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On 12/19/2014 5:38 AM, Roger Evans very_fuzzy_logic@...
[TekScopes] wrote:

P { margin-bottom: 0.21cm; }I recently was given a 7934 minusplugins
as 'spares or repair' and was surprised to find that innon-storage
mode everything that I have tried so far works perfectlyand the
bandwidth and triggering seems fine. The trace is sharp, but a
bitfaint on the fastest speeds (about 10kHz rep rate from an
avalanchepulse generator) but fine with the room lights dimmed.
I have to admit to never having used astorage oscilloscope! We had the
Tektronix 611 storage display unitattached to a mini-computer before
raster graphics displays wereaffordable (colleagues at NASA Goddard
had a colour graphics displaythat cost $250k) and later I used the
Tektronix 4010 computergraphics terminals quite often so I am used to
some of the artifactsthat you get with storage displays.
My question is whether my 7934 just hasan aging CRT or may need some
adjustment on the voltage settingsaround the storage control.
Currently the 'level' control on thetop right of the panel has its
spindle broken and I cannot adjust it,a replacement is on order but
will probably not arrive for a week ortwo. I don't claim to have made
any exhaustive checks of the storageoptions (largely since I don't
know what to expect!) but in generalterms I have a rather bright
uniform background in storage mode. Some combinations of settings will
darken the background but notuniformly (darker centre, bright edge)
and the trace and readout arereasonably sharp against the brighter
background but become blurredon the settings with better contrast.
Save and erase do what youwould expect given the bright background.
So basically, if the tube is on the wayout then I will give the 7934 a
wash and brush up and concentrate ongetting my 7904 100% right instead
of the 99% at present. On theother hand if there is the prospect of
improving the storageperformance with a bit of work then I can start
reading and learningabout storage tubes and do a check on voltage
levels while waitingfor the replacement pot to arrive.
Also a Merry Christmas to everyone,
Roger

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Practice, study, rinse & repeat, and ( only ) then maybe tweak some
internal adjustments.
I have two 7623A scopes and at one time used the storage extensively,
mainly for the setup and design of servo systems on large machine tools.
That was then and now I hardly ever use the storage on these scopes.
My 2232s/2432 scopes are just much better at it and the "playback" can
be adjusted for
the best capture on my phone camera.

The exception to the above is a few cases where the "variable
persistence" mode can be just the ticket.
You might just check the archives for more information.

[Non-text portions of this message have been removed]

P { margin-bottom: 0.21cm; }I recently was given a 7934 minusplugins as 'spares or repair' and was surprised to find that innon-storage mode everything that I have tried so far works perfectlyand the bandwidth and triggering seems fine. The trace is sharp, but a bitfaint on the fastest speeds (about 10kHz rep rate from an avalanchepulse generator) but fine with the room lights dimmed.
I have to admit to never having used astorage oscilloscope! We had the Tektronix 611 storage display unitattached to a mini-computer before raster graphics displays wereaffordable (colleagues at NASA Goddard had a colour graphics displaythat cost $250k) and later I used the Tektronix 4010 computergraphics terminals quite often so I am used to some of the artifactsthat you get with storage displays.
My question is whether my 7934 just hasan aging CRT or may need some adjustment on the voltage settingsaround the storage control. Currently the 'level' control on thetop right of the panel has its spindle broken and I cannot adjust it,a replacement is on order but will probably not arrive for a week ortwo. I don't claim to have made any exhaustive checks of the storageoptions (largely since I don't know what to expect!) but in generalterms I have a rather bright uniform background in storage mode. Some combinations of settings will darken the background but notuniformly (darker centre, bright edge) and the trace and readout arereasonably sharp against the brighter background but become blurredon the settings with better contrast. Save and erase do what youwould expect given the bright background.
So basically, if the tube is on the wayout then I will give the 7934 a wash and brush up and concentrate ongetting my 7904 100% right instead of the 99% at present. On theother hand if there is the prospect of improving the storageperformance with a bit of work then I can start reading and learningabout storage tubes and do a check on voltage levels while waitingfor the replacement pot to arrive.
Also a Merry Christmas to everyone,
Roger

Hi Michael,

Thanks for the recommendation on Bourns. I ordered enough to replace all of them in the scope since I have found three bad. I suspect the 50 ohm ones in the .2 mv circuit were killed by someone hitting the input with too large a voltage. In any event, they'll be new now.

