Power supply issue on Tektronix AWG410

[JazzCat007]

Hello everyone..  First post here at EEVblog forums.  I'm looking forward to getting more involved in this community.

I recently picked up a very reasonably priced Tektronix AWG410 with what appears to be a power supply issue.  (It was sold as for parts/repair, so no surprises) Many of the key voltages on the power supply unit as described in the AWG410 service manual are not preset, so I can only assume that there is a relatively fundamental issue that needs to be tracked down in the power supply or related to it. I can hear a relay activate when the main switch on the rear panel is turned on, but the front-panel power button has no effect.

So far I haven't found anything obvious in the power supply. (Checking for physical evidence like blown caps, basic in-circuit diode/transistor checks, etc...) What I could really use is a schematic for this power supply, but I'm getting  impression after some Google searching that this might be hard to come by. I believe this PS was manufactured by Fuji Microelectronics, part number FH0410. The Tektronix part number is 630-A006-00. Does anyone have any idea if a schematic can be tracked down for this unit?  If not, is there any experience base with these power supplies that might suggest common failure mechanisms to look for?

Sincerely,
Luke

[JazzCat007]

I forgot to include in my original post a question about the front panel ON/STBY button. Presumably, I should include the possibility that there is a fault in the ON/STBY button or its support circuitry. There service manual doesn't appear to provide any advice for specifically troubleshooting this path to the power supply. I assume that the PS must supply a standby voltage for the ON/STBY button function.  Does anyone know how to check this out in more detail?

BTW -- I misspoke slightly in my original post.  There is one meaningful voltage present on J3 connecter from the PS to the "A10 Connector" board.  Pin 16 measures around +4.2V.  The service manual indicates that it should be +5V.  Could this be the standby supply?

I also noticed that the J3 connector includes a "Remote On" at pin 9.  Is this for the ON/STBY function?  If yes, is there way to bypass the ON/STBY circuit at this connector to see if the power supply output will ramp up independent of the standby control logic? I don't know if it needs a jumper to ground, a voltage signal, or something else. (Again... I could probably answer some of these questions for myself if I actually had a schematic.  -alas- )

[lugaw]

I have a AW615 with a shorted main board. It also doesn't turn on like your unit. The only way I can turn it on is by removing the J1 J2 J4 connector and leaving the J3 connected. If this still not turn your unit on there is a problem with the power supply.

[Swake]

Is this your power supply? (Found these pics on Ebay). From the label on the picture I would say the 4.2V is not to be considered a normal situation.

Lets try answering the question if the issue is with the power supply or somewhere else. Unplug everything except mains and check if that standby voltage is normal now.
If you experiment connecting the remote-on pin to ground or elsewhere then make sure to limit the potential current using a resistor.

Do you have a means to power up the rest of the electronics using lab supplies? It seems you need 3 different voltages.

Careful poking in that thing, many electrons jumping around.

[JazzCat007]

Swake -- Yes...  That looks like the correct power supply unit.

I agree that 4.2V doesn't sound normal. (And to be clear, I don't really know if that's the standby rail either... It was more of a question on my part since it was the only connector voltage which seemed to be present and meaningful.)

So... I've got the power supply completely removed.  The boards are separated from the aluminum PS chassis box also so I can have an easier time following tracing and probing. When I provide power to the PS, there doesn't appear to be any improvement.  Pin 16 on J3 still reads low at 4.2V.  I also don't see any other correct voltages show up on J1, J2, J3, or J4.  Regarding your question about lab supplies, I only have a three-output bench supply at the moment...  (One is fixed at 5V, the other two are adjustable)  This isn't sufficient to power the full AWG410...  I see 7 different voltages on the J1-J4connectors, based on a connector pinout in the service manual.  (+12V, +5V, -5V, +8V, -8V, -2V, +3.3V)

I'll attach a quick photo to this post for reference. (I can share more as needed) The top board disconnects from vertical board thanks to a set of three connectors that can be separated.

Here are some of my initial observations:

• The mains plug feeds the top board which I can separate. Roughly half of this board appears to handle fundamental rectification and heavily filtering of the mains power. When I power this board by itself I can measure 164.5V DC on the output end.
• The other half of this board include 4 transformers and support components which I'm guessing make up four separate switch mode supplies.
• The bottom board includes two more transformers and a lot of inductors and capacitors.  Perhaps two additional switch mode supplies?  (That's six total at this point...  It's possible that one or more of these transformers have multi-tap secondaries -- I haven't sorted that out yet.)
• The horizontal board appears to be mostly full of voltage comparators -- based on looking up the datasheets for numbers found on the ICs. There is a 12V regulator IC on this board which is working.  I can measure 12V on it's output and many of the comparator ICs appear to be getting this 12V supply also.
• This three-board arrangement is gets more difficult to probe when they are all connected together.  I can get to the backside traces, of course, but it's hard to locate the components on buried on the inside to sort out what I'm looking at. I can't tell yet if each switch mode supply has fundamental dependencies to a neighbor board via the connectors. I could try to test the top board by itself, for example, but I'm not certain if I would be missing key portions of the circuits when it's disconnected.

Alright...  where to start?   
 

