I have one that is fully restored except for a few resistors in the vertical amp. Anyways about three years ago while using the scope the 5ADP1 CRT in it lost all ability to focus properly. I then tried several 5ADP* series CRTs all with the same problem, yet a 5CP1 works properly. I thought it was the CRTs, but one still has the QC sticker on it. I also tried the CRTs in an unrestored 304H and got the same or similar problem. I am gonna try and switch out the intensity and focus controls of the restored 304H (originals burned out and had to replace with regular variable resistors) with those in the unrestored 304H and see if my problem goes away. I kind of have to fix the scope as I have put $$ in restoring it before the problem occurred. I cannot use the 5CP1 as it is not flat faced even though that's the CRT listed in the manual. I'm thinking mine came from my local air force base as it has a sticker warning not to measure line voltage because the case is grounded and it has a flat faced CRT in it.

I am at my wits end with the scope. Other than removing the focus and intensity controls from the chassis and seeing if the problem is solved I don't know what to do.

Anyways about three years ago while using the scope the 5ADP1 CRT in it lost all ability to focus properly. I then tried several 5ADP* series CRTs all with the same problem, yet a 5CP1 works properly.

A few questions here:
1. You tried several 5ADP* CRTs .... how many? ... you might have a couple of bad tubes, but as the number goes up, the likelihood of them all being bad in the same way decreases dramatically.
2. Perhaps the operating specs for the 5CP1 are slightly different ?
3. Do you have a schematic for the scope?
4. Does the focus pot have any effect? Do you have to set it to one end or the other for "best" focus?

My best guess for now, is that one of the resistors in the HV divider string (which includes the focus pot) changed value while you were operating the scope, and a 5CP1 works ok with the changed conditions.
jr

For the last three years I have asked several times on different forums and no one can seem to pinpoint my problem. Maybe somehow there is some leakage to the chassis or the rear board (has most of the CRT components on it) that is enough to affect the 5ADP* series CRTs, but is still within specs for the 5CP1 CRT.

What is odd is the fact that the CRT looses focus whenever a waveform is displayed. I do notice that the HV to the CRT does drop by about 100 Vdc when the intensity is full CW. Don't know if it is normal or not.

I am wondering if it may be leakage from the focus and intensity controls to the chassis given the originals were mounted using an insulator to isolate the variable resistor case from the chassis and the knobs were mounted to a plastic shaft. I am gonna check for cold solder joints and places where HV leakage could most likely occur soon as I can get to the scope.

I say the 5CP1 works ok, but I ma not entirely sure as it's been ages since I have tried it. Still doesn't explain why the 5ADP* series CRTs do the same in the unrestored 304H. I can maybe take pics of the inside of the scope tomorrow if that will help.

Now I can connect another 5ADP* series CRT in parallel with the one in the scope and I get proper focus.

Here's a couple pics of my problem. The CRT installed still has the QC sticker on it. I did quickly remove part of the sticker though.

Normal (camera makes trace look out of focus when it is actually pretty sharp) although focus control (bottom right of pic) has to be nearly full CCW

http://i863.photobucket.com/albums/ab196/jmcinvale/304H%20scope/Scope.jpg

With waveform displayed (waveform very out of focus and focus control not moved from position in first pic and camera does make it look slightly worse than it actually is, but not by much).

http://i863.photobucket.com/albums/ab196/jmcinvale/304H%20scope/Scope2.jpg

"What is odd is the fact that the CRT looses focus whenever a waveform is displayed. I do notice that the HV to the CRT does drop by about 100 Vdc when the intensity is full CW. Don't know if it is normal or not."

I believe that I am seeing evidence of an oval spot in both pictures... note how the sweep line seems to "taper" off at each end.
I suspect that some drop of HV at full beam current (perhaps 100uA or so?) would be fairly normal.

"I am wondering if it may be leakage from the focus and intensity controls to the chassis given the originals were mounted using an insulator to isolate the variable resistor case from the chassis and the knobs were mounted to a plastic shaft."

I was about to ask about something like this... I have seen it before, a pot with HV on it may well develop leakage to ground, throwing the voltages in the divider string off considerably. Since the spot is quite oval in the Horizontal direction, perhaps the voltage on the astigmatism electrode is way off as well.

"Now I can connect another 5ADP* series CRT in parallel with the one in the scope and I get proper focus. "

Huh ? So if you connect two tubes at the same time to the scope they both look ok?
jr

Yes connecting both CRTs to the scope makes them look ok. I will remove the focus and intensity pots and isolate them from the chassis then I will see if the problem is fixed. If so I will use the intensity pots from the restored 304H. If not I will see if there is any leakage somewhere else.

Another thing I can do is put a 5ADP1 CRT in both scopes and measure the voltages on both scopes to see if there is any difference between the two scopes.

"I will remove the focus and intensity pots and isolate them from the chassis then I will see if the problem is fixed."

Be VERY careful with those floating pots... The primitive HV supply of these old scopes can easily deliver a LETHAL SHOCK!:eek: If the pot is leaky or arcs over you would NOT want to be touching it!
jr

What I will do is remove the knobs and set the pots with the scope on for decent intensity and proper focus. I will then power off the scope and remove the controls hopefully without disturbing their positions. I will then power up the scope and see if anything changes.

Again, be very careful! The HV caps can hold a charge long after the scope is turned off. Discharge them several times, and keep on hand in your pocket.... these primitive old HV supplies can be KILLERS!

jr

Yes I have been bit by the scope before. Not fun

Have you measured the focus voltage with no waveforem/focus good, 2 CRTs focus good, and 1 CRT waveform focus bad? This should give you a clue.

not yet. when I decide to mess with the scope again I will try the scope without the focus and intensity controls mounted to the chassis so I can rule out leakage from the pots to the chassis as the original pots had a plastic shaft with insulated mounting piece that isolated the variable resistor s from the chassis. I could just replace them with the focus and intensity controls from the second 304H I have that is waiting to be restored which I may just leave as a parts chassis for now.