Tektronics 2213 with no signal response

[highwayman]

I have a Tektronics 2213 that I have got for a good deal years ago.  I am getting back into electronics and have often been mystified by o-scopes.  Mine in particular worked since I got it.  One issue that got annoying was the power button was stuck 'ON'.  So...I open the scope to look at the on-off button.  I made no changes that I am aware.  Just opened the scope.  Realized that I could pull the on-off switch or stiffen the long plastic bar that controls the power.  I re-assemble the scope.

I turn it on.  No power.  Fuse is blown.  I buy extra as I anticipate some trouble.  After I change the fuse, the scope powers up without blowing the fuse.  But the transformer hums much more than before.  The screen show a beam that does not respond to any signal input.  The test point is good per my DMM.  AC, DC, GND does not change the screen.  In my attached picture, the top is CH1 and the bottom is CH2.  Beam intensity does not change the brightness.  The vertical position DOES work and the horizontal moves the left 'start' of the signal.  Focus blurs the signal.  Changing the time changes how fast the signal moves from left-to-right.

Aside from the service manual, any hints on where to start?

[tautech]

  The test point is good per my DMM. 

What do you mean by this? 
Have you checked all LV supplies are exactly to spec?

The Tek troubleshooting guide in the first post in this board can also be a useful companion to the SM.

[highwayman]

Thanks... Looking at the block diagram, I was too focused on A-H blocks in the block diagram and missed the "LV Supplies" on the side.

The test point is good per my DMM. 
What do you mean by this? 

The 2213 has a 100mv 1kHz test point.  I checked that point with a multimeter and confirmed that I had a reference signal that was present (160mv @ 930Hz).

Interestingly, I used a smartphone app and found that my hum is primarily at ~360 Hz.  Off to study the 2213 schematics.  I will  update as more is discovered or a new head-banging situation is encountered.  Thanks again!

[Alex Eisenhut]

Must be a fake scope. The real brand is Tektronix.

[highwayman]

Small update.  Here are the test points for the voltages from the supply.  Hope this saves somebody a little searching.

Point                           Voltage      mV Ripple      Resistance
TP501/Chassis           GND
TP500                       -8.6               10                     114
W985                          5                  10                     330
W975                         8.6                10                     95
W965                         30                 50                     905
W966                         100               200                   12K5

All of my voltages were 70-80% below what they should be.  The ripple was within spec.  All the resistances were good too.
Next step...Go probing in the preregulator with a different o-scope. 

I plan to order one, but have to ask: "How critical is an isolation transformer?"

[highwayman]

Another update ... and a few questions.

With the 2213 on an isolation transformer, I probed the indicated test points.  Since my serial number was below B02XX, I needed to probe the circuit without U920.  Points 26, 29, and 30 were spot on.  Points 27 and 28 were completely opposite.  In the schematic, point 27 is at the cathode of the 10V zener VR914.  Point 28 is on the anode side.  YET, the wave profiles show the peak of 28 to be GREATER than 27.  In my case point 27 looks like 28 in the manual.  Point 28 looks like 28, but with a 10 volt peak.  Based on VR915, point 28 should not exceed 10 volts with respect to TP915.  Error in the manual? 

I am also seeing issues on points 31 and 32.  instead of 80V, I see 60V.  I also see something completely unexpected!  Q940 and Q942 are triggering at 10kHz.  The wave is not square.  It is as if the transformer voltage can't keep up to achieve 80 volts.  The oscope did start making a higher pitch hum when all this started...

With all of that said...

I 'sense' that Q948, Q954, and Q956 are triggering the signals at points 31 & 32.  I do not see how the oscillation is happening or know enough to know if it should be 10kHz.

Considering VR915 (10V Zener) in series with VR914 (10V Zener), shouldn't point 27 have a max of 20V?  Wouldn't point 28 have a max of 10V?

UPDATE: As often seems to happen after I post anything..I come across a partial answer.  Section 3 discusses the theory of some of my circuit.  The 10kHz is expected.  The manual references 20kHz triggered by T940 and T942, but it is likely referencing point 33.  I will try to upload a screenshot of points 27 & 28 if I can.  The voltage across my C937 is 30V instead of the expected 42V.  Current Limit Board?

[DmitryL]

This version of 2213 (not 2213A) uses a lousy triac-based pre-regulator to make 40-ish volts from mains. Using isolation transformer can drive pre-regulator nuts.

Also, because of this triac thing and recovery processes in mains rectifier diodes it is possible to have nice and strong 100Hz spikes all around the circuit. It is especcially serious when you power it from 220V mains. In my case these spikes interferred with a trigger circuits. But this is another story.

[highwayman]

I inspected the current limit board.  The F937 fuse was blown.  Q938 had a resistance <8 ohms between the gate, source, and drain.  I even swapped leads to account for any integrated diode.  I replaced the fuse and the IRF730 (Q938) and the oscilloscope started to power up...then F937 blew again.
Before, because Q938 was shorted out, the scope continued to run.  This time, the scope shut down, but not immediately.  It went from lower to higher pitch like it was starting up...then nothing.

Before I start buying fuses by the dozen, any pointers on where to probe while the scope is powering up?

Pointer: For those near an Ace hardware, I was surprised to find that they stock various types of fuses, but not in quantity.  It is usually just a few individual fuses of different sizes.

[highwayman]

This oscilliscope has been on my bench taunting me for months.  I either need to fix it or sell it for parts.  Since it worked fine until I opened it to investigate the power switch, I can't let go of the idea that it can be fixed.

One thing that I think may be 'it' is the T940 frequency.  It should be 20kHz.  Instead, it is 10kHz.  I have isolated the page, but I just don't see why it would oscilate at half the frequency.  Some sort of interaction between Q940, Q942, and T942.

I am hoping that something blindingly obvious is the issue that I may have missed.   

[singapol]

You give up too easily...how to gain experience?

If the test points don't tally then suspect everything but through experience you will zero in on some components like electrolytic caps..why? When they have aged and are old they delvelope certain peoblems like esr (equivalent series resistance) and cap values not to spec. Examples like C951 10uf, C945 2.2uf. C947 33uf., C957 1uf.,or C917 2.7uf.  Either esr or cap value or both are out of specs.

[james_s]

Did you unplug anything to get to the switch? Any missing screws? Loose wires? Missing insulating spacer or anything like that you might have disturbed? Does it have a 120/240 voltage adjustment switch on the back? Did you knock a component loose or bend one over and short it on something when messing with the power switch? The humming power transformer and low voltages sound like there might be a short on one of the power supply rails.

[oldway]

I have repaired a lot of Tektronix 22xx and the first thing I always do is not to power it on but to first test the inverter and all the scope functions by feeding C940 in 2235/36 (C957 in your 2213) by an external power supply with 42V and current limited to 2A.
Current consumption must be between 1 and 1.5A.

[cnqhdszq]

Don't give up ! I had spend 2 years spare time on a old ACR8501  LCR (AIM 401 ).

The end is  :  repaired !  the only bad component is a resistor  10 Ohm turn to 70 Kohm  ?It cost me  2 year.   but I grasped it