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Mr.RC-Cam

Tektronix 2245A Oscilloscope, SMPS repair & re-cap

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I own three different o-scopes. My favorite daily rider is a Tektronix 2245A, circa 1990. I've had it about 20 years (purchased used) and have treated it like a little princess.  Aesthetically it looks remarkably good and has maintained its measurement accuracy over the years. It has been a reliable friend indeed.

But a few days ago I powered it up and she was dead. No lights, no screen trace, only a brief hiss sound at the moment of power-on. After a 911 panic moment I collected my thoughts and mentally prepared to troubleshoot the power supply.

Here's an image of the Tektronix 2245A scope (photo courtesy of a random web search):

tel2245_stock_photo.jpg

 

I'll share all the details in this blog of the scope's repair journey. If fixing old test equipment is something that interests you then hang around the virtual campfire and listen to my story as it unfolds. Hopefully this ride-along workbench adventure has a happy ending.

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Some background: Tektronix 2000 series scopes have a history of power supply failures. They use very high quality parts but these are old models. Aging components, such as the electrolytic capacitors, are near the end of their useful life. Especially those found in the scope's SMPS (switch mode power supply) board.

I found interesting online discussions concerning other TEK scope power supply repairs. But apparently the 2245A model has a robust SMPS design, so I was unsuccessful in finding any specific repair tips on it. That means I won't be able to lean on the shoulders of other 2245A troubleshooters.

But I have a schematic, test equipment, and motivation to fix this scope. So the journey begins.

The TEK2245A Power Supply schematic: TEK2245A_smps_schem.pdf

 

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It's a 15-minute job to extract the power supply out of the scope. Remove a couple dozen Torx screws, lift off the processor board (pull out the flex PCB ribbon cables), and disconnect the high voltage lead from the CRT. It's important to discharge the high voltage, even if the scope has been turned off for several days.

The scope's interior is looking a bit empty at this point.

scope_supply_removed_1200.jpg

 

Before starting the troubleshooting I marked all the electrolytic caps with their schematic reference numbers. The hand written ID's help cross-reference the parts while reviewing the schematic.

all_electrolytics_1200.jpg

 

You may have noticed that many of the silver colored capacitors (see upper left area of photo) have axial leads. But I soon discovered that all the electrolytic caps are actually radial packages (both electrical leads on bottom of cap). The top side axial leads you see are dummies and their PCB solder pad is not connected to any circuitry.

What is this phantom axial lead madness? I suspect that Tektronix was concerned about mechanical stability. The tall electrolytic caps with the unused axial lead rely on this extra appendage to support the component. After all, this is a portable scope that was designed to withstand hard use and abuse out in the field.

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With the power supply on the workbench it's time to start the troubleshooting. This always begins with a thorough visual inspection. I saw no bulging capacitors, no overheat discoloration, and found perfect solder joints. Other than a tiny bit of dust, the board looks like new. That's both good and bad news for me.

Now for a gratuitous public service announcement: Don't apply power to the SMPS board without an isolation transformer. That's because traditional switching power supplies expose dangerous/deadly mains voltage. The isolation transformer is not the holy grail, but provides some basic protection from the earth referenced voltages. NEVER use any AC powered measuring equipment without the isolation transformer.

Here's a photo of the 400W isolation transformer that I am using:

isolation_xfmr1000.jpg

The SMPS board has low voltages (5VDC-58VDC) mixed with very high voltages (130V, 3KV, 14KV) that can persist for several days after power is removed. So even with power disconnected, manually discharging the output capacitors before handling the board is a mandatory chore.

While power is applied it's important to work slowly and methodically. I usually keep one hand in a pocket as a reminder to be safe and to help avoid fatal electrical shock / injury as I troubleshoot.  Let me be clear, it's nerve-wracking to work on high voltage circuits. The currents can vaporize tools and kill you too.

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During my bench testing I discovered that the power supply briefly runs for a couple hundred milliseconds. Fortunately this is enough time to confirm the DC outputs have the correct voltages. But because the supply immediately shuts down the troubleshooting period is short lived.

The brief hiss noise that I hear is due to an event called the "chirp mode."  The faint noise originates from the magnetic component vibrations that occur when the supply starts up, fails, and shuts off.  The chirp mode is designed to repeat every few seconds (continuous shutdown & restart), but my supply only does it one time whenever it is powered-up. I don't hear it again until I remove power, discharge the caps, and then try again.

Time to study the schematic. There's a pre-regulator switching supply that bootstraps the output switcher. It is given about 100mS to begin running using the mains voltage that is stored on a 100uF cap (C2204). If the pre-regulator is successful in starting up, it generates an output voltage (43V @ 2A) that is routed back to the 100uF cap, which provides the ongoing voltage to power itself. This voltage is also supplied to the DC output's Inverter Power Switcher that does the heavy lifting (it generates the various low and high voltages that power all the scope's circuitry).

