International teamwork: Fixing Spotmatics

Jesito · Expire: 2015-01-07

I'm still impressed. Thanks to the new technologies (maybe not so new now), I've shared a nice experience of working togheter with another Forum colleague (Peter) to jointly fix the battery replacement problem with the Spotmatics.

This story had two starts. For one side, I got an old and apparently crappy Spotmatic SP "for parts" very cheap from Fotomachi (many thanks!). I was thinking on doing some surgery to take the film sprocket out and fixing the Chinon CS body. But after some trials, I saw that in despite the bad external appearence, the inside was fine and the camera could be used. Most of you know my feeling about cannibalising a working camera. It's out of my believings. So the donor turned to be the patient and the patient, the donor. If some spare should be needed, will be the Chinon who provide it, because it is faulty.

But before I could start doing any work (my time is limitted and my projects, many), I received an email from Peter asking for advice on the battery replacement of the Spotmatics. As you know, the Spotmatic was designed with the old 1,35 mercury batteries in mind, nowadays banned because they aren't fine for the environment. The replacements are the short-life zinc-air ones, used in hearing-aid devices that give 1,4 volt but last for one or two month.

Peter did try the Silver-oxide ones, that provide 1,55 volt. Unfortunately, the metering circuit inside the Spotmatic (a simple Wheatstone bridge) is not able to compensate this voltage increase, so Peter's pictures were underexposed.

Peter sent me a couple of links showing the internal metering circuitry of the Spotmatic SP, and after some brain humming, we concluded the modification that was proposed in one of the links could be done. (BTW, here you have the links, from Peter's first email: http://tinyurl.com/4nbwga and details of the meter circuit and the modification:
http://www.mypentax.com/Meter_sensitivity_mod.html ).

The mod is quite simple: Just to add a resistor in series with the galvanometer inside the Spotmatic, to limit the excess current because the new voltage, and to add a parallel resistor to keep the voltage across the ensemble the same that before. In this way, using the common, long-life Silver-oxide batteries could be done.

The second problem was to get the resistors. The computed values for them are not standard, and the person who did first the mod, used the closest standard values. But in my opinion, this variance could derive in a metering out of tolerance. Not so dramatic as not to doing any change, but if we could get the precise resistance values the result could be noticeably better.

Then I recalled an old trick to trim resistors that I had used in the past: Resistors used to be a tiny coal cylinder with two leads.

Starting with a higher value than we need, and plugging the ends of the resistor in a resistance meter, and filing the resistor body, we can reduce the diameter and then increase the resistance value. Nowadays the resistors are made of metal film, and I didn't know if the trimming would be possible, but being so cheap I tried it.

With a thin flat file I destroyed several of them (the value raising quickly instead of decreasing), until I moved to a round one. Then I was able to manufacture some precise resistors. Once trimmed, I applied a cover of nail veneer to protect the open area. I measured them again after 24 hours and the value was stable, so I sent a few of them to Peter.

That was the easy part for me. I'm better in electronics than in mechanics. Fully disassembling a Spotmatic body was far beyond my experience, so I was doubtful in even starting with the process. Peter was so kind of patiently explaining how to disassemble, first the bottom cover to show the battery contacts (I was afraid of a overflooded battery stuck inside the holder), next to disassemble the top cover.

In the meanwhile, Peter found another mod that was easier because it wasn't necessary to open the top cover: adding a Germanium diode to the battery, to drop the 0,2 excess voltage. So I looked for replacement and found another article showing the use of a SB160 Schottky diode. A quick research in the local shops, and I got a bunch of them. Sent some to Peter, who decided to try both solutions, one on each of their Spotmatics.

We were using email for the exchange, but it was not quite interactive, so we decided to move to chatting. I don't use to keep the chat client open, (because I do not chat a lot), but it has proven to me that it's a wonderful and fast alternative to email for such kind of projects.

Well, after this introduction, we (Peter and me) will be showing you the details of each of the mods, to allow you to mimic them.

The first shots from Peter, once the mods done, are impressive. (I haven't been able to try mine yet, I'm still pending of resealing the camera).

Could you please post some samples, Peter?
I'd appreciate very much if you add your own comments.
Thanks!.

The next posts will be covering the details, pictures and schematics.
Hope this would be useful for some of you.

