Just can't fix: tek 535a

Paul Han · **Posted:** Apr Mon 16, 2012 4:00 am

Hi,

The vertical signal output gives a clear square wave, but the display has no sweep and shows only a dot. All power supply voltages are good, most of the caps and electrolytes have been replaced. The horizontal stage is not creating the sawtooth wave forms that are supposed to be connected to the left and right deflection plates, and the def. plates are only charged to about 100 and 200 volts. All sockets have been cleaned.

I'm stumped

Retired Radio Man · **Posted:** Apr Mon 16, 2012 5:03 am

At minumum you need a schematic. A maintenance manual would be better. Then you study the circuits until you understand how they are supposed to work. Once you understand the functions you check voltages and signal trace as you would a radio or any other electronic device.

RRM

Paul Han · **Posted:** Apr Mon 16, 2012 5:15 am

I have bought the original service manual. I know that the power supplys are working and the horizontal amp is not working. I would like some help on repairing the horizontal amp

pehamel · **Posted:** Apr Mon 16, 2012 6:35 am

If you have only a dot, then the vertical amp is dead also (you say the vertical output signal is there).
It may be easier to troubleshoot the vertical channel looking for a lost voltage (which is probably common to both horizontal and vertical).
The horizontal circuit may be a phantastron which was always a challenge.
Good luck, and keep one hand in the back pocket.
Pat

arbilab · **Posted:** Apr Mon 16, 2012 7:20 am

How can the vert output be a square wave and the display be a dot?

Johnnysan · **Posted:** Apr Mon 16, 2012 9:47 am

You say that the power supply is working, so I assume you have the 100, 225, 350 and -150 volts called for.

Horizontal sweep can be stopped by a break in that circuit at many points. Since this is an all-tube scope, the tubes throughout the horizontal circuit need to be checked. You may just have a dirty tube socket or sweep timing switch.

If the scope was non-functional when you got it, someone may have been in there before you. Are all the tubes correct in their sockets? Has someone been resoldering?

You must resist the urge to move internal controls until you know the circuits are working. I have worked on many scopes that took a few minutes to find the problem, but hours to re-calibrate.

Tubenut · **Posted:** Apr Mon 16, 2012 9:54 am

Hi Paul, when you talk "square wave" do you mean the calibrator output. The vertical should have no signal unless you are feeding it with one. The Horizontal should have a saw to sweep the trace across the screen. If your vertical is working with no horizontal deflection. You will get a vertical line when you give the scope a signal.

Mikeinkcmo · **Joined:** Oct Sun 15, 2006 12:57 pm **Posts:** 3174

Are you certain the problem is the Horizontal amplifier, and not the sweep/ramp generator? If you can see dots at the left edge of the display I suspect the horizontal amp might be OK. Are you sure you have the Trigger controls in the proper positions? Do you have a second scope to aid in trouble shooting?

You might want to take a look at this TEK trouble shooting manual

Paul Han · **Posted:** Apr Mon 16, 2012 5:28 pm

Hi,

Sorry, I should have given more details.

I have been using a hp 1740a scope

The scope is set up according to the test procedure found in the maintenance manual.

The scope is connected to the square wave calibrator. I have checked the output of the calibrator

The dot is controllable by the horz and vertical controls

The horz control is unable to move the dot fully to the right

The vertical signal output is working, checked by my scope.

Johnnysan · **Posted:** Apr Mon 16, 2012 6:12 pm

I don't think your calibrator has anything to do with horizontal sweep.

Tubenut · **Posted:** Apr Mon 16, 2012 6:44 pm

Lets start at the plug in. Do you have another "plug in" you can try. (box that inserts into the face at the bottom left corner) You do have a plug in, I hope. If your "plug in" is faulty, this will also cause these symptoms.

Paul Han · **Posted:** Apr Mon 16, 2012 7:18 pm

Hi,

I am using a CA plug in.

I connected the calibrator to the input of the ca, in accordance the manual, and I know the ca is working because I get the calibrator square wave at vertical signal out. So something in between the trigger pick off and the CRT s wrong

Johnnysan · **Posted:** Apr Mon 16, 2012 8:33 pm

The biggest problem that you have is no sweep. You need to solve that first.