Sure glad I had my 465B for troubleshooting this scope. Now if I can just find the button flex circuit for my 2232, I'll be in good shape. I'm in contact with the original manufacturer of the part and hopefully, they will make an engineering run for me and there will be a few left over for others

Interesting on the oil. Makes sense too.

I appreciate the input. Sorry to hear of your health issues. I know that is not easy to deal with.

Thanks again for the help and suggestions.

Steve

n4bos@... [TekScopes] wrote:

UPDATE:
Well, the scope is MOSTLY fixed.... Turned out the reason I didn't have sweep was R740, (A Sweep Gain) pot was OPEN. I replaced it with another 1K pot and the sweep was back. Then I set about to check the calibration and I found that there was no difference in vertical gain between .2 and .5 mv on BOTH channels. That ALSO turned out to be two bad 50 ohm pots. With that, I took the 1K pot that I had removed apart and found that it had a lot of "oil" in it. I suspect someone used a cleaner that carried mineral oil and it contaminated the trim pots. Now I just have to find some of the little 50 ohm trim pots and I suspect the 2215A will be back among the living.

Thanks to all for the help. If anyone has a suggestion as to the best place to get the little trim pots (or a better replacement) I'd love to hear it.

The oil is to keep moisture out. My preference is genuine Bourns parts. They have the lowest failure rate that I've seen, and at one time I had to remove Beckman from our 'Qualified Vendor List' because they were shipping trimpots they were damaged in the board washer. We had to replace thousands of them, on new boards because water got inside, and they would drift after a board was calibrated. The problem was the tiny o-ring on the shaft was out of spec. They are supposed to have fixed the defect, but we weren't going to modify hundreds of B.O.M. again if we had no reason.

It sounds like you've made good progress since the last time I was online. :)

I am disabled, and I don't have a lot of good days between doctor's visits.

UPDATE:
Well, the scope is MOSTLY fixed.... Turned out the reason I didn't have sweep was R740, (A Sweep Gain) pot was OPEN. I replaced it with another 1K pot and the sweep was back. Then I set about to check the calibration and I found that there was no difference in vertical gain between .2 and .5 mv on BOTH channels. That ALSO turned out to be two bad 50 ohm pots. With that, I took the 1K pot that I had removed apart and found that it had a lot of "oil" in it. I suspect someone used a cleaner that carried mineral oil and it contaminated the trim pots. Now I just have to find some of the little 50 ohm trim pots and I suspect the 2215A will be back among the living.

Thanks to all for the help. If anyone has a suggestion as to the best place to get the little trim pots (or a better replacement) I'd love to hear it.

Again, thanks for the help.

Steve

Meh, in any case, if you get the dreaded trace deflected off the CRT and you can't get it back with the controls, the input dual FET might be bad. If it is, apparently 2x 2N3819 TO-92s glued together face to face do the trick, as long as you swap over the pins properly.
Balance is a bit off, but at least it displays properly again.
Maybe there are better FETs with a better pinout, I just shopped for parts in stock locally.

Thanks for the information everyone.

I'm fairly new to this group and it is amazing the level of knowledge here,
I have a lot to learn.

I picked up a couple Tektronix P6047 probes to match the 454 scope that I
bought recently for hobby level repairs.

Brian

I was recently helping a friend troubleshoot a Tek PS280 that he had bought on e-bay. In the process I found a fault which is design related and may be more common.

The symptoms were that the adjustable positive supply had no output, and the negative supply had regulation issues. Upon opening the supply, I found one of fuses (F102) from the transformer secondary was blown. As this feeds the output pass transistors directly, I figured there were many possibilities of root cause and troubleshooting could take some time. But a physical examination quickly uncovered the cause.

