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I have been working on getting the schematics for the TDS 420/460 converted
from production engineering schematics to a form which can be used without
trying to create a manual. I have no background in manual writing, just a
technical background working at Tektronix for about 30 years. I was able
to obtain the drawings from one of the engineers who designed the TDS
series, now passed on. I tried to post on TekWiki via Kurt but I am not
savvy enough to figure out the posting process. So, I have placed the
schematics on my personal ISP site for the time being.



I had sent a couple of the trigger related schematics to Tim in hopes it
would help him find his problem. Kurt at TekWiki has also been informed of
the existence of the schematics.

With the exception of the power supply, I believe the schematics to be
accurate. The power supply information is from Lecroy and has been used
with success but may not be 100% accurate.

Best regards,

Ferrous Steinka

My caps were backwards as well. Looking at W140.com I see that they were added at SN 6040.

Hello Chris,
I am afraid there is no reasonable way how to rewind the transformer. It is very small (just 3x3x3cm), has 6 independent windings and broken one is closest to the core. It is also completely potted by some glue so probably the only way how to disassemble is by acetone and it will likely dissolve everything except ferrite core and wires.

Here is the photo of that transformer: /g/TekScopes/photo/300091/3876209?p=Created%2C%2C%2C20%2C2%2C0%2C0 , see the size comparison with RJ45 plug.

If I am not able to find the replacement, I am considering alternative way to build separate small power supply to be put inside of scope. It needs to provide 12V and galvanically separated +/-15V.

Vladimir

It's quite hard to believe that every 130LC was drawn and assembled incorrectly, and not picked up during what appears to be one ofTek's longest runs of production.

I do recall a piece of Marconi test gear where a similar arrangement was near the (single-ended and not centre-zero) meter and the note to the schematic said "polarity of capacitor to be chosen during calibration".

To my mind 10-12V DC pulses are too high to be normally tolerable when they could be avoided simply.

Noted re HT on the meter - that became clear looking at the schematic: we're measuring small deltas riding on a large baseline value. Bit like measuring DC on the mains...

I wonder if 15V back-to-back Zeners might provide enough protection? I've used this on an input 'protector' for an AF sig gen and/or curve tracer I didn't want to blow up poking around in valve gear.

Replying by memory, I believe it is correct.? In most cases, if the fuse that blows is the .250 mA fuse, check the tantalum caps nearby. The culprit mostly will be the HV block. There was another thread where I wrote some in detail.

I don't recall for sure, but on my two 130s I think those caps were also reversed from the "proper" polarity, but it didn't seem to make much difference to operation. The net DC voltage on the meter movement is pretty small, although the AC applied to the meter circuit is fairly large, low frequency narrow pulses when it's near the bottom of the scale. You can look with a scope and see what's going on, then decide appropriate polarity - maybe backwards is better - I don't remember which way I picked.

I decided to put in about five to ten times more C for filtering to get less bouncing around and easier reading at low values below about 5% of FS. The tradeoff is overall slower response, but no hurry with something like this. I used good hermetic Ta caps.

BTW be very careful in poking around in the meter circuit. The meter floats on the B+ and the signal operates around it. The series R to the B+ there limits the current to a low average value, but the B+ filter capacitor stores enough energy to likely easily blow the movement's coil if anything is inadvertently shorted to ground. I had planned to modify the circuit for meter protection but didn't do it yet. The apparent easy solution of adding clamp diodes does not work here because of the large (around 10-12V I think) pulse voltage applied then averaged by the range filter caps. I did that first and it totally messed up the readings, until I figured out what was going on.

Ed

Hi,
not what you want to hear, but have you considered re-winding it ? Either on the same or a similar former. If you're very lucky, the primary winding might be the outer one, or a separate former ? (can't find my photos ATM). If you don't feel like counting turns, and if you still have your friend's T3 you could determine the turns ratio. Not a magnetics pro here but I assume with turns ratio, inductance, operating frequency and guestimating the current loads (should be fairly low) you can probably get pretty close ?

Chris

Bad SMD? Not likely due to age. However, for the long term every board should be pulled (only one or two at a time) and inspected anyway, see. See /g/TekScopes/photo/285143/3590895? Chemical hour meter will be leaking if still installed. Then the power supply needs attention, x2 y2 caps etc. on mains. I refer you to John Miles' notes which has detailed support info. Manuals are easily found by Google. Units are very maintainable too with parts available.

Bummer. It was worth trying.

On Wed, Jan 15, 2025, 11:02 AM Tim Wright via groups.io <al7ds=
[email protected]> wrote:

Just checked, and it does not. Different ACQ board altogether. I have a
540, and it is similar to that. Makes sense, same series.

Just checked, and it does not. Different ACQ board altogether. I have a 540, and it is similar to that. Makes sense, same series.

Same here, my caps were backwards. I also replaced a couple of the film caps there because they were DiFilm with high dielectric absorption which warped the low-scale reading. I found that polyester was too good, and ended up adding RC’s to simulate a selected amount of DA. That might be taking it too far but it was so satisfying to run the problem to ground at last.

Dave Wise

There are many ways to approach it. You should be able to do this with a 7904 + 7B92A in delayed trigger mode.

You'll need to display a complete frame with the main sweep and display the scan lines with the delayed trigger sweep. Just a few lines should do nicely.
You can input the RGBHV lines to the vertical channels as needed for viewing and selecting pulses of interest.

IIRC, you can put VSYNC directly into the main trigger EXT input and HSYNC into the delayed trigger EXT input and use both at the same time.
That might give you better triggering depending of the operational status of your 7B92A.

