Faulty Telequipment D1016

[heze1982]

Since a few year I'm the owner of a Telequipment D1016 oscilloscope. Working with it was always a bit tricky, but now the scope died completely...

The problem is that I do not have anything on the screen. When I hit the beamfinder, I see two horizontal lines (both channels) with a width of approximately 1 cm (0,4 inch) on the left side of the screen. When I pull the xy button, one spot is displayed on the middle of the screen. I think the timebase is not running at the moment and therefore I cannot see anything on the screen (except for the beamfinder).

On this forum I read that the switches are quite delicate. I inspected and cleaned the small wipers of the switches and luckily they were optically in great shape.

On this forum I saw other threads where people were assisted very well. I could not find a thread where the exact same problem occured, so I started a new thread.

I all ready own the service manual of this scope.

Can anyone tell me what to do? Are there parts which are extra delicate and have to be checked first?

Thanks in advance,
Regards Hendrik

[ronbryan]

Hendrik

Have you done preliminary checks to ensure the the +22V, -22V and -170V power supply voltages are correct?

Ron

[Skywave]

Yes, indeed: sounds like PSU trouble to me.

Al.

[heze1982]

Thanks for the quick response.

In my previous message I forgot to mention that I already measured the power voltages.
22V power voltages were off by 0.5V, but now readjusted to 22V and -22V.
The -170V power line is here -165V. Can I adjust this power line too (potentiometer)?

I followed this document to check the separate parts of the scope, and the document told me te following things:

http://www.sphere.bc.ca/test/tek-par...ing-scopes.pdf

'If you have only a dot for a display when you press the beam finder, the fault is probably located in the horizontal section' (page 22).

and in the horizontal section (page 59):
'Rotate the time/div switch to the x-y- position. You should have a dot near the center of the display. If this does not appear, the fault is probably in the horizontal amplifier. You can verify this further by pressing the beam finder and observing the display. If you get a centered dot, the problem may be in the timebase or in the crt-section. If you get a dot that is deflected to either side of the display, the fault is probably located in the horizontal amplifier'.

I tested this, and I will test this again after work, but according to the document I think the horizontal amplifier is faulty.

Does someone know which faults are common in this section of the scope?

Thanks in advance,
Regards Hendrik

[heze1982]

Luckily I tested it again and this is the outcome:
When I pull the xy switch, a centered dot appears.

This means that the fault - according to the document - has to be in the timebase or in the crt section.

Can someone help me troubleshooting this section, because I'm a bit puzzeled right now?

Thanks in advance!
Regards Hendrik

[ronbryan]

Hendrik

There is a 5V regulator IC5 on the 'sweep generator horizontal amp calibrator' schematic (diagram 6). This powers the 74 series logic on the timebase. Check that this regulator is working correctly.

When you checked the switches, can you confirm the 'Trigger Level' switch S3a,b,c and 'Trigger' switch S1a,b,c both had all three of their wipers still in place and making correct contact in all three positions. (I mention this in case one or more wipers may have fallen out of the housing and become lost).

I suggest you read through sections 6.4 and 6.5 of the manual circuit data section to get an idea of how the circuitry works.

If you have a logic probe or another oscilloscope, try the following.

Feed in a signal from the calibrator to Ch1 set to DC, 100mV for example, set the trigger level to Auto, trigger source to Ch1 and see if the trigger amplifier schmitt trigger circuit clocks 7474 IC3b pin 11. This should confirm whether the sweep circuit is being triggered and therefore if the fault is with the trigger circuitry or the sweep/X deflection circuitry.

Ron

[heze1982]

Ron Bryan,

Thanks for your reply!

The 5V line works well.
The wipers of the switches are fine.

The scope has no setting trigger=ch1, so I measured other situations:

When I select the trigger to 'line':
On pin 11 of IC3 I can measure a square wave signal of 50Hz with with amplitude 2,5V (independently of the sec/div switch). Rotating the trigger potmeter results in varying duty cycle.

When I select the trigger to 'ch2' and apply no signal to ch2:
sec/div = 5 us -->zero V signal with every 75us a spike of 2.5V.
sec/div = 10us -->zero V signal with every 130us a spike of 2.5V.
No change when I rotate the trigger potmeter.

When I select the trigger to 'ch2' and apply calibrator signal to ch2:
I cannot get it proper onto the screen of the second (working) scope.
Looks like 0V with sometimes a spike to 2,5V.

The biggest abnormality is the 2.5V. When the logic IC's are running on 5V, the signals have to be the same voltage...?

I will investigate this further, but help is still welcome!

Best regards Hendrik

[ronbryan]

Hendrik

The minimum logic threshold for the 74LS74 is 2V, so a 2.5V spike (from ground) is sufficient to triggger the flip-flop.

What is happening at the Q 'bar' output on pin 8 of IC3b (74LS74)? Is it stuck low, or is it changing state from high to low? This output controls the discharge transistor of the timing capacitors.

My manual is for my 20 MHz D1016A and I did not realise that there were differences when compared to the D1016 timebase; for example, as you pointed out, the functionality of the D1016 trigger selection switch is not the same. (see photo)

Hopefully there are not too many other differences!

Ron

[heze1982]

Apologies for the slow respons...

During measurements I discovered that when I pushed a part of the timebase PCB, that the trace came back so I soldered that area again, and now the timebase is running again!

