12. Laser Precision power meter and probe repairs.
Action Status Issue Model Function
Refurb Done Dim backlight Rj-7610 Single Energy Radiometer up to 1J
Refurb Done Dim backlight Rj-7620 Dual Energy Radiometer up to 1J
Refurb Done Dim backlight Rk-5710 Power Radiometer up to 30W
Repair Fixed Probe N/S RkP-575 + Rk-570C Shutter Probe CW 10W Pyrosensor
Laser Precision optical power/energy meters - replaced all dimly lit LCD backlights
Laser Precision Rk-5710 Power Radiometer up to 30W
Laser Precision Rj-7610 Single Energy Radiometer up to 1J
Laser Precision Rj-7620 Dual Energy Radiometer up to 1J
2013 eBay $14 Chinese EL backlight white 150mm x 50mm x2 for Rj-7610 & Rk-5710
2013 eBay $11 Chinese EL backlight blue 149mm x 44mm x1 for Rj-7620 power meter
2022 eBay $ 6 Chinese EL backlight white 6" x 2"(152mm x 51mm) + inverter x3 for all three meters
The electroluminescent backlights in these units are off by default and have to be turned on rather laboriously using the front panel membrane push-buttons, presumably to extend their life. The Rj-7610 has the same PCB as the Rj-7620 but is missing the extra components for the second channel. I found the backlight inverter and circuitry to enable it had been manually wired into to every PCB as a later addition, with different circuits for the Rk-65710 and Rj-76x0. All inverters run off the 5V supply and have an enable pin that must be logic 0 (0V) or floating to turn on the inverter (I assume it has an internal pull-down resistor). All instruments have an 8031 µC and the inverter is enabled via a PNP transistor with its emitter wired to +5V.
In the Rj-76x0 the 8031 µC I/O port on pin 14 drives the base of the PNP transistor. When when the port is driven to a logic 0 (0V) the PNP turns on and drives +5V into the inverter enable pin, turning it on.
I moved the wire from the µC port pin to its 0V supply pin 20, forcing the inverter permanently on.
The Rk-5710 design is more involved. Its 8031 µC drives a 74139 dual 4:1 decoder, and one of its outputs controls the inverter. Again, the inverter was added later, so its transistor is a manually wired circuit next to the inverter, but it differs by having a 7474 D-type latch configured as a toggle between the 139 and the PNP transistor, see circuit below. A capacitor clears the latch at power-up, forcing the 7474 Q- output to a 1 (+5V), turning off the PNP and disabling the inverter. The 139 output has to transition 0->1 to cause the latch to change state and turn on the inverter, and a second 0->1 transition turns it off. To force the backlight permanently on, I re-wired the PNP base to 0V.
However the intensity of the backlights was too low and in 2013 I replaced them. Initially I fitted a blue one to the Rj-7610 but it wasn't very bright so I fitted white ones to the other two instruments.
Below, the outside LCD area is about 145mm x 16mm. Inside, the LCD PCB is retained by four Torx 10 screws. The original backlight was soldered to the front of the PCB and two wires soldered to its back lead off to the inverter. Although the inverter output is floating ac, the black wire connects one lead to 0V which is also connected to the chassis metalwork.
Backlights can be cut to size from larger panels. The cheapest panels were from eBay China and in 2013 the smallest ones I could find that were long enough, were 148mm x 44mm. This was a good thing, because when I removed the LCD PCB I found the original panels measured 152mm x 21mm. If I had bought 16mm wide panels they would have misaligned. I used the original panel as a template and cut the new panel to the same size. The new panels shown below are the 152mm x 51mm ones I bought in 2022.
By 2022 I was having difficulty seeing all three LCDs. This time I measured light levels and found there were wide differences in brightness levels of identically sized panels. I also noticed once shining through the LCD, there was huge reduction in light intensity compared to a naked panel on the bench.
I replaced all three backlights as before, this time using white panels but they seemed little brighter.
At first I wondered if it was because like the last lot they were from eBay China and this time they came with an inverter yet were half the price I paid in 2013 despite a $/£ drop. I had hoped to buy them cheaper without inverters but now I was glad they were included, because the only easy fix I could think of was to run them at a higher voltage even if it shortens their life (they're not expensive after all) but I didn't want to risk doing this to the original inverters, and leaving those in meant I had a fall-back in case the idea didn't work.
That said, the onboard inverters run off +5V but the Chinese inverters run off +12V and there is no +12V supply.