Here's the pre-regulator circuit.

pre-reg_schem.jpg

So I learned that at power-up there's a split second situation where the power supply must run *before* it can run. A proverbial catch-22. I found it frustrating to troubleshoot given that only a fraction of a second is available to observe the signals before the supply shuts down.

I was able to confirm that the pre-regulator's switching transformer (T2203) is working during the startup period. It is providing about 60VAC to the pre-regulation rectifier (CR2202). So that helpful information suggests to me that the shutdown problem is probably caused by a failure that is triggering the protection circuits.

There's a current sensing circuit that will disable the pre-regulator if current draw is excessive. I used a Fluke peak reading multimeter and measured the current sense resistor's (R2201) voltage drop. It reported 0.072VDC max, which is an indication of safe currents. Spoiler Alert: I will soon discover this to be an incorrect/misleading observation.

There's also a over-voltage crowbar that shuts down the supply if the pre-regulation voltage climbs too high. I disabled the protection (remove Q2206) and sadly the problem persisted.

protection_ckt800.jpg

 

The final protection circuit is a thermal sensor switch (S2202) that is mounted to the pre-regulator's MOSFET driver (Q2201) heatsink. It shuts off the supply if the transistor gets too hot (Over-Temp protection). The ohmmeter reported the thermal protection switch is OK. For further confirmation its electrical contacts were temporarily bridged with a jumper wire, followed by a power on test. No improvement.

over_temp_sw800.jpg

 

So it seems I'm at the end of the rope. Time to use the shotgun approach. That is to say, check/measure all the components one-by-one to find the bad apples.

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Shotgun troubleshooting begins.

I decided to replace U2201, a MC34060P switchmode modulator IC. Long out of production, I was lucky enough to have a spare (date code 1986). Rather than direct soldering the replacement IC, it was installed using a Augat machine pin IC socket. Unfortunately the new IC did not fix the supply.

mc34060p_IC_1200.jpg

BTW, if you need a replacement MC34060P then the NTE1753 is a direct substitute. It too is out of production but gives you a second chance in your hunt for this rare part.

Using a DMM (Digital Multi Meter) to check components in-circuit is not a reliable way to confirm they are good. But I have good success if the parts are removed before measuring. It's a slow and tedious process, but remarkably effective in the right hands.

All the output caps were removed from the board using a de-soldering pump tool. Their capacitance and ESR measurements confirmed everything was OK. Too bad, since I was hoping to find a bad one. But since this is a repair AND refurbish project, I'll be replacing all the power supply's electrolytic caps.

With the forest of caps out of the way I had easy access to a long row of diodes. One lead on each diode was unsoldered, lifted, and the diode checked (via DMM's semiconductor test function). This provided some "good" news; CR2213 is shorted.

shorted_diode_1500.jpg

Why would I say this is "good" news? Because the shorted diode is the smoking gun I was hoping to find. It is a failure that would trigger the pre-regulator's over-current protection.

Replacement parts (diodes, caps) are on order and will be here in a few days. I'm optimistic that they will bring life to this dead scope. Come back soon to hear the rest of this campfire story.

 

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Parts are on their way. While I wait for their arrival I visually inspected all the PC boards. And this good deed punished me with more bad news; The 3V lithium battery is dead. It measures 0.3V, so I'm afraid that the stored Calibration Constants data has been corrupted. I won't know for sure until the power supply is repaired.

The battery is a Panasonic BR-2/3A that is mounted on the bottom of the Processor board. It looks like this:

battery_lithium1_800.jpg
 

Before I removed the bad battery I temporarily bridged a 3V coin cell into the circuit on the top side of the Processor board. If the cal data is still intact then the coin cell will continue to retain the data until after the replacement battery is installed.

battery_coincell_800.jpg


Fortunately the industrial BR-2/3A battery is not hard to find. It is also a kissing cousin to the popular CR123A cell (non-rechargeable version), which I already have on hand.

The CR123A battery is missing the through-hole mounting leads that are needed for PCB installation. I have a battery welder, so it's not a problem to replicate the leads on the new battery to mimic the BR-2/3A cell.

Here's the welder:

battery_weld1_800.jpg

It is a DiY MOT (microwave oven transformer) monster that I use to build 18650 battery packs for ebikes and FPV equipment. It makes nice welds that are much better than those provided by the "attractively priced" Chinese machines.

I duplicated the leads seen on both ends of the old BR-2/3A using 8mm wide nickel strip. Then welded them onto the new CR123A cell. The photo below shows the old and new cells.