[JazzCat007]

I need to correct one thing I said in my previous post. After some more measurements with the multimeter this evening with the PS powered up, I realized the comparator ICs on the horizontal board are getting a 22V DC supply. (The regulated 12V source I mentioned must be for something else.) I'm no sure where the 22V supply is coming from yet...  I found connections to both the top and bottom boards with this 22V DC supply present. I'll have to do more manual trace following to figure out which switch mode supply (or other) source this is coming from.  More signs of life. I guess it's not entirely dead.

[JazzCat007]

I had some more time to investigate the power supply today. One minor note is that I discovered that one of the four transformers that I claimed are on the bottom board is actually only being used as an inductor. It's even labeled that way on the silkscreen ("L" vs. "T) - I just missed that in my quick look over previously. I was able to track down the 22V switch mode supply to transformer T3 on the bottom board.  I'll start capturing a reference list as I figure this out:

Board	Transformer	Final-Output	Notes
--------	---------------	---------------------	------------------------------------------------------------------
Top	T1	?	Current sensor on output side of T1 goes to vertical board via connector CN33.
Top	T2	?	Strange looking output waveform on the scope...  Bad single-diode rectifier?
Top	T3	22V - 23V	Supplies comparators on the vertical board
Bottom	T3	?
Bottom	T4	?

Regarding my note for the T2 switch mode supply on the top board:  I starting using my scope to look at waveforms on the output side of the transformers. I could see a messy-looking AC waveform after the diode on the secondary side of T2. The diode tested okay in-circuit with the diode setting on my multimeter.  It reads 202mV forward voltage drop, which suggests to me that this is a Schottky diode.  Reverse bias doesn't register on the meter.  Seems okay, but this doesn't explain the waveform that I was seeing.  Incidentally, I recently picked up one of those cheep T7-H component testers (similar to TC1), and it thinks that the diode is a capacitor. Are these TC1 or T7-H component testers capable properly testing a Schottky diode?

Is it possible that a Schottky diode can measure good with a multimeter, but fail when operated at frequency?

I decided to look up the datasheet for this diode and hit another snag -- I can't find it.  The markings on the diode appear to read "00441" The final "41" is in smaller font for some reason. Does anyone know which diode this is based on the markings that I see?

[MathWizard]

For that messy voltage after the diode, is that going into into something like an CLC filter, if it is and it's working, it should be a lot cleaner after the inductor.


There's lots of IC in there, are any of them dedicated controller IC's

I'm guessing the overall output power of the rails would be fairly low, at least compared to a computer PSU. Do you have anything like power resistors to use as a dummy load ?

Today I'm having a look at a computer PSU I haven't touched in years, but I can still remember where a diode temp sensor has to be soldered back onto (1 side anyway)

I'm just going through it unpowered. IIRC this one keeps resetting itself, so pretty soon I'll have to scope the controller chips, see which one is doing the reset.

[JazzCat007]

MathWizard -- Yes there are dedicated switch mode controllers present.  They all appear to be located on small separate SMD-based PCBs which are edge connected to the main boards.  I've attached a picture of one such SMD board, labeled H13, mounted to the bottom PS board.

I've spent more time inspecting the connectivity to/from this H13 controller and its associated transformers, drawing up a schematic (still incomplete), and making some more measurements. The controller device is a M51995AFP Switching Regulator Control IC. After drawing up a partial schematic, I attempted to measure the Vout switching control signal. It doesn't look quite right to me, but I don't have much experience looking at switch mode control signals. I've attached my schematic of this location and two waveform snapshots of the signal at the primary side of T4. (Disclaimer -- I'm drawing this schematic based on visual inspection of the of the PCB...  There could be mistakes at this point.)

Here's what I noticed so far:

• The controller doesn't drive the FET devices directly. Instead, there is an intermediate transformer T4 which steps up the voltage before being presented to the two FET devices. The two FETs control the current flow into the primary side of the transformer T3. I'm not sure how common this control topology is -- perhaps others have seen this before?
• Note: The FET symbols could be wrong... It's difficult to expose the part numbers which a hidden beneath a clamping heat sink. I'm making somewhat of a educated guess here.
• The switching control signal at the primary input to T4 appears to have a slow 60Hz modulation which impacts the amplitude of the faster control switching frequency.
• I looked at the controller Vcc input (and all other pins on the controller board for that matter), and couldn't find a source for this 60Hz noise.)
• I zoomed into the control waveform to see the faster switching behavior. The switching control frequency is around 83 kHz, but this is below the oscillating frequency range of 170-207 kHz indicated in the M51995AFP datasheet.
• The switching control signal at the primary input to T4 also doesn't look as clean as I expected to see. Does this look normal?  Should it be closer to a square wave with duty cycle skew?

Q: Does all this potentially point to a bad switch mode controller, or should investigate other factors first?

Note: I don't think I can safely scope the signals on the 164.5 VDC supply area (Between T4 and T3) since I don't have an isolation transformer at the moment. Perhaps it's time to invest in one. I can poke around with the multimeter, but I wont be able to evaluate the AC waveform characteristics in this region. The circuits on the secondary side of T3 should be safe.