BUT...

Now I experience a new problem: the Y-amp is not accurate at all (for both channels)! This was already the case before the timebase-error occured.

I applied varying input signals (DC) to the scope and looked what happened on the scope. This are the results:

- The output on the screen is not equal to the input voltage.
- When I double the input, the output on the screen is not doubled.
- Doubling the V/div don't result in an output signal divided by two.
- Both channels have different transfers between input and screen...

Can someone help me further with this scope? I borrowed another scope from a friend, so I can take measurements at request...

Thanks in advance,
Regards Hendrik

The manual I have is also for the D1016A instead of the D1016. So I don't have exactly the right manual, but fortunately the most circuits are the same...

[ronbryan]

Have you cleaned the contacts of all the wafers of the Volts/Div switches? A little DeOxit applied to the contacts with an artists brush (to avoid it going everywhere) and then operating the switch a few/many times is one way of cleaning the switches, although I think Tektronix recommend only Iso Propyl Alcohol to be used on their higher performance scopes equipped with leaf switches. Cleaning the switches may fix most of your problems, other than differences between the channels - re-test to see if successful.

I suggest you measure the output of the calibrator and confirm it is 0.5V pk-pk and then use that as a test waveform instead of DC, as faults in the Y attenuators do sometimes only show up if the input is a square wave (for example excessive overshoot if the frequency compensation capacitors are dry jointed or faulty).

There is a calibration procedure in Chapter 4 of the manual for setting up the gain of the Y amps (Section 4.2.5.1), although it does require a few bits of test-gear. Once you have fixed any faults, this procedure will make the Y deflection of both channels equal. I find it helpful to clearly identify the pots used for the Ch1 and Ch2 adjustments, so that inappropriate adjustments to the wrong channel are avoided.

Ron

[heze1982]

Last night I had some time to further investigate the problem. I cleaned the v/div switches, and unbelievable, so much dirt on the wipers of the switches. Now they look clean again, but unfortunately the behaviour of the scope is the same as before the cleaning...

I don't tink calibration is the right thing to do right now, because the transfer between input to output not defined. Not only the voltage levels are incorrect, the gain is also completely not linear. Setting the v/div switch from 1 to 2 v/div does not results in half the sginal on the scope... And besides that I don't want to mess up the current calibration before I found the other errors, because then I cannot rely on anything anymore...?

What is the next thing to measure? I'm realy puzzled right now...

Thanks in advance!

[ronbryan]

Hendrik

Using the D1016A manual, which is all we have available, have a look at the "CH2 Vertical Attenuator" drawing 2 in the manual. I hope that is the same as your D1016 circuit. There are switched divider networks for the three decades of attenuation and gain switching of the vertical amplifier for the intermediate ranges in the ratio of 0.5-1-2.

For Channel 1 the gain of Vertical Amplifier stage IC601c / IC601d is switched according to the 0.5-1-2 requirement using switch S902-4R in switch positions 6-5-4 respectively. For gain setting '2' (switch position 4) just R611 (1k07 ohms) is in the 'inter emitter' circuit of the differential amplifier, for gain setting '1' (switch position 5) R916 (511 ohms) is switched in parallel with R611 and for gain setting '0.5' (switch position 6) R915 (221 ohms) is added in series with R511.

For the incorrect gain switching from 1V to 2V that you have observed, I suggest you check R916 (511 ohms) and confirm that it is being switched into circuit in parallel with R611 (1k07 ohms).

It would probably be easier to diagnose attenuator and Y amp gain problems if the deflection at all settings of the Volts/Div switch were checked, as the errors may form a pattern according to the switch position.

Ron

[ronbryan]

A correction to the 2nd paragraph of the previous post. Hopefully I have it right this time!

For Channel 1 the gain of Vertical Amplifier stage IC601c / IC601d is switched according to the 0.5-1-2 requirement using switch S902-4R in switch positions 6-5-4 respectively.

For gain setting '0.5' (Volts/div switch position 6) R915 (221 ohms) is switched in series with R917 (15 ohms) with the combination switched in parallel with R611 (1k07 ohms).

For gain setting '1' (switch position 5) R916 (511 ohms) is switched in series with R915 (221 ohms) and R917 (15 ohms) with the combination switched in parallel with R611 (1k07 ohms).

For gain setting '2' (switch position 4) the gain is determined by R611 (1k07 ohms) alone.

For the incorrect deflection that you have observed when switching from the 1V to 2V ranges, I suggest you check R916 (511 ohms) and R915 (221 ohms) and R917 (15 ohms) and that the combination is being switched into circuit in parallel with R611 (1k07 ohms).

It would also be useful to know if the deflection is linear for known input levels when the Volts/div range is not changed. If so, this is more likely to be a problem within the Y deflection amplifier than the attenuator section.

Ron

[heze1982]

Thanks for thinking along with me to find the solution to the problem of the D1016 oscilloscope. After a day of measuring I found all the errors:

- I already found a dryjoint in the timebase section
- After a while of measuring I also found a dryjoint in Amp1
- Half an hour later I discovered also a dryjoint in Amp2.

At that time I discovered that both channels were reacting on the input signal, but the calibration was off.

- I also discovered the adjusting potmeters being off completely (a present of the previous owner I think...). I had to recalibrate both channels and now everything is fine again!

Thanks again!
Regards Hendrik