Tek CRT DC Restorer Circuits

2018/12/21

Z-amp drive R1485
300 Vpp AC drive C1420
R1422 connected to -2450V Voltage source
CRT pin 2, the cathode on -2450V
CRT pin 3 is grid, control by this DC Restorer Circuits

Purpose of DC restorer Circuits

DC restorer is kind of brain burner. I pretty lost here, every time i come here, It's time to hack into it, figure out how it works. first of all, we need understand what it's doing and for what.

The first intending of the DC restorer here is keep the Grid as negative as cathode , for Tek 465 it's -2450 V. this is the starting point for all DC restore Circuits.

The second function is keeping the screen black. To stop any electron pass though the Grid, Grid voltage need lower than the CRT cathode a certain amount of volt, called cutoff voltage(Vco). DC restorer transfer the CRT bias to Grid thus lower the Grid potential.

The last purpose is couple a Z-amp's un-blacking signal to a very negative CRT Grid. The huge voltage difference between Z-Amp's output (+27V) and the Grid voltage (-2450V) is the main factor why use such a DC restorer.

While the trigger circuits detecting a valid level of signal, control logic kick the sweep circuits, then sweep circuits given a sweep start signal to control logic, then control logic sustaining a un-blacking signal level to Z-Amp(DC amplifier), while this level present, pass through the DC restorer, Grid voltage higher than Vco, we see the sweeping line on screen, while this un-blanking level dispear, ended, the sweep ended also, beam return back to start position ASAP and no beam on screen.

DC Restorer Principle

Diode Clamper ( not clipper) circuits is the fundamental build block for this DC restorer. Under standing this little beast make analysis the Tek DC restorer much easier than running circuits transient states in mind.

A DC bias added to diode, would shift the signal to a negative level, and the signal keep original amplitude.

One key point is the Diode prevent any signal level exceed the Vbias voltage. remember this.

A DC bias for the diode shift the signal further positive, and the signal keep original amplitude.

Similar as Negative shift Clamper, Diode here prevent the signal level lower than the Vbias.

Reference:

en.wikipedia.org Clamper)

How DC Restorer works

This circuits contains 3 DC clamper.

AC signal inject form 47pf capacitor C1420.
clamper 1) bias from the CRT BIAS POT, provide a voltage lower than Vco(CRT cutoff voltage), shift signal toward negative.
clamper 2) bias from DC Z-amplifier, amplifier's Intense control setup a Initial Z-amp output DC voltage. this voltage shift the signal toward positive.
clamper 3 bais from negtive 2450 voltage source. shift the signal way down to the CRT cathode potential level.

The clamper 1) and clamper 2) bias the signal toward two opposite direction, then what happen?

That is a tricky situation, but understandable. CRT bias generally shift the signal maximum -50V, and this also set up the AC signal's Upper limit. At same time Z-amplifier's Intense control potential set up a lower limit for the AC signal , and shift the signal DC up a certain amount, let assume +10.

Combine clamper 1) and 2), it's DC shift function total get -50+10 = -40V, And it's overall limit it's the AC signal no more than 50V and no less than 10 Volta. if AC signal peak to peak is 40, it's ok, AC signal is not been clipped.

But if AC signal is 40 Vpp, and the lower limit, typically for Tek 465 is 27volt, upper limit assume it's 47 V, the AC signal would became a clipped signal while it reach to capacitor C1488. look like following diagram.

Here we are, got a 20 Vpp clipped signal. Forget the DC level, from now on, this 20 Vpp signal going to driven the clamper 3) AS pure AC signal.

The clamper 3) DC bias to -2450V, from CR1488's perspective, it's going to say, after C1488, the potential there, no more higher than -2450V.

Ok, that's is just what the clamper did. now, we would got a AC signal at joint of C1488 and CR1488, which amplitude is -2470 TO -2450.

Now, what the CR1487 does ?

it's simple, CR1487 combine with C1487, form a negative peak detector or Rectifier ! Then we got a DC potential again at Grid, it's -2470 V.

Let's calculating again:

clamper 1) CRT bias shift DC level -47 V
clamper 2 ) Z-amplifier Intense shift DC level +27V
clamper 3) shift DC -2450V

so totally, we should got -47+27-2450 = -2470!!!!

Two different analysis method, Got same Result!

At the end of DC restorer analysis, we need to emphasis how the C1487 works, the 5)th signal path.

IF C1487 not connected to Tip 1486, instead, connect C1487 to ground, what happens? does it work in this way?

Yes, but lost something.

C1487 connected to TP1486 means it connected to Z amplifier a low internal resistance voltage source(compare to Grid, it's low Independence definitely). This mean TP1486 look like a Ground for C1487.

And we got a big performance increase, because while Z-amp output the raising edge or falling edge of the un-blacking signal, the high frequency signal couple to Grid directly via C1487.

DC Restorer: re-think it's function

think it in another way, we could see.

After power up CR1488 charging the C1488 to -2450V, setup the Grid bias.
R1486 22M and C1487 forming a RC changing circuits, time constrain is 33ms. After power off, it discharging capacitors, make it safe.

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