DSCN5351.JPG

 

The replacement battery fits like the original.

battery_installed_800.jpg

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The replacement parts have arrived. It's always a pleasure to be visited by the big brown truck.

As mentioned, this power supply repair includes a full electrolytic recapping. Keep in mind that I didn't find any bad caps. But this is a 30 year old scope and for long term reliability it seems like a good idea to replace the SMPS's aging capacitors. Plus it's the perfect time to do it.

This is the graveyard of parts to be replaced:

caps_graveyard_600.jpg

 



Here's the mouser.com parts list to re-cap the power supply (electrolytics only):

3 pcs 4.7uF 100V, 661-EKYA101E4R7ME11D

2 pcs 39uF 160V (150V), 647-UCY2C390MPD1TD

2 pcs 100uF 63V Non-Polar, 647-UEP1J101MHD

1 pcs 270uF 450V, 661-ELHS451VSN271MR3

4 pcs 470uF 25V, 667-EEU-FM1E471B

1 pcs 1000uF 50V, 667-EEU-FR1H102B

8 pcs 1000uF 25V (12V), 661-EKZH250EC310225S

The shorted diode is a 1N4936, 863-1N4936G
 



All of the replacement capacitors are smaller than the originals. Some have incompatible footprints (different lead spacing), so I'm prepared to get creative to make them fit.

Here's a comparison of the size difference on some of the caps:

output_caps1_1000.jpg

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The two 39uF caps in the DC output filter needed minor lead bending. Some electronic-safe RTV was applied to hold them upright. The other output filter caps were an exact fit.

caps_DC_output1_1000.jpg

 

The old 1000uF 50V pre-regulator filter cap (C2203) has a 3-lead pinout. So the much smaller replacement needs its two leads to be formed to fit the PCB. A 3D printed (ABS plastic) base mount helps support the new capacitor.

Cap_C2203_500.jpg

 

The photo below shows the newly installed C2203 capacitor. The bent leads under the cap prevent it from resting flat on the PCB. The 3D printed base is a nice solution since it acts as a rigid standoff. Plus it eliminates the need for a big blob of adhesive to secure the elevated cap.

Cap_C2203_Mtg_500.jpg


The old 270uF 450V mains filter cap (C2202) has four leads. The replacement has two leads so it needs some rework to make it fit. A 3D printed base helps with the installation.

cap_2022_lead_mod_400.jpg

 

The new C2202 is installed:
Cap_C2202_installed_800.jpg


BTW, here's the STL file for the two 3D printed plastic base mounts: TEK2245A_Cap_Base1.stl

The remaining parts went in without any tricks. The rebuilt power supply is ready for testing.

 

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The repaired supply is back on the isolation transformer. The brief hiss noise wasn't present when power was first applied, only silence. But the voltmeter provides much joy; The supply is running.

All DC output voltages are about 15% too high. But this supply's regulation is reliant on a proper load, so I'm not too concerned. As a sanity test a 15 ohm resistor was added to the +5V rail and the loaded output's voltage dropped from ~6.8V to 5.5V. Still too high, but the voltage will be further reduced when all the outputs are properly loaded.

After confirming the supply is working I shut it down, did the capacitor discharge bleeding chore, and re-installed the original MC34060P Vreg IC. Another test and everything is still OK, so the original IC will stay in the circuit.

Success, such a lovely thing. I'm on the home stretch.

 

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During disassembly all the hardware was organized to help identify where it belongs. So re-assembly was fast and easy. It's too soon to install the outer case because the supply voltages need to be checked. All voltages are measured on the power supply's main connector that is accessible on the bottom of the scope.

With a bit of trepidation, the scope was powered up. Good News: It's working! Bad News: It booted into the test menu splash screen. As feared, the dead battery has caused the loss of the stored calibration constants data.

But all DC output voltages are within specification. This is a good time to adjust the 7.5VDC rail (POT access is on the side of the chassis) and recheck all voltages.

Now it's time to address the nagging test menu screen. Fortunately loss of Calibration data is not as serious as I thought. The scope's built-in self calibration feature is easy to use and it stores new calibration constants.

Self Cal Measurements Instructions (excerpt from page 1-6 in the Operators Manual):

self-cal_instr_800.jpg


The calibration measurements function takes a couple minutes to run. There's some comforting status messages during the test too.  So sit back, relax, and let it run.

After the test I pressed the CH4 button to exit and return to normal operation.  Some typical workbench measurements confirm everything is back to normal. The scope is repaired and it feels great to see her alive again.

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Here's a beauty shot showing the repaired TEK 2245A measuring the front panel's Probe Adjust test signal.

repaired_scope_800.jpg

 

This is the end of the campfire story telling. It was a bumpy E Ticket ride with a splash landing. But everyone survived. Hopefully all your repair journeys are successes too.

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