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TG501 1NS Schottky


Rick
 

Hi All,

My TG501 1ns output died. I suspect it's CR515, the 4V hot carrier diode at the start of the 1ns section. It's just a rare sized barrier Schottky. If I understand the circuit correctly it adds some high frequency distortion by clipping the 2ns sine wave and then uses a series of LC circuits to filter out all but a clean 1ns sine wave. My question is how critical is the 4v reverse voltage and other specs in this application? Is there a suitable modern through hole replacement?

Thanks, Rick


Rick
 

So a new CR515 diode did not bring the 1ns signal back to life. Seems like the only thing left is the signal getting hosed up under the Ft Knox shielding on the other side of the board. I'm still thinking the problem has to be at the entrance to the 1ns circuit because the 2ns signal is there, albeit a little lower than it should be.

Does anyone have any tricks or experience removing the completely soldered on shield to get to the traces? It's heavily soldered on all four sides. I'd surely have to overheat it to get all the solder off. I thought about cutting an access hole in it but the filings from a Dremel would be impossible to get out. I'd have to snip the hole.

Rick


 

Hi Rick,

What are you using to look at this signal?

Regards

On 3/6/2021 11:11 AM, Rick wrote:
So a new CR515 diode did not bring the 1ns signal back to life. Seems like the only thing left is the signal getting hosed up under the Ft Knox shielding on the other side of the board. I'm still thinking the problem has to be at the entrance to the 1ns circuit because the 2ns signal is there, albeit a little lower than it should be.

Does anyone have any tricks or experience removing the completely soldered on shield to get to the traces? It's heavily soldered on all four sides. I'd surely have to overheat it to get all the solder off. I thought about cutting an access hole in it but the filings from a Dremel would be impossible to get out. I'd have to snip the hole.

Rick

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Joel B Walker
 

Remember that the TG501 must be terminated with 50 ohms. If not there may be NO output or distorted and low output. You can use a bnc "T" fitting and put a 50 ohm terminator on one side, and the scope on the other.


 

On Sat, Mar 6, 2021 at 05:23 PM, Tom Miller wrote:


What are you using to look at this signal?

In addition to what Tom is saying: You have to realize that you need a fast 'scope to observe the 1ns signal; It's barely visible on a good 400 MHz 'scope, like a 2465B or the like; after all, it's a 1 GHz signal.

Raymond


 

On Sat, Mar 6, 2021 at 11:32 PM, Raymond Domp Frank wrote:


In addition to what Tom is saying: You have to realize that you need a fast
'scope to observe the 1ns signal; It's barely visible on a good 400 MHz
'scope, like a 2465B or the like; after all, it's a 1 GHz signal.

This has been addressed a few times before in this group: Unless your 'scope is fast enough, it seems like there's no 1ns signal.

Raymond


Ed Breya
 

I think in the OP it was stated the 1 nSec output was "dead," but how dead?

If you're viewing on a fairly fast scope, say 500 MHz BW, you should still be able to see something of the 1 nSec signal if it's there, but you can't make any accurate assessment of the level, since it will be down the response curve rolloff, which should be not too dramatic. The 1 nSec is only one octave away from the 2 nSec, which apparently can be seen - but it already appears smaller than it really is, since you're at the scope's BW. With an analog scope, if you can see the 2 at reasonable amplitude, then you should be able to see something of the 1, if it's working. Check the amplitude spec for 1 nSec,compared to the 2. Chances are it's quite a bit smaller, but maybe enough to get by. The main thing is to see if it's there at all, and maybe it's OK, if only the scope could see it properly. Also, try to trigger the scope from one of the lower marker frequencies that's still synchronous with the 1 nSec.

Ed


Don Bitters
 

A tactic I have used when drilling sheet metal is to cover the point to be drilled with either double sided tape or folded duct tape to catch the waste material. The normal drill rotation pulls material from the hole. For the final penetration of the metal shield you could also invert the drilling - drill upwards. That would virtually guarantee no waste metal under the shield.
Don Bitters


 

Another approach to containing metal chips when drilling is to use a
putty-like material to catch the metal shavings.  The putty-like
material can be packed on the drill side of the sheet metal (either as a
toroid, which would allow for visual confirmation of the target hole
location, or on the "backside", as a "blob" to catch the shavings as the
drill bit pokes through.  After the hole is drilled, the putty-like
material is removed, along with the shavings that it has
caught/contained. I've heard of using chewing gum for this (but never
used it myself).  What I have used (successfully) for this purpose is
"Duct seal"- sold in electrical sections of hardware stores as a
permanently-pliable sealant for closing passage holes in sheet metal, or
masonry, that are drilled to allow for electrical cables or other
utilities.  It's not expensive, and it's quite tacky. Another option
might be "plumber's putty".

Disclaimer: I have not been following this thread to any great detail,
so if my comment is wildly inappropriate, I apologize.

Mike Dinolfo N4MWP

On 3/6/21 7:01 PM, Don Bitters via groups.io wrote:
A tactic I have used when drilling sheet metal is to cover the point to be drilled with either double sided tape or folded duct tape to catch the waste material. The normal drill rotation pulls material from the hole. For the final penetration of the metal shield you could also invert the drilling - drill upwards. That would virtually guarantee no waste metal under the shield.
Don Bitters




Rick
 

Thanks for all the quick feedback. From doing bandwidth tests I realized I wouldn’t be able to see much of a signal on the 5440 scope even the 5B44. I am able to verify the whole TG501 with my 5S14N Sampler plugin. Plus I should be able to count the 1gHz signal on my DC502. It’s dead as a door nail on both. Only the noise of the cable is their. Definitely using a 50 ohm termination.

Using something sticky sounds like a good idea. Drill upside down. I do like the idea of scoring a square in the shield with the slow Dremel, but don’t go all the way through. Then finish the cut with an Exacto. The way the circuit is designed with the board’s etching acting as the inductor makes it have continuity all over the place. Not easy to chase the signal. Seems like seeing that underside is the only choice. Maybe I should cut on the sides so I can see all I’d the underside.

I have 2901 that I decided to calibrate today. The procedure say that the 2ns signal is dialed in by 6 caps and not tho move them too far. Only one combination of settings produces the 2ns signal. I experienced just that. The signal drops out very quickly if you go a hair too much. I wonder if the same thing is happening with the TG501’s 1ns settings. Maybe they are all out I’d whack. The 2901 Time Mark manual has a hi res picture of all the adjustment cap starting positions so you can find your way back. I wish that was available for the TG501.

Rick