Tektronix 2246 - No beam!

[sync]

Already tested. There is 15-55V on 7G.
https://www.eevblog.com/forum/testgear/tektronix-2246-no-beam!/msg316438/#msg316438

[cs.dk]

These caps isolate the control grid (wehnelt) high voltage. If they are leaky then the control grid would be more positive than the cathode (-2.7kV). But the control grid is too much negative. So it's unlikely that the caps are the fault.

Please make photos of the area of the removed caps. So we can see differences between your unit and the service manual.

Then put the caps back in. Try adjusting the GRID BIAS pot (R2719) like oldway suggested. If this doesn't help then lift one leg of R2722 (yellow arrow). This will disconnect the 130VAC to the DC restorer. Without this the DC restorer can't work. You should get a very, very bright trace. If you get this the caps are not the fault.

I put the caps back in, messing around with R2719, no luck. :/
When lifting R2722 i've got a really bright trace, and the neons turned off.

Side question, what is wehnelt? I'll google that. EDIT: Googled that phrase, and become wiser

[sync]

Good. The caps are not the problem. Please check the missing CR2703. You don't have to find it. It's connected to C2711 and C2712. Make a diode test at the leads of the caps (yellow arrows).

[cs.dk]

Good. The caps are not the problem. Please check the missing CR2703. You don't have to find it. It's connected to C2711 and C2712. Make a diode test at the leads of the caps (yellow arrows).

Thanks.
It did measure "nothing" both ways, with the diode-test. Even at 20M-scale, the leads of the caps are open.

Because of trouble with soldering these bastards in again, i lifted C2711, i thought it was a cold joint - Guess what, there is something hiding under there, more correct it is hiding under C2724 hidden by C2711.  Could this one be the problem?

Why are this thing so hard to solder? One end of both caps went fine, the other is just impossible. I did use flux, the solder just don't want, what i want it to. Need to clean up with some wick, and try again, when the cap need to be soldered in again.

What diode do i need for replacement?

[sync]

That's a hidden problem!
And a bad design. The diode is likely broken due mechanical stress from the cap.

Yes, it could be the only problem. I done a simulation with an open CR2703 and it shows to much negative voltage. The problem of your scope.

Regarding the diode replacement type. According to the service manual it's a 225V 200mA signal diode. A BAV21 would be a close match. An UF4004 is also good. Any fast silicon diode with >=225V and >=200mA should work. A 1N400x is too slow.

For replacement I would cut the rests of the broken diode from the board. Put back the caps and solder the new diode across the cap leads. Cathode to the smaller C2712 and anode to C2711. Just another bodge Not worth the trouble to get the new diode between the caps. Fortunately this is the low voltage side of the caps.

About the soldering. I don't know. Maybe the leads are oxidized?

[cs.dk]

I've ordered some UF4004, with some luck they are here thursday.

[cs.dk]

It's alive!!! 

I did nip one of the ribboncables though, so i miss the main-menu.  It's the one on the side, i didn't noticed it, so i cared about them in top-front. All measurement menus are there, so it must be fixable, maybe one pin is not correctly seated. I'll have a look at that.

I did find an old Compaq hotswap server-PSU, an got some parts from it. I could not wait for the new diodes to arrive. I took a chance, and installed a EGP20D 200V 2A diode, it seems to work. Worst case the smoke would let out, and i'd have to wait for the new UF4004.

The horizontal is a little of scale, the vertical seems to be pretty close.

Thanks alot folks, without your help it was not being a scope again

[BravoV]

I have been following this thread closely, its just I dont have anything to contribute, just glad to see a happy ending. 

Congratulations ! Great job and very well deserved  ! 

[sync]

Congratulations! Glad it's working again.

About the horizontal. The calibration output is not very precise (1kHz +/- 25%). Do a self calibration and check it with a accurate source.

[ElectroIrradiator]

Glad to be proven wrong about the pair of HT caps.

Now get the iso out and clean her up a bit, she deserves it.

Congratulations and enjoy.

[cs.dk]

I'll test with a more stable signalsource, to see if it needs adjustment.

Yeah, some cleaning would not harm her

Now i just need a new set of probes. Does all Tek probes feature the "auto 1x-10x" function?

[alm]

No, look for the 'readout' function in specs or datasheet. For example, in this list it's indicated as "R.O.". Note that 1x probes will never have a readout pin since 1x is the default.

[pgross]

Glad to see that you managed to get the scope fixed. Kudos to you and to this forum 

I 've recently restored my 2246 and now its good for another 25 years...
My scope is completely recapped (mainboard and PSU) with some fresh electrolytics from Panasonic and Nichicon.
The backup Lithium battery (mounted on the solderside of the readout PCB) is also replaced.
Now your scope is alive, have you tried to run the self test from the setup menu?

One thing I have noted is that the readout on my 2246 screen tends to flicker - more or less in accordance with the sweep time.
I dont know if this is a normal behaviour - I have tried to adjust the readout jitter for optimum performance, but somehow the flickering can't be completely eliminated.

Please feel free to send my a PM if you have any comments or questions, preferably in danish 

/Peter

[cs.dk]

No, look for the 'readout' function in specs or datasheet. For example, in this list it's indicated as "R.O.". Note that 1x probes will never have a readout pin since 1x is the default.

Thanks. I'll have a look at Ebay for some probes.

Glad to see that you managed to get the scope fixed. Kudos to you and to this forum 

I 've recently restored my 2246 and now its good for another 25 years...
My scope is completely recapped (mainboard and PSU) with some fresh electrolytics from Panasonic and Nichicon.
The backup Lithium battery (mounted on the solderside of the readout PCB) is also replaced.
Now your scope is alive, have you tried to run the self test from the setup menu?

One thing I have noted is that the readout on my 2246 screen tends to flicker - more or less in accordance with the sweep time.
I dont know if this is a normal behaviour - I have tried to adjust the readout jitter for optimum performance, but somehow the flickering can't be completely eliminated.

Please feel free to send my a PM if you have any comments or questions, preferably in danish 

/Peter

Nice - With a recap, it will sure last a long time.
If self-test is equal the Diagnostic, it says Passed

Mine is also flickering, not much though. I haven't noticed the sweeptime vs. flickering yet.

[viperrepair]

HI just want make correct Sync's suggestion to solder the replacement diode's cathode to c2712 it is actually supposed to go to the resistor under c2712 not to c2712. almost made that mistake luckily I caught it.
Regards.

[viperrepair]

"And a bad design. The diode is likely broken due mechanical stress from the cap."

I believe its a result of the plastic shield being installed incorrectly....with force.