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Tektronix 465,NO Trigger
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john@...
Greetings All, I recently purchased a 465 on Ebay UK, having now cleaned it up it's looking good, It seems to be working with a good sine wave on both channels from the calibrator, but the only way I can trigger the waveform is by very careful adjustment on the TIME/DIV control and the little red Knob in the center, I don't have a Trigger light ( the lamp in OK) so I presume that I have a Trigger fault. I have checked all the voltages and all are in spec.
Although I am fine when repairing Ham Transceivers I know nothing about Scopes. Can any kind expert give me some suggestions as to what to test first ?? Thanking you in advance John mi0dfg |
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Hello John,
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As a general rule of thumb, a thorough check of the Low Voltage power supplies, both for voltage on spec and ripple are both important and yet, an easy check. Not just because problems at the power supplies may cause, at times, unpredictable problems that are hard to make sense of (even odd ones such as everything working BUT syncing), but also because finding an off-spec power supply that has not yet caused a serious damage, can save you from watching the inevitable magic smoke, and the need for a much more complicated repair. Having made sure the power supplies are healthy, the next step (in your case, where the scope seems to be working to a great extent) is to get acquainted with the operation of your scope, to be sure that the position of the controls are all correct for the operation that you're attempting and for the signals you're applied. More specific to the symptom you mentioned, here are the basics... 1. Exercise all selector levers, push-buttons and pots a dozen or more times... to make sure that you cleared any false contact that may be present, due to oxidation of contacts, or dried lubricant crud. 2. Make sure Horizontal Display mode ganged push buttons is set to A sweep (button "A Lock Knobs" pressed), 3. That the A and B Time/Div dials are locked (that both the clear dial skirt and the gray knob are aligned and moving together) and a suitable sweep speed is selected. 4. Trigger mode ganged push buttons is set to AUTO (button AUTO pressed). 5. That the A Trigger level pot is set to middle range. 6. Vertical signal level has some healthy 4 or 5 graticules in height (and that you use only one channel (for now) 7. A trigger coupling lever is set to AC 8. A trigger source lever is set to NORM. From this starting point, if it doesn't sync right away, then there must be some issue at play... and it will become a matter of narrowing it down, by fiddling with the controls, one at a time, in a combinatory fashion... For instance: A. From the starting point above, try to move (slowly) the A trigger level pot, from CCW to CW. If no avail, put if back to center. B. Try to change the Source lever, from NORM, to the specific channel where you have your vertical signal fed to (CH1 or CH2), and repeat sweeping the A trig level from full CCW to CW. C. If still no avail, change the Triggering coupling from AC, to HF REJ, to LF REJ and DC... for each possible coupling, try the steps A and B above. D. If neither work, use a BNC "tee" and a coax cable, and try to feed the same vertical signal to the A EXT input, in combination with changing the the Source lever to "EXT" or "EXT÷10". At this step, try playing with all combinations of the Coupling lever while sweeping the A trigger level pot from CCW to CW. E. If neither work, then you can still try to feed a vertical signal from the AC mains (or the output of an AC transformer), select an appropriate sweep speed and change the Source lever to "LINE". While at this step, exercise again the Trigger coupling lever and the A trig level potentiometer... As an extra resource of information, while at each of the lettered steps above, you can try to press the trig view switch. Under normal conditions, on a working oscilloscope... while pressing the TRIG VIEW push button, you should see on the screen (instead of your Vertical signal), a "fac simile" of the triggering signal that is coming out from the A trigger amplifier, past all the coupling, source and level controls....It should **normally** display much larger on the screen than your original vertical signal is showing. While you press the TRIG VIEW button, rotating the A TRIG Level potentiometer should make the signal cross the screen from top to bottom (while turning the A trig level from CCW to CW). If you see the trigger signal while pressing the TRIG VIEW button, and if it reacts to the A trig level control, and the scope still doesn't sync, then you have a problem on the triggering circuitry past the triggering pre-amplifiers. If you don't see the trigger signal while pressing the TRIG VIEW button, or it stays stuck at either the top or the bottom of the screen, then you have a problem on the trigger input circuitry or the trigger pre-amplifier. Hope this helps you move further... Rgrds, Fabio On Thu, Aug 16, 2018 at 05:38 PM, <john@...> wrote:
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Harvey White
On Thu, 16 Aug 2018 10:18:01 -0700, you wrote:
