Tektronix Oscilloscope problem

[sido]

Hi Wm6. a bit of update. I got a response from the Tek forum and i am being pointed to the vertical amplifier section also

Here is a quote from a fellow on the Tek forum.....

"Assuming all the low voltage points are within specification then you next suspects are in the vertical output amp section ...starting with Q254-Q257 and working backward. IF any of the low voltage points are out of spec those must be dealt with first"

So there seams to be a problem there wm6 What i have noticed so far is the measured voltage between r266 and r271 at the 6 x 150 ohm resistor voltage divider network. +30v is present in the input of the divider network, but the upper vertical deflection output voltage is +21.7 volts and the lower is +15.1v with unit turned on...They should be both +8.8v. The supposed +8.8v X 2 measured voltages go to the CRT, but do not look to good so far.

The measured on board resistance between each resistor 150ohm's x 6 are all 141 ohms. I think its within spec because me multimeter might not be that accurate at so low ohms readings.

I have done other tests on the sub boards from the front panel and all are good. so it looks like i am in the right part of the main circuit board.

Hopefully i get it going soon. I intend on refurbishing the power supply section to new updated equivalent parts when all is working ok so i get longer life and hopefully problem free for years to come.

Cheers Sid

[WM6]

I have posted the fault on the Tek forum. Waiting for reply's.

Good choice, sido, they should know better than we here.

But one more suggestion. If all with power supply is ok and you can not get satisfying calibrating signal on screen, then we need to go back to Y-Amplifier stage.

There is a circuit (or IC chip) in vertical amplifier stage that commutate both chanel signal together (something like in past popular DIY circuit called commutator that convert one-channel scope to two or eve multi channel scope - the same functionality). Try to search faults (to measure voltage first) around those commutating circuit (IC).

[sido]

Hi Wm6

Are you tried to connect scope calibration signal (from scope calibration output) to both of scope channel input? It is a square waveform signal. After connecting try with different setting of Volts/Div knob to se what happen. Read manual first.

Yes i connected internal calibrated square wave signal. It is present but very small amplitude and varying the volts/div i cannot expand signal on display.

You say power supply is ok and that you measure all voltages. Does this mean that you measure all voltages on power supply output according service manual? If not, then do this step.

I measured all power voltages in power supply section as per Service manual. all are within Spec.

You can try with disconnecting all other electrical consumers in the house and the test scope on safety switch trip, or try to connect your scope power cable to other mains power point in house and see what happen.

Tested and everything is good.

I have posted the fault on the Tek forum. Waiting for reply's. In the meantime i will check all other low power voltages to each sub-section's as per schematic. The schematic's have other low voltage check points and hopefully i can narrow it down to a particular area in the circuit.

Thanks for your help Wm6.

Cheers sido

[SVEN1]

Here's the Tektronics user group worth joining, 6200 members and everything you ever needed to know on servicing your Tek, along with parts sources, etc.

http://tech.groups.yahoo.com/group/TekScopes/?yguid=334758051

Very active group average 1400 posts a month.

[WM6]

Yes, vertical position circuit (or Y position circuit) belong to vertical amplifier.

Are you tried to connect scope calibration signal (from scope calibration output) to both of scope channel input? It is a square waveform signal. After connecting try with different setting of Volts/Div knob to se what happen. Read manual first.

You say power supply is ok and that you measure all voltages. Does this mean that you measure all voltages on power supply output according service manual? If not, then do this step.

All parts are there for reason, so you need to replace faulty capacitor with new good one.

Regarding safety switch, we now know that reason to safety switch trip was in filter module.
But, not necessary, that this module is someway faulty. Safety switch is working on difference between in-coming and out-coming current (sense possible leak of current to protective ground). Power filters need some leakage current to work properly and can put mains installations conditions at safety switch current limits (about 30 to 300mA depend on factory switch settings) which can be "deceived" trough filter back EMS on scope switch OFF in connection with filter capacitor discharging (so delay in your safety switch trip).

You can try with disconnecting all other electrical consumers in the house and the test scope on safety switch trip, or try to connect your scope power cable to other mains power point in house and see what happen.

