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 Post subject: Re: CRT circuit question
PostPosted: Jun Thu 07, 2018 1:49 am 
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Posts: 1262
Location: Baton Rouge, Louisiana
Norm Leal wrote:
R14 etc prevents loading of the signal going to deflection plates. If centering controls were toward either end signals to deflection plate would be shorted out.

I see (said the [still blind] man)*
Thanks

*as he picked up his hammer and saw

_________________
Mark


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 Post subject: Re: CRT circuit question
PostPosted: Jun Thu 07, 2018 1:46 pm 
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Location: Warner Robins, GA
kc5gym wrote:
There is a guy on the web that had a hardware problem with his crt


In this instance the clocck was designed for scopes with a square crt as that's what's commonly available.


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 Post subject: Re: CRT circuit question
PostPosted: Jun Sat 09, 2018 1:44 am 
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The 500K dual gang pots and the 2.2 meg resistors came in today. Will install them tomorrow and see how it works.

If everything goes as planned the circuit will work good.

I will then just need a 5K pot and spst switch to reduce the size for square screens.

If it does work I'll have to shave down one side of the pot shafts to fit the D shaft knobs.

Will no longer have burn in protection, but that's not a problem as I only run a CRT bright enough to be seen good based on the room lighting.



EDIT:

Tried the AC coupling of the deflection plates with the 5UP1 position control circuit and it worked, but when displaying the digital clock face the display would occasionally move slightly then return to its position which I am assuming is due to the DC voltage on the couplig cap changing.

Did the same with the menu only it was each time the selected item flashed on and off.

That is unacceptable to me so I'll leave it as is.

That said the designer of the clock circuit did get back to me and said he'd make a note of it and look into it at some point.


The good thing is I can now re-visit my DuMont 292 and use these parts to do it right as a few years ago I had just used a couple dual gang 5 meg pots with no series resistors not knowing any better.


Just got bit again by the B- so I'm going to re-secure the driver board then close it up and call it done. No more experimenting with that one. Too dangerous given the current I have behind the B- supply which is filtered by a .3uF cap after the B- resistor. Plus I've already got the circuitry worked out for the other two CRTs anyways.


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 Post subject: Re: CRT circuit question
PostPosted: Jun Mon 18, 2018 11:54 pm 
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An update.

I've had the clock running around 8 hours a day for nearly a week and it is working great.

No part of the case feels the least bit warm except the plastic cover over the rear of the CRT which is to be expected.

I do think that I will find or make some small TO-92 heatsinks as I don't like how warm the transistors get even though they operate just fine.

I think of it as extending the longevity as the transistors will be a little cooler.

Best part is the image is pretty much where it should be when I turn the clock on (plugged into the same power strip as my stereo so it turns on and off with it) and is only around less than 1/4" off from where it normally is.

That is a far cry better than the original scope circuitry even though the image isn't quite as perfect as it was with the scope circuitry, but it isn't noticeable unless using the binary clock display.

Now if the designer of the clock circuit can work up a special firmware that makes the round clock faces fill the round CRT when it is adjusted to properly display the calibration square I'll call this circuit finished.


What I might do is get a solderless breadboard and build a duplicate of the deflection amp circuit so that I can use it to easily further optimize the circuit to provide an even better display and will also permit easy testing of other CRTs.


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 Post subject: Re: CRT circuit question
PostPosted: Jul Wed 18, 2018 1:31 am 
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Clock is still running great and keeping reasonably good time.

The 3" version I built in the OS-8 scope is working great as well.

I have one more driver board and I'm thinking of maybe building another using a 5ADP5 provided I can find the proper variable caps.

Once I use the last driver board I may experiment with the circuit using a different transistor that has a greater bandwidth.

Perhaps that will lead to the development of my own circuit that works well.

Would be a good way to put scopes that are pretty much worthless for measuring signals (such as a scope with the 3AP1 or cheap scopes) to use as a functional clock.


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 Post subject: Re: CRT circuit question
PostPosted: Jul Wed 18, 2018 3:21 am 
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Location: pensacola fl
Hi. Those scopes also make great am modulation monitors. You could build a 3 incher in with a transmitter. Set it up waveform and or trapezoid pattern. Good way to tune up accurately. Just a thought.


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 Post subject: Re: CRT circuit question
PostPosted: Jul Wed 18, 2018 4:01 am 
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That's definitely another use.


