Tektronix 2445A: Repair/Restoration/Calibration

11393407_504943222988196_8837509473214264700_oThis is the Tektronix 2445A on my workbench, underside (belly) shown. This is the A1 board containing the attenuators for Ch1 and Ch2, and preamps for all 4 channels.

I’m doing voltage and voltage ripple measurements, as per Service Manual. So far, all voltages were within tolerance, though the 5V ripple is on the high side and exceeds what the Service Manual recommends. But even though, this higher ripple voltage shouldn’t cause the problems I’m experiencing with the scope (wrong reading for Channel1 measurements.)

Reading the Service Manual, I noticed there is a way to switch the scope into Diagnostics mode. This is done by pressing Delta-V, Delta-T and Slope buttons all at the same time.

Repeatedly doing these steps, I noticed my scope not reacting or doing anything. According to manual, the scope should display certain user prompts on the screen but on my scope, they’re not showing at all.

I know the buttons for the Delta-V and Delta-T works, because I’ve been using these cursors in the past to make voltage and frequency measurements. The Slope button, I admit I haven’t really used. It’s always shown (+), and doesn’t toggle to (-) when the button is pressed…. Hmmm……


Restoration & Repair of Tektronix 2445A

I have a 4-channel Tektronix 2445A I bought sometime around 2008(?)  Basically, this scope is almost a 30-year old equipment, circa 1987 based on my research. I like this 2400 series scope compared to the Tek 2000 series  it replaced, the 2445A being a 4 channel scope and having the ability to easily measure Delta-V and Delta-T (voltage and time/frequency using movable currors).

There are many variants of the Tektronix 2445 scope. There is the plain 2445, a 2445A, and a 2445B.  There are other scopes that look similar, like the 2465A, which looks exactly like a 2445A. It’s very confusing sometimes when downloading Service Manuals so make sure the Service Manual you have is specifically for your equipment. There may be slight variations in schematics, or board layout/parts placement.

I noticed problems with my Tek scope when attempting to do voltage measurements using Channel 1. The results I’m getting were not making sense (i.e. scope reading was larger than the actual voltage I’m feeding into it’s BNC jack!)  Switching the probe to Channel 2 showed the correct readings,  so I know the problem is specific to Channel 1.

At this point, I’m debating between buying another replacement scope from eBay to replace this 2445A, buying a brand new (and spending more) on new scope, or attempting to fix this 2445A.

If I buy another *used* scope from eBay, it will be another gamble since I will not know beforehand the actual and true condition of this equipment.

Buying a *new* unit will cost more than $1000 dollars, for a name-brand 2-channel unit.

And trying to fix this scope, I’ll be without a working scope during the time this scope is disassembled. Plus… how can I fix a scope, if I don’t have another scope to use to probe things around?

In the end, I found a good deal on a brand new Rigol DS1054Z digital oscilloscope.  This DSO is fully featured, 4 channels, and best of all costs only $399. A discount code further lowered it to $375 with free shipping… + a free $15 gift card.  Did I say it’s fully loaded with features? Though some of the features are only  on Time Trial… but fear not, a “hack” can be found on the Internet and just by entering a few hex numbers, you can “upgrade” this scope to a 100Mhz, fully-loaded with all the bells and whistles options.



The nice thing about this scope are the “statistics” about the waveform you’re viewing. Plus, it’s a storage oscilloscope so you can capture events and review them later  in more detail.  But this post is not about the Rigol scope. This post is about my Tektronix 2445A and my desire to repair it, restore it and bring it back to a fully working condition.

The Service Manual

The first thing you should do before attempting to repair/restore/calibrate your test equipment is find it’s service manual. Do not attempt to repair complex test equipment like scopes, signal generators, etc. without a service manual, as it will be almost impossible to know what you’re doing. There are several trimmers and test points on the board, and you won’t know just by casual observation of the board which is which.

