Tek 2445A: No Chanel 1 or 2 Vertical

Unless you've forgotten to throw the soft switch from GND to DC or AC for those channels you may have found this instrument's end of life failure. A Tek 2445 scope is worth only a few hundred dollars on e-bay.

That's what I am afraid of. I am reading the service manual, and I currently suspect batteries and the tables they maintain in the main CPU board. I have looked at those units on fleabay and I might buy one for parts if I can get a clue as to what it is I need to replace.

Otherwise, I will have to dump this thing on fleabay for a handful of magic beans and just buy a new one...for hundreds of beans.

frankd20 reminded me that the most common culprit in electronics failures are capacitors shorting out. Trace your input circuitry lines for any signal capacitors that may have shorted out, including decoupling capacitors. You might get lucky.

Good Luck

I opened it up and looked for blown caps already, and I found none. However, I did not open the input modules to ch1 and ch2 as I would have to disassemble the unit much further than I wanted to at the time. I want to get more clues before I tear it down again.

Not sure I can help you, I have a Tektronix 475 that when I went to turn it on one day the fuse blew and did the same with a replacement fuse. After some digging it turned out to be a filter capacitor on the power supply. It was physically difficult get to the board but after replacing the cap it worked just fine and still is.

The fact that Ch1&2 are dead and Ch3&4 are not may be a common component that has died.

You can download the service manual from tek.com

https://www.tek.com/manual/2445a-and-2455a-service-manual

Figure 10-4 on page 249 (pdf physical page) has a block diagram.

On page 263 the Ch1 and Ch2 analog inputs are processed. They share a common amplifier U450 (A&B) component. But this appears to be a trigger signal. See if the power for U100 U200 is a problem (big preamp custom ICs). U160 and U180 do some sort of analog offset into the preamp component, and share common amplifier sections.

If that is true, then I know its the hybrid U600. I can buy one on fleabay for about $11 beans.

I meant U100 not U600.

Channels 1 and 2 have slightly different controls to 3 and 4:-

Do they fail miserably on both DC and AC.

Also it looks as thought the circuitry is harder to get to for 1 and 2

I wonder if there are any electrolytics in the power supplies for those two channels which may have dried out.

My previous search was specifically aimed at finding those electrolytic caps. I found none so far that have blown or puffed or leaked. The unit is remarkably clean inside.

I also re-seated all connectors and socketed chips. Looking at those circuits was like a time capsule! Brought me back to my early days...

The unit boots up fine and does not throw any codes, so I'm inclined to think I may have failure in both of those little silver boxes feeding into the hybrids and not the main board. Those are where I would expect to find decoupling capacitors. However, It seems to me those caps would not be polarized because the input from the probes can be either positive or negative with respect to ground, and an electrolytic cap in there would be reverse-biased. Then again, they could be on the other side of the amplifier, in which case they would be inside or just past the hybrid. The hybrid does not look like it would have those types of caps in it. Also, in my own work with sensitive high-frequency devices, ceramic and mylar caps were preferred.

Hello Deefburger, I would start by checking the voltage on common components, from what little I have seen, try U160 and U180. These 2 op-amps are common to both Ch1 & Ch2. They are common TL072 and TL074.

See if the volts are on both these chips, if not, trace the power feed to them. Maybe a burned out resistor caused by a shorted Cap.

Rod

Will do.

Hi,

We had a number of these and they all died the same way. I found that one of the Tektronix proprietary parts was the issue. Unfortunately they no longer make this. I was on a hunt for a while and even emailed the guys at Tektronix. I did a quick search to see if I can find the P/N I had found. It's 155-0241-02. Just google that P/N. There are some on ebay I believe but obviously no guarantee. We ended up retiring the units. Here is something else you might want to look at;

http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Thanks for the tip. But OUCH I could buy a whole parts unit for what people want just for that IC!

Maybe I should just scrap the old girl while she's still showing 4 traces!

How is Chanel #5 doing?

Channel #5 has the news at 11:00

Another thought Deefburger. Are you able to insert a signal to channel 1 or 2, then use either channel 3 or 4 to follow the signal through the system. Where it was last seen, the fault will lay between there and the next stage.

I hope this is possible on your machine.

Rod.

I had to re-read your idea a couple of times but that is not a bad idea. I may give that a try. I'll have to study up on the service manual and see where I need to test and see if I can remove the coverings from the ch1 & 2 input sections.

Hello again Deefburger, I am afraid that I do not make myself clear at times (most of the time), but as an old fault technician, I like to get to the component that has failed. I hate to see things discarded, so I go down to component level. I downloaded the manual for this CRO so I might be of help. Inject the signal at the input, via the CRO lead if you wish, no need to pull the channel compartment apartment apart at this time. Inject the signal, look at the circuit and find the first place the signal passes through, outside the channel 1 & 2 compartment, if the signal appears there, go to its next point on the circuit and check there, follow it through the system until it is lost, then the problem is cut down to a small section. Please post anything you find. There are probably many others on the forum who can help as well. If further help is needed then I will look at the circuit and help guide you through the system.

Regards,

Rod.

Ahaa, Physician, heal thyself

If you decide to move into new technology, this is an excellent, yet frugal, brand new 4 channel oscilloscope. It is a lower bandwidth but 25 Mhz is fine for most home use.

I have considered one of these before I bought the TEK a couple years back but they did not have the Linux Software for it at that time, only Winders.