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

[jack m]

Hi Mike,
 
This certainly is a comprehensive analysis you have described to the list.
 
I received a parts list from a list member which has added to our resources.
 
I took a closer look at the bad bulb, and I discovered that the "GE-A6" I claimed earlier is actually a "GE-AG", which ties in with your findings from the Glow Lamp data book.  Now, for which AG, there may be a clue with the color of the dot(s).  Looking very critically at the dot(s) complex using greater magnification, it "resembles" the appearance of a Venn diagram, with what may have been three color dots originally daubed on the glass.  
 
The (most obscurred) one on the lower left of the triad is either black or green (probably black if I had to guess), the one on the lower right has only an outline remaining of what was most likely brown, and the one on top is probably red.  Looking inside of both the NE-2 and the AG, there is no obvious appearance of doping on either element.
 
I sure wish I could do surgery, but the one lead is broken off up in the base.
 
I checked the part numbers in the SB-610 and SB-620 manuals, and the (similar) neon lamps for each of these kits are completely different, although the SB-620 lists it's two neon lamps as NE-83's in the description.  No description for either the SB610 or SB-630 lamps - just Heath part numbers.  A check of the usual Heath parts sites has nothing listed for neon lamps in the 412-series of part numbers.
 
This is turning into a real detective story!
 
All help is greatly appreciated.  What a resource this reflector is!
 
Regards,
 
Jack W3RU
 
> Date: Tue, 7 Dec 2010 14:26:33 -0600
> From: Mike.McCarty at sbcglobal.net
> To: oldvette at hotmail.com; heathkit at mailman.qth.net
> Subject: Re: [Heathkit] SB-630 information 2nd request - problem discovered!
> 
> 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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