Minolta 7000: Memory backup lithium battery, main batteries, functionality, 7000 or 9000, disintegrated plastic, cleaning contacts, voltages

Andreas Thaler

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Completely unexpectedly, my neighbor gave me an obviously unused Minolta 7000 with two equally new Minolta AF zooms, complemented by a Minolta Program 2800 AF flash, which works just like all the other devices.

Sometimes things just come to you on their own


Today I looked at the built-in 3 V lithium backup battery, about which the following information is provided in the user manual for the 7000:



massimoscottinelweb.com


This means that the battery lasts for ten years.

Assuming the camera was manufactured in 1985, that would mean an end date of 1995. So 30 (!) years ago.

Since I didn't see a battery warning (flashing ISO display) on the LCD, I assumed that the battery still had enough voltage.




The battery plate in the handle of the 7000 after I removed the battery holder (screws are already loosened).




6 x JIS 000 cross-head screws






The two cables with the contacts to the main batteries are soldered to the plate.






1 x CR2016L






The battery has an overvoltage of 3.25 volts, as if it had just been installed, new and unused.

That's unbelievable, because it is constantly under load when the main batteries are not inserted.

And I don't think that this 7000 was stored with batteries inserted.

Remains of crystallized battery electrolyte in the battery holder suggest that there hasn't been any voltage here for a long time.

I also don't believe that the memory battery has been replaced.

This reminds me of the backup battery in the Canon T90, which apparently also lasts forever; at least I don't have any of my >10 T90s with a dead battery.

Very impressive!
 
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Paul Howell

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The back up battery in my Minolta 7000 is dead, I have not taken the time to replace it, I guess I should. A great gift.
 
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Andreas Thaler

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Insert backup battery with correct polarity

After inserting the backup and main batteries, all LCD displays flashed for a few seconds. I turned the camera on but it didn't respond to any inputs.

I cleaned the contacts on the battery plate, inserted the backup battery with the polarity reversed, again no response.

It worked the third time, probably a contact error.

Since the backup battery fell out when I removed the battery plate, I couldn't determine the correct polarity, and there is no marking.

But the polarity, i.e. how the battery has to be inserted, is determined by the contacts on the battery plate and on the main batteries holder:



The straight arrow points to the negative connection of the main batteries holder, whose potential is identical to the potential of the battery plate (ground).

The three white circles mark the contact points for the backup battery and are also grounded. This means that the backup battery must be inserted with the positive pole (engraving) facing downwards.

The double arrow symbolizes the connection between the negative pole and ground.




Here you can see the bottom of the battery plate. The red arrow points to the positive connection for the main batteries holder.




The connections on the main batteries holder confirm these considerations.
  • white arrow: negative connection,
  • red arrow: positive connection,
  • polarity determined with the multimeter.
 
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Andreas Thaler

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I removed the backup battery again and reinserted it. This time it worked perfectly. Again, all the displays on the LCD flashed for a few seconds.

The open circuit voltage is now 2.42 volts (-0,83 volts) probably the measurements put a significant strain on the battery (rather unlikely) or I short-circuited the battery with the tweezers or its capacity is fragile. Whatever.

In any case, the backup battery maintained the previously set ISO value after removing the main batteries. Whether it continues to do so is irrelevant, as I will anyway be replacing it.
 
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Andreas Thaler

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There is a separate SPT Journal for the Minolta Maxxum 7000, most likely written by Larry Lyells, to whom we owe most of the technical publications on photographic equipment.

The technology of the 7000 in detail with troubleshooting and disassembly instructions:

SPT Journal 1997

Issue dedicated to the Minolta Maxxum 7000
learncamerarepair.com
 
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Andreas Thaler

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Protection against wrong polarity

This section of the circuit diagram should answer the question of whether inserting the backup battery with the wrong polarity can cause damage to the electronics:




By switching a diode in the direction of flow of the current, the circuit is protected.

If the polarity of the backup battery is wrong, no current can flow against the reverse direction of the diode (SPT, page 39).

I am putting this posting up for discussion, hence in grey. See comment by @Nicholas Lindan below.


+++

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

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CMOS circuitry and a display "sleep" may have reduced current draw on that coin cell to effectively zero for the past forty years. Either that, or your camera was in some kind of time warp for all those decades it spent in the top of a closet or back of a sock drawer.

More seriously, I've seen coin cell powered devices that came with a plastic pull tab that removed an insulator from one battery contact -- if that camera was actually new in box, that tab may have been present until pulled by whoever sold/gave it to you.
 
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Andreas Thaler

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I rule out an LCD power saving circuit, as the backup battery only saves digital values, but does not supply the display with voltage.

In any case, Japan's secrets are kept here in Mount Rokkor
 

Sharktooth

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It's "interesting" that Minolta chose to use a limited life battery that was not intended to be user replaceable. That really defies all logic, so I consider it engineering "stupidity". I've actually seen an even worse example in the Fuji GX680. The first and second versions used a 1/2 AA lithium battery that was soldered in to the back. When the battery dies, you're expected to take it back to Fuji for replacement. That obviously isn't available now. You can replace that battery yourself if you're capable with a soldering iron, but it really was a bone-headed design. At least they finally figured it out for the third version, and used a user replaceable battery.
 

Nicholas Lindan

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The diodes in the above schematic are 'auctioneering diodes' that cause power to be supplied by the higher of the two battery voltages - either the 3V lithium battery or the 4.5V 3xAA battery pack.

A diode is needed in series for another reason: connecting a non-rechargeable lithium battery to a higher voltage will cause the lithium cell to burst into flame. The diode prevents current flowing back into the battery.

Panasonic/Matsushita batteries are the best available and are very long lived, however 30 years is pushing it. The battery may test at 3V but it won't be able to supply much current.

