[Heathkit] SB-630 information 2nd request - problem discovered!

[Mike McCarty]

jack m wrote:

[...]

> So I figure this is NOT an NE-2 in the SB-630.  With the NE-2, the
> ID/buzzer just goes on and stays there, but can be released with a
> plastic tool prying up on the relay.  Without the NE-2, the reset
> button will pulse the relay on/off.
>  
> Upon magnified inspection, I see a "GE-A6" cast into the broken
> bulb's base, whereas I see a "GE" and "2" on my ten neon spares.

I've got a copy of the GE Miniature Lamp catalogue, and it doesn't
list a GE-A6. So, I pulled my copy of the GE Glow Lamp book, and
looked in the specs section, and there is no GE-A6. It gives
both the "old" and "new" designations.

> Just a copy of the parts list would be very helpful, as well as any
> insight to the neon bulb used in the relay circuit.  Any pages from
> 29 on back would also be appreciated.  Many thanks to those who have
> helped so far.

It's probably not going to be helpful. It'll probably just
say it's a selected neon bulb with a dot.

Let's look at the circuit and try to figure out what it does.

C23 and C24 start out discharged, and the lamp is off. The
caps charge up through the 144 Meg resistance until the voltage
across the lamp is sufficient to make it fire. The sudden
flow of current causes the voltage at C22 to jump, and this
gets passed along to the tube grid, pushing it into conduction,
and energizing the relay.

When the relay pulls in, the charge on C23 and C24 gets dumped
to ground through current limiter R28, with a time constant of
4 ms, that is, nearly instantly. It would be about 20 ms to
complete discharge. At that time, the voltage is mostly
from the 220K R33, hence only about 1/2 % of the
voltage on the pot. At this time, the voltage from the pot
through the 220K resistor must be less than the maintaining
voltage of the lamp.

So, the lamp is part of a relaxation oscillator. It's probably
a selected one, with a known breakdown voltage and resistant
to "dark effect", possibly with a coat to imcrease leakage
resistance (it's in a 140Meg circuit, after all).

The 0A2 regulates somewhere between 140 to 168 V. That means
that the range of voltages on the pot are

Vreg.	Range on pot	Across Bulb
------	------------	-----------
140V	54.6 to 57.4 V	82.6 to 85.4 V
168V	65.5 to 68.9 V	99.1 to 102.5 V

So, we need a bulb which is pre aged and has some radioactive
material for stable operating characteristics, and likely
a coat for low leakage. It must be able to fire reliably
on less than 82.6 V, and must extinguish reliably when
the pot voltage is applied through 220K. The latter requirement
is a little harder to figure, since the supply has an
impedance I haven't figured, but is something on the
order of 90K. We'll just look for lamps with an extinguishing
voltage close to the maximum strike voltage (reduced operating
range).

Looking in the GE glow lamps, I find these as candidates

	Initial Dark BV	
	---------------	Extinguishing
Bulb	Min	Max	Voltage (250K)
----	---	---	--------------
3AD	65	80	50		edgy high BV, good candidate
3AG-A	60	80	50		edgy high BV, good candidate
3AG-B	65	73	50		sel. 3AG-A, excellent candidate
3AG-C	68	76	50		not as good as 3AG-B
5AB-A	60	80	50		cheapest preaged, good candidate
5AB-B	62	72	50		selected 5AB-A, excellent candidate and low cost
5AB-C	68	78	50		selected 5AB-A, excellent candidate
5AG	64	80	50		good candidate
5AG-A	68	76	50		excellent candidate

My best guess is that for the lowest cost most likely to
succeed lamp, try the 5AB-B, if you can find one. Next best
is probably the 3AG-B. Anyway, this table should get you
in the ballpark for a lamp which will work.

Try not to handle coated bulbs very much. Oils etc. from
your hands may compromise the leakage resistance.

HTH

CC: original poster

Mike
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