Inside the Minolta Program Back Super 70: Contact problems resolved

[Andreas Thaler]

Since this control back for the Minolta 7000 AF quickly forgets the time and date settings and showed low voltage for both fresh LR44 batteries, I opened it to have a look.

One of the two batteries in the second battery compartment had leaked and it could have damaged the circuit board behind it.

It still opens without clip fasteners and there are a few screws and a handful of gold-plated and spring-loaded contacts that are loosely inserted.

Electronics from the 1980s, I saw one or two ICs under the LCD and lots of discrete SMD components.

I will clean the contacts and the board, more I cannot do.

 

[Andreas Thaler]

What could be the reason that the time and date settings are lost and the battery warning appears/dissappears even though the batteries have sufficient voltage?

Perhaps a loose connection or resistance in the power supply, perhaps due to a bad soldering point?

 

[Andreas Thaler]

The program back is reassembled, and also the spring-loaded gold contact pins for data exchange with the camera are back in place.

Number 1 in the row on the far left is a metal pin which obviously has no contact function.

When the pins fell out of their holes, I could no longer identify the right hole for the metal pin, but a faint circular pressure mark on the gold contact plate below gave away its position.

I checked all the battery contacts and their connections on the circuit board for continuity, cleaned them with electronic cleaner, cleaned the batteries thoroughly with Zippo lighter fluid, removed as much dust as I could from the circuit board and examined it with a magnifying glass.

I couldn't see the area under the LCD because it is partially glued on and soldered with jumpers.

The date and time are set.

Let's see if it stays that way.

 

[koraks]

What could be the reason that the time and date settings are lost and the battery warning appears/dissappears even though the batteries have sufficient voltage?

The first thing that comes to mind is leaking capacitors, in particular a leaking tantalum cap somewhere. But there are other possibilities as well. The answer is likely in the area you haven't been able to access, underneath the LCD module.

 

[Andreas Thaler]

The first thing that comes to mind is leaking capacitors, in particular a leaking tantalum cap somewhere. But there are other possibilities as well. The answer is likely in the area you haven't been able to access, underneath the LCD module.

Thanks!

I am now learning to accept boundaries before I cross them and make myself unhappy

 

[koraks]

Ah, yeah, this is challenging for sure!

 

[Andreas Thaler]

The program back retained the time and date settings overnight, which is progress.

I used lighter fluid to remove some tenacious coating on the negative poles of the two LR44s that were in the second battery compartment for these settings.

Perhaps this caused contact problems and the settings were reset when the input voltage fell below the minimum level. The battery warning that appeared in between also fits into this picture.

Fingers crossed!

 

[Andreas Thaler]

In order to understand how contamination on the contact surfaces of the batteries affects the voltage conditions, I made an equivalent circuit diagram.

Here, 3 volts of battery voltage supply the program back circuit, which is assumed to be a load (resistance) with 1 kiloohm.

100 ohms are added as the internal resistance of the batteries.

This leaves 2.73 volts in the voltage divider to supply the load, which should still be sufficient for stable operation.

The values are only assumed and are intended to illustrate the relationships.

An additional resistor, formed by the dirty contact surfaces of the batteries, is now added to the series circuit.

At 806 millivolts, it takes up a large part of the battery voltage.

Only 1.99 volts remain for the program back circuit (load), which is too little for stable operation. The battery warning appears and the time and date settings are reset.

Due to the unstable battery contact, the voltage conditions in the chain divider have changed again.

More current flows because the batteries contact has improved again, that means less resistance.

So there is now 2.65 volts of input voltage for the program back circuit, the time/date display starts again and remain stable until the next reset.

---

This is just an assumption and simplified.

But it helps to establish connections that provide guidance when electronic troubleshooting.

EveryCircuit: Animated interactive circuit simulator

Interactive real-time circuit simulation and animated visualization inspired students and engineers to design 2.6 million circuits online and in mobile app.

everycircuit.com

 

[Andreas Thaler]

I think the problem is solved, no further resets occurred, the program back keeps the settings.

So there were contact problems with the batteries/connectors, maybe the dust on the circuit board also contributed, who knows.

 

[koraks]

Sounds plausible; good to hear it's likely fixed!

 

[Andreas Thaler]

The program back keeps the settings, the problem is solved