Paul Han · **Posted:** Apr Mon 16, 2012 9:34 pm

In the horizontal amp, is B386 (NE-2) used as a oscillator with v384B (6dj8)? Mine looks blackened and shows a sign of sputtering so I was wondering if that being faulty could cause a lack of sweep.

Thanks,

Paul

Tubenut · **Posted:** Apr Mon 16, 2012 10:06 pm

The bulb is probably gettered. I have a drawer full of gettered neon bulbs. I have two 547's, two 541's, two 545's, a 524AD, a bunch of 531's I think ..... A lavoie Tek equivilant etc....But I sold my 535 some time back.

I was thinking I could look in the bottom to see if I could see a gettered bulb, but the scope is gone. At any rate, you should see a saw in the hor time bases. Have you tried both time bases A and B? Are the they both not functioning? Have you checked the Voltage's in the supplies listed on page 4-15 of the service manual? Oh ya, by sputtering do you mean flashing? If so, the bulb will visually flash at slower sweep rates, which would mean the oscillator is working. How the bulb works....The voltage will climb until the bulb ionizes, at his point, it conducts and drains the timing cap, and the cycle starts over. This is how the ramp is created. Function: When the ramp starts at the bottom, the trace is at one side of the screen. As it climbs the hill, the trace moves across the screen. When it's at the top of the hill the trace is at the opposite side of the screen. When the ramp falls to the bottom again, the trace starts at the other side of the screen again and the cycle starts over.

Mikeinkcmo · **Joined:** Oct Sun 15, 2006 12:57 pm **Posts:** 3174

Paul,

Try this, connect the calibrator out, at about 5 volts or so, to the external trigger input, set the trigger source to external mode to Auto trigger, and see if you get a line across the CRT. Are the little Neons above the CRT blinking?

Do you have the wave forms shown in Fig. 1-1 on page1-4? Most important is "sawtooth A", if you don't have that, the timebase isn't running, whether its due to triggering or something else is to be determined. Have you looked at any of the wave forms shown on the schematics?

grid2 · **Joined:** Jun Sun 14, 2009 10:44 pm **Posts:** 102

Perhaps this may/not have bearing: Recently on 585a that I'm working on, I had the symptom of no-sweep and a dot (2 dots if piping in a sqare wave) that could only go from left of screen to the center-line using the horizontal (left/right) control.

After scratching my head and going through the manual troubleshootings steps to no avail :cry:

the problem turned out to be: Bad Tube Contacts!! I powered up and took a really good looked inside and saw some darkish tubes. They were also cold to the touch! I pulled the tubes, deoxit the socket and pins - then reinserted. Powered up and then wiggled to make sure the heaters came up. Yep - that fixed the problem! And they indeed were the tubes in the horizontal amplifier (6cl6, etc. that mount on the underside of the top chassis - which you can't really see well unless scope is on the side and the door is open).

In hindsight, this stands to reason as the scope had been stored for some while w/o tubes in the socket.

Dave Wise · **Posted:** Apr Tue 17, 2012 10:26 pm

The 530/540-series Teks use neon bulbs, but not as oscillators, only to obtain voltage drops, or protect tube grids from transients. The timebase(s) use a pentode tube wired as a "Miller run-up" circuit. The plate is capacitively coupled back to the grid, and a negative source drains current from the grid. This cause the plate to rise, which supplies a balancing current through the cap. Except for the small change in grid voltage from start to finish, it's a perfect ramp. The rate (dV/dt) is I/C. There's a cathode follower buffering the plate, with two neons in between, one dropping the voltage, the other protecting the buffer's grid.

B386 is a grid protector, not an oscillator. It should never be lit except while the power supply is going through its warm-up delay.

Dave Wise

Tubenut · **Posted:** Apr Tue 17, 2012 11:52 pm

Dave you are correct about the Miller circuit being the oscillator. In this case the Neon Bulb's are part of the Miller runup circuit and will ionize. The description is on page 4-5. In order for the bulb to aid the circuit (B167), (B171), it must ionize in this application. In older scopes, the 884/885 or just a simple neon is all the sweep needs to create the saw. (in this scope the circuit is a little more advanced in order to get fast sweep speeds.) The simplified schematic is below.
Good Luck

Just can't fix: tek 535a

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