Google is your friend here, but beware because Google may bring you results that are not exactly for the equipment model/batch/variant you have. A 2445 scope is not the same as a 2445A scope, and a 2445A scope is not the same as a 2445B scope. Yes, some functionalities, schematics, parts, boards may be shared between these different scopes, but it’s better if you get the exact Service Manual for your equipment.

Another issue I encountered is some of the PDF Service Manuals you may download are just “image scan” of the original manual. Therefore, finding specific “text” or “keywords” in this PDF manual is impossible… and this can become a hassle as you’re forced to read the entire manual, or at least go through all the pages and get yourself become familiarized with it.

There are companies out there that sell OCR’d (Optical Character Recognition) scans of the PDF manuals. This makes the PDF searchable for keywords, which is a very useful feature! The downside is you have to pay for these kind of PDF Service Manuals. The good news is it doesn’t cost much… $15 typically. It’s well worth the money given the ability to do full text search on these PDF Service Manuals!

On the next post, I’ll show the guts of the Tektronix 2445A scope.




Add Secondary Windings to your Toroidal Transformer for Additional Voltage Outputs

So here’s the scenario… you have a power transformer with a voltage output of 25V + 25V AC.   Let’s say, you’re using this power transformer to supply power to a power amplifier project.  So it’s all good.

But you also need 5 Volts DC to power some logic circuits (a microprocessor, Arduino, PIC microcontroller or similar) within the same project.  Nowadays, incorporating microprocessors in DIY projects is common… thanks to the popularity of Arduino. You may be using this microprocessor to integrate an LCD screen, or perform temperature monitoring, or perform speaker overload protection.

What are your options?

1. You can use a voltage regulator and step down the high voltage DC output of your power supply to 5VDC. You can use a 7805 regulator, or an LM317HVT 3-terminal regulator. But this is wasteful, and will generate a lot of heat!  25VAC rectified to DC will be approximately 35VDC, give or take a few. If you regulate this down to 5VDC, you’ll be dissipating 30Volts in your regulator as heat! This will require a big heatsink.  In this example, we’re using a 25+25VAC transformer. But what if your transformer is bigger, say 50+50VAC output, you’ll need to bring down 70VDC (after rectification) to 5VDC.  Yikes, you need to dissipate 65Volts of energy.  Not to mention, this may be exceeding the Safe Opearting Area (SOA) of your regulator. (See datasheet on maximum voltage differential, Vout – Vin allowed by your regulator.)

2. You can install an additional small power transformer in your project. This is feasible, and one can do this instead. The smaller transformer can be another toroid, or even a small PCB mounted transformer. The cons of this though is the cost.  Buying another transformer for your project will be expensive. If this is a PCB mounted transformer, then you need to design a board to accomodate this transformer.  Not to mention, the additional transformer and PCB will take up valuable real-estate space in your already cramped chassis.

Hmmmm…. adding that Arduino to your power amplifier project seems to be a lot of hassle. Now, you’re thinking of giving up the idea  of having a cool-looking LCD display and push buttons and rotary encoders on your project.

But wait… there’s another solution!  How about just installing additional secondary windings to your existing toroidal transformer?

It’s very easy, let me show you how:

So let’s say this is your power transformer:



What we need to do is just wrap another layer of wire around this transformer. So you’ll need magnet wire (check out eBay for sellers selling magnet wires, so you don’t have to buy several thousand feet of spool.)

I used the same magnet wire thickness just to keep things simple.



Step 1: Wrap 10 turns of magnet wire around your power transformer.  I used masking tape to keep the wire in place.



Step 2: Strip the insulation from the ends. It’s coated with lacquer so you can either use a sandpaper or blade to scrape off the insulation and get to the bare copper. I personally used a thermal wire stripper since I have one.



Step 3: Measure the AC voltage you got from your 10-turn secondary winding.

In my case, I got 2.74Volts AC from my 10-turn winding.



