The Minolta 7000 AF repair marathon

[Andreas Thaler]

I unscrew the broken bracket.

Overview

The intact lock is attached to the housing as a spare part.

There is also an almost new back door from the spare parts box.

The new back door opens and closes as it should.

I remove the burr of melted plastic with the knife. The cutting disc had worked here.

My spare parts for the 7000.

Cleaning all accessible contacts with electronic cleaner.

When testing the camera, the aperture closes unevenly.

The cause of this is this slightly bent nose on the aperture ring.

Here is the correct position of the nose on the mirror box of a disassembled spare part 7000.

I adjust the nose with the flat-nose pliers.

Now the aperture closes correctly.

The contacts for the shutter release.

In my opinion, the touch switch works in such a way that two contacts are closed via the finger, which so activate the light meter.

 

[Andreas Thaler]

The gearbox is running dry. Where I can reach the gears, I apply some grease.

The top cover is also replaced and connected to the camera by soldering the cables from the hot shoe to the flexible circuit board.

Cleaning the eyepiece.

The service manual does not tell me which side the eyepiece is inserted into the housing. So I'm going back to my disassembled spare parts 7000 for orientation.

I fix the eyepiece with some Pliobond.

I also found a nicer name plate among my spare parts.

There is also an almost new handle.

Removing the focusing screen and cleaning together with the underside of the prism and the mirror. I use the blower and the fine lens brush for this.

The resurrected 7000 in a new look and fully functional again

All functions seem plausible, I will later check the shutter speeds with the Reveni Labs Camera Tester and the light meter.

AF, aperture and shutter sequence are OK.

 

[Andreas Thaler]

This is what the dysfunctional Candidate Yellow looked like before.

I am amazed.

The previous owner had fixed the shutter release bracket, which had obviously become loose, with adhesive tape.

Why he had not used some glue for this remains a mistery.

Only the chipped eyepiece frame and the worn base plate are reminiscent of Candidate Yellow's former external condition.

I had no replacement here, or if I were to exchange the 7000 I would give it a new serial number, which would take away its identity. And mess up my repair log.

Oh, there are still two screws missing!

Exactly five hours of work.

It was great fun again!

Mission achieved

 

[Andreas Thaler]

Conclusion

• For the next time, I now know how to release a blocked lock for the back door. Maybe I would have figured it out by studying the service manual, but the decisive action with the Dremel was quick and caused little collateral damage. Of course, spare parts must be available, so it was no shame to replace the already rotten back door. And I always have fun with the Dremel
• Once again, the causes of the faults were banal. A bent coupling for the aperture and contaminated contacts. No broken ICs or other failures of electronic components.
• I know of two types of aperture rings in the bayonet for the 7000 (post #93). One is made of steel, the other is made of aluminum or an alloy that deforms easily, as was the case here.
• Even if an SLR is worn out and dirty, that doesn't mean that its interior is in the same condition.
• No repair shop would have accepted this 7000. And if they did, you would have to pay for around five hours of work plus spare parts. That is not realistic given the low prices for the 7000 on the market.
• It is a shame about every 7000 that remains in the cupboard or is thrown away because of errors like this. That does not have to be the case, as you can see.

+++

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

 

[Andreas Thaler]

Addendum to Candidate Yellow - Repair damage with Sugru

When I cut open the back door with the Dremel to remove it because the lock was blocked, there were two small damages to the housing.

The cutoff wheel left traces here. This means that the back door may no longer be light-tight in these places. Otherwise, no essential parts of the camera are affected.

This can be quickly repaired with Sugru.

The halberd spudger with the soft blade is also a great help here when applying and shaping it.

By the way this is also an ideal tool, together with tweezers, for changing focusing screens on the 7000.

Done.

Let harden overnight.

Then rework and clean.

Sugru needs to be used up as quickly as possible once the pack is opened.

Here most of it is left over, so it would be good to have some jobs done together to work economically.


Serial number exchange on the base plate

The base plate of this 7000 is also very worn. A nicer replacement from a 7000 for spare parts is available.

To ensure that this 7000 retains its identity, I am exchanging the serial numbers.

The labels made of soft plastic are self-adhesive.

The label fraud is complete

Now this 7000 has a nicer base plate after replacing some parts.

 

[Andreas Thaler]

Shutter speed test

The Reveni Labs Camera Tester shows that the two shutter curtains do not move at the same speed. The second shutter curtain (C2), which closes the film window, is a little slower.

This results in uneven exposure of the film with overexposure in the upper part.

The curtain travel time should be 7.2 milliseconds (relative to 24 mm for the vertical shutter):

1/2000 second

1/500 second

1/8 second

The curtain travel time is set mechanically directly on the shutter and cannot be done from the outside.

To do this, I would have to remove the mirror box again and then find out how to test the shutter with the camera disassembled.

