[Heathkit] Final update on the SB-630 Monitor

[Mike McCarty]

jack m wrote:
> The SB-630 is now timed to the gnat's eyelash - 12 repetitive
> timeouts with the stopwatch reading 2 hours and 4 seconds on the last
> one!

Nothing succeeds like success. Great job! Congrats!

> One final modification was made worth noting to those interested.  I
> noticed that the relay was sluggish, and sometimes did not return to
> full NC which caused the buzzer to not drop out without repeated

One possible cause would be too much idle current; maybe the
pulse coupling cap is leaky. That would be C22.

Another possibility is a problem with the "hold up" circuitry,
that is  C25, R32, and R23.

[...]

> After measuring all voltages in the associated circuits and
> substituting a brand new NOS 6EW6 relay control tube with no effect,

If the voltages are correct, then the idle current is likely
ok, so I'd investigate the hold up circuitry. Have you measured
the resistors?

> I placed my resistor substitution box across the 220K ohm resistor
> between the cathode and screen grid of the tube.  I started with a
> parallel 220K and worked down to 68K.  The 68K caused relay dropout
> too quickly, but it sure was solid.  I finally settled on a parallel
> 220K resistor, which resulted in 110K across the tube pins.

Well, if it works, it works.

> Everything is solid now with a 1-second delay every time.

Ok, then the hold up circuitry is probably ok.

> I also noticed that the neon light flashes only when the 6EW6 is cold
> and during it's warm-up cycle when using the reset button.  I never
> did get to visually "catch" the pulse of the 10 minute intervals, so
> I can't verify that.  But the pre-6EW6 warmup period (about 15
> seconds) may be an easy way to verify operation of the neon light.

I'm not surprised. The lamp is only on until the relay closes, and
dumps the charge on C23 and C24 through R28. The time constant
there is 4 ms, so even five time constants to complete discharge
is only 20 ms. That's a very brief flash. However, it only needs
to discharge down to extingush voltage, that is 50 V, when the
other side is just at BV, which is only 60 V or so for the 3AG.
So, just to drop 10 V from about 150 through 1K is not long.

Lessee, the initial discharge rate is 150 mA or so. With only
4 uF of capacity, the initial rate of change of voltage is

dv/dt = i/C = 0.15/4e-6 = about 3kV / sec.

That's going to be practically no time at all to go down
only 10 V or so. 10V / 3KV/s = 10Vs/3000V = 3 ms or so.

A 3 ms flash is pretty brief, especially when not very bright,
and not repetetive.

Anyway, outstanding job!

Mike
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