Restoration Tektronix 5111

I met this oscilloscope in Delta hamfest,  I am glad to saw a tektronix again in a swap meet. I have no plan starting to equip lab with bulk device, i kind of seeking small devices instead. I still prety love to access tektronix, though. 

I re-start pursuing my interestsf from searching vintage radio shop or electronic store. When the S.P.A.R.C appears, I know exactly where I need to go, I drive there and met this oscilloscope again. I decided to restore this scope, connect with it, to experience it fully, but I don’t need to have it.

This scope have serveral unfunctional function, the major progblem is the scan line may stay on top half of screen could not go down below the midle screen. The waveform bottom dispeard and became a flat line, just like amplifer peak clipping，but at bottom. 

Preparation befere make mesurements

preoblme description: both channel have same problem, all get clipped in right the midlle of screen, switching the two vertical amplifer is not help at all, the most hard part, it is not always been clipped, some time you wilggle here and there, touch somethere wil recover the waveform to fill full CRT.

Because both vertical channel behavior same and swap them did no help at all, beside i clean the golden finger, nothing better. So i assume the problem must be in a common module for both channel. And the scan line stay only half of CRT means, one of vertical deflate plates missing it's drive voltage! it must be like this.

This analysis imediately get me a plan for a checking list:

• is one of the high voltage supply for vertical deflate is failure/missing?

• any voltage in drive DC differentail amplifer off scale?  there must be one plate's voltage abnormal! otherwise the problem is won't there.

Starting from final vertical amplifer is good idea.

Measurement

The schematic shows that, both vertical and horizontal deflate palte driver is almost same structrue and they are on same PCB, as show in the following pictures. We must have a ground alligator clip for ground and a test tip isalated most of the metal and leave only the very tip bare, that is for safty and protect the ciurcits, prevent ground any high voltage components lead.

Checking 200V high voltage, it's both ok on left or right deflation, the hot end of R104 and R114.  Mesureing the volt present on Q114 collector, I fainnaly notice, when the problem hit, collector of Q114 stay on 200V won't change, even you tune the vertical position knob. the another deflation transistor Q104 will chang it's voltage from 200V to 50V also, that's the normal behaviour. 

Now we konw on the Q114 side there is somthing wrong. So the 4.5 V on Q114 is wrong, it's -0.5V instead.  Base of Q114 is also not right it's amost same voltage on either of R112.  

Before i can sort out those beheviour, I notice a very funny thing. IF i mesurment the Q114's collector voltage, the CTR will recover to normal for a while! just at the moment my multimeter's probe touch the collector, that's wired and funny. In a radio receiver this might means a oscillation exist, for somewere a broken components, i never meet. 

And furthermore, I toucht the transistor can do same thing, this is not strange because the metal can is just the collector of this transistor. but, while i just use the probe, and this probe is not coonected to the multimeter at all, it's a fly wire, it does same thing to the oscope!! it triger the scope CRT recover to work too!

Bingo

After play about one hours the funny transistor, I realize the transisor should not get conducted, thats why it's collector voltage stay at 200V and it's be not  conducted so that 4.5V on emmiter is missing. that's must be like this, multimeter test 2 be junction confirmed it's BE is dead open. The wire trigger on the high voltage must make that BE junction works again for a while.

One vertical channel's gain tune POT shaft connetor is broken,  usePC super epoxy glue should works well.