BG7TBL GPSDO 2021 - repair help - circuit diagram or ideas? - Page 1
Forum section: electronics / repair

author: LB6B
author_rank: Newbie
timestamp: March 22, 2025, 10:52:00 am
content: Hi I have a BG7TBL GPSDO with display - 2021-06-10 PCB design.It had been sitting on the shelf for some months, but connecting it yesterday nothing happened.I opened it up and found that one of the electrolytics (middle one C22) was bloated and was leaking so I replaced that and the two others, but still no success.The unit is pulling far too much current so the lab powersupply turns off immediately when connecting power.Does anyone have a circuit diagram or any idea to where the problem might be?Thanks for your help and time!IngersU2= MP1584ENU5= GH17M

author: Swake
author_rank: Frequent Contributor
timestamp: March 22, 2025, 05:31:20 pm
content: If a component consumes lots of current it will heat up very quickly. Try observing the board with a thermal camera, if you don't have one you can detect heating components with IPA or lighter fluid. Confronted to heat it will evaporate in few seconds.Search for a (near) short and verify the known diode junctions.

author: LB6B
author_rank: Newbie
timestamp: March 22, 2025, 08:23:18 pm
content: Thanks for your valuable input Swake.I DO have a FLIR IR camera, so I to do that and ramp the power level of the powersupply up to 4.5A (MAX) @12V, but it trips after less than a second so really no difference on the pictures before and after on the FLIR camera.Next I tried soldering out (see picture) first the L6 and connected 12V. No power consumption then. So I soldered L6 back again.Then soldered out L7 and connected 12V. Still no power consumption. Soldered back L7.After that it tripped in less than a second like before...I also measure the the diodes, and they look OK.My guess would go towards U2 at the moment... It is a MP1584EN. Possibly U5 which is a GH17M I need to get one of each of those and give it a check, I think...??But it could of course be something after those pulling things down... I find it hard to find these details without a circuit diagram :-)Thanks again Swake! Let me know if you have any more inputs or ideas!

author: Swake
author_rank: Frequent Contributor
timestamp: March 23, 2025, 08:20:57 am
content: Is D1 ok? It seems to have a small bump, but I'm not sure as the image is not entirely clear.Quote from: LB6B on March 22, 2025, 08:23:18 pm'trips' ? You mean it is going to constant current mode and voltage collapses?You don't want to pump it that hard, that is over 50W and can consequently damage a lot. Set the power supply current limit to something low, let's say 500mA and 8V during the debugging. Without an oscillator oven on the board I would set it to something like 200mA, but that oscillator will consume quite some power during the first minutes to heat itself. If the board of the BG7TBL has a short then the power supply will go into current limit mode and voltage will collapse, but that is 'fine', current still flows. Don't keep it on for too long at a time just in case. 20 seconds or so, repeat if necessary. This gives enough time to find a culprit with a FLIR.Quote from: LB6B on March 22, 2025, 08:23:18 pmInteresting. Are these connected in series? Or one on the positive side and one on the negative side?Disconnect the power supply and measure between the power input pins, is it a short? If so, you're not far away from the issue.Check the bigger ceramic capacitors for a short. C56 in particular, but also C2; C71; C73. If you are unsure, then remove to measure. This should also show up on the FLIR very quickly.Quote from: LB6B on March 22, 2025, 08:23:18 pmThere is a third regulator on the other side of the board LT1764. I guess it is to power 5V to the oscillator.You could remove them one by one. If one of them is blown, it is high probability that it is caused by something else that went short and resulted in over current.Quote from: LB6B on March 22, 2025, 08:23:18 pmTime for paper and pen (or Kicad) and start drawing....There is an application schematic in the datasheets of the MP1584EN that might help somewhat.

