Telequipment D75. Trace problem.

[londoner]

here is another

http://m.youtube.com/watch?v=tjWHACLEESU

[maninashed]

Londoner, you seem to be pressing EVERY button you can find in a completely illogical fashion!. Set the scope up as in post #8. Leave it set like that. You need to select A ONLY on the S2A. It looks from the photos you have attached and the utube video that you have the DELAYED TIMEBASE running (A delayed by B) which is controlled by the small knob inside the clear time/div timebase knob. You DON'T want the DELAYED timebase running for the moment. Otherwise, we are just going round in circles, and the results are meaningless.
Also, you need a proper BNC-BNC coax cable, or a proper scope probe, or you will feed in signals that you don't want and you will think there is another fault.
Bill

[Radio Wrangler]

OK, I'd seen enough a couple of minutes into the second video.

That's looking promising.

The X position controls are working, the timebase is running. The Y side of things seems fine.
You've found the timebase speed (time/div) switch and that does indeed control the speed of the spot.

What's missing?

The timebase doesn't sweep the full screen.... this is relatively trivial and is the last thing you need to fix because other things may change it.

The sweeping stops after some time.... OK there are bad components to be tracked down and replaced.

The intensity control doesn't seem to work.... leave for the moment. The timebase has a system to overrule the intensity pot, to darken the spot while it returns to the left, as well as an arrangement to run both timebases (you have two!) with one sweeping the spot while the other brightens the trace to show you what part of a waveform you will zoominto when you switch timebases. Your intensity is fine for getting the X amplifier fixed and reliable.

You aren't showing a normal waveform because you haven't got the timebase triggering.

If you've got a known working Tek 465 there, maybe getting some practice in driving its trigger system would show you what the telequipment scope should be doing?

I don't think you're terribly far from having a good scope.

It's proving that all the expensive or irreplaceable bits are OK

It does look like there's a bit of RF pickup. I too had assumed it was the connection.

I'd suggest a scope driving lesson on the 465 to explain triggering and dual timebases.

David

[londoner]

iI did it on purpose just want somebody to see how the wave form reacted after pressing tbe buttons.
iI will reset as per post #8.
iI do have a unterminated bnc to bnc cable and tektronics 10x probes.

[maninashed]

Post a picture, set as post #8, but ensure 'A ONLY' is pressed in, and ALL the others are out. SWEEP should be on REP, Trigger AUTO, Source INT. There should be a horizontal trace, with the time/div transparent control set midway. Use one input channel only.
Bill

[londoner]

I reset it as per #8. no sweep at this time just a dot.
it was so unstable I did not know what happened. It was so discouraging
I measured voltage on x amp again.

base voltage on tr4 is 17.8V( out of specs) and tr5 is 11.5V.
what controls the base voltage on tr4. that is what I am suspecting now.

[maninashed]

My first instinct would be to remove the S2A time-base unit from the scope, then power up and re-check those voltages but position the spot in the centre of the screen using the horizontal position control and the Y position control on the V4 vertical unit. Then re-check the voltages. If there is still an severe imbalance, then you are going to have to check that X amp pcb once again because you have missed something.
Basically, TR4 and TR5 are phase splitters in a balanced push-pull amp, if the input voltage swings one way, the output swings the other way. TR3 and its associated components set the base conditions for TR4, so you need to re-check these.
Bill

[micheal]

You have time base knob A (main T/B. grey with red top) in the wrong position,
set to 1m/s (vertical)
T/B A (main timebase knob grey with red top)
T/B B (is delayed timebase, large clear knob)
Mike

[maninashed]

Micheal, you are right. The centre smaller knob is the A time-base control.
Set both knobs to a central position, and you should have a trace.
Sorry for the confusion.
Bill

[londoner]

I have both knobs on s2a in the midway(one big clear plastic and another is grey with red in the center).
I injected 100Khz signal into channel A with a straight through bnc to bnc cable.
I have a sweep this time but not being triggered correctly. waveform moves constantly. I tried trigger level and did not stop it from moving.

How does it look:

the sweep/trace can move up and down by adjusting the position knob on V4
the sweep/trace can move freely to RHS but not LHS. feels like the spring hit the wall.

waveform can reacts with frequency of the input signal.
I adjusted attenuator and wave form elongates on vertical side.

overall : sth wrong on triggering.


I will post pic later.

[maninashed]

Yes, that confirms that the V4 is working, but the fact that you can only move the trace to one side indicates that the problem lies with the X amp, as we suspect. This can be confirmed by removing the S2A and observing results. The triggering is another matter, but that can be addressed later after the X amp is repaired.
You need to re-check that X amp pcb and find the problem. If possible invest in a good transistor/diode tester if you are not confident in determining your results.
Once you have the voltages on the pcb, correct to the manual, the output should swing either way, depending on the setting of the horizontal control.

Bill

[londoner]

I tested all diodes and transistors, all good. I did not find anything wrong at this time.

what about the resistors and capacitors?

[maninashed]

Check that you have good +24v at 150/8 and -24v at 150/7. From your findings so far I suspect there is a problem around Tr3, check R19/R16, and other resistors in this area. You have obviously missed something. Checking resistors is easy, checking capacitors is more problematic, as they will sometimes test ok with a DVM, but fail short when a high voltage is applied. So if in doubt, replace with new components. In my experience, the ceramic capacitors are mostly reliable, but electrolytics and paper capacitors are most , likely to fail. There is only 1 electrolytic on the board in a decoupling position, and is unlikely to cause your symptoms. The circuit will still operate without C14 in place, so you can remove it temporarily. The other capacitors on the board are colour banded 'tropical fish' types, these are polyester types and are generally reliable.
So you need to go over the board again carefully, it would be best to remove it for service, but draw diagrams/take photos so you know where all the connections fit.
Bill