Step 4. Compute how many volts per turn we got.

2.74Volts / 10 turns = 0.274 Volts per turn

Using this information gathered from our transformer, we can now compute how many turns we need for the voltage output we want.

I also measured the length of wire needed to make that 10-turn winding. For my power transformer size, it takes about 5.5ft of wire to make that 10-turn winding.

Step 5. I want to use 5 Volts DC to power my Arduino… and I want it regulated.  So we need at least 7 Volts DC rectified so we can get 5Volts regulated output.   So let’s shoot for 5Volts AC for our secondary winding add-on.

That means, we’ll need:

5VAC / 0.27 Volts per turn = 18.51 turns, round up to 19 turns

In this case, I decided to round up to 20 turns for a nice round number.

And we’ll need approximately 11 feet of magnet wire to wind 20-turns.

…. so go back to step 1, but this time winding 20-turns around your power transformer.

I had some extra wire left, so I just went for another extra turn… 21 turns total.   Doing some math, we should get around 5.67 Volts AC (21 turns x 0.27 Volts / turn).

After doing this, I measure the output voltage and I got 5.8Volts.  Very very close to our computed 5.67 Volts.



Perfect…  so this means our 5.8VAC rectified to DC will be about 8.2VDC, which is more than plenty enough for our project.

Now, all we need to do is rectify this AC voltage to DC, and regulate this down to 5VDC. And we’re done.


High Voltage, High Current Power Supply

I plan on building a solid state power amp, but don’t know which one yet. In the meantime, I figure I can go ahead and start making the PSU for my future amplifier.

I tried iTead Mall to place my order for (5) PSU boards. So far, I’m pleased with iTead’s quality, though there was some miscommunication with them. They didn’t update my order status for 2 weeks, it still says “Processing” on their website, when in reality, they’ve already shipped my order but didn’t email me a tracking #. Anyway, once I submitted a ticket, they’ve been very responsive and apologetic. Then by coincidence, I got my PCB in the mail the day after I complained. I will be using them again in the future.

So basically, I want the PSU to be able to cater to higher voltages, like +/-80 or 90VDC… so I made sure there’s enough room on the board to allow the use of bigger capacitor, 100+V rated or more.

The PSU board can accept either spade connectors, or terminal blocks.

I’m using Ultra Fast Recovery Rectifiers, bypassed with Panasonic Safety Class X2 polyester film caps (250V rated). I have heatsinks on the rectifiers but they don’t need it. They’re cool to the touch.

There are green LED indicators under the fuse on the DC output, so if the fuse blows, the LED also goes out and you can quickly see the problem. There’s also a couple of AC fuses.

It’s basically just a simple power supply, albeit using high current/high voltage parts.

Step 1: Bare PCB … ready for population and build.


STEP 2: Insert 0.1uf / 100V ceramic caps. These are bypass capacitors for the DC output. If you need to add more bypass caps, you can solder them underneath the board across the capacitor terminals.


STEP 3: These are the current-limiting resistors for the LED power indicator. The 3mm LEDs are located under the fuse on the DC output.


STEP 4: Fuse holder clips…. 2 fuses for the AC primary side, and 2 fuses for the DC side. Depending on the VA size of your power transformer, pick the appropriate/correct fuse rating.


STEP 5: These are the 4x Ultra Fast Recovery Rectifiers, rated 600V Reverse Voltage, 10Amps. Added TO-220 heatsinks just to be sure. Yes, I know… it’s overkill.


STEP 6:  I made a mistake of trusting the datasheet… the component size is correct but the lead spacing is wider than I expected. So here’s a temporary fix to make it work with the PCB.HVPSU-Step5

STEP 7: These are the (4x) Safety Polyester Film Capacitors, rated 250Volts, Class X2. Each capacitor is connected across each Rectifier Diode.


