# HG changeset patch # User David Douard # Date 1461100506 -7200 # Node ID fdee5b2913ce3b4f6d85a25155cfc12ae56bc671 # Parent 26e34edd2984952f20caff44bdda389d121bbd6b [tek] new blog on tek2445 diff -r 26e34edd2984 -r fdee5b2913ce content/TeK2445_3.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/TeK2445_3.rst Tue Apr 19 23:15:06 2016 +0200 @@ -0,0 +1,203 @@ +========================================== + Tektronics 2445 scope PSU repair: part 3 +========================================== + +:Author: David Douard +:Category: Electronics +:Tags: Tektronics, 2445, analog scope, repair, test equipment +:series: Tek2445 PSU repair +:series_index: 3 + + +After having mostly `destroyed the PSU of my Tek 2445 +<{filename}/TeK2445_2.rst>`_, I've decided to be a bit less foolhardy +and I've subscribed to the `yahoo Tek group`_ and discovered that such +a failure has been reported there, also after a full recap of the +preregulator and regulator of the power supply. + +I found that I was not so wrong when I built my completely guessed +LR1060. + +It seems to be built on a 75Ω resistor. Mine was not so far, but I +decided to rebuild one with closer specifications, so I took a new 68Ω +resistor and I wrapped some 34 gauge wire. The result is a bit +cleaner, from: + +.. image:: {filename}/images/tek2445/lr1060.jpg + :alt: first attempt of a hand made LR1060 chocke. + +to: + +.. image:: {filename}/images/tek2445/lr1060_v2.jpg + :alt: a better hand made LR1060 chocke. + + +But I was stuck because of my dead TR1050. Due to the lack of sucess +getting the main oscillator starts, I ended by suspecting TR1050 +suffered when Q1050 exploded... And I was right: one of the two +primary coils was short. + +I've found a reasonnably cheap replacement part on `QService +Electronics`_ shop. It took a bit long to arrive (12 days from Greece +to France, but to be honest, it seems the longest part of the transit +was in France), but it arrived. So yesterday evening I've been able to +replce it. + + +The TR1050 transformer +====================== + +This transformer is a small input transformer with 4 coils around a +common ferrite core: + +.. image:: {filename}/images/tek2445/tr1050.jpg + :alt: The TR1050 input transformer. + +I was not sure of the specifications of each coil, since mine as (at +least on one coil) defective. From the schematic, it looked natural +that both the primary windings should have the same charactritics. So +when my replacement transformer arrived, I took a few measurements. + +The result is: + +.. list-table:: TR1050 windings + :widths: 15 10 30 + :header-rows: 1 + + * - Winding + - R + - L + * - 1-2 + - 4Ω + - 6mH + * - 4-5 + - 4.5Ω + - 6.5mH + * - 6-7 + - 0Ω85 + - ~ 10µH + * - 6-7 + - 0Ω8 + - ~ 10µH + +I've also quickly checked the voltage ratios using a 42kHz signal: + +.. image:: {filename}/images/tek2445/tr1050_ratio_measurement.jpg + :alt: Measuring the turn-ratios of the TR1050 transformer. + +I use CH2 of my scope to probe the signal produced by the `HP890A4 +<{filename}/hp8904a.rst>`_ linked to pins 1 and 2 (thus a primary +winding), and CH1 on a secondary winding (pins 6-7), which resulted +in: + +.. image:: {filename}/images/tek2445/DS1Z_QuickPrint17.png + :alt: Measuring the turn-ratios of the TR1050 transformer. + +.. image:: {filename}/images/tek2445/DS1Z_QuickPrint18.png + :alt: Measuring the turn-ratios of the TR1050 transformer. + +As one can see, the turn ratio seems to be 10. Both the 2 secondary +windings (5-6 and 7-8) which provide power for the preregulator +control (U1030) and the inverter drive (U1062, U1064 and U1066), are +identical to each other. + +Testing the new T1050 transformer +================================= + +After having soldered the "new" T1050 transformer, I followed one part +of the Power Supply Troubleshooting Procedure from the Service +Manual. In the diagram, there is a path in which there is: + +- Unsolder and lift the end of W1060 near pin 7 of T1060; + +- Connect the primary test load between the lifted end of W1060 and + the sources of Q1060 and Q1070; + +- Power up and check for 120V accross load (plus a 1Vpp triangle + wave). + + The test load consist in: + +.. image:: {filename}/images/tek2445/psu_test_load.png + :alt: Primary Test Load. + +So I built one and connected it: + +.. image:: {filename}/images/tek2445/psu_with_test_load.jpg + :alt: The PSU with the test load in place of T1060. + +Was a little anxious when I switch the PSU on, but it did not +exploded. And the voltage across the dummy load was just fine! + +So I resoldered W1060, reinserted all the jumpers (between A2 and A3), +plugged in a dummy load on J303 (unregulated 5V), a tried again. The +PSU was starting... it was not completely stable though, but I was +pretty sure this was due to the fact the load was not correct. So I +reinstalled the PSU in the scope, and gave it a try: + + +.. image:: {filename}/images/tek2445/back_from_the_death.jpg + :alt: It's alive! + +At last! + + +Summary +======= + +I've spent a lot of time trying to understand and fix this PSU. I +think I've lost a lot of time and effort because I've been too hasty, +and I should have taken more time reading the service manuals. By the +way, the service manual for the Tek 2465A is a better source of +documentation, for example the question I had about the voltages with +respect to "REF" on the schematic is answered in this later manual: +there are indeed 2 references, as I suspected. + +The main lesson from this is: never test a switching power supply +unloaded. I'm pretty sure I blew up the power transistors, the +transformer and a few other parts because I was switching the +*unloaded* PSU on and off. It's probably a lesson 101 on switching +power supplies... never too late to learn. + +A the end, I've replaced many parts on my A3 preregulator board. Some +are probably not required: + +- T1050, +- Q1060 and Q1070: I replaced with IRF820, +- Q1050: replaced with a IRFP450;the higher gate capacitance seems not + to be problematic finally, +- Q1022, Q1030, Q1040 and Q1063: replaced with 2N4403 and BC337 (the + transistors I found at my local components store) +- almost every capacitor, +- U1029 and U1040 optocoupler , +- U1066: replaced by a DS0026CN bought on ebay +- CR1040: replaced by an unknown germanium "similar looking" diode, +- R1060 and R1070, +- R1069 + +So I'm done for now with this puppy. It should be completely checked, +but that will be another story. + + +Post Scriptum +============= + +For the fun: + +.. image:: {filename}/images/tek2445/side_by_side.jpg + :alt: Side-by-side: DS1054Z vs. Tek2445 + +Despite the fact the Rigol DS1054Z has an incredible number of +features for which the Tek cannot compete, I still like this +old puppy a lot. Its front panel is much easier to use: every control is +directly accessible. And yet, I don't think the Rigol can measure both +the rise and fall time (with a decent precision) of a square wave: + +.. image:: {filename}/images/tek2445/rise_and_fall.jpg + :alt: Side-by-side: DS1054Z vs. Tek2445 + +Beautiful double time-base! + + +.. _`yahoo Tek group`: https://groups.yahoo.com/neo/groups/TekScopes/info +.. _`QService Electronics`: http://www.qservice.eu/ diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/DS1Z_QuickPrint17.png Binary file content/images/tek2445/DS1Z_QuickPrint17.png has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/DS1Z_QuickPrint18.png Binary file content/images/tek2445/DS1Z_QuickPrint18.png has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/back_from_the_death.jpg Binary file content/images/tek2445/back_from_the_death.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/lr1060.jpg Binary file content/images/tek2445/lr1060.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/lr1060_v2.jpg Binary file content/images/tek2445/lr1060_v2.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/psu_test_load.png Binary file content/images/tek2445/psu_test_load.png has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/psu_with_test_load.jpg Binary file content/images/tek2445/psu_with_test_load.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/rise_and_fall.jpg Binary file content/images/tek2445/rise_and_fall.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/side_by_side.jpg Binary file content/images/tek2445/side_by_side.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/tr1050.jpg Binary file content/images/tek2445/tr1050.jpg has changed diff -r 26e34edd2984 -r fdee5b2913ce content/images/tek2445/tr1050_ratio_measurement.jpg Binary file content/images/tek2445/tr1050_ratio_measurement.jpg has changed