From
http://repairfaq.ece.drexel.edu/sam/rball.htm by Samuel M. Goldwasser
See also Notes on the Troubleshooting and Repair of Electronic Flash Units and
Strobe Lights at
http://repairfaq.ece.drexel.edu/sam/strbfaq.htm
Patient: Three-part unit for one of those teeny-tiny Minox spy cameras. :-)
Symptoms: Dead on both AC and rechargeable battery power.
Testing: Not applicable - done via email. :-)
I was contacted about this rig via email and attempted to walk Stan through the diagnostic and repair process. With such an old device, any rechargeable battery was almost certainly dead and any electrolytic capacitors would also be highly suspect.
Since the unit acts totally dead both from its battery as well as the AC adapter, I first suggested replacing the NiCd cells. Having accomplished this, and letting it charge overnight, there is now at least some quite whining to indicate that the inverter is running. However, the ready light still does not come on even after several minutes (cycle time should be under 10 seconds).
I next suggested that Stan should attempt to measure the voltage on the battery as well as the energy storage capacitor to determine how far - if at all - it is charging. I warn Stan to take extreme care around the cap - those can be lethal!
The report isn't promising. On battery alone, the battery voltage is stable at 2.45 VDC and the cap only charges to 48 VDC or so; on AC to 170 VDC. Even the latter is much less than the expected minimum of 300 VDC. What is going on?
Since Stan noted that the capacitors retain their charge for hours, it is unlikely that they are bad - leaky - but just in case, I suggest just replacing them with any sort of capacitor with similar uF rating and at least equal voltage rating (for testing only) - those from disposable cameras would be most appropriate and FREE if you know where to ask!
No change at all. At this point, Stan suggests that he is over his head on this one and is about to give up. So, I volunteer to look at the unit if he pays shipping both ways.
A few days later.....
The flash head itself is about as big as the entire camera with the power supply and charging adapter being somewhat larger. Actually, the power supply is a lot larger. I don't suppose spies generally like to use electronic flash in covert operations too often anyhow!
My first step is to reverse engineer the circuit. I don't expect anything particularly unusual but this will make any troubleshooting a lot easier
Note: The BAY90 rectifiers cross to 1000 V, 2.5 A general purpose diodes.
The Flash Intensity switch, S2, selects between 12 W-s and 24 W-s. There are actually three positions. Apparently, you are supposed to pause in the middle one called "Hold 1 Sec" when switching between power levels for at least 1 second (surprise, surprise!) to allow the capacitor voltages to equalize! I would assume that the reason for this is to prevent damage to the switch contacts.
The flash head is separate from the power supply and appears to be very much like any of the other strobes. However, note the adjustment for the ready light!
I was not willing to completely disassemble this unit so some of the actual components and wiring were guessed
The wall adapter/charger provides both the current to charge the 2 cell NiCd
battery and a high voltage AC output (CCAC) to power the flash when plugged into
an outlet regardless of the state of the batteries. When operating from the wall
adapter, D1 and D2 in the power supply unit in conjunction with C4 form a
voltage doubler that takes the 130 VAC (>80 V peak) output of the adapter and
produces over 300 VDC to charge the energy storage capacitors
Some interesting features I was not aware of previously - which might have helped to narrow down the problem (and possibly give up - as you will see):
Since the capacitors (and other sources of leakage) have been eliminated, on a wild guess, I decide to replace the high voltage rectifiers. The are marked "BAY90" which crosses to 1000 V, 2.5 A (probably less but that is what my ECG book says). Replacing with suitable diodes and.... No change at all.
There is only one other thing that can prevent the capacitors from charging from the AC adapter - an open inverter transformer secondary. This would not be fun. Indeed, all efforts check its resistance failed. The transformer is bad. Can it be repaired? I don't think so - not unless the break is at the end of the winding on the outside. No such luck. In fact, after unwinding all 1,950 turns of #46 wire, I never did find it - probably one of the 50 or so times I thought I broke this super fine wire in the process, it was already broken. :-( No way to get that back together anyhow and the ferrite core was in several pieces as well....
So, go to plan B....
I tell Stan to see if he can locate another similar unit at auction or elsewhere to use for parts - even if it doesn't work. Within a half hour, he replies that an eBay on-line auction lists an identical model - with the identical symptoms - and it was still available at $37. No way I said, it may have the same problem as well and thus not be repairable! (That unit finally sold for $83 - Yikes! - and it could be a dud.)
So, I volunteer to perform a transplant......
There is nothing particularly unusual about the requirements - charge some large caps to around 300 VDC. Any vanilla flavored pocket camera needs to basically do this. However, just any unit would not necessarily work:
Checking the schematics for each of the other battery powered flash units in the document: "Notes on the Troubleshooting and Repair of Electronic Flash Units and Strobe Lights and Design Guidelines, Useful Circuits, and Schematics", it seemed that the one described in the section: "Photoflash circuit from Keystone pocket camera" might work if it could accommodate the 2.4 V NiCd rather than 3.0 V Alkaline battery it expects. This is often the case since Alkaline voltage is not really constant at 3.0 V but drops gradually as they are used up (NiCd voltage is nearly constant until the charge is exhausted). Therefore, it probably should work down to about 2.0 V (but with longer cycle time). And, the lower effective series resistance of NiCds would partially offset the lower initial voltage. I have a couple of the Keystone units so it is easy to try.
A simple test jumpering 4 wires confirmed functionality. The actual inverter portion of the Keystone flash occupies a volume of about 1" x 1-1/4" x 3/4" or just slightly more than that of the original dead inverter transformer! Some quick action with a hacksaw and nibbling tool resulted in a cute little circuit board that could be tucked into the available space. Some electrical tape assured that there would be no nasty short circuits. The chopper transistor was left exposed so any heat from it would have somewhere to go.
The excised circuit was attached to the positive terminal of the battery, the negative (center) at the switch, and the two secondary leads of the inverter transformer, taking care to get the polarities correct (the waveform out of the inverter is asymmetric and it would not work well if reversed). Except for T1 (dead) and C2 which I removed, all other components were left in place since they shouldn't affect anything.
It seems to work fine on both power settings and on battery or AC. The voltage on the energy storage capacitors stabilizes at about 315 to 325 VDC in all cases. The battery charges fine. What more can you ask? :-)
At first, I thought there was one slight problem: When plugged into an AC outlet with the power switch in the 'on' position (meaning the inverter is also running - the flash operates from AC with this switch off), I was afraid the voltage will eventually climb beyond the safe limits of the capacitors. Then, about 3 AM the next morning I realized there was a missing plastic piece that Stan had not sent me to prevent the switch from being moved into the 'on' position with the adapter plugged in (or vice-versa).
Comments: There is no doubt this unit would have hit the land-fill had it not been for my curiosity in determining what exactly was wrong - since the behavior didn't make sense given my (initially incorrect) understanding of the basic design. Once the nature of the AC adapter was revealed, everything fell into place. Had stand tested the transformer and found it to be open, I would have probably just suggested a nice funeral. :-(
Fortunately, the original inverter was so huge compared with the replacement that space was not a problem. I also believe that the cycle time is now about half of what it was originally so that is an added bonus. I bet the unit will produce more shots on a single charge as well.
Virtually all small battery powered electronic flash units use circuit designs that are very similar. Over the years, parts - particularly the chopper transistor and transformer - have improved greatly, thus the decreased size and increased efficiency.