Jes.

peterqd · Expire: 2014-01-04

Thanks Jes. Can I first say it's been a real pleasure for me to work with you on this. Your idea to use MSN Messenger was brilliant!

Up till last October I'd been using 1.35v MRB-400 WeinCell zinc-air batteries in my two Spotmatics. Both meters were accurate when the batteries were fresh, but without a constant air supply the voltage begins to drop, and the capacity of these batteries is quite low, as Jes says.

We'd planned a trip to Paris in October and I wanted to take a Spottie with me, so I needed to do something about the battery. I found a paper on the Yahoo Spotmatic group which said the electronics on these cameras is capable of coping with the higher 1.55 voltage using silver oxide cells, so I got some 387S s/o cells which have the distinctive plastic insulating collar like the original PX-400 mercury battery, and after putting one in the camera and loading it with Kodachrome 64, off we went.

The results were a deep disappointment:

I took care with both exposures, metering away from the sky and making sure the needle was centred. I was really sure this was going to be the answer, and I even told Carsten about it. Embarassed

Dave and Andy advised me to tune up the ASA dial to get over it. Using an accurate meter and other cameras, I found I needed to set the dial on 1200 for a 400 film to fool the meter, 1? stops out. That worked fine, but it was unsatisfactory as a permanent cure. That's when I found the article Jes has posted. I first asked Jes for help to buying the correct value resistors and from that point on I knew I was in safe hands.

After we finished I shot a quick roll of cheapo Kodak 400 in each camera and every single picture was correctly exposed! I was delighted.

First, this camera has been fitted with two new resistors near the prism: (Spotmatic SP with Tamron SP28-80)

and this one is from the camera with the diode next to the battery compartment (SPII with Jupiter-9). I believe this camera might be slightly over-exposing now, so once Jes has tested his camera we are planning to experiment with the adjustable meter-calibration resistor under the rewind crank.

Farside · Expire: 2013-12-27

Excellent news. I need to do the same with a couple of mine now. I think I'll go the diode route as it's probably easier.

peterqd · Expire: 2014-01-04

Jesito · Expire: 2015-01-07

LucisPictor · Expire: 2013-12-03

Amazing! Simply amazing!

Farside · Expire: 2013-12-27

Thanks, guys. I've been using the OM1 sans battery recently and find it's ok with a handheld meter, but in changing light situations it's nice to have an onboard meter again. The other one I really need it for is the P6 metering prism - I bought that purely for the prism, but it seems a good thing to have metering working on it.
Of course, these two things being wildly different from the Spotmatic, probably have a whole other set of problems in trying to insert resistors.

Jesito · Expire: 2015-01-07

Lets look first at the diagram schematic and pictorial of the original Spotmatic circuitry. Since the links provided by Peter where the resistor mod pictures are heavily copyrighted, we will use our own pictures and schematics.

The picture is an excerpt of the Pentax service manual and shows the schematic:

The meter circuitry shows a Wheatstone bridge, kind of a square with the four sides being four resistors the battery connected between two opossite corners and the meter to the other two.

According the info from Pentax, the meter (the circled M uppercase on the schematic) has an internal resistance of 3 Kohm. They also say that the needle is centered when a current of 3uA (microAmpčre) traverses the meter.

To keep this same current (to keep the needle movement the same) with the new battery we should put a resistor in series with the meter of 444 Ohm, and across the ensemble of the meter and the new resistor, a new one (in parallel) of 23,25 KOhm (kiloOhm).

That's how the modified circuit will be:

These values are not available in the market. There are two approaches. The one taken by the original article writer of chosing the closest upper standard values (470 Ohm and 27 KOhm), or the way we chose: Making our own ones. The accuracy would be better with our ones.

We will depart from standard lower values, since trimming the resistors increase the resistance, so we'll start with standard 390 Ohm and 22 KOhm ones. I got a few precission resistors of 432 ohms, that are not usually stocked in the shops, so in my case I started to a closer value, but the principle is the same.

We will connect the resistor to a VOM in the resistance measuring scale. Then with a round file, we'll slowly start filing the resistor body near the middle part, keeping an eye on the resistance meter, to stop in time.

and got the desired 444 Ohm value:

After the trimming is adequate to cover the bitted area with some nail veneer to protect and isolate the resistor. Please keep it overnight and measure it next day. If the measure is consistent, use the resistors. If not, trim some new ones. Resistors are cheap.

