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7844 power supply

Peter Gottlieb
 

A couple of weeks ago I mentioned a 7844 which instantly blew the line fuse but which had previously worked.

It was suggested I look at the line rectifier and bulk caps.

I finally finished the project on my bench and put the scope there. The problem turned out to be that the two main switching transistors were shorted.  After replacing them all was not well though.  The supply kept trying to come up, making a weak rattling sound.  I could see it was trying.  Then ever once in a while it would come up, fan run, and scope start, but then within a few seconds go back to rattling.  Then in the midst of considering where to look next it just stopped.  Now, nothing.  HV is on the main caps and the neon is rapidly flashing, but nothing else.

These supplies are infamous for being a pain in the neck.  Has anyone seen this and have any hints for what I should look for?

Peter

Ed Breya
 

The last time I fixed a 7844 PS, it was fortunately just two or so rather large (100 or 220 uF 20V) Ta caps shorted, on the pre-regulator supply board, for the +/- 15V inside the PS module - the one inside the cage, just below the outer, upper, linear regulator board. It's a PITA to get to, but doable - if you take enough stuff apart, you can run it while opened up and make measurements. The fix is fairly straightforward, but there's very little space, so you'll have to get creative with alternative cap arrangements. I manged to do all the work from the component side, avoiding having to remove the board, which is even more a PITA.

If it's not those or something else like another Ta cap in the scope circuits, it could be the dreaded HV winding failure on the main transformer. Hope for the lesser grief.

Ed

Peter Gottlieb
 

I looked again and one of the switcher transistors failed again so replaced that. Now back to burst operation.

The switcher shutoff is coming from the control chip. It seems to be triggered by large spikes coming in on the balance input and those seem to be coming from the current sense on the HV side. This might be caused by a shorted HV diode or the multiplier. I am hoping it is not the HV winding as that would likely mean scrapping of this otherwise mint scope, but the indication is promising (at least right now).




Peter

On Nov 18, 2019, at 12:50 AM, Ed Breya via Groups.Io <edbreya=yahoo.com@groups.io> wrote:

The last time I fixed a 7844 PS, it was fortunately just two or so rather large (100 or 220 uF 20V) Ta caps shorted, on the pre-regulator supply board, for the +/- 15V inside the PS module - the one inside the cage, just below the outer, upper, linear regulator board. It's a PITA to get to, but doable - if you take enough stuff apart, you can run it while opened up and make measurements. The fix is fairly straightforward, but there's very little space, so you'll have to get creative with alternative cap arrangements. I manged to do all the work from the component side, avoiding having to remove the board, which is even more a PITA.

If it's not those or something else like another Ta cap in the scope circuits, it could be the dreaded HV winding failure on the main transformer. Hope for the lesser grief.

Ed


Peter Gottlieb
 

Well, I now highly suspect the shorted winding.  I disconnected the HV winding from circuitry and am getting overcurrent on main transformer drive.  Since I also disconnected the LV rectifiers and am still getting the overcurrent, something in there is shorted.

Unless I get lucky and find a parts 7844 or a presumably unobtanium replacement transformer, this scope seems like it's destined for the heap.

Too bad because it has the beautiful blue phosphor and a really strong CRT with low hours on the scope.

Peter

On 11/18/2019 12:50 AM, Ed Breya via Groups.Io wrote:
The last time I fixed a 7844 PS, it was fortunately just two or so rather large (100 or 220 uF 20V) Ta caps shorted, on the pre-regulator supply board, for the +/- 15V inside the PS module - the one inside the cage, just below the outer, upper, linear regulator board. It's a PITA to get to, but doable - if you take enough stuff apart, you can run it while opened up and make measurements. The fix is fairly straightforward, but there's very little space, so you'll have to get creative with alternative cap arrangements. I manged to do all the work from the component side, avoiding having to remove the board, which is even more a PITA.

If it's not those or something else like another Ta cap in the scope circuits, it could be the dreaded HV winding failure on the main transformer. Hope for the lesser grief.

Ed


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