I don't want to go to that trouble here and will end the revival of Candidate Yellow with this.

Since I only photograph on negative film with hybrid processing, the exposure deviation should be manageable in image processing.

I will try this out next week on a walk through Vienna and report back here.

It is always good to have cameras for spare parts, also for study and comparison purposes.


+++

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

 

[Andreas Thaler]

No luck with Candidate Black

The first test started well, all functions were OK.

Only the autofocus was acting up. The lens focused briefly, then there was a grinding noise and the AF clutch turned by itself. It only stopped after removing the battery holder.

After removing the front panel, I saw that two transistors had become so hot that they had melted the plastic.

Now I could also interpret the strange dents that I had noticed on the outside of the panel before.

The C & C Troubleshooting Guide lists the error pattern. Either a problem with the AF motor, which is difficult to turn (and causing a high current flow), or certain defective transistors.

Lubrication is recommended for the motor and the gearbox. To do this, I would have had to remove it using the well-known method of dismantling the mirror box.

I looked at a 7000 that had already been disassembled for spare parts, but I couldn't find any place on the motor that I could have greased.


So I tried my luck with the transistors

It was not clear whether the two transistors had been overloaded by an internal short or a defect somewhere else in the circuit. They were probably defective, as comparative measurements of the resistance at the connections showed.

I noticed that a lot of current was flowing because the batteries quickly got warm. So there was definitely a short.

I desoldered all transistors on the front circuit board and replaced them with transistors from a spare 7000.

I can't say whether these transistors fit, there are several versions of the board. But I tried it.


Unfortunately

this brought up a new error pattern. „Film“ and the frame counter were flashing on the main LCD.

The camera did not respond to any inputs, so I could not test whether replacing the transistors had solved the autofocus issue.

So a problem with an incomplete shutter cycle following the C & C Guide. I turned the motor axis as instructed with the compass tool, but to no avail.

More spare parts and it was good practice in desoldering and soldering SMD components

---

Conclusion

• Even with troubleshooting instructions, there is no guarantee of success. If the error cannot be fixed with them or a new error appears, you are on your own.
• There is not much you can do without instructions if there is an electronic defect. Check the circuit board and the components on it for any abnormalities, check the cables and soldering points, but that is it.
• There is no detailed circuit diagram in the service manual and for a comprehensive check, the camera would have to be dismantled in order to see all the parts of the circuit board. But it cannot be tested in this condition.
• But no reason to be frustrated. Every project takes you further and increases your experience

+++

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

 

[Andreas Thaler]

Checking the removed transistors

I wondered whether they were all transistors at all, the SMD construction is not always clear.

According to the circuit layout in the service manual, this is the case, see all components with „Q“.

I will test the already removed transistors with a component tester to find out more about their function.

 

[Andreas Thaler]

I test the transistors with the component tester and the multimeter in diode test mode.

These are PNP and NPN BJT transistors.

For one of the transistors, the component tester shows a short circuit between two test leads, but this is not actually the case.

This means that there is a low-resistance connection between two of the three connections (base, collector, emitter) in the transistor.

This should not be the case, however, as the connections must either be high-resistance or have a measurable resistance, depending on the polarity.

This transistor is therefore defective.

A measurement with the multimeter confirms this result. The diode tester shows no voltage drop between base and collector in either polarity direction.

The ohmmeter shows a corresponding resistance of 0 Ohm.

The transistor was probably destroyed by overload.

To compare the voltage drop on an intact transistor, between the base and the collector and emitter, approx. 0.7 volts drop in one polarity direction, in the other the diode tester shows no value.

The ohmmeter shows a corresponding resistance in the 100 kilohm range, with the opposite polarity the path is high-resistance, depending on the transistor type PNP or NPN.

---

Conclusion

• One of the eight transistors that were desoldered is defective.
• The transistor was probably destroyed by too much power consumption.
• One cause could be a stiff AF/motor or a short circuit.
• In order to check this, the mirror box of the 7000 would have to be removed, without being able to test the circuit in operation and without a circuit diagram. This effort is not justified.

+++

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

 

[Andreas Thaler]

I am not an expert in transistors, a discussion is welcome

 

[forest bagger]

Hello Andreas!

I desoldered all transistors on the front circuit board and replaced them with transistors from a spare 7000.
...
I am not an expert in transistors, a discussion is welcome

Did you replace each single transistor with the same transistor from the other 7000?
Besides, the two duo diodes in the circuit layout could be defektive, too.

 

[Andreas Thaler]

Hello Andreas!
Did you replace each single transistor with the same transistor from the other 7000?
Besides, the two duo diodes in the circuit layout could be defektive, too.

Yes, I did it exactly. But there are different versions of the circuit board and the printing on the transistors is different.

