| Electronics > Repair |
| Tektronix TDS2024 Oscilloscope - Repair Log |
| << < (5/5) |
| sean0118:
Okay, good news the LED backlight kit arrived (PCB model number GYD-09). I was hoping to modify it to only need +6V. The board uses a DF6113 LED driver IC and I managed to find a schematic online. The datasheet reference design is a fairly simple boost converter, however this board has repurposed the IC to make an inverter circuit. I don't think they are even using the feedback pin (FB) as it's being pulled down to GND. The current sense pin (CS) is used instead to regulate the current through the LEDs. From testing it drops out and stops switching the MOSFET at about 8V. I thought I could lower this my changing the inductor value, but that just changed the switching frequency, it still drops out around 8V. I'm not sure what determines the voltage it drops out at? Anyway, I decided to give up on that and power it from 10V (using the +6V and -4V rails) which worked. I modified the board a little bit, changing the 220uF input capacitor to a low ESR type, I also added a 10uF electrolytic across the LED strip. I'm not sure if this is really a good idea, but older board revisions do have a footprint for one there. I also changed the current sense resistance (R4 & R7) to 2.5 Ohms (four 10 Ohm 1/4W in parallel, looks dodgy but does the job ;D). This limits the maximum output current as by default it can draw well over 1A, which is too much for the power supply in the TDS. I also added a trimpot across the ENA (3.3V), DIM and GND pins, which allows the brightness to be reduced, but I didn't end up using this and left DIM at 0V (max brightness). With this setup the display is bright and I'm measuring only 90mA across the sense resistors which seems pretty reasonable. I cut the LED strip down to the correct size then just used double sided tape to attach it. I then stuck some sticky tape and foil over it to reflect any escaping light back to the screen. A little bit of a hack but works well. I still need to go back and replace the capacitor aray which failed, I suspect it isn't critical though, I'll put four 0402 capacitors in its place. I'm also going to mount the LED driver board properly (it's only attached with double sided tape atm). Also interesting, checking the error log there's heaps or errors there: 7 DI ADG421 Diag Failed, 1 SL SL_stopAcq: Proc B, 2 SL SL_stopAcq: A!=B. Only error "7 DI" is listed in the manaual, with the cause listed as ADG421 diagnostics failed / main board or power supply failure. I'm glad I can call this one a success because I had my doubts there for awhile. ;) edit: I forgot to mention, I'll probably use plastic hardware to mount the PCB and put some kapton tape under it as the board GND is now at -4V. I'll go back and replace the sticky tape covering both sides of the foil with kapton as well as it should last a lot longer. I also measured the power supply rails to make sure they weren't being loaded down (-4.3V, +3.4V, +6.4V, +31V). edit 2: I also forgot to mention that it successfully passed the self cal without error. :D edit 3: Just occurred to me the input voltage required for startup is probably set by the forward voltage drop of the LEDs, maybe some large resistor in parallel would allow it to start switching at a lower input voltage? |
| pasta2018:
--- Quote from: sean0118 on August 15, 2021, 04:39:50 am ---Thanks. Good news though, I went through and measured the resistance of every pin to GND and compared it to the other EE48AC (Excel spreadsheet attached to save others the time lol). There were a few pins, the corner tabs and the bottom pad which were only 13 Ohms to GND compared to 188 Ohms. I found a capacitor array (C403) on the bottom of the board which connects to the bottom pad. I pulled this of and the resistance for both EE48AC went to 500 Ohms! I booted it up and the cal output now goes to 1KHz, so I guess the EE48AC might be undamaged. :D But, I can't see anything on the display, the backlight is off but even when I shine a light into it I can't see anything... --- End quote --- I have a tds2024,and it reports ch3 and ch4 failure,I measured U700,the 180ohm is only 3 ohm which connected to C703,maybe the same problem. Do I have to install a new one or just swap it ? if replace it with a new one ,what is the value of ccapacitor array |
| pasta2018:
--- Quote from: pasta2018 on April 07, 2024, 01:38:33 pm --- --- Quote from: sean0118 on August 15, 2021, 04:39:50 am ---Thanks. Good news though, I went through and measured the resistance of every pin to GND and compared it to the other EE48AC (Excel spreadsheet attached to save others the time lol). There were a few pins, the corner tabs and the bottom pad which were only 13 Ohms to GND compared to 188 Ohms. I found a capacitor array (C403) on the bottom of the board which connects to the bottom pad. I pulled this of and the resistance for both EE48AC went to 500 Ohms! I booted it up and the cal output now goes to 1KHz, so I guess the EE48AC might be undamaged. :D But, I can't see anything on the display, the backlight is off but even when I shine a light into it I can't see anything... --- End quote --- I have a tds2024,and it reports ch3 and ch4 failure,I measured U700,the 180ohm is only 3 ohm which connected to C703,maybe the same problem. Do I have to install a new one or just swap it ? if replace it with a new one ,what is the value of ccapacitor array --- End quote --- It's about 100nf 0603*4 |
| factory:
The info from this thread was very helpful, in the repair of a TDS2014 with a 12 DI fault (channel 3 diagnostic failed), other symptoms were power up fail for channels 3 & 4, probe cal signal measured at approx 3kHz and a strange offset for both channels 3 & 4 when the position control was used. C633 was to blame, one section now identified as a 2.7 ohm resistor. I've replaced it with a Yageo 100nF 50V array, CA0612KRX7R9BB104 from Mouser (two intact sections of the old part still measured at 100nF), while it was apart I took the opportunity to replace the Lelon caps with Nichicons. Full repair documented here; https://www.vintage-radio.net/forum/showthread.php?p=1675757#post1675757 David |
| Navigation |
| Message Index |
| Previous page |