STEP 8: Terminal Blocks with screws. Heavy-duty, rated up to 32Amp, and can handle up to AWG#10 gauge wire. AC side accepts dual secondaries from power transformer. DC side outputs split voltage, V+, V- and GND.


STEP 9: 10,000uf capacitors for DC Filtering. Since this is a split power supply, there’s 20,000uf for each V+ or V- voltage rail. Use the appropriate voltage rated capacitor matched to the power transformer, and DC output you need. The higher the capacitor voltage rating, the more expensive it gets. There’s enough room on the board to accept bigger diameter/higher voltage capacitors up to 100Volts DC rated or more.


Another view of the finished High Voltage Power Supply.


Size comparison of finished High Voltage, High Current Power Supply compared to an iPad Air. PSU weighs in at 11.1 ounces.


Power Supply Testing.


Ripple voltage is 11.5mVolts peak-to-peak, 25V+25 AC input, 35.4VDC + 35.4VDC output.




The above screenshot for ripple is wrong. I’m basically measuring noise in the environment, instead of the actual PSU ripple. Here are updated screenshots.

There you go, that’s more like it… 2.5mVpp ripple.



And we know it’s the ripple from the PSU, and not some environment noise because the frequency is around 120Hz…. ripple from a full wave rectifier.


Crown XLS 202 Amplifier Repair – Part 5

I found out you can flip the main PC board (saving me from removing all PSU and ribbon cable wires)!  This is very convenient and just made this repair task so much easier and faster.


These are the locations of the Channel 2 Power Transistors.


Doing some resistance testing using a multimeter, I confirmed that one of the NPN transistor was shorted (due to it’s very low reading).  But since we’re already here, I decided to just replace ALL Power Transistors of the affected channel. The NPN/PNP power transistors were only $4.60 each at Mouser.com, so it’s better to replace them all now while I have the PCB right in front of me.

A Hakko Desoldering gun is a godsend for this job. I just use the desoldering gun to remove the Power Transistors soldered to the board, then unscrewed the heatsink from the board. powertransistors

Heatsink successfully removed from board. Now, it’s just a matter of unscrewing the TO-3 transistors, cleaning the heatsink, applying some fresh thermal compound, and installing new transistors back. mj15025

So after replacing all Power Transistors in Channel 2 (again, only (1) NPN transistor was bad, but I just decided to replaced them all and give it a “fresh start”), put everything back together, and power up the amp.

Fingers crossed…. the amplifier stayed on, and didn’t shut down! No more FAULT lights. Yay! The amplifier seems fixed.


In my excitement, I didn’t bother connecting it to a signal generator and oscilloscope, I just went ahead and connected it to my speakers and fed it some audio signal.

YES! It’s fixed.

The final step is replacing all the tiewraps I cut, making sure all the cables and wires are secure inside the amp, double check all screws are tight, and replaced the cover shut.  Done.

Thanks for reading!

Crown XLS 202 Amplifier Repair – Part 4

My hunch is there’s DC output voltage triggering the DC protection circuitry and automatically shutting down the amplifier.

So we need to measure if there’s any DC voltage at the output of the amplifier.   Note, you cannot do this check by sticking your meter at the output binding posts… because those are connected to the relay terminals — and you won’t be able to get any useful reading.

We need to measure where the RED arrow is pointing in the schematic below.  (Of course, the negative or BLACK probe of our meter should be connected to Ground. There is a common ground point on the amplifier, just right below the power transformer location.)

Continue reading Crown XLS 202 Amplifier Repair – Part 4

Crown XLS 202 Amplifier Repair – Part 3

On my last post, I have a theory that DC protection is kicking in for this Crown XLS 202 amplifier and that is what’s causing the FAULT condition.

So now, it’s time to open up this amplifier and verify that theory.

NOTE: You need star-security bits to open up the cover on this amplifier. Good thing I have some of that special screw driver heads in my shop.

They basically look like this:

Continue reading Crown XLS 202 Amplifier Repair – Part 3