开云体育

I'm working on a 7b92 with what appears to be three apparently
unconnected problems (though the second and third may be related).

1) Will only trigger in "AC HF Rej" mode.

2) The "Runs after DLY Time" switch appears to work in reverse!
That's to say that when it is in, the delayed trace jumps from one +ve
going signal to the next, but when it's out it behaves as runs after
delay time!

3) Some sort of problem with the Z and Y axis controls.

a) The "non-intensified" portion of the dealying sweep is not visible
at all at any brightness setting of either the mainframe or 7B92
intensity controls.

b) When operating in Alt mode potions of the waveform are absent
(though they sometimes re-appear for a few momements) and the
delaying sweep is distorted somewhat.

I've posted a picture of this at:



Thanks in advance
Dave

Further information the following part of the problems:

3) Some sort of problem with the Z and Y axis controls.

b) When operating in Alt mode potions of the waveform are absent
(though they sometimes re-appear for a few moments) and the
delaying sweep is distorted somewhat.

Test condition is with the 7B92 set with both sweeps set to 1mS per division, a 1kHz input signal, and ALT mode active.

Looking at U380 pin 3, I see (NOT)SWEEP_GATE +ve going pulses from about +2V to about +6V at about every 12mS.

I think I should see pins 5 and 6 alternating between 0V and about 6V with the changeover at every pulse on pin 3.

What I actually see on (say) pin 5 is an approximate square wave (so far, so good), but the changeover is every 1mS, rather than every 12mS or so.

If I change the input signal to 100Hz, the changeover is now every 10mS, and to add oddness on top of oddness, the (NOT)SWEEP_GATE signal on pin 3 is now a square wave with a switchover every 10mS!!!

Dave

I've been working on the triggering problem with this.

Looking at TP720 when the TD triggering is disabled (in AC HF REJ mode), and the sweep gate circuit is fed from a Schmitt trigger. In this mode I can get a good locked trace from an LF signal.

In this case when using internal triggering, and a 2 division 50kHz sine wave, the signal at TP 720 is a 2 volt p-p sine wave when I've adjusted trigger level for the largest signal. If I then look at the emitter of Q702 in this condition, I see -0.25V for 20uS, a step up to 0V, a 1.5V +ve immediately, followed by five more at 20uS intervals, and then a few uS later the whole pattern repeats (time to repeat about 120uS).

The signal at the anode of CR750 is similar, but the +ve going spikes are only about 100mV.

Unfortunately the 7B92 service manual doesn't give any waveforms etc, so I can't tell if this is right or not.

Now to the non-working case.

When I switch to normal AC triggering (thus using the TD at CR720 to create the signal feeding the sweep gate) I see a 0.5V p-p signal at TP720 that looks like a sine at the top and botton, and like a square wave in the middle. There is also a "spiky" bit in the middle of the verticals - this suggests to me that the TD at CR720 is probably good, but the level of the signal is rather low.

Looking at the emitter of Q702 I see nothing except a small (-25mV) pulse every 108uS, and pretty much the same at the anode of CR740 and CR750.

I can't look readily at the base of Q744 (which tests good) as that is back of board, but it is connected to the emitter of Q702 by a 150R resistor. AFAICT, the base of Q744 is driven in this case via the capacity between screen and core of the coax on the back of the board, and one end of the anode connected cable is left open???

Questions:

1) Can I test a TD with an ohm-meter in diode test mode, or will I kill it? Does it look like a normal diode when it is good?

2) What else should I look at to nail the guilty part?

Thanks a lot
Dave

OK I've been looking at this further. To recap:

1) At pin 3 of the D-flop (U380A) Pin 3 I see a pulse of just under which goes up to about 5.8V pulse at about 12 times the time per division. The rest of the time the signal sits at 2.2V. If I look very carefully I see a small twitch at half the fundamental frequency of the input signal on the 2.2V level, but this is SMALL.

2) I see a square wave at exactly 1/2 the fundamental frequency of the input signal on pins 5 and 6. This square wave goes from 0.2V to +5.8V (note 5.8V NOT 5V).

Looking at pins 1 and 4 (Set, Reset), I saw a few mV of the square wave superimposed on the +5V at 1/2 the fundamental frequency. I added some small (0.22uF) decoupling caps between these and ground. Now the bounce of these pins is in time with the (NOT)SWEEP_GATE clock pulse on pin 3. But pins 5/6 still flip at the same rate.

U380 is a 7474 TTL D-Flop, is the tiny twitch likely to be enough to clock it given that it is riding on 2.2V?

If it is, then that explains what's happening to some degree, though I still can't see how a 7474 supplied from +5V can flip its outputs between 0.2V and 5.8V.

If the above explains what's happening, would I be right to be checking the resistors and diodes around Q322 and Q326?

Thanks
Dave

Hi, have you ever managed to fix the missing portions in the alt/intensified modes? My 7B92A never shows the alternate (delayed) sweep. It just shows the intensified portion of the delaying sweep. Any ideas? Thanks!!

Correction. I finally got my 7B92A to show both traces (delaying and delayed). I had the wrong combination of CRT intensity, B Horiz Intensity and 7B92A intensity/contrast settings. However what I am missing is the non-intensified portion of the delaying sweep. If I push some of the intensities to the max I can see it very, very faintly. This is obviously not working properly because at those settings I am certainly in the CRT-burning range of intensity. Any ideas? Thanks!!!

On Sat, 21 Jul 2018 13:09:29 -0700, you wrote:

Hi, have you ever managed to fix the missing portions in the alt/intensified modes? My 7B92A never shows the alternate (delayed) sweep. It just shows the intensified portion of the delaying sweep. Any ideas? Thanks!!

Possibly the first thing to think of is if it's in that mode at all?
You might consider looking at the outputs of the switches (or where
they show up) to see if they are proper. Switches can fail rather
easily. Given that those outputs are correct, you need to look at
connections and semiconductors next.

Harvey

On Sat, 21 Jul 2018 13:55:29 -0700, you wrote:

Correction. I finally got my 7B92A to show both traces (delaying and delayed). I had the wrong combination of CRT intensity, B Horiz Intensity and 7B92A intensity/contrast settings. However what I am missing is the non-intensified portion of the delaying sweep. If I push some of the intensities to the max I can see it very, very faintly. This is obviously not working properly because at those settings I am certainly in the CRT-burning range of intensity. Any ideas? Thanks!!!

Isn't there an intensified beam setting?

I seem to remember one on the plugin.

Harvey

If by "intensified beam" you mean the mode where the "selected" portion of the delaying sweep is brightened, then yes, there is. The problem is that I am only seeing the brightened part but not the rest of the sweep (unless I turn the intensity to dangerous levels, then I can see it very very faintly).