Category Archives: Repair Work

Tektronix 2445A Repair – Part 2

UPDATE: The scope is 100% repair, unfortunately I didn’t update this blog while I was repairing the scope. So I’m going to look back at photos and videos I took and try to sum up my thoughts on what I did. So this is a retrospective look at my repair job.  Here we go…

While fiddling with channel 1, I noticed that when I wiggle the BNC jack hard enough that I can make the scope display appear correctly. On this video, the horizontal cursor markers mark what should be the proper height of the waveform. But as you can see, if I wiggle the BNC jack, the amplitude of the signal changes.

So this is the cause of my Channel 1 inaccuracies! In a way I’m glad because it’s not a defective preamplifier or A/D chip or something complicated like that. That would take a lot of debugging and probing. Now I know the problem is BNC related.

So my next question is… maybe the BNC jack isn’t making good connection? or maybe I need to replace the BNC jack?

I see that the BNC jack for Channel 1 and 2 are unlike the BNC jacks for Channel 3 and 4. On channel 3 and 4, the BNC jacks are screwed to the case and are wired to the motherboard. For Channels 1 and 2, the BNC jacks are part of a module in a silver looking box. Later on, I learned these are called attenuators. And this is basically a discrete/SMD hybrid part made by Tektronix. Inside each attenuator are 4 relays that control the signal path (and therefore attenuation).

On this next video, I’ve determine the problem is inside Channel 1 attenuator. How? By tapping on the attenuator with the palm of my hand, I can change the signal I’m seeing on the scope. There’s definitely something wonky going on with Channel 1 attenuator.

So it seems, to fix my scope, I need to replace Channel 1 attenuator. So off to eBay we go…

The Tektronix part# for Channel 1 and Channel 2 attenuators are different. So when buying on eBay, make sure you’re searching for the correct part# for your scope, or look at the pictures closely and see which side the connecting wires are coming out from (left side or right side).

In a way, I’m pleased that I’ve found the problem. But a little worried also. To fix this problem, I have to remove the main motherboard of the oscilloscope. And that is NOT a light proposition! Lots of wires to desolder, lots of screws, and just lots of things that could go wrong every step of the way if I’m not careful.

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.

ds1054z

 

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.

 

 

 

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.

pcbflip

These are the locations of the Channel 2 Power Transistors.

channel2

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.

ampfixed

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

Crown XLS 202 Power Amplifier Repair – Part 2

On my last post, I agreed to take a look at an XLS202 power amplifier brought home by my daughter.  It belongs to a friend of hers, and it needs fixing.

First, we need to know what the problem is with this amplifier. So I plug the power amp, and turned it ON.   It came to life, power lights on, fans running at full speed, all LEDs also turned on, including the SIGNAL, CLIP and FAULT light, and then everything turned OFF automatically.

Okay…. So I tried turning it on again, and this time it’s dead… nada. The fan didn’t even moved.  Hmmmm….

Continue reading Crown XLS 202 Power Amplifier Repair – Part 2