In memory backup applications the drain on the battery is in the nano to pico amperes - less than the battery's internal leakage current. The battery will last as long installed in the camera as it will in it's unopened blister package.

Matsushita batteries are available from DigiKey, Mouser, Element 14/Newark, Arrow, et al.. Batteries sold on Amazon by other than Amazon, and, of course, on ebay, have a good chance of being counterfeit.
 
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Andreas Thaler

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Thanks for the hint!

I was wondering what the dashed line around both diodes meant and understood it as one component.

I was able to edit my post and put it up for discussion, see comment there.
 
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Donald Qualls

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Chan Tran

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The memory backup battery in my Nakamichi CR-7a cassette deck is the Sanyo CR2032-FT10

SANYO CR2032-FT10

Replacement for CR2032-FT10 (for SANYO) and others Batteries Lithium
www.interlightus.com

It's just the CR-2032 with solder tab and since it's soldered on to the PCB I didn't feel like replacing it until it's 25 years old and when I took it out it still had more than 3V output.
 
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Andreas Thaler

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Review of three more Minolta 7000 AF

Today I removed and measured the backup batteries from two more Minolta 7000s.



Both were inserted in the manner described, i.e. with the negative pole facing upwards and in contact with the metal cover plate.

I assume that the batteries have not been replaced since the cameras were manufactured, but I have no proof of this.









The voltages were 3.29 volts and 3.26 volts, and all batteries are in perfect external condition.

I checked the correct function of the backup battery after inserting it as follows:
  • set any ISO number except 100 (which is obviously set after replacing the backup battery), e.g. ISO 320,
  • remove the main batteries and insert them again,
  • check whether the previously set ISO number is displayed unchanged.
 
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Andreas Thaler

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Main batteries



There are two battery cases for the main batteries:
  1. BH-70S for 4 x AAA and
  2. BH-70L for 4 x AA, which also offers an enlarged handle.



Here you can see the battery cases that came with my 7000.

All of them show corrosion on the contacts due to battery electrolyte.






The big advantage of both battery cases is that leaking batteries do not come into direct contact with the battery contacts on the camera.

This protects the camera electronics behind the contacts from damage that can only be repaired by dismantling the camera.

I remove the corrosion in the battery compartments with a bath in Durgol descaling solution.
 
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Andreas Thaler

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Functionality: a sample



Of four Minolta 7000 AFs, three work perfectly, one has a defective aperture control. This means that only the smallest value is ever generated for all aperture settings.

There are repair instructions for this:

I fixed my Minolta 7000 (aperture magnet problem)

groups.google.com




The LEDs are 2 : 2 - two are intact and two have leaked at the edge, which does not limit functionality.


I tested all cameras for plausibility of functions (except for the one with the defective aperture control):
  • aperture control,
  • AF,
  • shutter,
  • operating modes,
  • exposure and AF sound signal,
  • backup battery,
  • LED displays in the viewfinder and on the housing.
They are now ready for the test with film.

The camera with the defective aperture control is intended as a repair project.
 
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Andreas Thaler

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Minolta 7000 AF or Minolta 9000 AF?



Apart from the capabilities of both cameras, I would choose a 7000 when it comes to being ready to take photos.

Apart from a defective aperture control and external LEDs that occassionally are running out („bleeding“), I am not aware of any other typical weak points with the 7000.

The 9000 suffers over time or occasionally from a sticky shutter, a defective aperture control, misalignment of the mirror, a blocked shutter and virtually all external LCDs are affected by bleeding.

Repairs, except the misaligned mirror and external LCD, are only possible by removing the mirror box, see current threads.

Both Minolta sisters suffer from disintegrating plastic parts on the housing (grip surfaces).
 
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Andreas Thaler

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Removing battery electrolyte



Soak in Durgol Descaler undiluted for 30 minutes and then rinse with water.




This will remove the battery electrolyte, which progressively attacks metal.




Damage to contacts that has already occurred cannot be removed in this way.
 
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Andreas Thaler

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Coating on the handle, sticky and disintegrating plastic



The whitish coating can be wiped off best possible with isopropyl alcohol, which also reduces the stickiness.




Plastic parts that break off on the handle can be glued with Pliobond.

Missing parts can be recreated with Sugru:

Post in thread 'Minolta 9000: Checking shutter and focusing screen, adjusting the mirror, replacing parts, cleaning'
https://www.photrio.com/forum/threa...-replacing-parts-cleaning.210960/post-2857893

Post in thread 'Minolta 9000: Checking shutter and focusing screen, adjusting the mirror, replacing parts, cleaning'
https://www.photrio.com/forum/threa...-replacing-parts-cleaning.210960/post-2858854

Repair work: Sugru, the multi-purpose mouldable glue

For small repairs on cameras, I like to work with Sugru, a mouldable adhesive that can be kneaded and shaped. After hardening, it replaces plastic. https://sugru.com/ https://en.wikipedia.org/wiki/Sugru Here are two projects where I used Sugru. Alignment of a battery contact on a Canon...
www.photrio.com




Alternatively, the plastic can be removed completely, as with the Minolta 9000 AF:

Post in thread 'Minolta 9000: Checking shutter and focusing screen, adjusting the mirror, replacing parts, cleaning'
https://www.photrio.com/forum/threa...-replacing-parts-cleaning.210960/post-2858854

However, I don't know what the handle looks like under the plastic on the 7000.


+++

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

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Andreas Thaler said:
You had a generous budget, the Minolta measuring devices were certainly not cheap
Click to expand...

I used a beautiful Minolta color spectrophotometer for measuring color of molded plastics and paint. This was about 30 years ago. I remember it had a focusing screen that looked like it was borrowed from an SLR for aiming the measuring sensor. Pretty neat.
 
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