In the service manual - obviously an early version - the problem with overheating is already listed and the installation of an additional diode is described. There is more on this in the other documents, where major modifications are described.

Perhaps it really is the AF motor that is no longer turning easily. Let‘s see if I have another 7000 in the lot with this problem.

 

[forest bagger]

And what about the two duo diodes D2 and D11, named A4?

 

[Andreas Thaler]

And what about the two duo diodes D2 and D11, named A4?

Both with approx. 660 mV each at the anode on the right (one connection) and two cathodes on the left (four measurements in total).

With reversed polarity the connections are high resistance.

So the diodes should be OK.

 

[Andreas Thaler]

Motor power transistors
Revised for greater current capability.
If you encounter transistor damage from overheating, replace Q12 and the defective transistor, Fig. 31.
To prevent the problem from reoccuring, Minolta suggests adding diodes to the power transistor (diode - 9361-163111). In the winding-motor circuit, connect a diode between the emitter of Q4 (cathode) and the cathode of D2, Fig. 31, and a diode between use collector of Q1 (anode) and the emitter of Q3.
In the AF motor circuit, connect a diode between the collector of Q12 (anode) and the emitter of Q14. Also connect a diode between the emitter (cathode) and collector of Q13.

SPT Journal, Minolta Maxxum 7000 Service Manual, Winding-base Plate Set, issue 1987


I assume that these diodes unload the collector-emitter path by connecting them in parallel?

 

[Andreas Thaler]

I only have wired TTH diodes, that might work with the space.

However, I don't know the specification of the given diodes. If these are power diodes, the space will be tight.

 

[forest bagger]

I assume that these diodes unload the collector-emitter path by connecting them in parallel?

No, Andreas, anti-parallel.
These old simple transistors include no diodes to short-circuit negative voltage peaks which are generated by the inductances of the motor windings.

 

[Andreas Thaler]

No, Andreas, anti-parallel.
These old simple transistors include no diodes to short-circuit negative voltage peaks which are generated by the inductances of the motor windings.

That means that the overload is caused by the inductive voltage peaks? Freewheeling diodes were unknown at the time? Or do the two A4s take over this function?

That surprises me, because such a voltage peak only lasts for a short time and I am surprised that the transistors get so hot that the casing melts.

Isn't it the case that too much current flows through the transistors because the motor is having difficulty turning? That is how I understand the note in the SPT Journal.

 

[forest bagger]

There are two possible causes for transistor burnout:
If the current flow is too high, the collector-emitter path is overloaded, becomes very hot and burns out, which either leads to a short circuit between the collector and emitter or to evaporation, and then there is no longer any measurable contact resistance in any direction.
If a negative voltage spike causes the collector-emitter path to short circuit, then too much current flows and the transistor gets hot.

 

[Andreas Thaler]

The C&C Troubleshooting Guide states that if you have problems with overloading these transistors:

• Check lubrication on M-2 [AF motor] and gears.

• High resistance solder connections or defective transistors. If current is high lube bearings.

C & C Associates troubleshooting guides - Electronic Troubleshooting the Minolta Maxxum 7000, p. 16.

---

I took a closer look at the AF motor; it is located on the left under the mirror box.

It is held in place by two screws.

Motor with gearbox.

Gearbox removed (two screws).


Apart from the note in the technical documentation that removing the motor will interfere with the adjustment of the AF CCD unit to the right, I would not know where to lubricate the motor?

 

[forest bagger]

When you can turn the pinion on the axis of the motor manually only with effort, put some thin oil on the sintered bearings. If that and lubricating the gears does'nt help, I quit.

 

[Andreas Thaler]

When you can turn the pinion on the axis of the motor manually only with effort, put some thin oil on the sintered bearings. If that and lubricating the gears does'nt help, I quit.

Thanks, maybe the problem will come up again, there are still a few candidates in the box.

 

[Andreas Thaler]

Cemented bayonet screws

Today I wanted to replace the aluminum aperture ring on a 7000 AF with a steel one because it was slightly warped in the coupling area.

To do this I had to remove the bayonet ring, or rather try to, which so far had not been a problem.

This time I could only loosen one of the six screws with a screwdriver. Acetone, a soldering iron, drilling and unscrewing - nothing helped.

So I grabbed my Dremel with a tungsten carbide cutter and removed the screw heads. After that the ring could be removed; I have a replacement.

However, I was unable to remove the screw remnants.

I could grip them with my Engineer PZ-57 pliers, but the screws are cemented into the hole with adhesive. It didn't work. Drilling didn't work either.

At least I was able to free one hole with a butane torch, which dissolved the adhesive. But there is too much heat, which damages neighboring components.

I've never experienced anything like this with Minolta.

This is even worse than with Nikkor bayonet screws!

For parts.