Greetings All, I recently purchased a 465 on Ebay UK, having now cleaned it up it's looking good, It seems to be working with a good sine wave on both channels from the calibrator, but the only way I can trigger the waveform is by very careful adjustment on the TIME/DIV control and the little red Knob in the center, I don't have a Trigger light ( the lamp in OK) so I presume that I have a Trigger fault. I have checked all the voltages and all are in spec.Triggers are derived from the appropriate channel, or externally. They're processed, and that signal is used to trigger the sweep. AUTO triggers the sweep at a fixed rate in the absence of a trigger. If it works on auto, then the sweep should be able to be triggered. That you can trigger, but only at a specific spot, suggests that the trigger amplitude is *really* small somewhere. This is a case where you'd love to have a scope to test the scope. I'd get the manual, then look at where the signal comes from the vertical amplifier (there's a switch in there). Start tracing the signal from the vertical amplifier through the trigger circuits, observing voltages and voltage limits on controls. If everything is OK, and you get the right signals, then check the input to the sweep generator, that might be bad. Harvey Can any kind expert give me some suggestions as to what to test first ?? |
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"It seems to be working with a good sine wave on both channels from the calibrator"
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??? Is this the calibrator on the scope? If so, it should be a nice square wave. Are you using a scope probe? Regards ----- Original Message -----
From: <john@...> To: <TekScopes@groups.io> Sent: Thursday, August 16, 2018 1:18 PM Subject: [TekScopes] Tektronix 465,NO Trigger Greetings All, I recently purchased a 465 on Ebay UK, having now cleaned it up it's looking good, It seems to be working with a good sine wave on both channels from the calibrator, but the only way I can trigger the waveform is by very careful adjustment on the TIME/DIV control and the little red Knob in the center, I don't have a Trigger light ( the lamp in OK) so I presume that I have a Trigger fault. I have checked all the voltages and all are in spec. Although I am fine when repairing Ham Transceivers I know nothing about Scopes. Can any kind expert give me some suggestions as to what to test first ?? Thanking you in advance John mi0dfg |
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john@...
Thanks Harvey, all suggestions are very helpful as I said before I am useless when it comes to scopes and it a big learning curve!!!!
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From all the tests I have made there seems to be a complete lack of any trigger action, and No trigger lamp indication, will carry out the tests you have indicated and once again thanks for taking the time to offer help 73 John On 17 August 2018 at 00:18 Harvey White <madyn@...> wrote: |
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John Stoole <john@...>
Thanks for the very comprehensive reply Fabio I will start to carry out the suggestions you have made, and will let you know how I get on
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73 John On 16 August 2018 at 22:02 Fabio Trevisan <fabio.tr3visan@...> wrote: |
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Hi John,
No problem about the time... It's a relief for my mind to solve those puzzles. Some comments further down, just after your answers... Regards, Fabio On Fri, Aug 17, 2018 at 07:16 AM, John Stoole wrote:Hmmm... Both things above together, may mean that the triggering signal, through the triggering input and pre-amplifier stages may be so much offset in D.C. that the triggering signal coming out of the pre-amplifier into the triggering discriminator (Tunnel Diodes or Tek Custom IC yet to be clarified) is never crossing the 0V (which is a must, for the trig. discriminator to operate). **Please try this one more thing** While pressing the Trig View, press also the Beam Finder (you should see the trace either stuck near the top or the bottom of the screen). While keeping both pressed, move the A Trigger Level, to see if the trace shows any tendency to move towards the center of the screen... This will give you some hints if the trigger preamplifier is really stuck at some DC level, or if it still reacts to the A trig level pot. 2. If I switch the A COUPLING to DC, then as I step the SOURCE switch from NORM to EXT10 the trigger lamp flashes proving the lamp is OKThis is good! It gives us some indication that the triggering discriminator is working... It's probably detecting the triggering signal crossing 0V, when you're changing selections, which is probably causing some change in the DC levels at the input stages... 4. the TRIG HOLD OFF control works well and helps to lock the waveform when movedExpected... The Sweep control logic seems to be working, otherwise there wouldn't be a trace whatsoever. 5 Tried 5 v AC into the external trigger but that would not sync the waveformThis indicates that whatever the problem is, it's probably at or after the trigger pre-amplifier input selector (e.g. it sort of rules-out that your problem is at the pick-off circuitry). In time, the pick-off circuitry are the pieces of circuitry of the Vertical pre-amplifiers that "taps" the vertical signal from Channel 1, Channel 2 or from after the channel Alternating circuit (the NORM pick-off) and routes it to the trigger preamp. If the problem would be at the pick-off circuitry (only), you would still be able to trigger from an EXT input. the rest of the scope seems to work very wellTo further help you from this point on, we will need to know: A. What kind of Trigger pre-amp board your 465 has. I`m not even sure if this "doubt" applies to the 465, but I used to have a 464 and I know that (for the 464 at least) there were two versions of Trigger boards, one that uses Tunnel Diodes, and another one that uses a Tek custom IC (as the triggering signal discriminator). Many thanks again for the help |
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John Stoole <john@...>
Hi Fabio, I carried out the following test .....
