Standard light for camera repair dimmer stability question - Page 1
Forum section: electronics / projects

author: bottlegardener
author_rank: Contributor
timestamp: June 19, 2025, 02:40:27 pm
content: Hi,This is a luminance standard box designed to provide a calibrated light source at two levels for adjusting a camera's light meter (built in the early 1970s). The original advertising said that it should maintain a stable light output even in a drifting line voltage but it doesn't do a very good job.I was wondering if anyone has any suggestions as to which components in this device might be causing the voltage to drift.I already replaced the diac, triac and the two 400v 0.1uf film capacitors. My next hunch would be to replace the front panel control R1 (1kohm 2 watt carbon pot) with a multi-turn wire-wound trimmer.Thanks in advance for any tips.Edit: I just realized I didn't label the transistors (which I believe are for the indicator light circuit, where a neon bulb lights up when it's at the calibration voltage)Q1, Q2 = 2N2923 NPN; Q3, Q4 = 2N1305 PNP

author: Grandchuck
author_rank: Super Contributor
timestamp: June 19, 2025, 06:16:47 pm
content: R1

author: MariuszD
author_rank: Regular Contributor
timestamp: June 20, 2025, 10:32:58 am
content: Quote from: bottlegardener on June 19, 2025, 02:40:27 pmThis is a typical dimmer with a triac. There are no feedback or feedforward circuits here. There is no reason to expect a stable output voltage. You need to adjust it manually if the mains voltage changes.I haven't seen the manual. I don't understand how to use the circuit that lights up the indicator. It can probably serve as a guideline on how to set R1

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 20, 2025, 01:16:20 pm
content: I don't understand this circuit. What's the diff pair doing? There does seem to be some kind of feedback going on but I've no idea how to adjust it or how well it might work. I'd probably trash it and build a new circuit with a photodiode to monitor the output and stabilize it.

author: bottlegardener
author_rank: Contributor
timestamp: June 20, 2025, 10:31:54 pm
content: Thanks for looking at this folks.I'm attaching a better scan of the schematic plus the instructions, parts list, and calibration notes.The instructions claim "The illumination level has a virtually undetectable variation at voltage levels from 110 to 135 volts" which is not the case for me.Perhaps they were being bold in their claims? National Camera sold a more expensive unit alongside this one later in the 70s; I found a clipping from a trade journal (attached) where it said the upgraded unit features stabilized DC voltage.I wonder if there is an affordable 50-60vdc adjustable PSU? It's basically powering a string of 5 car indicator lamps. On the "low" setting, it's a string of #67 lamps in series (8 watts x 5); on the "high" setting, it's a string of 5 #1073 lamps in series (24 watts x 5).Edit: I should say I haven't yet had a chance to upgrade R1 as Grandchuck suggested. Bournes 2-watt wirewound pots are available at a local surplus shop and I'll try to get one Monday.Edit 2: I should also add my mains voltage doesn't vary too much (~ 118 to 121 volts) as far as I've been able to measure.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 23, 2025, 04:50:16 am
content: Now I understand. A properly trimmed transistor circuit operates as a window comparator. There is no automation here, before each use, you need to adjust R1 so that the indicator light shines the brightest."The illumination level has a virtually undetectable variation at voltage levels from 110 to 135 volts" True, but you must adjust R1 every time when voltage changes.

author: bottlegardener
author_rank: Contributor
timestamp: June 24, 2025, 01:26:56 am
content: Thanks so much for that very helpful explanation!

author: bottlegardener
author_rank: Contributor
timestamp: June 29, 2025, 12:23:55 am
content: Hi everyone,Just as an update (in the unlikely event that someone else is using this unit or cloning its circuit): for R1 (the front panel dimmer control), I replaced the old Allen-Bradley single-turn carbon pot with a 10-turn Bournes wirewound type. That made all the difference in the world.My target luminance (512cd/m2) is set when R1 is trimmed to 14kohm @120VAC. Testing on a variac, the 10-turn R1 gives me a 10 o'clock to 2 o'clock adjustment range to hit the calibrated luminance level with 122 to 117VAC line voltage.Thanks again for the tips and circuit explanation.