On 09.10.2017 03:55, wrote:
I have this old fluorescent desk lamp from the 60s at the latest
that I'm fixing. It weighs a ton, has been passed down from my
father-in-law, and it just retro-cool. According to him it's always
worked, except the F15-T8 bulbs are possibly original and very worn
out, and only one bulb ever lights. I've been told this has always
been a problem, and holding the start button for a long time will
sometimes get both to start. I've never been successful but I don't
like this and want to fix it.

Upon reverse engineering it I come up with this schematic (it's
rough, drawn in Paint!):


https://imgur.com/a/0YYKV

I'm no fluorescent light expert, but how the start button is
connected to both lamps seems odd to me, a little like directly
paralleling LEDs or neon bulbs after the limit resistor. If I
disconnect one bulb, the remaining always starts. I doubt this
design would ever work very well....would it?

....


This circuit is indeed rather unreliable, one lamp will always fail to
start. The reason is that the filaments have a rather low resistance and
as soon as one discharge is ionized, it will steal the remaining start
pulse from the other lamp through the filament resistance. It will also
overload the lamp that works because additional current will be driven
into it through the other lamp's filament.

There are at least 3 ways to fix this (roughly in order of usefulness):

1. Change the SPST start button to a DPST one, use one pole per lamp.

2. Keep the SPST button but add a DPST relay to start the lamps via the
relay contacts and use the button to energize the relay coil.

3. Put a center-tapped inductor between the lamps as shown he

https://imgur.com/a/lCoNK

This will present a low impedance to the preheat current (inductor has
opposite-phase windings, being essentially shorted) but a much higher
impedance to the ignition pulse (inductor has in-phase windings, being
essentially open to high frequency components from a pulse). The 1kV
varistor is only there to protect the switch contacts from arcing.