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While pressing the Trig View, press also the Beam Finder (you should see the trace either stuck near the top or the bottom of the screen). While keeping both pressed, move the A Trigger Level, to see if the trace shows any tendency to move towards the center of the screen... This will give you some hints if the trigger preamplifier is really stuck at some DC level, or if it still reacts to the A trig When pushing the Trig View, /Beam Finder, I get a short trace near the bottom of the screen, but moving the A Trigger Level, has no effect at all John On 17 August 2018 at 13:38 Fabio Trevisan <fabio.tr3visan@...> wrote: |
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Hi John,
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Ok... So it seems there's a strong DC offset pushing/pulling the A Triggering pre-amplifier way out-of-range. There can be several causes to it. My own 464, for instance, had a defective FET on the Tirggering pre-amplifier input buffer. From this point on, I can't guide you much further without knowing what version of Trigger board you have... So that I can grab the correct schematics and give you objective instructions of where to try to measure things up. If it's convenient for you, take a picture of the board and send me PM, and I can post the picture on the Tekscopes photo area. Rgrds, Fabio On Fri, Aug 17, 2018 at 01:17 PM, John Stoole wrote:
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John Stoole <john@...>
Good Morning Fabio,
I have carried out the tests to the U640 as you suggested for both the A trigger and the B trigger here are the results A TRIG 14/15 20.4 mv 2/3 -7.9 volts !!!!!!!!!?????? B TRIG 14/15 100.7 mv 2/3 42.6 mv I noticed that all of the voltage tests in the Service Manual are carried out with the NORM button pushed, so the tests above had the NORM button in. I think I may have a problem with the FET's ?? Thanks for all the help John |
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Good morning John,
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Yey! Seems like you've made some progress there! A reading of -7.9V on the pins 2,3 of U460 is not normal, if the input is set to EXT and the input jack is grounded... the output of the input buffer must be 0V or close to it. There's a chance that it's a FET problem, but it also can be anything like an opened resistor or diode... so you`ll have to break it down and isolate the possible causes. First thing, I would start by removing the FETs (they're socketed, so it's easy), connect a resistor of - say - 1k between pins 2,3 and ground, and measure again the voltage at pins 2,3... Voltage there should show a low voltage (my guess is that with a 1k resistor it will be something around -0.1V). This test is only to rule out that it's not U640 itself who's pulling the voltage down. Now, if the voltage at pins 2,3 is low as expected, then you know there's nothing wrong with U640, and you can probe voltages around the FETs, and measure the components around there, to see if there's nothing shorted, or opened, or anything of that sort. By looking at the initial symptom, of -7.9V at pins 2,3, my hunch is that the upper half of the input buffer was open circuited (either the FET itself, or R623, or R622). Keep hunting and let me know... Rgrds, Fabio P.S. If you find one of the FETs is damaged, you can restore almost full functionality to the scope by "stealing" the FETs from the B triggering input buffer (until you can find a new pair of FETs). The whole B trig. input and preamp only see some work when you need to use a "post delay" triggered operation (i.e., if your B triggering source lever is set to anything but "B STARTS AFTER DELAY"). The "post delay" triggered operation is not very often used. The regular use of the Delayed B sweep (where B sweep is simply triggered by the end of the delay) don't use any of the B triggering Input selector, preamplifier and even the tunnel diodes' trigger discriminator (just FYI). On Sat, Aug 18, 2018 at 08:09 AM, John Stoole wrote:
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John Stoole <john@...>
Well Success at last Fabio !!!!!
