577 Curve tracer ringing CRT and noisy step generator


DW
 

With the variable collector at 0 or very near 0 there is a ringing sound, I listened carefully and the ringing is coming from the CRT, weird. With the ringing sound there is a interference pattern with the trace near 0 where the solid line becomes a speckled line. With the higher collector voltage everything is fine, CRT stops ringing and the speckled trace becomes solid.

Another symptom I have is the step generator. When switched to repeat there is a flickering pattern around the trace that occurs at the same rate as the step generator, noisy step generator basically causing some trace interference when switched to repeat. Switching step generator polarity reduces the interference where it is not as bad but is still there.

Some thoughts I tried is re seating some IC's and ribbon cables. I noticed some trim pots on the step generator board, maybe they are dirty and I can work them around noting their position. Perhaps the power supply board at the rear of the instrument could have it's capacitors checked but I am not sure if power supply ripple would cause the CRT to ring with 0 or near 0 collector voltage and a noisy step generator.


DW
 

I posted pictures of the symptoms the step generator is causing, you can find this in the photo library of this group under Tek 577

With no load, variable collector at 0, display set to magnified mode, collector supply set to AC 6.5V, notice the left and right elliptical patterns on the display, picture was taken in a dark room


DW
 

I will check the power supply for ripple and invest in a LCR meter to check the capacitors. It seems they make 100 kHz LCR meters you can check capacitors in circuit, maybe I will take a look at that.


DW
 

Power supply measurements not bad

+12V measured +11.98DC 0.001AC ripple
-12V measured -11.73DC 0.001AC ripple
+200V measured 211.4VDC 0.386AC ripple
-30V measured -29.97DC 0.001AC ripple
+30V measured +30VDC 0.001AC ripple


chipbee40
 

Having repaired several of these, apart from PSU problems (low value caps, o/c diodes etc), most problems are with the really bad IC sockets, which in worst case you will need to replace. Other than that the op-amps can be just weird, many faults can be cured by replacements. I had one that had uncurable glitches on the trace and added 0.1uF caps across the power rails right on some of the logic chip power pins, can't remember which, sorry. One had a collector transformer breaking down, eventually burned out, not fun. That was fine until collector volts turned up...........then arcing.


DW
 

Thanks, I will definitely keep that in mind.

I tried exercising the pots on the step generator board, no luck. I might try replacement IC's as now that I think about it I could have a partially failed / noisy socketed IC.


DW
 

https://groups.io/g/TekScopes/photo/86536/3?p=Name,,,20,1,0,0

I included a diagram of the step generator, check the link above for review

I found these socketed IC's on the step generator board
U570 Microcircuit Li Low input offset current
U530 Microcircuit Li Low input offset current
U520 Microcircuit Li Low input offset current
U380 Microcircuit Li Operational amplifier
U350 Microcircuit Li Dual operational amplifier
U360 Microcircuit Li Operational amplifier
U430 Microcircuit Di Quad 2-input Pos Excl Gate
U268 Microcircuit DI 2 input and or invert gate
U270 Microcircuit Di Monostable Mv TTL

I am considering replacing the op-amps and keeping the old ones around. When you say bad IC sockets do you mean loose, broken, or corroded pins? Thanks


DW
 

Here is a nice website to look up Tek part numbers

http://vernonianorthern.com/TekParts/Tek156Page.php


Dave Casey
 

The bad IC sockets are due to corrosion plus a design flaw. The sockets in
question were made by TI and are used very heavily in Tek gear. They
contact the IC pins on the narrow edges instead of the broad edges and thus
contact area is significantly reduced and more susceptible to corrosion. If
you have these sockets and they are showing signs of corrosion/high contact
resistance, it is advised to replace them.