However all meters have a LM340T/7815 TO-220 +15V 1A IC regulator for the analogue circuitry.
I measured the maximum current taken by the new inverters at 12V( 50mA) and 15V (80mA) along with backlight intensity for both a full size 152x52mm panel as well one cut to 152x21mm for the LCD, and checked the +15V supply was unchanged (+15.23V). I concluded the extra 80mA should not be a problem.
I also measured the inverter output voltage from the original inverter and a new Chinese inverter and found they were similar. I only measured one each but when I measured light levels I realised there must be quite a wide tolerance between different inverters.
Original inverter at 5V no panel ~162Vac, with panel ~ 109Vac
Chinese inverter at 12V no panel ~148Vac, with panel ~ 108Vac
Chinese inverter at 15V no panel ~190Vac, with panel ~ ADD Vac
The Chinese inverters aren't marked but I confirmed a working voltage of ~108Vac at 12V, much the same as the 5 Volt ERG LPS-05-2-1 inverter fitted to the Rk-5710. Increasing the inverter input to 15V produced ~160Vac, doubling the current and increasing light intensity of one panel by 32% and the other by 49%. I only measured one inverter; I expect they also have a loose output voltage tolerance.
I added the new inverters and wired the backlights to them instead. Running them at 15V instead of 12V gave a 30% boost in brightness, and I could now easily see the front panel. I stuck the new adhesive-backed inverters onto the metal side panel on each meter to dissipate any heat, and wired them to the +15V regulator MLC de-coupler.
Rk-5710 Rj-7610 Rj-7620
Original New inverter Original New inverter Original New inverter
Above, the meters inside before and after the new inverters were added and wired to 15V.
Below left, 2022 meter intensities before refurb: Rk-5710 ( 7 Lux), Rj-7610 (12 Lux), Rj-7620 ( 4 Lux)
Centre & right, meter intensities after refurb: Rk-5710 (38 Lux), Rj-7610 (14 Lux), Rj-7620 (25 Lux)
The Rj-7610 is greyed out because at this point it had not yet been refurbished:
Later in 2022 I refurbished the Rj-7610 too (the sticker is reminding me to disable its onboard inv):
TEST RESULTS
I had a spare blue full panel from 2013 for comparison and measured similar intensity at lower current. However I don't think this is significant as I found great inconsistency across all panels & inverters.
2022 152X52MM ON BENCH 2013 149X44MM ON BENCH 2013 PANEL FROM LCD ON BENCH
WHITE #1 WHITE #1 BLUE BLUE WHITE WHITE
PSU Full panel Full panel Full panel Full panel 21mm wide 21mm wide
Volts PSU current Intensity PSU current Intensity PSU current Intensity
3V 0.5mA 22 Lux - - - -
5V 15mA 81 Lux - - - -
12V 80mA 433 Lux 60mA 633 Lux 30mA 578 Lux
15V 110mA 567 Lux 80mA 839 Lux 60mA 864 Lux
2022 152X52MM ON BENCH 2022 152X21MM IN Rk-5710 2022 152X21MM ON BENCH
PSU WHITE #2 WHITE #2 WHITE #1 WHITE #1 WHITE #2 WHITE #2
PSU Full panel Full panel 21mm wide 21mm wide 21mm wide 21mm wide
Volts PSU current Intensity PSU current Intensity PSU current Intensity
3V - - - - - -
5V - - - - - -
12V 80mA 381 Lux 50mA 34 Lux 50mA 592 Lux
15V 120mA 479 Lux 80mA 38 Lux 80mA 791 Lux
FINAL RESULTS
2022 backlight intensity before refurb: Rk-5710 = 7 Lux, Rj-7610 = 12 Lux, Rj-7620 = 4 Lux.
2022 backlight intensity after refurb: Rk-5710 = 38 Lux, Rj-7610 = 36 Lux, Rj-7620 = 25 Lux.
Right - Lastly I removed my earlier mods to force the onboard inverters on, partly to save power but also because they might not like running without a load.
Left - The lids warp outward unless nylon washers are placed on the mounting screws between the lid and frame, and I finally got round to gluing all 24 in place.
Laser Precision RkP-575 + Rk-570C Shutter Probe CW 10W Pyrosensor
RkP-570C no output signal on arrival.
15µF 20V tantalum capacitor C201 short circuit on +15V supply line. Replaced, unit operational.
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