The main regulator board was loose near the top of the instruments. There were also the broken pieces of the thin nylon standoffs used to support it at the bottom of the instrument case. I could see with the standoffs broken, it was possible for the trace side of the circuit board to touch the upper support bracket, shorting something to ground. Closer inspection of the bracket showed a slight burn mark, with a corresponding burn mark on the circuit board.

Examining the broken brackets shows they are inadequate to support this board. The ones used are the thin “snap in” type which do not use separate screws. The bottom of the board is retained in a large slotted channel that it slides into, which serves the purpose. The board contains several relays, large capacitors and other heavy components. When exposed to shock such normal shipping handling, the thin standoffs which the board is mounted with can and do break. As the unit has no other obvious dents or damage, I would suspect that the drop was not excessive. I would also guess that the vendor who Tek OEMed this instrument from in Taiwan never did a standard shipping drop test, or they would have found this design flaw.

I replaced the standoffs with more robust 1/4” diameter, 3/8” long nylon standoffs which have a 4-40 tread to accept a screw. Nylon is necessary because some of the mounting holes pass through board traces. McMaster Carr part number 96110A560 is a perfect fit. The board and bracket have mounting for 4 of these spacers along the top, but it appears that only 3 of the thinner ones were actually installed in this unit.

I replaced the fuse and powered up the supply and found it totally functional. I did not check the circuit to see what had shorted, but in this case it looks like something before many semiconductors, as the 5 A fuse protected everything from additional damage.

If any readers have either the PS280 or PS283, I would suggest you removed the cover and check to see if the main regulator board in your unit is securely mounted. If not, I would recommend using the more robust standoffs to replace the inadequate ones in the original instrument.

- Steve

I have replied to similar messages in the past. Peter’s reply is right on. To get the best performance, higher bandwidth scopes with 1 Mohm inputs need the probe to be designed to counteract the effect of parasitics in the front end. Most of these effects are seen in the first 2 nanoseconds, so the practical breakpoint where you should really stick with the suggested probe is about 200 MHz.

The effect shows up as an overshoot and/or ringing in this area of a pulse after a fast clean transition. However, another rule I like to say is that passive probes are really only practical for looking at frequencies below 200 MHz, right at the breakpoint where you should use the suggested probe for the particular scope model. Regardless of how good the probe’s performance is, frequency components greater than 200 MHz will probably be greatly distorted by the circuit under test acting on the severe loading the probe is imposing. The load comes from the capacitive element, not the resistive.

For lower frequencies below a GHz or so, the probe load can be estimated by calculating the capacitive reactance of the specified input C at the frequency of interest. (capacitive reactance, Xc, = 1/(2*pi*f*c) . So, for a high quality 200 MHz probe with an input C of 10 pF, the load would be about 80 ohms at 200 MHz. So ask yourself, would the circuit I am probing change its properties if I placed an 80 ohm resistor from the point I am probing to ground? Well, that is what you are effectively doing, but only for frequency components around 200 MHz. As the frequency goes down, the loading decreases until you hit the resistive limit, which is usually 10 Mohm.

So if you really need to see the high frequency components in the circuit, you should use either an active probe (which has less than one tenth the input C), or a transmission line probe. The latter has a higher DC resistance, but very little capacitance.

With higher frequency active probes (BW > 1 GHz), simple capacitive reactance may not represent the loading, as the inductive parasitics enter as well – forming LC resonate circuits.
------
In general, I am not a fan of switchable 1X/10X probes. In my opinion, all of these l have crappy high frequency performance. The reason is that the attenuation switch is in the wrong place. Putting it in the probe head adds several parasitic elements in the sensitive very high impedance portion of the probe circuit. This would not be a problem if the switchable element were on the lower end of the attenuator, with the switch itself on the grounded side. But this would require the switch be placed in the comp box at the scope input, which is inconvenient for the user. Thus probe designers compromise and put the switch in the probe head, which really screws up the high frequency performance.