Dave Casey

On Tue, Mar 12, 2019 at 8:55 PM <wilson2115@outlook.com> wrote:

https://groups.io/g/TekScopes/photo/86536/3?p=Name,,,20,1,0,0

I included a diagram of the step generator, check the link above for review

I found these socketed IC's on the step generator board
U570 Microcircuit Li Low input offset current
U530 Microcircuit Li Low input offset current
U520 Microcircuit Li Low input offset current
U380 Microcircuit Li Operational amplifier
U350 Microcircuit Li Dual operational amplifier
U360 Microcircuit Li Operational amplifier
U430 Microcircuit Di Quad 2-input Pos Excl Gate
U268 Microcircuit DI 2 input and or invert gate
U270 Microcircuit Di Monostable Mv TTL

I am considering replacing the op-amps and keeping the old ones around.
When you say bad IC sockets do you mean loose, broken, or corroded pins?
Thanks




DW
 

For U380 (installed NEC C1458C) U350 (installed 156-0158 RCA) U305 ((installed 156-0158 RCA) I will be replacing with MC1458P from digikey

As for U360 (installed 1560067-02) this proves to be a more difficult to find IC however I did find VA741TC from the website listed above, if anyone has any advice please let me know.


chipbee40
 

U360 is a plain vanilla 741 opamp


DW
 

Thanks

Also I checked and I definitely have the TI IC sockets with the narrow pin inserts, I will look them over

And a thank you to all who commented here, your input is helpful and appreciated


DW
 

296-1403-5-ND‎ x 3
LM741CNNS/NOPB-ND‎ x 1

I believe I will give this a try from Digi Key

If this doesn't work I might look at replacing all the socket for the IC's on the step generator board


chipbee40
 

Have you worked through the adjustment procedure, checked the waveforms against the manual? I usually find any faults/oddities that way, The step generator waveforms should be very clean. Getting the sockets out is very time consuming, I avoid it if possible. Some Deoxit on the ic's and a couple of insertions cures almost all but the worst sockets. Also deoxit into the pushbutton switches cures a lot of problems.


DW
 

Good idea, I have an oscilloscope, I will put it to good use


DW
 

https://groups.io/g/TekScopes/album?id=86536

I tested a Toshiba A1962 PNP transistor to assess the overall condition of the step generator. It works as you can see from the link, however the trace becomes garbled with lower step sizes of the generator and higher vertical sensitivity. I am using 15 inch clip leads from the test fixture so that is probably not helping. Finally I have a strange reading when I set the step generator to 0.05 and 0.1V


DW
 

https://groups.io/g/TekScopes/album?id=86536&p=Name,,,20,1,0,0

Updates to the photo gallery, probing the base of the 177 test fixtures reveals small parasitic spikes, I believe this is what is causing the garbled trace I am experiencing, now to trace where that is coming from


DW
 

https://groups.io/g/TekScopes/photo/86536/11?p=Name,,,20,1,0,0

Probing pins 2 and 6 of U220 reveals square waves that are not symmetrical, should I be concerned and is there a adjustment


Rick Doersch
 

The looping you are showing on the pictures with transistor curves are normal for all 577 and all other analog curve tracers. They increase as you decrease drive and increase vert sensitivity. Here is a post that talks about the reasons for it. The reason you don't see it on the fast setting of generator is it makes 2 steps per half wave of collector supply. One on rising side of of wave then trace is blanked and another on the falling part of wave.

https://groups.io/g/TekScopes/message/152053?p=,,,20,0,0,0::relevance,,577+looping,20,2,0,27803072


DW
 

A big thanks, that certainly answers the loops in the transistor trace in which there is nothing actually wrong with the instrument or transistor

Now that just leaves me with the spiking interference when I turn on the step generator

When set to repeat and the selector switch to off on the fixture and the collector at 0 volts a little spike appears on the right or left of the dot, it flickers at the rate the step rate is set at. If I push single I see the spike briefly appear and pushing it again the spike appears. My best assumption so far is I have a leaky IC or bad socket which I hope not. I bought some op amps which I will replace in hopes of fixing the noisy spiking step generator.