Repairs & Cleaning

Inside the Mind of Minolta – an Investigation of the Internals of Minolta’s SRT 303 – By Bob Janes

October 23, 2020

As I mentioned in a previous article in which I dissected an Old Canonet, I have occasionally taken interesting but non-functioning cameras apart – mainly to investigate, but there have been the occasional times when I’ve managed to fix something.

What I intend to do with this article is to show how fiendishly complicated an SLR camera can be by taking apart an old Minolta SRT 303b from the 1970s.

The body in question

This particular 303b is a functioning camera: the shutter fires and seems to give reasonable times; the lens stops down; and the focusing screen allows the lens to be focussed properly.

On the down side, it has suffered some reasonably severe crushing of the pentaprism, it is also dirty, is missing the plastic tip of the wind-on lever , operation of the mirror lock-up seems erratic and the meter is not working. It was also missing the battery cap, although that could be scavenged from or shared with another camera. In fact (as pointed out by one of the commenters below), it may be a 303 that has been given an old 303b top-plate – but in any case, its days of being a desirable camera are past.

The right tools for the job

Japanese cameras after about 1968 tend to use cross head screws, but those are not ‘Frearson’ or ‘Phillips’ screws as they might be in Europe or America, Instead they are JIS  (Japanese Industrial Standard) screws. The JIS screws are designed to give a better hold than you get with the other cross-head screws – you can use a standard set of Phillips precision screwdrivers, but you need to be careful that you don’t ‘cam out’ and damage the screw head. To be honest, I can only identify the JIS sockets in my set by reading the little engravings, but if you are going to do this sort of thing regularly yourself (and particularly if you have ambitions to put things back together and use or sell them afterwards), you might want to treat yourself to a suitable set of JIS precision screwdrivers.

Objectives

I’m intending to strip down this camera to:

  • Remind myself how the SRT cameras operate ‘under the hood’ (I last did this stuff some years ago) and to supply information for this article. I’m particularly keen to show what I remember as complex linkages to position the ‘paddle’ for the match-needle exposure and also to show what the ‘Aperture Direct’ viewing system looks like under the hood.
  • See if I can see why the meter isn’t working, whether the impact damage has upset anything else and whether any bits can be salvaged – particularly if they might be used in one of my working SRT 101s.

I’m not intending to put this camera back together – the only thing that would change my mind is if I saw something obvious and simple that was causing the camera meter not to work, in which case I might try a fix and put a film through what would then be the ugliest camera I’ve ever shot with.

Removing the top-plate

It is a good idea to set the shutter speed to 1000 and note ASA (so it goes back in the same place if there is a need to re-assemble) before undoing the little off-centre screw on the shutter speed dial.

The dial top will be pushed off by the spring underneath.

The shutter speed dial then just lifts out.

You can remove the rewind knob by turning anti-clockwise while you put a bar across the forks inside the back to stop the spindle turning. The disk under the rewind knob has notches either side of the spindle – they can be turned using a pair of dividers or needle-nosed pliers (see inset picture).

Remove the collar around the shutter release by turning anti-clockwise. This one came off easily with finger pressure. If it is tight, use a pair of pliers across it but maybe pad the edge of the collar with a strip of cardboard or similar to protect the chromed finish (this one shows past damage).

Note how the washers sit under the collar before lifting the wind-on lever off.

In addition to the obvious cross-head screw around the top-plate there is also a slot headed screw that hides under a little red plate that also acts as a mark to line up mounting SR lenses. Gently prise off the red plate to reveal the slot underneath – those other two slot-headed screws (which would normally hide beneath a ‘Minolta’ logo) also need to come off before the top-plate will come away. Note that the switch to cross-head standard screws, introduced in the late 60s, mainly seems to apply to visible screws. We are going to see more slot-head screws from here…

Examination of the insides

What looked like catastrophic damage to the top-plate ‘hump’ does not seem to have been transferred to the inside components. Note the two separate CdS sensors, each of which samples a different bit of the subject and which together allow the CSC metering system to do its stuff.

Here you can see the Aperture Direct window, which projects the aperture set on the lens into the top of the viewfinder. The image from the aperture ring is reflected off a little mirror on an arm, then down into a prism cemented onto the main pentaprism.

This is a view through the back of the pentaprism after it has been removed from the camera – in this case the Aperture Direct window appears above the viewfinder.

Looking at the back of the camera, you can see the cable runs (indicated by the red arrows) which go via a bunch of pulleys to allow the camera to set the ‘paddle’ in the viewfinder in the correct position to indicate proper exposure – this cable affects its position with regard to set shutter speed and ISO.

In this shot you can see how the cord from the shutter speed dial wraps around the large upper pulley, while the (larger) lower, silver coloured one has another cord that transfers information about the position of the aperture ring on the lens (this is read by the body via a ring attached to the little knob visible next to f/8 on the lens).

The positioning of both pulleys is transferred by an arm and pivot that is attached to the ‘paddle’ (arrowed in red and highlighted in magenta). Note that the pivot moves as the shutter speed dial is turned. The inset picture shows the meter, pivot and paddle from the other side after they have been removed from the camera.

Fiendishly complicated! The top pulley would normally be held flat by the disk around the spindle.

With the pentaprism removed you can see the focussing screen clearly. Note that there are quite a few slot-head screws around here, despite this particular model being released in 1975. Fans of waist-level viewfinders may note that this camera is still functional.

With the baffle removed it is easier to see the paddle and the meter needle. You can also see the low and high limit marks of the meter and the battery check index (when you set the battery check, the meter needle should deflect to that tab).

The cord attached to the aperture reading ring.

