465/475 Volts/Div range LED swap problem.


Dave Peterson
 

Hi All,

Been working on replacing 465 Volts/Div range bulbs with LEDs. Has anyone here actually done this before?

I've tried searching here and EEVBlog forum, but I've either gotten too many results or no details. Searching for "465 LED" is too general and there's too many off topic results, and "465 volts/div LED" returns two results that yield nothing. Same on EEVBlog. People seem to do it, but don't seem to be doing much beyond straight swap (with current limiting R), or don't reveal what they're doing exactly.

I ran into a surprise, and have worked out a fix. But I never came across this issue looking into it. Does everyone just live with it, or is there another scenario or solution that I'm not finding or thinking of? There's a peculiarity to the 465/475 1X/10X indicator circuit that results in an issue with a straight swap that I'm not willing to live with.

"Atta' boy/girl" for anyone who knows the issue I'm talking about. I'll reveal, but I'm getting my ducks lined up. I want to share my solution including LTSpice schematic/simulation. I think it works nicely, but I'm a classic Rube Goldberg solution expert. I.e. I seem to try the most difficult solutions first. Am I over thinking this? I tried several alternatives before building what I have.

Would like to know how far into the weeds I've gone before I expose myself to withering ridicule.

Thanks,
Dave


Michael W. Lynch
 

I am sure that more than a few people here and elsewhere would like to hear more about this. I am a devotee of Mr. Goldberg myself.

--
Michael Lynch
Dardanelle, AR


Dave Peterson
 

Here's a hint:

https://www.eevblog.com/forum/index.php?action=dlattach;topic=45906.0;attach=157509;image


What's wrong with this picture?

Dave

On Saturday, February 20, 2021, 09:46:09 AM PST, Michael W. Lynch via groups.io <mlynch003=yahoo.com@groups.io> wrote:

I am sure that more than a few people here and elsewhere would like to hear more about this.  I am a devotee of Mr. Goldberg myself.

--
Michael Lynch
Dardanelle, AR


Dave Voorhis
 

On 20 Feb 2021, at 17:54, Dave Peterson via groups.io <davidpinsf=yahoo.com@groups.io> wrote:

Here's a hint:

https://www.eevblog.com/forum/index.php?action=dlattach;topic=45906.0;attach=157509;image

What's wrong with this picture?
It’s too small to see?

Sorry to be grumpy, but I’m not finding your guessing-game approach much fun. Could you share the problems you’ve found and the solution you’ve developed and, with respect — and unfortunately there’s no write to say this as politely as I would say it in person — please spare us the theatrics?


Ozan
 

The picture is too small to see what is happening but looking at the schematic X10 light bulb supplies the base current of Q396. Either X1 is not lighting up (too small current) or X10 is lighting dimly when X1 is selected depending on what is connected for the LED circuit.

A properly sized resistor in parallel with the X10 resistor should fix it.

Ozan


--------


Here's a hint:

https://www.eevblog.com/forum/index.php?action=dlattach;topic=45906.0;attach=157509;image


What's wrong with this picture?

Dave

On Saturday, February 20, 2021, 09:46:09 AM PST, Michael W. Lynch via
groups.io <mlynch003=yahoo.com@groups.io> wrote:

I am sure that more than a few people here and elsewhere would like to hear
more about this.  I am a devotee of Mr. Goldberg myself.

--
Michael Lynch
Dardanelle, AR






Ozan
 


A properly sized resistor in parallel with the X10 resistor should fix it.
I meant a properly sized resistor in parallel with the X10 LED should fix it.

Ozan

--------

Here's a hint:

https://www.eevblog.com/forum/index.php?action=dlattach;topic=45906.0;attach=157509;image


What's wrong with this picture?

Dave

On Saturday, February 20, 2021, 09:46:09 AM PST, Michael W. Lynch via
groups.io <mlynch003=yahoo.com@groups.io> wrote:

I am sure that more than a few people here and elsewhere would like to hear
more about this.  I am a devotee of Mr. Goldberg myself.

--
Michael Lynch
Dardanelle, AR






Dave Peterson
 

Sorry Dave.

The link was to the thumbnail. The full picture can be expanded by finding it in the thread:

https://www.eevblog.com/forum/repair/tektronix-465/msg697587/#msg697587


Not so much theatrics as impatience on my part. I'm cleaning up my LTSpice simulations to upload to the files area, and need to build up a photo album. In the interests of saving on the TekScopes group space usage I need to resize all my pictures by quite a lot.

No need to apologize. Pardon my impishness and impatience.