BTW, in case you are wondering, I worked as an engineering manager for 4 years in the probe design group at Tek.

- Steve


---In TekScopes@..., <valvesruleok@...> wrote :

All scope input attenuators/amplifiers have unique transmission characteristics due to parasitics that are (or should be) compensated for by the manufacturers recommended probe. A third-party probe cannot do this so will not be as accurate over the passband because it can only be designed for a theoretically perfect R//C input over a range of C. But how much difference this makes in practice would take quite some testing and gear to discover. For hobby purposes it's probably not worth worrying about.

Peter

Hi Tom,

At this point, I have trigger but the actual sweep seems to be a bit flaky. I had a nice trace for a while but now I have just the trigger as seen by just flashing dots on the screen. Now I get to chase a missing sweep problem but at least I have the trigger figured out. I'm picking up a higher voltage transistor for the bad one but I doubt that will affect the missing sweep. Stay tuned.....

Steve

Hi Steve,
Congratulations on a successful step in your 2215A repair!
I do not have enough knowledge to properly comment on the transistor
question, but my guess is that the choice of transistor is not critical
as it appears to be just and on-off switch for the sweep waveform.
Hopefully one or more of the Tekscopes members who actually know
something will comment on this transistor question, and we will all
learn from the answer(s).
I'm glad you brought his missing sweep subject up, and caused several of
us to dive into the service manual. I have a 2215A which has something
not quite right going on in the horizontal sweep circuitry, and I can
use your sweep problem knowledge as inspiration to fix it.
tom jobe...

Bump...
Did you repair the 1A4? I have to repair mine now... I need some success stories.

Thanks Tom.... I guess I needed a nudge to go back through that section. I had before, checking voltages and resistances and everything LOOKED close..... Well, thanks to your suggestion, I went through in greater detail and found that the voltages on Q701 looked a bit odd. The base and the emitter were at essentially the same voltage. I checked the transistor (you can isolate the base by pulling connector P9700) and found to be VERY leaky base to emitter. The transistor is an SPS8246 and I wasn't able to find a data sheet so far but I sub'd in a 45 volt NPN I had and ..... I have a trigger.

I THINK a 2222A should work as the highest voltage there is 30 volts but, I'm not sure about the needed gain. The transistor simply clamps the trigger caps to prevent charging so I THINK a 2222A should be fine. Any thoughts on that.... or maybe you know the specs for the SPS8246 compared to a 2222A?

My sweep is a bit narrow now but I suspect I can calibrate that back. I haven't finished checking yet but I wanted to give you an update.

Thanks to all for the help (so far). It was your friendly "nudge" that got me there...

Steve

I just happened to see your post and then eBay item number 121520632728
right after. It is a 576 that is being parted out. Maybe the transformer
is still available.

DaveD

Hello Dave,

I'm the guy from germany with the HV transformer at fault in an otherwise quite well working 576. If you happen to have one of those rewound transformers left, I'll be interested of course.

cheers
Martin

Hi to All,


Just received a DL-11 Delay-Line. On this moment I'am looking for its Operation/Service Manual to make it complete. I can only locate the 070-7051-00 DataSheet of it. Anybody who has acces to the other documentation?


Greetings,


Egge Siert

All of the trigger inputs (internal, line, and external) should activate the
trigger light when a trigger is present. If P-P Auto triggering is active, it
should be difficult *not* to have a legitimate trigger and this goes double with
the line trigger.

I need to take a closer look at the trigger to sweep circuits. Keep in mind
that the 2213A, 2215A, 2230, 2232, 2235, and 2236 all use the same basic design.

I have a scan of a Huntron 1005B-1 manual (37 pages) if anyone needs it.Has parts list & schematic.
?HankC, Boston
WA1HOS


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Have you tried using a external trigger input?
Mike

Hi Steve,
How about starting your search where the A sweep waveform originates on
page 3-10 in the "Theory of Operation" section.
It clearly describes how the waveform is generated.
tom jobe...