Under the base-plate – there are no obvious signs of corrosion or disconnected wires that might account for the meter not working.

The focussing screen lifted clear.

The screen and condenser taken out.

A view of the pentaprism from underneath, showing the front CdS cell.

The prism from the 303b with the CdS cells removed is on the left, with a prism from an SR 7 (an earlier Minolta without TTL metering or an Aperture Direct Window) on the right for comparison. You can see how the CSC metering and the Aperture Direct prism really complicate the shape of this precision engineered prism.

To sum up

I could not see any sign of what was preventing the meter from working on this particular camera. If it was related to the damage, it certainly was not obvious. It is notable that it was the electronic parts of this camera that failed. Even stripped down the mechanical functions controlling film transport and shutter actuation were still functional; If I had re-attached the bayonet and put a lens on it, it could still have taken photographs.

Habeas corpus

Mechanical SLR cameras are complex machines. It is astounding to think of the work involved in designing and developing such a device, particularly in an era before Computer Aided Design.

Support & Subscribe

35mmc is free to read. It is funded by adverts. If you don't like the adverts you can subscibe here and they will disapear.

For as little as $1 a month, you can help support the upkeep of 35mmc and get access to exclusive content over on Patreon. Alternatively, please feel free to chuck a few pennies in the tip jar via Ko-fi:

Become a Patron!

Learn about where your money goes here.
Would like to write for 35mmc? Find out how here.

11 Comments

  • Reply
    Peter
    October 23, 2020 at 5:05 pm

    A wonderful anatomy lesson! Thank you!

  • Reply
    Alexander
    October 23, 2020 at 6:00 pm

    Hi & thank you for your report. As you mention “operation of the mirror lock-up seems erratic”, I’m wondering: was there any original 303b having a mirror lock-up feature? At least, I’ve never seen one. Even many 303 (w/o “b”) were sold in Europe without any mirror lock-up, AFAIK, so it’s really a surprise what you have here. — Maybe there was some former repair where the top has been exchanged?

    • Reply
      Bob Janes
      October 23, 2020 at 8:15 pm

      Good spot! You are right that the info in the Rokkor files (https://www.rokkorfiles.com/SRT%20Series.htm) suggests that the mirror lock-up disappeared later in the run of the 303 (ie before the ‘b’) – and the cosmetic damage to the ring holding on the wind-on lever and shutter release showed definite signs of it having been taken off in the past… With that in mind it is perfectly likely that someone has made one good out of two iffy cameras.

      Having said that, the 303 only predates the 303b by two years, but I’ve edited the title and put a note in the article.

    • Reply
      Christopher James
      October 23, 2020 at 10:37 pm

      This is a wonderful article thank you! As a Minolta SRT shooter myself I’ve taken the bottom plate off mine to lubricate the mirror return but never looked under the top plate. These are – like all SLRs of the era in sure – incredible works of art and engineering. I absolutely agree with you about how much work must have gone into designing this. I’m also continually amazed at how well tiny strings and pulleys have held up over 50 years. Thank you!

      • Reply
        Bob Janes
        October 25, 2020 at 10:00 am

        Where in the bottom of the camera do you lubricate the mirror return? I’ve seen a number of cameras with a mirror that won’t return but had never thought of it being down to lack of lubrication…

    • Reply
      Daniel
      October 26, 2020 at 4:44 pm

      Great article! I just want to add that I have a n SR-T-303 bought in Switzerland in March of 1974, and it definitely has a mirror lock-up. A couple of years ago, I started using the camera again occasionally, and it works perfectly. There is a description of what it was like to work with film again after many years of shooting digital in the Photography section of my web site.

  • Reply
    Kodachromeguy
    October 24, 2020 at 4:18 am

    Nice job of explaining and illustrating. These 1960s and 1970s SLR (as well as mundane rangefinders of that era) cameras were amazing pieces of engineering, metallurgy, and precision assembly. And to think, 50 years later, many of these cameras still work well. I wonder how many of the contemporary uber-computerized, uber-featured DSLRs will be working in 5 decades?

    • Reply
      Bob Janes
      October 24, 2020 at 9:35 am

      Even a few years on from the 303 cameras were far more electronic and far less repairable. I remember my father fault-finding on my brother-in-law’s Pentax ME Super and him not being able to get far into it due to the flexi-cuircits plastered over the pentaprism.
      I recently delved into both a Pentax MX and a Minolta XD7, both of which were stone cold dead without their electronics – not only flexi-cuircits with no visible detaching points, but also covered with tiny potentiometers.
      Worth mentioning though that both those later cameras also featured cables – in each case to transfer the shutter speed info into the viewfinder.

  • Reply
    Giuseppe
    October 25, 2020 at 8:43 am

    Devi usare anche un lembo di gomma o silicone per rimuovere la ghiera che copre la la leva di avanzamento della pellicola sennò non riesci ad aprire la macchina,ho già riparato la mia Minolta Srt 393b

    • Reply
      Bob Janes
      October 25, 2020 at 9:56 am

      Google translate: “You must also use a rubber or silicone flap to remove the ring that covers the film advance lever otherwise you can’t open the machine, I have already repaired my Minolta Srt 393b”
      Yes, if you can’t grip with fingers to take off the retaining ring for the wind-on lever, a grippy surface such as an eraser can help.

      Would love to know some details of the repair you did – these topics are learning opportunities for us all.

  • Reply
    Sroyon
    October 25, 2020 at 2:20 pm

    Nice demonstration! Are you in the Facebook group called Learn Camera Repair? Eugene Pate has posted lots of videos, guides, service manuals, etc. and he seems to specialise in Minolta SLRs.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.