Finally, here you have a sketch of the internal printed circuit board showing the place inside the Spotmatic where to place the new resistors:

Now that we know what to do, the next step will be opening the Spotmatic and place the resistors in.

Jes.

Farside · Expire: 2013-12-27

Jes, excellent work, thanks. I'll have a go at that sometime.

Jesito · Expire: 2015-01-07

peterqd · Expire: 2014-01-04

Jes, you're doing a brilliant commentary. I only have one thing to add so far, about removing the rewind knob. The knob screws onto the thread at the top of the spindle (normal RH thread) seen in the last pic above. To remove it, prevent the spindle turning with a small screwdriver in the fork inside the film compartment and unscrew the knob by hand.

Here a word of caution is needed. The rewind spindle is the only way to open the back of the camera. It needs to be pushed down in its tube to undo the nut under the knob, and it's very easy to push it down too far to pull it back up again from the top. If this happens with the camera back closed, it is very difficult, almost impossible, to open the camera again. Therefore the camera back MUST remain open while the rewind kob is removed. The back is not removable, so for safety I put some masking tape over the catch to prevent it from locking shut.

Jesito · Expire: 2015-01-07

luisalegria · Expire: 2018-01-18

This is great work !

All this needs to be organized in a more permanent archive for reference. Nobody else in the world is going to give better advice on this kind of thing, and it will be a pity to have it buried in messages.

Attila · Expire: 2025-11-18

Many,many thanks for sharing this!

Jesito · Expire: 2015-01-07

peterqd · Expire: 2014-01-04

Attila · Expire: 2025-11-18

I will publish on main site, thank you!

Farside · Expire: 2013-12-27

peterqd · Expire: 2014-01-04

Jesito · Expire: 2015-01-07

I think I understand why the battery voltage does affect, but I need to check it with figures.

The basic Wheatstone bridge works in a way that when both legs of the bridge have the same resistance, the current flow through the galvanometer is zero.

But in the cameras, this is not true. One leg is made upon the photoresistors that measure light. The other leg is the speed dial knob resistor. When they are in sync, i.e. the knob marks the adequate speed according the current exposition, the galvanometer doesn't show a zero reading as it should be in the Wheatstone bridge, because the Spotmatic manufacturers decided to show the "in syc" position as the center of the scale. This means the brigde is not balanced, because it does need some bias offset to show the sync situation. to move the galvanometer needle to the center of the scale.

This bias offset is the key. Whith a standard balanced bridge, the current flow through the galvanometer is zero, meaning that both legs are equal, so any change in the voltage across the bridge won't affect the measure and the galvanometer will still show zero.

But in the camera there will be some voltage to show the needle in the middle position. If you increase the voltage across the bridge, the voltage across the galvanometer will also have an increase, and that matches with Peter's experience!.

Jes.

peterqd · Expire: 2014-01-04

Here at last is the next stage in the process, removing the top cover of the Spotmatic. Jes has let me do this bit myself Wink

The pictures below are of an SPII but the other models are generally similar. I'll describe any differences between the models as we go along.

Before you start, prepare a clear worksurface and spread out a cloth to work on. It helps to prevent things rolling if you drop them. Get the tools ready and find a container or small tray for the keeping the parts safe while they're loose. I use the lid of a small biscuit tin. You'll need a jeweller's plain screwdriver about 1.2mm wide (and a small Phillips screwdriver if you're working on an SPII or F), a larger screwdriver about 3mm wide and three special tools for undoing slotted nuts and screws. I've described these later.

Firstly remove the lens and fit a body cap if you have one, and remove the battery. Set the shutter speed on B and the ASA on 100 - these are just easy-to-remember reference numbers in case you need them during re-assembly. B is the easiest setting to identify while the speed dial is removed.

Without changing these settings, remove the small aluminium screw in the centre of the shutter speed dial. This screw is normal RH thread. You'll need to find or make a tool with two pins that fit the two small holes in the screwhead - I used a small pair of adjustable dividers with the pins ground down slightly. Be very careful not to let the tool slip and scratch the top of the screw.

Keep downward pressure on the speed dial as the screw is loosened. There's a spring underneath that will jump out and fly across the room if you're not careful!

With the screw removed the speed dial and the ASA dial are released and can be removed. The spring and the body of the knob can also be lifted off. There might be a thin brass spacer washer on the top of the central shaft. If so, be careful not to lose it. The washer seems to have been omitted on later models. Don't worry about remembering the position of the dials, everything goes back only one way.

