Earlier this week I managed to rescue a bunch of oscilloscopes which were destined for the dustbin. They were declared faulty were thus due to be smashed into small pieces by a big hammer.
One of them is a Tektronix 2445A 4Ch 150MHz machine. It's a really nice bit of kit of mid 1980's vintage. This one includes "Option 5" TV triggering (it was from a television station), user manual and service manual.
Upon power up there was a very blurred barely visible image. Good, I thought, at least there is beam current. I fiddled around a bit, but couldn't get an intelligible trace or menu. I left it on for a while and when I came back the image had improved and the screen displayed an error code. TEST 05 FAIL 40.
I referred to the handy service manual which revealed that some voltages were too high. I took the back off of it and found the test points.
The -8V rail was reading -5.7 and the -5V rail was reading -3.8, no good. These symptoms reek awfully of PSU capacitors to me. Teardown time.
Here is the power supply module in the middle.
Got it all out.
Someone had already had a go at this in the past. However, they had replaced the original 105ºC capacitors with 85ºC ones. One of theirs had a big dent in it.
Another one had an ESR of more than 50?.
I set about testing and if necessary replacing all of the output capacitors. I put red marks on my new 105ºC capacitors. Only two capacitors (factory originals) had a good low ESR, so I put them back in.
I put it all together only to discover that the power switch was now sticking. Stripped it all down again, fixed the switch and re-assembled it again.
It powered up straight away, no errors.
Here it is connected to my Off Air frequency standard. My frequency standard doesn't have very fast logic in it, so the waveform is not very clean, but it is accurate.
10MHz into Ch1.
1MHz into Ch4.
Looks spot on to me.
Now one more thing to fix. I later discovered, much to my annoyance, that although the instrument would turn off, the power button continuously indicated an on status.
These push buttons work in rather an interesting way, but unfortunately the only way that I can currently think to describe the way it works is not appropriate for this type of forum. A lot of Tektronix gear of the era employs this style of on / off indication.
It turned out that my pushbutton was broken. I did however manage to get hold of a dead Tektronix 1720 vectorscope which kindly donated it's pushbutton to my cause. Everything now seems to be OK.
For my next little repair project I will be having a go at a Tek 2445. I wouldn't be surprised if it is also suffering from moody capacitors. I shall start a new thread for that one and another for the 465B which I also managed to rescue from destruction. The 465B seems to be working OK so far, so perhaps a restoration project will be more of the order.
