The Pentax Electro Spotmatic: Reverse-engineering a marvel of 1971's Japanese electronics [Part 4].
Channel: Edwin Steiner
Date: 13/06/2023

The Pentax Electro Spotmatic was the first SLR camera with automatic shutter control.
In these videos I share what I have found out by reverse engineering this camera
and I go through a systematic trouble shooting and repair of the electronics of
a Pentax Electro Spotmatic.

This is part 4, where we test the solenoid and the comparator and switching circuits,
characterize the JFET buffer and the discharge curve, and build and trim a new discharge
network.

Part 1 is here: https://youtu.be/GrQI07X9Zi0
Part 2 is here: https://youtu.be/jkEPIudCGIo
Part 3 is here: https://youtu.be/HOnj1KhhlGY

Please contact me in the comments or via email if you want to repair an Electro Spotmatic!
I can provide schematics for the camera and test setups and design files for the breakout system.

ERRATA: at 41:23, the lower plot's y-axis should read "V / time-doubling" instead of "mV / time-doubling".

00:00 Intro
00:59 Testing the solenoid: resistance
02:05 Solenoid functional test
04:20 Switch test: Pin 3 to GND
07:46 REF Pulling pin 3 high
09:15 High pin 9 voltage test
10:10 Medium pin 9 voltage test
11:09 REF (Not) probing pin 3
12:09 REF Pin 9 to pin 3 path
13:31 REF Testing considerations
14:43 REF Setup for JFET characterization
15:47 JFET results
16:35 REF JFET operating range
18:37 REF JFET buffer revisions
19:28 REF Comparator revisions
22:31 REF Unpowered JFET buffer
23:13 REF Setup for discharge curve measurement
27:06 REF Discharge curve theory
30:10 REF Theory: Discharge slope, phases
32:29 REF Theory: Diode properties
34:02 REF Theory: Discharge current
34:50 REF Discharge network revisions
37:01 REF Theory: Trimming discharge
40:41 REF Comparison to measurements
41:21 Results for the discharge curve
42:55 REF Better discharge trimming setup
45:21 Results for the discharge curve (2)
46:22 Building a new discharge network
51:53 Testing the new network
52:29 Problems with the VR1 trimmer
53:49 Removing the trimmer
54:10 Testing the trimmer
54:41 Installing the new network
55:43 Final results for discharge
56:36 Dedication

Pentaxforums entry about the Electro Spotmatic: https://www.pentaxforums.com/camerareviews/pentax-electro-spotmatic.html

Video explaining the ideality factor of diodes: https://www.youtube.com/watch?v=BC1E13CKf8g