Khz (since you can get that from an audio generator), to the primary and see what happens. If the transformer is ok it should transfer the waveform to the secondaries while drawing very little current. Note that sometimes intermittent shorts only appear when the high voltage windings are used close to their operating voltage.
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Old 7th August 2009, 09:36 AM   #6
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Allright, I had a bit of time this morning. I tried to follow Eva's advice.

I fed 8V of 15khz into the primary through r458.

6.5VAC developped accros r458.
2.5VAC developped accross the primary.
81VAC appeared on pin 9.
26VAC appeared on pin 10 and 8.
nothing appeared from pin 3 to 6.

The xformer could be in better shape than I thought
This evening, I'll try to start it with a bulb in serie.


Thank you a lot for the advices so far, I really appreciate it.
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Old 7th August 2009, 10:53 AM   #7
Eva is offline Eva  Spain
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Could the winding between pins 3 and 6 be open or shorted?

This winding is used to estabilish self oscillation and even for current limiting (by detecting transformer saturation). Q458 will be on forever if the winding is shorted, and it can turn on too due to parasitic currents if the winding is open.
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Old 7th August 2009, 04:54 PM   #8
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I don't know how I measured 1.3meg yesterday between pins 3 and 6... they're definitely reading a short now.

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Old 7th August 2009, 06:34 PM   #9
star882 is offline star882  United States
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It might be normal for the winding to read as a short. Try putting a signal into that winding and measure voltages on the others.
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Old 7th August 2009, 08:02 PM   #10
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Putting same voltage @15khz through that winding (connection from the other side of r457 to the connected side of r455) does result in the entirety of the voltage being absorbed by r457 and nothing appearing on the other outputs.
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