author: LB6B
author_rank: Newbie
timestamp: March 23, 2025, 09:00:06 pm
content: Dear Swake.Thanks for your great description, thoughts and ideas. Also thanks for your help and patience!Having followed your description. After having disabled the powersupply overload protection I set it to 8V and 500mA. First picture is without any voltage applied. (There are some reflections on the board that FLIR recognizes as higer temp, but these are just spuriouses.Applying voltage to the board made voltage drop to a little over 3V and the two next pictures.As you see L13 flares up. So does the input diode (reverse polarity protection).Both have been measured. Diode is ok and L13 is 10.3uH on my Keysight U1731C tester.Removing L13 from the board stops any current consumption so the shortcircuit is definately behind that.I notice the GNSS chip is warming up as well. Not sure this is part of the problem(?), just an observation.I also did a test on the other side of the board and the LT1764 regulator. It does not show any heathup.I also measured the capacitors in the area on the board, and found no shortcircuits there. Input resistance for the 12V is a around 2.5MOhm...So still a little lost, but lots of learning. Thanks again Swake!I'll have a closer look tomorrow and start sketching up a little circuit diagram :-)Thanks again!

author: Swake
author_rank: Frequent Contributor
timestamp: March 23, 2025, 09:53:48 pm
content: I'm a bit confused nowAccording to its specs the BG7TBL eats up to 15W, but that will only be the case when the heating in the oven of the TCXO is maxing out.Initially you wrote:QuoteThis indeed indicates a short or similar and also over 50W of consumption, albeit for a very short time only, but nevertheless 3x higher than spec.With the power supply limited to 8V/500mA, you now write:QuoteThat is pretty different! This is only 1.5W.... ok, the BG7TBL is probably not doing much because of the low voltage. But it clearly is not a short.Add a little more current, let's say 1A and if it then stays at 8V or very near then I have the feeling that your issue might not be in the BG7TBL but come from the power supply (and more particularly the overload protection).What exactly is the power supply you're using?You might want to measure the voltage at the output of the 3 regulators and verify if that is stable.When a component is shorted it will show up on a FLIR at like 50°C or more within seconds. It does not feel like that was the case here, correct?

author: LB6B
author_rank: Newbie
timestamp: March 23, 2025, 10:27:12 pm
content: Sorry for the confusion and again thanks for superquick response, Swake!My bad on the 4.5Amp. I set my powersupply to this value and the overcurrent protection kicked in. The overcurrent protection is now turned off, but I found it was set only to 1A in the settings. Sorry, my bad!My powersupply is a Velleman LABPS3005DN.Applying 8V the power consumption is a little over 1.8A.See the enclosed picture. L13 is quickly getting past 100 degrees C. but I cant really see any other components past that increasing the temp, at least not anywhere close to that rate.

author: Swake
author_rank: Frequent Contributor
timestamp: March 24, 2025, 06:55:26 am
content: Good morning, for the ones on this side of the planetThat makes sense now8 V x 1.8 A = 14,4 W this is in spec if we can believe the sticker on the box.You can up the voltage to the specified 12V. Current will reduce accordingly to about 14.4 W / 12 V = 1.2 AThe coil should be able to sustain the 100°C. It will not be very happy with this, but it should not harm it as I believe most coils are rated to something like 125°C.Measure the output of the BG7TBL please, because I think it is working.The power consumption will reduce somewhat after a couple minutes. This is when the TCXO has heated up and does not need that much power any longer to maintain the temperature.

author: LB6B
author_rank: Newbie
timestamp: March 24, 2025, 07:49:33 am
content: Good morning Swake!Spot on. You are very good!!When I first cranked up the voltage to 12V I had a limit on 2A and strangely the voltage pulled down to around 8V.I went back to 8V and turned back on again only to see that the power consumption was now back to 1.8A.Then back to 12V (and still 2A current restriction) and Voila, it pulled 1.2A.Looked at the L13 with the FLIR camera. At that stage it was 132 degrees, but current started to drop back and stabilized at around 511mA.Display was also back and the led on the board started blinking - indicating GPS lock. All good!Not sure what happened in the first place? I assume there could be something on the power input connection??Anyway, thanks a million for your kind help and sharing your brains and experience! Highly appreciated.I now know a LOT more about the device as well as troubleshooting such problems. I am also pretty happy that I didnt have to reverse engineer the circuit diagram :-)I'll put it all back in the box and hopefully it will work OK now.Thanks again for your kind help, Swake!

author: Swake
author_rank: Frequent Contributor
timestamp: March 24, 2025, 09:07:30 am
content: Good newsRepairs and R&D bring a lot of experience.I had a small advantage in that sense that I have been building my own GPSDO with distribution amplifier and have gone through similar power consumption behaviour.