A 5" Multi-Purpose Monitor

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Re: A 5" Multi-Purpose Monitor

Postby Klaas Robers » Mon Aug 25, 2014 6:44 am

Dear Steve,
marvellous. The circuit diagrams are almost identical. I lowered the amplification by means of the 470 ohm resistor from kathode to cathode. This brings the bridge a little bit out of balance for equilibrium, but is was only 3 volts at the grids and that can easily be corrected by the pos control. Otherwise I could have used two resistors of 220 ohm.

As you can see I designed at a somewhat higher anode current, so the anode resistors can be lower (22k) than the 47k you use. I hope this makes the HF cut-off frequency higher, but that depends also of the mutual capacity of the deflection plates of course. The EF80 can have that higher current. In fact 10 mA, which can occur when the deflection is statically at one end for some time. Even then the max. dissipation of the tube is not exeeded.

Presently I am thinking about the EHT generation........ Then I can fire up the CRT and see how sensitive it is for magnetic stray fields.
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Mon Aug 25, 2014 4:59 pm

Klaas, it all depends on the application I guess. Here I'm assuming we're talking about ramp/timebase waveforms. Somewhere I read in some valve/tube literature that for timebase-type deflection the bandwidth required is 10 times the sweep rate. I thought that a bit low, but anyway. Applying that rule-of-thumb to standard 625 TV means a horizontal bandwith of around 160kHz is required. (Still seems low to me).

Say the CRT deflection plates, its socket, stray wiring capacitance, the EF80 (or whatever) socket and the EF80 itself amounted to 20pf (a very rough guess). With an anode load of 22k the -3db point is around 360kHz which easily meets the requirements of the above 'rule'.

For NBTV at 400Hz I think we can say it's sufficient. For one of my examples posted yesterday which had 100k anode loads the bandwidth comes out at about 80kHz all things being equal - for a pentode in pentode mode. It will be less for a conventional triode due to Miller effect. There are two ways around this, first use the triodes in grounded-grid mode, second drive the triodes from a low source impedance, say an op-amp output (or a cathode-follower) to swamp the anode-to-grid capacitance. That capacitance is still there though and needs to be added to our fictitious 20pF.

Yea, the downside of CRTs - the ruddy EHT. What I'll be using on this 5" monitor is frankly lethal. Voltage doubled/quadrupled from standard 50Hz transformers to generate +1400V and -1400V.

What type of CRT are you planning to use?

Steve A.
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Re: A 5" Multi-Purpose Monitor

Postby Klaas Robers » Wed Aug 27, 2014 3:39 am

Dear Steve, I am going to use a 5BP1 that was laying around here for tens of years. But is is a nice tube, for me the archetype of an oscilloscope tube. It needs 2kV voltage for a3, which electrode I plan to connect to the + 350 V tube supply. The DC-voltage on the deflection plates will be + 220 mean value. Then I need 'only' -1650 volt on the cathode.

In the past I made an SSTV monitor with the 7BP7 magnetic deflected tube, but that one needed 5 kV. I hope that I can use the same type of transistor EHT generator, based on an 88 mH toroid coil. Due to the high frequency small capacitors can be used in the voltage multipier, so no high currents are to be expected and that lowers the danger.

I want the monitor to use as an oscilloscope as well, so the bandwidth of the deflection should not be too low. For the sawtooth wave forms I plan to use resistor D/A converters driven from digital counters. That makes the difference between a monitor and an oscilloscope even smaller and there are no intrinsic linearity problems.
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Wed Aug 27, 2014 11:47 am

The 5BP1 is a good representative of tubes from this era and quite easy to drive. The advantage(?) it has is no PDA electrode so no high positive volage is required. The 5CP1 is from the same era and very similar but does have a PDA.

I also used a resistive D-A in the 48-line display I built some years ago. Although I used 1% resistors (and matched/measured them) the results were dissapointing. (see left of composite photo below).

I eventually conceded and fitted a 'proper' D-A, a DAC08 if I recall correct. The result is shown on the right-hand side of the photo. (Please excuse the lack of camera focus). I reasoned that it was the slight difference between the logic gates output levels/impedance causing the non-linearity, but I never thought to replace the chip to confirm or otherwise. (A 74HC4040). The resistive D-A was of the R-2R variety so each logic output saw the same load.

The tube used in the photo was a DG7-32. (7cm/3" approx.).

Steve A.

In the left photo it's easy to count the lines, with the 'bunching' in fours, there are 12 'bunches', 48-lines, that's including the sync line at the bottom.

As an afterthough I have added a back-end photo of said display device. It may be such that the cluster of resistors in the centre of the lower horizontal board are the offenders!
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Timebase Linearity 1.jpg
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Re: A 5" Multi-Purpose Monitor

Postby Klaas Robers » Wed Aug 27, 2014 6:32 pm

Steve, this is interesting. It looks that just one resistor needed tweaking of its value.

To start with, the resistors should be high ohmic enough, for an R-2R network i think of 10k and 20k or even higher. Generally the deviations are best visible in the centre, where the value goes from 7F to 80, but this isn't the case in your example. It is in bit 2 (the third), where the value goes from 011 to 100. If you see that you could have tweaked that 2R resistor somewhat to eliminate the visibility. We made even resistor DACs with 1 2 4 8 resistors in which the deviations from the sawtooth are invisible, e.g. in the PIC RGB decoder.

However, it is good to know of this practical experience of yours. May be I have use DACs of the type DAC8 from the beginning.
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Re: A 5" Multi-Purpose Monitor

Postby Steve Anderson » Thu Aug 28, 2014 11:39 am

It only takes a slightly dry joint, or the IC not quite sitting correctly in its socket to produce a result like the above. The R-2R ladder was based on 10k as shown in the photo below. These are being matched for a different purpose but in essence this is what I did for the DG7-32 display. I measured each resistor three times on three separate days and averaged the results to hopefully reduce errors. Then selected them into groups. The actual absolute value isn't that important, just the matching between them. Use the same meter each time which also eliminates calibration errors.

Once you get to six-bits or more it becomes critical, six-bits equals 1/64th, or about 1.6%. any more and it's pro-rata, eight bits is 0.4%.

Steve A.
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Re: A 5" Multi-Purpose Monitor

Postby hasham12 » Fri Jun 05, 2015 11:02 pm

thats really awsome i like this concept
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