Old Sunpak Auto 26SR photo flash - Page 1
Forum section: electronics / repair

author: Rhombic Drive
author_rank: Newbie
timestamp: April 04, 2020, 07:12:13 am
content: I recently picked up this flash on eBay for $6. It was in "used" condition, and not labeled as "not working/for parts" so I thought it might work. It does not! Obvious evidence of leaked batteries. I took this as an opportunity to spend a little extra money and a lot of extra time, to try to fix something I could otherwise replace with a modern working device for little cost. Should be fun.After cleaning the battery mess with vinegar, I tested the unit and the photo flash capacitor is not charging, or at least I didn't hear it charging and none of the LEDs or test lamp came on. The batteries felt a little warm when I took them out so there is some current flowing somewhere. I ended up disassembling everything and figuring out the schematics (attached as GIF). I won't call this "reverse-engineering" yet since I don't fully understand everything in this flash.I'm guessing the problem could be the inverter transformer, which would be unfortunate since I've read they are difficult to replace with a similar part. This one is only labeled "138 5ZA" and all I know is the secondary coil is around 2.1 H, 107 Ohm. I can't get an inductance measurement on the primary coil or feedback coil tap with my GM328 transistor tester (which should be able to detect > 0.01 mH) but their resistances are slightly under 1 ohm each. Assuming the transformer takes 6 V from the 4 AA batteries and puts out around 350 V, the inductance ratio should be about 3400, so the coils should have inductances near 0.5 mH. Not feeling optimistic about this possibility.All the resistors and capacitors seem to be near spec and working ok. The main photo cap is labeled as 650 uF, 350 SV, but I measured it at around 780 uF, right at +20%.I'm also not sure about some contact pads at the top flash lens zoom (seen in the schematic), which were covered with glossy scotch tape. This flash should change the LEDs on the back depending on the zoom position, illuminating the zoom level and changing the recommended power settings on the LED boards. No idea why they're covered up as I don't see how they can do anything in this state.I was also thinking about what to do in the case of needing to replace the photo flash cap. They are quite hard to find in the right size and capacitance. CDE makes the 7P601V330K042L ($30 bucks at Mouser, unstocked), there's the 687FSB330XGDG from Illinois Capacitor, the FK series from Rubycon (not in stock anywhere I could find) and possibly the LQS2V561MELB45 from Nichicon. I was thinking about other possibilities like the Nichicon LLG2G681MELB45 or UC-C ESMQ351VSN681MR45S. They are not photo flash caps but have similar dissipation factor and ESR.I did a quick calculation assuming 1/1200 s per flash, 350 V, 780 uF. The photo caps are typically rated at X number of flashes, continuously every 30 s. While the power during the ~50 J flash is in the 10s of kW, averaged over the rated flash interval this averages out to about 1.5 W. The normal caps are rated for max continuous ripple current, which for the ones I mentioned above is gives a power of around 1.7 W. If the caps aren't specifically rated for rapid discharge then I'm still pretty wary about using them, but thought I'd ask for opinions here anyway.Basically the components that will be difficult to replace without completely reengineering the circuits are probably: 1) photo cap 2) inverter transformer 3) trigger coil 4) thyristor array (Mitsubishi SA02), and possibly 5) flash tube. If anyone can suggest or verify ID of the components I've highlighted in red in the attached list, that would also be helpful. Maybe I'll rebuild everything from scratch as a hobby project. I have to admit I have no idea how precise all the component properties (BJT gains, potentiometer positions, photo cap capacitance, transformer ratios etc.) need to be in order to produce light of a desired intensity and duration. Not sure how I would calibrate it at this point...Thanks for any input! Apologies if I've left anything important out. First post and all.

author: meese26
author_rank: Newbie
timestamp: May 30, 2025, 08:32:21 am
content: Hey mate, I'm new to working on electronics (cameras and flashes specifically) and i was scouring the internet trying to find a service manual for the SunPak auto26SR until I came across your post. Do you have any photos of when you were putting the camera back together? a bunch of the metal fell out (that I'm guessing is a connection from the batteries to the circuit) and I'd love an reference to what it's meant to look like.