Play with it and see. I've used similar setups to watch live video on my 7904A.

The TDS520 component level service manual is available, not sure if it
applies to the 420.
Radu.

On Wed, Jan 15, 2025, 9:36 AM Tim Wright via groups.io <al7ds=
[email protected]> wrote:

All SMD caps replaced, board cleaned and inspected, via's reflowed if
showing corrosion signs. Still getting trigComparatorTest FAIL, ERROR ID
151 diagnostic test failure trigComparatorTest, TRIGA status after trigger:
exp = 1, act =0. ** alpha trigger (XTRIG), on the RS232 Debug port.
This tells me that there is still an open via somewhere. Since schematics
appear to be un-obtanium, I am wondering if anybody with experience on
these scopes can at least identify the part of the ACQ board I should look
at closer under the microscope. What chips handle the triggering? should
be close to the EXT trigger BNC connection on the PC, Yes? Only reason I
am fooling around with repairs on this "Antique" (Not so antique as my 511A
though) is for the challenge. It has become a big one. It would be great
if someone, ex-Tek repair tech, had the schematics on these things, but I
am not holding my breath.

All SMD caps replaced, board cleaned and inspected, via's reflowed if showing corrosion signs. Still getting trigComparatorTest FAIL, ERROR ID 151 diagnostic test failure trigComparatorTest, TRIGA status after trigger: exp = 1, act =0. ** alpha trigger (XTRIG), on the RS232 Debug port.
This tells me that there is still an open via somewhere. Since schematics appear to be un-obtanium, I am wondering if anybody with experience on these scopes can at least identify the part of the ACQ board I should look at closer under the microscope. What chips handle the triggering? should be close to the EXT trigger BNC connection on the PC, Yes? Only reason I am fooling around with repairs on this "Antique" (Not so antique as my 511A though) is for the challenge. It has become a big one. It would be great if someone, ex-Tek repair tech, had the schematics on these things, but I am not holding my breath.

When I first got this 7904 it has been stored unused for years. I turned it on and the scope, with some plug-ins, entered tick-mode. I spent months trying to figure out what was wrong to no avail. I could not see any over-voltage or over-current conditions. Over time the crackling sound went from unregular and rather modest to louder and louder combined with a hissing sound. Eventually the noise got unbearable.

I got a multiplier unit from a gentleman in the UK and replaced the bad unit. Now the scope was back in operation. I think the explanation is that when the internal HV multiplier arcing occurs the feedback signal to control circuit U1275, pin 10 and 11, phase detector is so distorted that the circuitry goes into tick-mode.

I know it is a nightmare going into this power supply. Some cables are thin and stiff. However, your situation is precarious, you have not much of alternatives. I would suggest that you try to replace the multiplier unit.

Good luck
G?ran

I read Alan Hampel's article on restoration of the above.

He asserts that "Surprisingly, electrolytics C99 (5?F) and C100 (25?F), factory originals, were installed backwards! Not surprisingly, they each had only about 10% of their rated capacitance and were very leaky". (These caps are in parallel to meter shunt resistors on 3p and 10p ranges).

The original Tek schematic does not label the meter +/- but it is clear that the RED wire (sorry you need the schematic to follow this) is the +ve. I have confirmed for sure that the meter operates correctly, with circuit disconnected, when the requisite ?A are applied to the this terminal of the meter

Having looked at my (original) 130, the electrolytics do indeed have their +ve going to what is declaratively the NEGATIVE of the meter as well. My S/N is 120xx

Has anyone been through this restoration? I'd be comforted for someone more experienced to confirm this apparently repeated build and documentary error by Tek. Thank you.

Hi, I read many of your posts on the 465m blowing HVPS fuse...I actually acquired a 465m looking almost brand new with the saga of the fuse blowing.. I just have one question, what is the voltage readind on your 465M at the 32v unreg post, on mine it is 56 vdc with almost no ripple..DO YOU HAVE THE SAME READING !!! By the way, I just found out about this group and I find it VERY useful when trying to fix Tek equipement, !!
Best reguard's
Richard

Thanks for the advice. Re. the nick in the HV lead, I have not used dielectric grease before, does it set or would I need some particular variant for the voltages involved? Also, would a better solution be a dab of grease to fill up the nick, then to contain the grease with a short length of 4:1 heatshrink such as this:



The product linked appears to have 36kV dielectric strength. I reckon one cm of this (16/4) would slip round the Alden connector (I know because I have some 14mm and that just gets round the leg of it , but it is 2:1 so sadly no use). The black HV cable is 5.5mm diameter in case anyone wants to know.

@G?ran; were the symptoms of the U1615 a quiet crackle, almost like a ticking at first but not regular? I can elicit the odd crackles from the HV ultor pink cable by poking it with a glass fibre rod but they have a different quality of sound and are not as quiet as the noise I'm concerned by. R1604 appears fine and looks original to me. Maybe if there is something wrong in U1615 it is not as advanced as yours was yet? Do you think I should swap over the 7 x cap multiplier for my spare just in case? I know its not clever to change without good reason, I can't even claim to be certain that the spare unit works (though it should do given provenance). My only reason would be to not have to dig into the PSU for a while after completing this particular task. Opinions/judgements on this anyone?

I am wondering

I have a recently acquired 465M with option 93. I can't seem to find and info on what that option is. Does anyone here know?

Evan

Just post them. Everyone and is dog expects to have to modify 3D printer files.

TIA

EJP