Yes, the 10x diode stays on weakly. I tried all manner of remedies, but the basic issue is that Q386 needs enough base current to drive the 1X LED, and that current is always enough to cause the 10X LED to start illuminating. I'd just ordered a batch of 100 2N3904's to replace one missing in the scope. So decided to wire one parallel to the 10X LED. I was really trying to minimize the complexity of this solution. This was the best I could come up with.

Posting the LTSpice files now. Want to/will add a writeup PDF too.

Dave

On Saturday, February 20, 2021, 10:11:48 AM PST, Dave Voorhis <voorhis@gmail.com> wrote:

On 20 Feb 2021, at 17:54, Dave Peterson via groups.io <davidpinsf=yahoo.com@groups.io> wrote:

Here's a hint:

https://www.eevblog.com/forum/index.php?action=dlattach;topic=45906.0;attach=157509;image

What's wrong with this picture?
It’s too small to see?

Sorry to be grumpy, but I’m not finding your guessing-game approach much fun. Could you share the problems you’ve found and the solution you’ve developed and, with respect — and unfortunately there’s no write to say this as politely as I would say it in person — please spare us the theatrics?


Dave Peterson
 


Chuck Harris <cfharris@...>
 

When faced with that problem, I usually add a diode
in series with the emitter of the transistor switch.
That allows a "0" to turn the switch off better.

-Chuck Harris

Dave Peterson via groups.io wrote:

Sorry Dave.

The link was to the thumbnail. The full picture can be expanded by finding it in the thread:

https://www.eevblog.com/forum/repair/tektronix-465/msg697587/#msg697587


Not so much theatrics as impatience on my part. I'm cleaning up my LTSpice simulations to upload to the files area, and need to build up a photo album. In the interests of saving on the TekScopes group space usage I need to resize all my pictures by quite a lot.

No need to apologize. Pardon my impishness and impatience.

Yes, the 10x diode stays on weakly. I tried all manner of remedies, but the basic issue is that Q386 needs enough base current to drive the 1X LED, and that current is always enough to cause the 10X LED to start illuminating. I'd just ordered a batch of 100 2N3904's to replace one missing in the scope. So decided to wire one parallel to the 10X LED. I was really trying to minimize the complexity of this solution. This was the best I could come up with.

Posting the LTSpice files now. Want to/will add a writeup PDF too.

Dave

On Saturday, February 20, 2021, 10:11:48 AM PST, Dave Voorhis <voorhis@gmail.com> wrote:

On 20 Feb 2021, at 17:54, Dave Peterson via groups.io <davidpinsf=yahoo.com@groups.io> wrote:

Here's a hint:

https://www.eevblog.com/forum/index.php?action=dlattach;topic=45906.0;attach=157509;image

What's wrong with this picture?
It’s too small to see?

Sorry to be grumpy, but I’m not finding your guessing-game approach much fun. Could you share the problems you’ve found and the solution you’ve developed and, with respect — and unfortunately there’s no write to say this as politely as I would say it in person — please spare us the theatrics?












Ozan
 

decided to wire one parallel to the 10X LED. I was really trying to minimize
the complexity of this solution. This was the best I could come up with.
See my reply https://groups.io/g/TekScopes/message/179044
A 470-ohm from collector of the Darlington to ground (across the LED+current limiting resistor) is all you need. No need for ten LEDs in parallel, and it is not the most robust solution either.

Ozan


Ozan
 

decided to wire one parallel to the 10X LED. I was really trying to minimize
the complexity of this solution. This was the best I could come up with.
See my reply https://groups.io/g/TekScopes/message/179044
A 470-ohm from collector of the Darlington to ground (across the LED+current
limiting resistor) is all you need. No need for ten LEDs in parallel, and it
is not the most robust solution either.
I now see the solution, 10x was just the label, not the number of LEDs. No need for extra transistor, 470-ohm is all you need. Sim might have worked OK but in the proposed solution base of the new NPN is not pulled to ground strongly so noise could couple to the base easily.

Ozan


Dave Peterson
 

There are many facets to this experience for me:

One is that I want this info in the groups messages in the hope that it might help others in the future. So pardon if this is rudimentary for the more experienced of the group.

Another is that I wanted to reeducate myself in BJT circuits. I haven't dealt with them since school. This has been a great experience.

It was actually the thought of adding a parallel resistor to the 10X led/resistor to provide an alternate current path that led me to using a NPN. I wasn't sure what was going to happen when Q382 tried to drive the 10X LED. Wouldn't the low R win over the LED and keep it weak/off? I didn't have LTSpice setup yet. So I thought, since I'd just gotten this load of NPNs, why not? And down the Rube Goldberg path I went.

The NPN does end up with an elevated base voltage when 10X is on and 1X is off. It doesn't effect the 1X diode, but it is dangerously close to turning on.

