Tektronix 2440 A16 power supply issue

[eevblognut2021]

Hi all,

I am new member to this site,

Please can you help,

 I purchased a Tek 2440 scope on ebay some years back the fault listed by the seller was noisy trace but all self tests passed. anyways last year during lock down I took a look inside and found the Rifa X class caps bulged on the power supply so I changed them for the same make and rating ( at the time I had not realized the Rifa Madness as Dave would say) so after installing the scope was working 30 mins later Snap Crackle Pop a lot of smoke, so killed power and waited for the smoke to clear, I open her up and found the Rifa C816 blown out. lesson to learn should have found Dave's video on Rifa madness before replacing!

So I'm into second lock down here in Guernsey thought I would take another look. so I am going to change the line Rifa caps with a good replacement but the resistors in series are burned, The R809 is simple as its a 30 ohm 5% .5 watt, but the resistor next to C706 (RT805) seems to be a thermal resistor of 5 ohm 10% Part number 5DA5R0K-270 SIL or Tek P.N. 307-0157-00 but I cant seem to find any thermal resistors that are less the 1K? let alone OEM parts!

So questions are as follows;

Can anyone shed some light as to what alternatives can be placed in this position (RT805) and do I need to worry about it as I am on 240 Volt line not 120 volt? 

Subject to the above having a successful work around. Can anyone advise on where I need to look for power supply ripple that could be causing trace noise? ( I have an old school Iwatsu 40 mhz scope that I can use to check)

I plan to change out the Dallas NVRAM chips with new and install sockets when the time comes, but as they are working and the scope passes self test, I am thinking best left alone for now.

Many thanks all and sorry if this has been covered already.

Kind regards

Denzil

[bdunham7]

It's just an NTC inrush current limiter.

https://www.mouser.com/ProductDetail/EPCOS-TDK/B57236S0509M000?qs=AKDv8POSxR3vP%252B1oSg4b5Q%3D%3D

For ripple, scope each rail of the PSU and see what you get.  If you find one noisy (the manual should give specs IIRC) the obvious place to look is the filter capacitors for that rail.  It helps to figure out the exact type of noise you are seeing, including the frequency.  It is not uncommon to see the PSU switching noise(20kHz or so) on the screen if all of the shields and covers are not in place.

[eevblognut2021]

bdunham7

Many thanks for that, will order the current limiter and check the PSU rails and compare to the manual specs, Thanks again

[eevblognut2021]

bdunham7

Sorry just a quick thought. just pulled up the manual is it possible that the resistor in the image (original post with yellow arrow) has being shown transposed from the actual board I have? If so the resistor that I should be looking up is R505? if this is the case then, R505 is a 100k resistor, which would only come into play if 115 volt is selected.

I hear the TEK sometimes made mistakes in there manual, I am new to electronic repairs so did not pick this up till I saw your TDK in rush current limiter on mouser, then thought I have seen something similar in the area.

Unless I'm missing something

Thanks again

[bdunham7]

I didn't look at the photo at first.  The burnt thing that the yellow arrow points to is not the NTC inrush limiter, just a regular resistor.  It's a bit hard to believe that a 100K resistor would burn up in that spot and by your diagram, it is shorted and not in play if the switch is set to 115V.  It would take 250V across the resistor to burn it up. 

What is the serial number of your unit?  Is it in the range serial numbers stated by the manual?  Tek liked to redesign their scope power supplies as often as possible.  You may need to trace the circuit on either side of that resistor to verify where those traces actually go. 

[eevblognut2021]

Hi bdunham7.

Your theory is correct re the 100k resistor and power supply redesign, Looks like Tek have redesigned the power supply. The S.N. of my scope is higher than the online service manual supports. Mine is S.N. B016167.
I emailed Tek last May about the manual and they told me that the online manual on Tektronix website was not the correct one for my scope S.N. and so they sent me an older manual which they said had no S.N. effectivity. unfortunately they didn't send me section 9 of the manual. I have asked for this in my email to them.

 Having looked at the power supply again today I have determined that this resistor is additional to the service manual lay out figure and this resistor appears to be between the Rifa cap C706 and the current limiter RT717.
 

So the unknown resistor has one blue and one gold stripe left so its 6 ? ? 5% and measures in circuit and out of circuit at 27 ohms.  I am thinking 60 ohm 5% tolerance? see picture below

 I have asked the question to Tek in my email this afternoon.

I will post here with the answer.

Thanks again

Denzil 

[bdunham7]

As far as I can tell, the later serial number schematics are lost to history.  The only known version of that manual appears to lack them and AFAIK there isn't a 'next' model that you could look at and see if it matches.  I've had to fix many a Tek scope with a schematic that was close but not perfect.  Those little details can be hair pullers.

Tek also likes to shuffle the parts around a bit with their 'redesigns'.  I looked at an A16 board from a 2430 and it has 3 resistors near that NTC, just with all 3 of them to the left of the NTC.  Can you desolder the other RIFA cap and see if it is blown up or shorted?  If so, the resistor next to it may be the equivalent of R713 in your older diagram.  This is a 68R 5% resistor and yours looks just like the one in mine, just in a slightly different spot.  I'd recommend you desolder both parts and beep around with a continuity checker and a strong flashlight so you can figure out where each of those traces go.

[eevblognut2021]

Hi bdunham7,

Thanks for that you are correct. I did a con check and found your resistor R713 68R 5% to be correct so I have ordered some with the rest of my supplies. certainly a hair puller!

Once I get to the point of testing voltages at test points. Is the test points the place where I should check for ripple? Is it possible to power the board on the bench out of the scope? I'm thinking the scope motherboard will need to turn on the power supply for some of the rails.

Many thanks for all your help will report back on here once done.


Regards

eevblognut2021

[bdunham7]

Once I get to the point of testing voltages at test points. Is the test points the place where I should check for ripple? Is it possible to power the board on the bench out of the scope? I'm thinking the scope motherboard will need to turn on the power supply for some of the rails.

Yes, you test the ripple right at the test points.  A dual-reading DMM or a DSO will suffice.  I'm not sure, but I don't think you can power it up without being fully connected.  I wouldn't try it.