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 Post subject: Re: CRT circuit question
PostPosted: Apr Fri 05, 2019 4:18 am 
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So someone posted in the April finds and losses that they got an OS-8 oscope.

I PMd them about the scope so as not to derail the topic and the conversation turned to oscilloscope clocks.

He posted this website

http://www.sgitheach.org.uk/scope3.html

That is a real nice scope clock from what I saw perusing around the website and is much more functional than the Dutchtronix clock board I used.

What drew my interest was the deflection amp circuit that clock uses which is based on the deflection amp of a Tektronix 221 oscilloscope.

Here's the circuit I may use.

https://www.dropbox.com/sh/rldue6seyzty ... tracking=1

I will need to redo the B+ supplies so that I have + and - 12 Vdc and will need to double the winding for the 270 Vdc B+ so that I can get a high enough voltage to where I can use a resistor then use three 100 volt zeners in series to produce 300Vdc B+ which the board requires.

This deflection amp will replace the whole CRT driver I currently have installed and should have better bandwidth which looking up that model scope shows it has a bandwidth of 5MHz so it should be more than adequate enough for the clock.

I talked to the designer and it will work with the Dutchtronix board as the deflection board does have external inputs for the X and Y.

The idea for now is to use just the deflection amp.

In the future given the dutchtronix board is no longer available as a kit, I may buy the full scope clock kit and use either the scope I had with the 5BP4 or a B&H model 34 scope chassis and install the whole clock circuit power supply and all.

That clock has many more features than the Dutchtronix clock does.

EDIT:

If the deflection amp works good and the board is small enough I will install another in the clock I made with the OS8C scope using the 3RP1A CRT.

EDIT 2:

Here's the modified power supply.

Attachment:
Power supply.png
Power supply.png [ 19.02 KiB | Viewed 221 times ]


The isolated DC-DC converter will be good as it keeps the extra 6.3 Vac winding isolated from ground and maybe it won't develop the problem the other power transformer did or if it does the maximum isolation voltage of the DC-DC converter is high enough to withstand the voltage.

My original thought was to put the 45 volt winding in series with the 212 volt winding and run the clock either off the input of the DC-DC converter or off the +12 volt output of the DC-DC converter. Figured it was best not to do that as in the 3" version of the clock I used the +/- 5Vdc version of the converter and I had to reduce the current through the LEDs to keep a small voltage drop on the +5V rail from happening each time the green LED turned on and shifting the image slightly.

I used 100uF caps for the HV B+ doubler to ensure I have a pretty stout supply. Not sure yet what series resistor I will need as that will depend on the current drawn by the deflection amps and the resistance needs to be such that B+ does not drop below 300Vdc under normal line voltage variations.

I fully intend at some point to build a scope clock using his whole kit and the 5BP4 CRT I have.

Eventually I'll do the same with a 5ADP* series CRT and a B&H model 34 scope with a 5DEP1 CRT.

Only thing I am not too fond of is his clock uses a remote control and far as I know doesn't have any buttons.

Not really a problem, but it is one more thing I have to keep up with.

EDIT 3:

Here's the deflection amp.

Attachment:
Deflection.png
Deflection.png [ 22.94 KiB | Viewed 213 times ]


The X POS pot originally operated off +/- 5Vdc along with the bias for the OP-AMP which operated on -5Vdc

Now it looks to me like the bias voltage through the 100K resistor may be to offset the DC output of the DAC. If that is the case then that may solve the issue of needing to reduce the vertical and horizontal level pots when displaying square menus and I can simply use a DPDT switch to select whether a pot is in parallel with the size pot on the clock board and a pot to set the bias. The size pot would be adjusted to where the calibration square will properly fill the screen while the bias pot would be adjusted to make the display centered.

Now I'm thinking of ditching the front panel vertical and horizontal size pots and making them the panel mount screwdriver adjust type of pot and if I do that I would do the same to the size pot of the clock board along with the second size pot and bias pot.

That way I have those adjustments available without removing the side panel of the clock. I could add more standard controls which would use more of the original knobs and make it look more authentic, but those would be easier to accidentally change the setting.

I could also do the position pots and focus and intensity pots the same way, but I do like the look of the knobs so I'm not sure exactly what I'll ultimately do.


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