Minolta Hi-Matic GF: Corrosion fixed in the battery compartment, impressions from inside the camera

I already had this small Hi-Matic on the table last year.

The previous owner had probably forgotten the two AA batteries in the battery compartment, which had leaked.

The two batteries were so swollen that I could only pull them out of the compartment with force using combination pliers and a hook:










The cleanup was successful so far:










However, it turns out that residues of battery acid are still active on one of the two battery contact springs.

That's why I'm going to tackle it thoroughly this time and remove the contact spring for cleaning.

Once installed, the soldered/pressed-in spiral springs are difficult to work on because they avoid the tool - as springs they are




The contact plate in the battery cover, which was very dirty at the time, is clean.




The lower contact spring „blooms“ due to the crystallized battery acid residues.

The process is progressively. If not cleaned, corrosion will continue and can eat away the spring.

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Before you open the camera, please consider this:

• You should only open flash units or cameras with inbuilt flash if you know exactly what you are doing. The flash capacitor of even small compact cameras like the Hi-matic can charge to a dangerous voltage.
• The measured voltage on the electrolytic capacitor of this Hi-matic is 170 VDC.
• Such high voltage is dangerous!

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The lower base plate can be disassembled after removing three screws.




There are three more screws under the leatherette.




The leatherette is self-adhesive and can be easily removed.




A screw holds the side panel of the battery compartment.

The contact spring is much easier to reach now, but I want to get full control of it.






Dismantling the upper half of the housing.

The rewind crank can be easily unscrewed from its axle.








The upper deck




The flash capacitor with its two electrodes.

I'll load it later and measure the voltage.

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Be careful not to touch these two contacts or bridge them with the screwdriver.

Dangerously high voltage can be present here!

The same applies to the complete circuit board.

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The contact spring is soldered into the circuit board.




Apply some flux to a piece of desoldering braid ...




… and place the tinned soldering tip on the desoldering braid above the soldering point - the spring falls out of the board.






No solder is left behind.




Overview






The corroded spring is placed in undiluted Durgol descaling solution for about a quarter of an hour.

Meanwhile I look at details of the circuit.

I can see the ignition transformer for the flash tube on the left in the picture.

Since I am not familiar with the circuitry for the electronic flash unit, here is a brief explanation of how an electronic flash unit works:

Electronic flash device [photo encyclopedia]

The electron flash device emits flash light through a glass tube containing a rare gas (xenon). A capacitor is charged with electricity from a landline, batteries or accumulators. It stores the energy until the flash is ignited: an ignition coil generates a high voltage of around 10,000 volts and ionizes the xenon (removes electrons). The discharge causes light to be emitted.

The performance of an electronic flash device depends on the voltage of the capacitor (V), its capacity in microfarads (C) and the efficiencies of the flash (lm/W) and reflector.

Click to expand...

Translated from German into English, source:









The Hi-Matic is cleanly constructed and has a pleasantly high-quality combination of metal and plastic.






The lens can be focused manually.

Four symbols for different distances are indicated on the locking focus ring.










The shutter speed is fixed.

The amount of light entering the lens can be regulated using an upstream screen aperture.

To do this, predetermined weather symbols are screwed into the aperture ring.






The film sensitivity can be set to either 400 or 100 ASA.

Here too, a diaphragm regulates the incidence of light.

Full lighting on the sensor for ASA 400, reduced lighting for ASA 100 - or vice versa, depending on the wiring of the photoresistor.






The extendable flash

In the meantime, the time for the spring in Durgol has expired.

Rinsing in Aqua Purificata and ...




… quick drying with the Dremel Versatip butane gas burner.






Since the corrosion has not only attacked the coating of the spring, but also the metal underneath in some places, I rework it with the stainless steel brush on the Dremel Stylo.

Anything that cannot be reached by the Dremel can be removed with a file.






I cover the sanded areas with conductive silver so that they do not oxidize.

I trust that when there is a high current flow through the battery (charging the flash capacitor), there will be no significant heat development caused by the conductive silver (resistor).

However, when I calculate it, so

P = U^2 / R

There will be a fair amount of power loss releasing heat assuming

P = 3^2 V / 10*10^-3 ohms
P = 900 W

So I will have to measure the temperature after the charging process.








The spring is back and soldered in place.

Subtle correction of the position by bending.




It will be exciting

The flash capacitor is charged, the readiness indicator - a glow lamp - lights up.




The multimeter shows 170 volts DC on the flash capacitor.

A voltage that should not be discharged through the body, as it is potentially dangerous.

So I treat this area with corresponding respect.




A final check into the now completely clean battery compartment.




The Hi-Matic GF is ready for operation again.




Small spring - big effort


+++

All information provided without guarantee and use at your own risk.

Andreas Thaler said:

Since the corrosion has not only attacked the coating of the spring, but also the metal underneath in some places, I rework it with the stainless I cover the sanded areas with conductive silver so that they do not oxidize.

I trust that when there is a high current flow through the battery (charging the flash capacitor), there will be no significant heat development caused by the conductive silver (resistor).

However, when I calculate it, so

P = U^2 / R

There will be a fair amount of power loss releasing heat assuming

P = 3^2 V / 10*10^-3 ohms
P = 900 W

So I will have to measure the temperature after the charging process.

Click to expand...

Done

After 2 x (5 x flash charging) in a row, the flash reflector becomes warm.

The two contact springs in the battery compartment come onto ...




… 30.6 °C (87.08 F) for the spring treated with conductive silver, and …




… 30.3 °C (86.54 F) for the untreated spring.


This is how precisely it can be measured with the thermocouple connected.

So I don't see any problems with overheating, although I'm interested in that from the electronic side.

Since the conductive silver does not completely cover the surface of the spring, the current probably runs through the untreated areas, which have lower resistance.

Hence the only slight warming.

At least that's how I explain it.

And here is the big sister

Hi-matic AF2

with autofocus:



Everything is very high quality with metal, advanced exposure control and a faster lens.


A nice family, although not complete yet:

The Minolta service manual for the Hi-matic GF is available here:

Excellent write-up! And to think I just use vinegar to clean up stuff like that and call it a day.

KerrKid said:

Excellent write-up! And to think I just use vinegar to clean up stuff like that and call it a day.

Click to expand...

Thank you

Vinegar is a good remedy for battery acid. But the aggressive stuff likes to crawl through cables and onto circuit boards. While it's cleaned out in the battery compartment, it often continues to rage inside the device.