Need help at troubleshooting my Tektronix 222 A

[128er]

Hi!

I recently bought a broken Tektronix 222A mini scope. You turn it on and it beeps. But the CRT stays dark. At  a cold start it draws about 583 mA, within 30 seconds it settles down to 552 mA at 12V. No battery is installed. It's powerd via the external power connector. I use the Tek 222 service manual, couldn't find anything useful for the 222 A model.

Link to the manual: http://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_222

The battery charger SMPS seems to work fine (sits on the main board). It delivers the 9,8 V, as mentioned in the manual. The CPU board seems to work, too. I can measure the X/Y signals, also the Z signal is there. 

The fault at my unit, I think, is very obvious at the A4-Power Supply Board.

The pages 26 and 70 in the manual are quite useful. Page 26 shows a block diagram of the power supply section and a little bit about the theory of operation. On page 70 you can find the interconnect diagram of all boards.

On the A4 PSU/CRT board is a TL594CD PWM controller. It gets the DISPLYE signal that is required for the IC to start working. Could not figure out if the feedback circuit is working correctly (But I assume this is a very important thing  ) . The FET's Q401 and Q402 are switching, but they are really hot (~90°C).

There is nearly nothing on the output rails Vcc, Vee and AVcc.

Here are the measured values - with respect to GND - on the CRT connector (with CRT removed):
1 - Grid: -269V
2 - Filament: -233V
3 - Cathode/Filament: -246V
4 - Focus: -162V

Measured resistance at the CRT filament pins: 6,2 Ohms

Anyone here who had a similiar problem, or can give me a good hint?

[tautech]

There was a very recent thread on the 222:
https://www.eevblog.com/forum/testgear/tek-222-portable-scope-schematics/msg527059/#msg527059
It was for display problems too.

[Andy Watson]

The only source I could find for manuals with full schematics for the 222A was Dave at artek:

 http://artekmanuals.com/manuals/tektronix-manuals/

The voltages on the cathode and grid are too low - they should around -1600V.

[c4757p]

The voltages on the cathode and grid are too low - they should around -1600V.

Are you sure? That's close to what they should be for a full-size scope!

[Andy Watson]

The voltages on the cathode and grid are too low - they should around -1600V.

Are you sure? That's close to what they should be for a full-size scope!

Yep! Beyond the range of all  my DVMs - I had to resort to the trusty AVO 8 to measure it.

The manual says:

High Voltage Power Supply

The 800 V and -1600 V voltage for the CRT is developed by a voltage multi-
plier circuit consisting of CR418, CR419, CR420, CR422, and associated
capacitors and resistors. The negative 1600 V is applied to the GRID through
Intensity adjustment R413. Focus voltage is tapped off of Focus adjustment
R415, which is tied to the 800 V supply and a resistive divider network. The
Astigmatism adjustment R424 controls 0406 to control X and Y axes conver-
gence.

Z-Amplifier - The Z-amplifier is a class B stage that is nonlinear, since it
only needs two output states: blanked and unblanked. When on, the output of
the amplifier is about 0.2 V; when off, its output goes to 119.3 V. This pulse is
coupled through a restorer to the grid of the CRT to turn the electron beam on
and off for blanking. The grid voltage of the CRT is about -1550 V and the DC
restorer circuit allows the Z-amplifier to take the grid 119 V below that level to
blank it. Blanking times are under control of the processor and are held to less
than 10 ms. Z-axis information is amplified to 120 V and added to the high DC
voltage grid through C414.
[\quote]

[128er]

Andy was faster . . . 

Yes, about -1550V. Even my differential probes are not capable of that voltage. My P5205 is only capable of 1300V.

But I think my first step will be, to replace the TL594. It's reference output is only about 4,09V. Should be stable 5V with 1% tolerance.

I don't like these "one part orders" . . . .