Tektronix 2246 Z Axis Issues

[mosspig]

My 2246 has no z axis intensity under normal operation. If I adjust R2719 (grid bias) up, however, I can get the display working (not a fix, however, as grid bias needs to be adjusted back down for normal operation). I've gone through all the test points in the Z-axis schematic and marked it up. It can be viewed here: https://i.ibb.co/0K6JBtx/schem2.png   Many values are out of range. I have already replaced CR2701/2/3/4 but no change in values. Can anyone look over this schematic and point out suspect components?

[rf+tech]

mosspig,

From your measurements, the voltage at the right end of R1001/Q1001 emitter is essentially invariant and appears to be held at a logic-high level.

Check for missing (active low) A INTEN GATE pulses at the right end of R1001/Q1001 emitter. All four INTEN GATE signals originate from U602, found on schematic (diamond)-4, located at grid coordinate G6.

RF+ Tech

[mosspig]

U602-17 and U602-18 (A INTEN GATE & B INTEN GATE) are both stuck high at 4.5v. No sort of waveform. No change as I exercise the respective control knobs. U602-20 does show a 500mv square wave on it but it does not vary when exercising the RO INTEN knob. It will drop to 0v when the knob is all the way CCW but that's it.

I also probed all the test points. Those waveforms are shown here: https://imgur.com/a/eDLAg84    7B 7C 7D and 7E are out of spec. 7F appears in spec.

[rf+tech]

Next thing, check for Vbb3, from U604 pin 11. U604 is an ECL device, MC10116. From the datasheet, Vbb should be 1.25 to 1.35 Vdc less than Vcc2 on pin 16. Referring to your annotated schematic, Vbb3 connects to the left end of CR1005, just above the red +4.5 annotation.

Beyond this, all of the logic inputs to the left side of U602 should be checked, to determine how much U602 functionality is actually operational or impaired. Does Beam Find or Bandwidth Limit work?

I've seen corrosion damage caused by battery electrolyte leakage, on at least one 2246. Both cell holders were trashed, several tracks and a section of ground plane eaten away and multiple through-hole parts damaged.

RF+ Tech

[mosspig]

Vbb3 (U602-11): 3.8v
Vcc2 (U602-16): 5.1v

All other functions on the scope work including BW Limit and Beam Find. The unit passes the pot test for the 3 affected controls (A INTEN, B INTEN, READOUT) - displayed 00 to FF.

No physical damage present on the board.

[rf+tech]

Since the intensity gate signals are stuck high, waveforms 7(x) are not expected to be correct. This is not the place to be searching for the fault.

Attention needs to be focused on U602 and U600.

The intensity gate signals originate in the lower half of U602. U600 supplies control signals to this lower half of U602. Check waveforms 4A through 4I. Of special interest would be TH0, HD0, HD1, BLANK, /ZEN, DS, /MGE.

Check /SIN, DATA, A0 and A1. Two address lines provide for four possible addresses, check for actual DATA aligned with addresses 00 01 10 11.

RF+ Tech

[mosspig]

4A-4I as well as SIN, A1, A0, and DATA shown here. https://imgur.com/a/7JKzTTo 

All 4x look wrong except G and H. Not sure about the other 4 points you requested.

[rf+tech]

No signal at 4A requires fault finding on schematic 3.

See pages 3-12 through 3-16 for detailed theory of operation on the A and B Trigger System.

RF+ Tech