Now that I have the simulation set up I was able to test the 470 ohm resistor, and sure enough, it works great. See, I knew I was making it more complicated than need be. Simplicity is definitely what I was trying to accomplish.

I spent a good amount of time yesterday and this morning coming up to speed on LTSpice. For example, I needed to figure out how to add a Darlington pair for Q382. As well as the simple things too. In fact I started with two 2N2907's as my Q382 at first. Worked pretty well, and got me up to speed enough to tackle the next level exercise of adding a third party model (and make it portable too).

I'm not very good at learning a new tool in a purely academic way - even if it is very similar to what I've used before. I need something practical to apply it to. This was a perfect circuit to cut my teeth on. Thanks to all who suggested it before. Was that you Michael? It has now also been a good exercise in finding minimal solutions. I also now have a better model to test on than a bread board with too few components.

I don't think I'll ever be up to speed on these scopes and circuits as so many of you. But I am learning, and very much enjoying myself.

Thanks to all who tolerate me and help.
Dave


On Saturday, February 20, 2021, 11:14:07 AM PST, Ozan <ozan_g@erdogan.us> wrote:


decided to wire one parallel to the 10X LED. I was really trying to minimize
the complexity of this solution. This was the best I could come up with.
See my reply https://groups.io/g/TekScopes/message/179044
A 470-ohm from collector of the Darlington to ground (across the LED+current limiting resistor) is all you need. No need for ten LEDs in parallel, and it is not the most robust solution either.

Ozan


Ozan
 

Hi Dave,
I look at the messages between tasks and usually on a small screen so my replies are usually on the short side. I hope it doesn't come out as terse. I like these puzzles, keep them coming.

Wouldn't the low R win over the LED and keep it weak/off?
The bulb must be consuming >50mA and your LED is probably consuming ~ 20mA so we can burn another 10mA without overloading the Darlington. On the other hand R has to be small enough to keep the LED off. Exact value is not critical but 470-ohm would work.

I spent a good amount of time yesterday and this morning coming up to speed on
LTSpice. For example, I needed to figure out how to add a Darlington pair for
I remember sending you an example LTspice schematic. Looks like you find it useful too, it is a great free tool.

Ozan


Dave Peterson
 

Ozan,

Ah, you're who sent the LTSpice reference. Yeah, thanks so much. It's simplistic, but does everything it needs to do. I can live with rotate-mirror-rotate vs. vertical-flip. BFD. I love simple.

It also correlates with measured very well.

I have not mastered concise. I admire terse that gets the message across. Your input is always spot-on. Keep it coming!

Dave

On Saturday, February 20, 2021, 03:09:56 PM PST, Ozan <ozan_g@erdogan.us> wrote:

Hi Dave,
I look at the messages between tasks and usually on a small screen so my replies are usually on the short side. I hope it doesn't come out as terse. I like these puzzles, keep them coming.

Wouldn't the low R win over the LED and keep it weak/off?
The bulb must be consuming >50mA and your LED is probably consuming ~ 20mA so we can burn another 10mA without overloading the Darlington. On the other hand R has to be small enough to keep the LED off. Exact value is not critical but 470-ohm would work.

I spent a good amount of time yesterday and this morning coming up to speed on
LTSpice. For example, I needed to figure out how to add a Darlington pair for
I remember sending you an example LTspice schematic. Looks like you find it useful too, it is a great free tool.

Ozan


Dave Peterson
 

Yep, just found the horizontal amp you sent. Too perfect. This is exactly what I was working toward:

I have the 465 Wes Bolin was working on for his friend in the garage. I'm going to try my hand at resuscitating it, and having a simulation to get to know the circuit will be a huge help. Starting with a simple circuit like the Probe Indicator was a perfect introduction.

Wes did a single PS cap fix, but I have a left over set of adapters and caps from doing a PS cap update on another parts scope. So I think first I'm going to do the full swap on it first. It'll be a bit before I can share the goings on with the horizontal amp. But I will when I get there.

Dave

On Saturday, February 20, 2021, 03:09:56 PM PST, Ozan <ozan_g@erdogan.us> wrote:

Hi Dave,
I look at the messages between tasks and usually on a small screen so my replies are usually on the short side. I hope it doesn't come out as terse. I like these puzzles, keep them coming.

Wouldn't the low R win over the LED and keep it weak/off?
The bulb must be consuming >50mA and your LED is probably consuming ~ 20mA so we can burn another 10mA without overloading the Darlington. On the other hand R has to be small enough to keep the LED off. Exact value is not critical but 470-ohm would work.

I spent a good amount of time yesterday and this morning coming up to speed on
LTSpice. For example, I needed to figure out how to add a Darlington pair for
I remember sending you an example LTspice schematic. Looks like you find it useful too, it is a great free tool.

Ozan