636 Flash for SX-70 Sonar

Stephan Wagner has converted the flash from a Polaroid 636 into an external flash for his SX-70 Sonar

Translation by George Holderied. The german original text is here.

The flash has a flashbar socket and can be removed from the camera.
No changes need to be made to the camera.

Flash features:

• The flash is supplied with power from the film pack battery inside the camera. Because of this it can only be used with Sonar-AF cameras which have four extra contacts at the rear of the flashbar socket.
• Recycle time is ca. 1.5 seconds.
• Camera shutter is blocked until flash is ready.
• Flash recycles after image is ejected and shutter button is released.
• After pressing the shutter button flash recycling is inhibited until shutter is released and focus is back to park position.
• When focusing manually, the shutter must be kept pressed until the picture is ejected. Here I cannot inhibit recycling automatically because the contacts give me no information whether the eject motor is still spinning. (The battery does not have enough power to eject / focus and recycle the flash at the same time.)
• The flash can be switched off (to conserve battery power and while it is plugged in or removed).
• The most difficult thing was to find a suitable connector that fits the flashbar slot, as it also uses the rear contacts. I've used a print from a defective Polatronic 1 flash and some Vero-Board for the rear contacts.
  Contact strips from used flashbars could be modified as well.

List of components:

• 1 Polaroid 636 camera or similar with working flash.
• 1 resistor 10 Ohm 0,5 W
• 1 resistor 4,7 k Ohm 0,25 W
• 1 resistor 10 k Ohm 0,25 W
• 1 resistor 10 k Ohm 0,5 W (only used to discharge the flash capacitor)
• 1 electrolytic capacitor 100µF 350V
• 1 electrolytic capacitor 33µF or 47µF 350V
• 1 capacitor 470pF
• 2 diodes 1N 4148
• 1 Mini-switch
• ca.1m stranded wire for connection. Using different colors helps to keep track of connections.
• I've bought all components at www.Reichelt.de for about 20 DM (10 US$)plus shipping
• I've snapped the Polaroid 636 up at ebay for 20 DM
• WARNING Inside electronic flashes there is dangerous high voltage !

Conversion:

1. First try the camera with an empty 600 film pack. Only the flash needs to work. The rest doesn't matter
   After testing the flash you can start to dismantle the camera:
2. Remove the film pack.
3. Open the flash 45 degrees. Now you can unhook the hinges.
4. Cut the ribbon cable.
5. The flash unit can be opened from the bottom side. There are two holes next to the hinges. Push the tabs in and lift the lid carefully.
6. Discharge the HV capacitor (The big one)through the 10k Ohm resistor 0,5W for at least 10 sec. Hold the resistor with an insulated tool ! Danger High Voltage! The resistor becomes hot!
7. Remove the print circuit board, (pull flash upward).
8. Unsolder the HV capacitor, TH1, Q9 and the ribbon cable connector. Q9 is needed later the rest can be discarded.
9. Cut the corner for the switch and cut the PCB trace (see PCB picture below)
10. Mount the 100µF capacitor where the flash capacitor was (Polarity!). Keep the wires at the PCB bottom as short as possible. Mount the33µF capacitor in parallel (see pictures below). This results in a total 133µF and fits well into the flash enclosure. One can also use a 47µF cap instead of the 33µF. I found this results in slightly overexposed images, and space gets tight inside the enclosure.
11. Modifications of the flash bottom. File a hole for the switch and cut a slit for the flash socket connector (see pictures). The flash connector is glued in place with epoxy so that it protrudes about 1mm on the inside.
12. Now solder: (contact # are numbers at the ribbon connector)
13. From # 3 to the emitter of Q10 the 10k Ohm 0,25 W resistor . Emitter Q10 is "(3)"
14. From # 3 and R1 at the side over #3 the 270pF-capacitor .
15. Unsolder the collector of Q6 and bend it upward. Stick the anode of a 1N4148 diode in the hole where the collector was. solder the anode to the print and the cathode to the collector of Q6. Solder the 4,7 KOhm resistor to the solder joint between anode and collector.
16. Solder the second 1N4148 diode in #1 cathode towards the PCB. Leave enough space in between PCB and diode, that you can solder the collector of the removed transistor Q9 there. the anode of that diode is "(2')"
17. The base of Q9 goes to the other end of the 4,7k Ohm resistor.
18. The emitter is "(8')" (I've used a BC 556 instead of Q9, because I didn't know yet if Q9 is used in its original place.)
    (In the pictures below there is a BC556 in the place of Q9). As Q9 (2N3906) and BC556 c and e are the other way, the BC556 must be mounted upside down. (see circuit diagram).
    
19. After connecting the cables to the socket connector it is crucial to add 3-4 layers of insulating tape over the solder joints so that the contacts cannot touch the PCB. Right in the center of the PCB where I placed the socket connector are the 330 Volts which, if they made contact the socket that would destroy the camera.
    After insulating the contacts the PCB is put back into the enclosure.. The switch and the 33µF capacitor are fixed with hot glue. Now put the cover back and the flash is finished.
20. The resistor 10 Ohm goes between #2 and "(6)"
21. connect #6+#5 to "(8+7)"
22. connect #7 to "(1')"
23. connect the new switch at the two cut traces where the original switch used to be:
    the outer trace goes to the center of the switch.
    the other trace goes to one outer contact of the switch. the other side of the switch goes to (7')"

The meaning of the numbers in brackets:
(1)- flashbulb 5 NC
(2)- flashbulb 4 NC
(3)- Power supply from Flash IC (+6V)
(4)- flashbulb 3 NC
(5)- flashbulb 2 NC
(6)- flashbulb 1 flash trigger
(7)- connect to GND for flash mode (camera)
(8)- GND

The contacts with ( ' ) are opposite to the respective contacts
(1')- + voltage when focusing(shutter button slightly depressed)
(2')- + voltage when taking (shutter button fully depressed)
(7')- + voltage when camera unfolded
(8')- + voltage when camera focus not in park position

Contacts of the ribbon cable:
#1- + voltage inhibits recycling (input)
#2- + voltage when flash is ready (output)
#3- triggers flash when connected to GND (input)
#4- stops the flash when connected to GND (input) The 636 has a computer flash function. When there is enough light reflected from the object, the flash is stopped by discharging the HV capacitor through TH1.
As the SX-70 couples the aperture to distance in flash mode there is no need for this function here.
#5+#6- GND
#7- + supply voltage of the flash circuit

Why have you changed the HV capacitor??

The 636 camera's computer flash electronics are in the camera body, not in the flash unit.
The SX-70 needs a constant power flash (such as flashbulbs in the flashbars) and sets its aperture accordingly. The original flash capacitor has a capacitance of 406µF. My first test picture with 600 film was grossly overexposed so I reduced the capacitor's size until I got correct exposure with 600 film. One could also try to reduce the 330V high voltage, but there is no easy adjustment, one would have to change the Z-Diode. It is not sure if the flash lamp would still ignite at the 110V necessary.
The original capacitor is almost enough to expose a SX-70 film, if the exposure wheel is set towards 'brighten'. I don't know if the battery can supply enough energy. (But it does in the 636)

Circuit diagram

Sketch of the complete 636 flash circuit.

Technical description:

1. Original function of the 636 camera 636:

2. Function SX-70 Sonar with the 636 flash:

Do I have to emphasize that I cannot be held responsible for any damages?

Pictures

Underside of the PCB Cut the track at the "=" . The"+" on the picture "+" corresponds to the "+" in the circuit diagram.

A picture of Stephan's daughter Sara, taken with the 636 flash.