These are the components of the shutter speed/ASA knob in order of reassembly:

Now the film advance lever and frame counter. Using the small screwdriver, slacken the three tiny setscrews in the edge of the cap over the frame counter dial and lift off the cap. Make sure the setscrews aren't in any danger of becoming fully unscrewed - you'll never find one again if it falls out.

Use the larger screwdriver to undo the screw holding the frame counter dial in place. This screw is LEFT HAND thread.

Take great care with this screw, it's very fragile and shears easily. Don't try to force it - if it won't turn one way, try the other. It doesn't need to be very tight, just the slightest amount of torque will hold the dial in place when you reassemble.

With the dial removed you'll see a slotted nut holding the base of the frame counter dial. To undo this you'll need a special tool, such as a lens ring spanner with spade ends. If you don't have one of these you can make a tool to fit. I cut a small square shape out of stainless steel sheet and formed the teeth to fit the nut with a fine file.

On earlier cameras (serial numbers up to 3,000,000) this nut is RH thread but on later cameras it was changed to LEFT HAND thread.

Under the frame counter base, the winding lever is fixed with a spring washer hooked under three bayonet wings on the central shaft.

First unscrew the three small screws fixing the washer to the lever. In this picture two of the three screws have already been removed. Now work the washer round until the bayonet wings become disengaged.

It might help to operate the lever while preventing the washer from turning. When the washer is free the winding lever can be lifted off, and then remove the plastic washer sealing the hole in the cover plate. Again, this only goes back one way.

These are the components of the winding lever and frame counter, again in order of reassembly.

TAKE NOTE: The next job is to remove the film rewind crank knob and the dials beneath it. The rewind spindle is the only way of opening the camera back and at one point it might need to pushed down inside its tube to undo the nut. If the camera back latches shut at this point it is virtually impossible to open it again! So, for safety, at this point stick some insulating tape over the latch and check that it can't close properly before you remove the knob.

Now you can safely remove the rewind knob. Prevent the spindle from turning with the larger screwdriver in the fork inside the film compartment and then unscrew the knob by hand. This is normal RH thread.

The dials beneath the rewind knob are held in place by another nut with two holes, which needs a different special tool to undo. I used the opposite corner of my piece of steel.

You might need to push the spindle down in its tube to reach the holes in this nut. Make absolutely certain the camera is open so that you can push the spindle back up again!

With the nut removed, all the dials can now be simply lifted off the camera. The SP, in the upper picture above, has a simple film-type reminder dial which turns through 360° and a thin spacing washer between this and the top cover. In the lower picture, the SPII and F have an aluminium ring with two wings which controls the film type and exposure reminder in the front window. The larger outer dial is a switch to select either FP or X synchronisation for the hotshoe. On these cameras there is a spring washer under the aluminium ring, but no washer under the main ring. Try to keep this complete assembly together when you lift it off as this makes it easier to reassemble, but it doesn't matter if it separates.

The SPII has a ball-bearing detent for the sync switch under the large dial. Thankfully there's no spring under it, but be extra careful not to lose it. A magnet is helpful here.

These are the components at the rewind end of the SPII, again in order of reassembly.

All that's needed now is to remove the screws fixing the cover to the main camera body. Each model has a different arrangement and number of screws, and the screws are different sizes, so take note of the type and size as you remove them. The single screw on the SP is a plain slotted type, but later models have several Phillips type screws.

With the screws removed the cover lifts straight up. Take care not to drop the loose stud inside the shutter release button, and don't forget to replace it when the cover is refitted!

The sync switch on the SPII consists of two spring contacts which are forced together by the probe under the dial. The cover has a spring under the hotshoe which contacts with the bar above the viewfinder, top right corner in this picture. The switch, contacts and the green wires are missing on the SP.

The next stage will be the fun part, modifying the electronics and soldering the two resistors. Watch this space.

zewrak · Joined: 12 Apr 2008 Posts: 1212

Get going already, tomorrow my Spotmatic arrives. Wink

Excellent work!

peterqd · Expire: 2014-01-04

Mal1905 · Expire: 2011-11-18

A fine bump, Peter - this is brilliantly documented work from you guys Wink

Articles like this should be stickied, or highlighted in some other way.

Esox lucius · Expire: 2011-11-18