[sido]

Cannot adjust to here.

Cheers Sido

[sido]

Hi WM6,

I snipped the faulty capacitor and the unit fired up as in the picture below.

I measured all voltages as per schematic below and all are spot on. The only voltage i did not measure is the 2Kv going to the CRT as i do not have a meter capable of do so, but seeing the CRT works, it must be ok.

I tried the switch off, like you suggested from fuse. The safety switch did not trip.

I have the unit running now for 2 hour and power supply seams ok.

I Still have a problem as i cannot get the baseline to center in the middle of both channel traces on the display and it seams that the input channels are not responding to a input signal source? I can only adjust both channels one graticule division only (as marked in red in picture).

I measured the horizontal amplifier regulator 79l05 for a input of -8.6v which is present for a output of -5v as per schematic. So that is working ok.

I cannot adjust both trace's to be in the center maybe because the vertical amplifier has a problem? Not sure where else to check.

Would that not working capacitor i snipped be a problem? I doubt it.

I have had a look at the schematic, but not familiar in operation.

Cheers Sido

[WM6]

Slip - Correction: not PID but FID switch.

[WM6]

Or do you mean that if the safety switch trips, then i should check the filter section for faulty parts in case neutral power wire is shorting to earth?

Cheers Sido

It can be faulty part or too high humidity in isolating parts (due long storage) in connection with back EMS occurrence inside filter module.

After long storage it is advisable to adopt such device to working conditions in warm and dry place a couple of days.

But PID is very sensible switch and some other reason, not directly connected with possibly faulty scope parts, can occur also.

[WM6]

Do you mean remove fuse, then apply power with switch on? to see if the safety switch drops? If not then check this filter section with power disconnected for faulty parts?

Cheers Sido

No, first apply power switch ON (and left it at ON position) and then "switch OFF" scope by scope fuse removing only.

If after about 3 sec PID switch not dropped, you need to check filter module (section between power cable connector on scope housing and scope power switch) for faults.

Of course if filter board is not potted with epoxy resin.

[sido]

Or do you mean that if the safety switch trips, then i should check the filter section for faulty parts in case neutral power wire is shorting to earth?

Cheers Sido

[sido]

Sorry about the picture quality. Hope this is much clearer.

Yes the house safety switch is of PID type.

Do you mean remove fuse, then apply power with switch on? to see if the safety switch drops? If not then check this filter section with power disconnected for faulty parts?

Cheers Sido

[WM6]

You have filter between switch ON/OFF and cable and this can cause main house safety switch turn off. Main house safety switch work on a little different way than usual mains fuses (of course, if we are talking about same thing here called PID switch).

You can try this:
switch scope ON and then take main scope fuse out of the scope (without switch OF using scope main switch). In this case main house safety switch should survive, which mean that cause is to find between scope main switch and power cable connector on scope housing (mean in filter module).

Please, post pictures of schematic in a little better resolution.

[sido]

Hi WM6, i assume you mean to have the power switch in the "ON" position without power connected and measure the resistance across active and neutral.

Well i did so and there is no short showing up either active to neutral, active to earth or neither neutral to earth.

Still wondering why the Oscilloscope fuse did not blow under the excessive load generated from the shorted capacitor??. I turned off the Oscilloscope power switch, with power cable still connected to power, but a few seconds later the house safety switch turned off.

Maybe the slow time it took the excess current draw took to build up in the scope caused excess heat to the power cable on the AC wall plug which happens to be a big run of cable to the fuse box at the front of the house. You would think that the safety switch would have activated well before i switched the unit off.

At least we can say that is a good sign, no short on input power.

I am eager to snip that Cap lead, but will have to wait till tomorrow.

Cheers Sido

[WM6]

Interesting thing happened though......Upon switching unit off, 3 seconds later my house safety switch tripped and i lost AC 240 Volt to my power points.....HOW Can that be?

Look at schematic for Switch ON/OFF position. It would be redundant to measure resistance on your scope power cable at switch ON position (not connected to main power point of course) first. Check on possible shortage at main input, you have to measure value of resistance of primary windings of power supply main transformator.