Electronics > Repair

Tektronix 2445 repair.

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Kibi:
A couple of weeks ago I demonstrated how I repaired a 2445A that I had found. At the same time I also got hold of a 2445 which is the older brother of the 2445A. This one also includes option 5 (TV triggering) because it is from a television station.

This 2445 powers up, but although there is a beam it is horribly scattered. The entire screen lights up dimly but it does respond to the intensity control. When I took the back off of it my heart sank when I discovered that the fan impeller would not turn indicating that the motor had seized. Unlike the 2445A, the 2445 does not employ a "PC" type fan. It uses a special type of brushless motor with hall effect sensors in it. The fan motor secured along with the capacitors on the power supply.



I got the fan assembly out.



I then removed the motor and dismantled it.





Unfortunately, the shaft cannot be removed without unwinding the coils. I could have done it, but I knew of an easier way. Earlier in the week on my travels I had rememberd seeing a Tektronix 475 which seemingly had suffered a face plant at some point.



These use the same motor, so as this unit seemed to be loosing parts day by day, I revisited its last known location to see if the motor was still inside. I was lucky, it was still where I had last seen it and the fan motor was still inside.



So out it came.



Back home again, I soldered the 475's motor in, but it would not turn. It transpired that the transistor array which is actually an MPQ2222 had one of it's transistors with an open junction and the 2N2907 can was also open circuit. the MPQ2222 had got really hot in its lifetime, probably more recently due to trying to turn a stalled motor. The heat had affected the PCB badly and although I was careful, I still lifted some tracks getting it out.
I replaced the 2N2907 with it's plastic counterpart and the MPQ2222 just got substituted with four PN2222's.



Once this was done the fan started turning.

Next, whilst the power supply was out I inspected the capacitors. One of the input filter capacitors did not look too great, so it and it's brother were replaced.



I then swapped out all of the output capacitors. I already had these capacitors in stock from the 2445A repair.



I put the oscilloscope all back together again with only a feint hope that it would work.
Alas, as I kind of expected the problem was still present. The screen was still lighting up all over but no visible trace. On the bright side, at least the fan was turning!
Fearing the worst I headed straight for the horizontal section. I set up the 2445A (which I still hadn't put back together at this time) with the same input signal to use as a reference when making measurements.



I could measure the horizontal signals all the way up to the ubiquitous U800 horizontal amplifier. The U800 drives the horizontal plates directly and one plate had no signal and the other had a signal but it was clipping and riding at about 40V instead of about 25V.
This is the very reason my heart sank when I first discovered that the fan had been stationary. The U800 is usually the first to "do one" when the ambient temperature gets too high.

To prove my findings, I disconnected the horizontal plates and just let them flap about in the breeze to see if I could get the beam to strike the phosphor.



Success! We can now see the beam displaying the input signal from the generator only without any horizontal deflection of course.



Even the menus appear to work!



Here the 2445A is displaying the same signal as the 2445 is but with horizontal deflection (and some attenuation dialled in on it's V/Div selector).
The 2445 is displaying the voltage pretty accurately though which is good I suppose.



Now I have to go off and find a replacement for U800. This will not be easy unless I can find another 24xx 'scope to pull it from.
On my travels some time ago I did see a smashed 2465 lying around somewhere but it has since disappeared unfortunately. I shall have to keep my eyes peeled and perhaps explore a bit harder to find one.

electronics man:
Thanks for that Great repair.

grumpydoc:
Nice work - when you've got these 'scopes out on the bench the usual advice is to make sure you have a fan directed at the hybrids to make sure they don't overheat.

qservice.tv claims to have a U800 in stock, although they want $139 for it.

There's a whole main board listed on ebay including the hybrids. Again not cheap - £135 (inc postage)

Good hint about the fan, didn't know that.

Kibi:

--- Quote from: grumpydoc on January 19, 2014, 10:21:14 pm ---Nice work - when you've got these 'scopes out on the bench the usual advice is to make sure you have a fan directed at the hybrids to make sure they don't overheat.

qservice.tv claims to have a U800 in stock, although they want $139 for it.

There's a whole main board listed on ebay including the hybrids. Again not cheap - £135 (inc postage)

Good hint about the fan, didn't know that.

--- End quote ---

Thank you for the advice.
If that smashed 2465 was still around I would have had the A1 board out of it. I'd also have had to repair it's front control panel and use that with the A1 board to turn my 2445 into a 2465.
I will also keep an eye on eBay for something more reasonably priced. It may be worth paying a bit more for parts if it means a bandwidth upgrade although a higher bandwidth control panel on eBay is unlikely to include TV triggering, so I would loose that. All swings and roundabouts at the end of the day.

BravoV:
About the U800 replacement, if you feel adventurous and in the mood of diy -> Tektronix 2465/2467 series U800 IC replacement design  ;)

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