Wednesday, 21 November 2018

Fixing my Pentax K-30 exposure problem: #4 more details on solenoid

I spent a jolly day thinking about the exposure solenoid and doing a few checks.

Everyone has a theory about why the green solenoid is problematic.

I've tried to work out how it operates...
...which is a bit tricky without a circuit diagram.

The solenoid assembly consists of a metal base plate, twin coils wound onto a former (bobin), a permanent bar magnet (3.5 x 1.5 x 1.0mm), and a horseshoe shaped actuator (plunger).

K-30 K-50 K-70 Pentax Ricoh exposure problems

The permanent magnet normally holds the horseshoe in place.

When the shutter button is pressed, a 10ms pulse with an amplitude of approximately 400mV is applied to the solenoid coil.

K-30 K-50 K-70 Pentax Ricoh exposure problems

This pulse, plus the downward force on the horseshoe applied by the driven pin in contact with the horseshoe, is enough to release the horseshoe and (presumably) unlock the aperture.

The solenoid control seems to rely upon feedback from the driven pin position. If the solenoid is uncoupled from the pin, the control circuit does not produce the necessary solenoid pulse length.

K-30 K-50 K-70 Pentax Ricoh exposure problems
a very short pulse is generated if the driven pin is not in the correct position

Therefore, simply hanging weights on the solenoid and operating the shutter with the solenoid disengaged from the driven pin does not work.

K-30 K-50 K-70 Pentax Ricoh exposure problems
testing the solenoid release force

When the solenoid is fitted in its normal position, it holds the driven pin slightly upwards. So if this test is repeated while applying slight upward pressure to the pin, the solenoid receives a full width pulse, and only a small load is needed to break the horseshoe away from the solenoid body.

Initial tests with the white (good) solenoid indicated that a load of only 3 quarters of an ounce would release the horseshoe. Due to the limitations of available loads (i.e. quarter, half & 1 oz), the best I can say is that 14g was not enough, while 21g was certainly sufficient for the white solenoid to operate and release the load. (Note: 1oz = 28.35g).

When tested with the green solenoid, almost 3oz (over 80g) was needed to release the horseshoe.

other points

i) I also put the horseshoe from the green solenoid into the white solenoid. This did not seem to degrade the white solenoid.

ii) Testing the release/break force with a d.c. supply appears to be a waste of time, as the two solenoids are similar when checked this way.

K-30 K-50 K-70 Pentax Ricoh exposure problems
my solenoid test rig (no expense spared!)

iii) The horseshoe mating metal stubs on the green solenoid have a groove on one face.

K-30 K-50 K-70 Pentax Ricoh exposure problems solenoid

If this wasn’t caused by manufacture, it may be the result of the horseshoe crashing back into the solenoid (8,000 times in my case). However, I think this is more likely a manufacturing flaw.

iv) The white solenoid (from the K110D) has blobs of glue on the wires at the solenoid end. This provides some degree of cable strain relief. The green solenoid does not have this, so the wires break very easily when flexed.

v) It seems to me that a possible exposure failure mode could be wear on both the driven pin and the pin slot on the horseshoe. At some point the horseshoe may not provide the necessary pre-load on the pin, resulting in failure of the control circuit to provide an adequate pulse to operate the solenoid.

This post is linked with part #1, #2 & #3


  1. Replies
    1. I 'think' replacing the solenoid fixed the exposure problem, but the camera developed another problem; the shutter would sometimes operate, even when the camera was just sitting on a table. This random shutter operation just got worse & worse.
      Even putting the old solenoid back did not fix it. So either this was a chance, unrelated problem or maybe it was zapped by static electricity while I was trying to repair it.

      I was very fond of my K30, but there is clearly a design fault which Ricoh refuse to admit to. However, I have since switched to a micro 4 thirds Olympus camera, which has completely re-charged my interest in photography...I love it!