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 Post subject: Scope clock modification
PostPosted: Apr Mon 22, 2019 7:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20061
Location: Warner Robins, GA
I have two scope clocks I used a deflection amp circuit in. Anyways recently a forum member posted a couple links to a scope clock.

One clock I really liked as it has a lot of functionality and one can buy the clock board, deflection board and power supply together or separately.

My thought is that since I have two clocks with the Dutchtronix clock board I would just update the deflection amp with the better amp. The next build I will use the whole clock kit.

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Now here's the power supply for the first clock.

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Power supply.png
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I can do the vertical and horizontal size two ways.

1. Keep the existing vertical and horizontal size pots on the front panel.
2. Move the size pot that is on the clock board to the front panel and ditch the separate size pots.

Which is the best option?

The deflection amp has its own size pots which I left on board given they are in the feedback circuit of the op-amp so as not to risk introducing any problems.


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 Post subject: Re: Scope clock modification
PostPosted: May Fri 10, 2019 2:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20061
Location: Warner Robins, GA
For the clock with the 3RP1A CRT I tried powering the clock board from the +12 volt output, but it didn't work so good and I wound up damaging a couple chips. I pulled the two chips from my other clock and tested with them. The clock board worked normally, but the 5V B+ line was only 3Vdc and the regulator was getting warm. I found out I had the regulator backwards and a new regulator properly installed took care of that.

I didn't like the board powered from the +12 volt line as the +/- 12Vdc regulator only supplies 125mA maximum so I decided to try powering the clock from the + end of the 10,000uF capacitor that connects to the 4.7uH inductor. With the clock running I get a B+ to the 5 volt regulator of 8.9Vdc and the output is 5.1Vdc. I varied the rheostat in series with the power transformer primary and the voltage stayed above the minimum regulator voltage.

Now I'm a little confused on how the B+ is as high as it is. I suppose the other half of the 6.3Vac winding going to the connection of both the 10,000uF caps has something to do with it.

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Power 3.png
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Will post pictures of the much improved display when I'm finished with it.

Just have to run three more wires to the power supply board, mount it then connect the wiring to the deflection board.

The deflection board is a little bigger than the board I had used, but this board is much better far as bandwidth is concerned.


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 Post subject: Re: Scope clock modification
PostPosted: May Wed 15, 2019 2:29 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20061
Location: Warner Robins, GA
Here's some pictures of the display

First shows very light burn in and that somehow the deflection plates were reversed, but I don't know if that happened when I did the original deflection circuit or when I did the current deflection circuit.

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Now given the CRT had to be rotated 90 degrees could that be causing this problem?

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Now some pictures of the display.

Notice the barely visible lines. That seems to suggest I may need some compensation in the amp circuit. I don't fault the circuit as I am using it with a different clock board than it was designed for and perhaps the way the intended clock board draws the display, it doesn't require the deflection amp to have as much bandwidth. That said I just used regular wire to go from the board to the CRT socket. Wire is maybe 6" long. I do know that moving one wire seems to have an minor effect of some sorts on the display so maybe I need to try some 300 ohm twin lead which some scopes use to connect the deflection amps to the deflection plates and perhaps I need to then route it a specific way. The board does have places for four compensation caps, but they were optional and the designer didn't think they were necessary with the 3RP1A CRT. Other than the extra faint lines, the geometry is perfect. As I found with the other CRT driver, each CRT has to be compensated differently so I am thinking that's all I need to do here.

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I had a picture of the binary display in a dark room, but when I looked at the picture on my computer it was out of focus.

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 Post subject: Re: Scope clock modification
PostPosted: May Wed 15, 2019 4:38 am 
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Joined: Aug Tue 28, 2018 9:22 pm
Posts: 786
Location: Sanford Fla 32771
Here is mine. Not a Duchtronix but I have the Dutch kit ready to build someday.


Attachments:
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 Post subject: Re: Scope clock modification
PostPosted: May Wed 15, 2019 10:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20061
Location: Warner Robins, GA
Very nice.

EDIT:

Interestingly enough I noticed that when the clock is first turned on and set to half or less brightness the display is brighter then after a few seconds it starts to dim some.

Is that just a function of the CRT as the heater heats the gun up or is that a function of the 4X voltage multiplier I used or is that another issue?

It starts to display a brighter image at 11 seconds after turn on and at 30 seconds I see the image dim some. I'm thinking it may be that the current drawn by the gun electrodes increases slightly as the CRT warms up fully.

I will try the twin lead to connect the deflection plates to the board just to see if that fixes the slight issue which although it does not affect the geometry, it does leave some leading edges on the desired displayed image which would otherwise not be there. I did check it by using an external scope connected to the deflection plates with 10X probes and confirmed it is the deflection amp which just needs a slight compensation which the twin lead might take care of. If not I will experiment with caps in the HF compensation cap location to see what that does. I'll first try fixed caps then if that doesn't work I'll go with variables which will have to be tweaked with a plastic adjustment tool given the caps are across the collector resistor for each final deflection transistor.


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 Post subject: Re: Scope clock modification
PostPosted: May Wed 22, 2019 7:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20061
Location: Warner Robins, GA
So I decided to order four .22uF 2KV caps to use two in parallel between the output of the voltage quadrupler and ground and two in parallel after the 100K resistor and ground. I also put one of the .22uF 1KV caps in parallel with the first .22uF 1KV cap that feeds the quadrupler. That increased B- to around -1.93KV which allowed me to increase the regulated B- to -1.673KV.

I varied the AC from 110-120 and B- did not vary more than 20 volts.

The image itself is a bit sharper and the issue of the focus being perfect in the middle or edges is gone. Now it has great focus over the whole CRT.

I would have preferred a little higher B-, but the zener string would not regulate at 110Vac line voltage if I went 150 volts higher on the zener string.

What I wonder is will the HV winding on the transformer handle the increased B- current?

Given the HV secondary is a tapped winding to provide the center tapped section for the 6X4 rectifier and the 135 volt winding for the CRT I would assume they just wound that secondary with the same gauge wire and as such it would be able to handle the B- current especially since I no longer have any tube filaments on the 6.3Vac winding.

My only other concern is burn in of the CRT.

I've only had the clock operating a few months and as seen in one of the pictures, there is already slight burn in.

If burn in becomes a problem I'll either stock up on 3RP1A CRTs provided I can find them for a reasonable cost or I will try a 3RP1 and see if I can live with the CRT face not being flat.

EDIT:

Here's the final schematic of the power supply.

I added a NE-2 indicator to the B+ line after the regulator as I had an extra one and wanted a visual indication that B+ is present given the capacitance in the B+ supply.

I do need to change out the original pilot light socket with a better one as the lens doesn't let enough light through from the NE-2 in the B- circuit and I most likely have a better one somewhere.

Having to used a resistor to drop the primary voltage to the power transformer so that B+ did not go over 450Vdc worked out quite nicely as it reduces the inrush current plus the transformer operates cooler given it is not running at 120Vac.

Attachment:
Power.png
Power.png [ 68.29 KiB | Viewed 141 times ]


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