I did not realize that the FET's were in sockets under the heatsinks, I decided to change them and I still had no Trigger, as they were out I tested the FETt's on my component tester and there was one faulty on the B TRIG and one on the A TRIG so I used the good ones on Trig A and found that as I turned the Trigger Level control to about 3 o'clock it locked and I had Trigger sync, and the trigger lamp is on. The voltages on the U640 chip 14/15 is 0v but 2/3 is 179 mv I have had a look around and have not found the reason for this ? Have you any thoughts for a suitable replacement for the FET's?, something I might get locally in UK We are getting there thanks to you John |
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Bert Haskins
On 8/18/2018 1:33 PM, John Stoole wrote:
Well Success at last Fabio !!!!!I'm really surprised that no one else has mentioned the infamous tunnel diode problem. This often shows up on a lot of the older Tek products and the are many, many previous discussions on this and other lists. The problem summery is that as the TDs age their characteristics change/drift,etc Some people have managed to obtain replacements that are as good or even better than the Tek original parts. This includes me. I had this on a 465B and some other Tek stuff. My first solution was to swap the pairs from the B timebase with those from the A section. This worked for a few years but course, the B timebase was now bad. Later I had problems in some 7K units and my good old lugable Cosser scope. To shorten a very long story, I ended up spending a lot of money on 4,7 and 10 ma. diodes from a ?Russian? ebay seller/s. I "selected" the trigger diodes using a combination of several test methods and used the ones that if liked in my 465B. My 465B now triggers solidly well above 300Mhz. Now the dirty part is that I have had several old TDs that looked good on the curve tracer but did not work well in any instrument. I really don't know the real why of this but my stuff works!. GOOD LUCK, Bert |
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John Stoole <john@...>
Hi Bert
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No its not the Tunnel Diodes that are faulty its the Trigger driver JFET's have blown, in fact one of them was cracked which I was unable to see until I removed the Heatsink, so I hope someone can suggest a substitute Regards John mi0dfg On 18 August 2018 at 19:07 Bert Haskins <bhaskins@...> wrote: |
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Bert Haskins
On 8/18/2018 3:49 PM, John Stoole wrote:
Hi BertIf you feel like doing a lot of work, you can look up the part number and cross-reference it to find what else it is used in. Some Tek parts are common and some are made out of unatatium, hope I spelled that right. Are these dual FETs? Bert On 18 August 2018 at 19:07 Bert Haskins <bhaskins@...> wrote: |
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Hello John,
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I'm glad you made progress in so little time. On the tekwiki website, down there is a reference material section and a Tektronix semiconductor design catalog. Link is here: http://w140.com/Tektronix_Xref_sm.pdf There you will find the specs of those FETs, They are selected matched pairs of 2N4416. Those are out of production but they're not hard to find. Ideally, you would need to buy a bunch of them and try to find between them, a pair that are similar to each other and are as close as possible to the target specification. IIRC, those are selected to have Idss between 10 and 15mA, while the plain 2N4416s are anything between 5 and 15mA. The voltage offset you're getting is due to the only two last FETs you got working are not matched. The A level triggering center trimpot is able to compensate for up to 200mV so, you' re at the edge of the cliff but still workable. Still, with the offset so high, they are so unmatched that they will not track well one to another with changes in temperature. You may be lucky to find the matched ones at Sphere Research, or Qservice (the latter probably better choice for you as they're in Greece).. Those are two very well known traders of Tek parts, that many in this group can endorse (me included, for Sphere, I never dealt with Qservice). Good luck, Rgrds, Fabio On Sat, Aug 18, 2018 at 06:04 PM, Bert Haskins wrote:
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