Electronics > Repair
Solartron 7150plus refurbishing and keypad repair
philpem:
I'm not sure if this counts as "test equipment", "projects" or "insanity" but I figure it's worth a post :)
I own two Solartron 7150Plus benchtop digital multimeters. They've both had the Schaffner AC filters replaced (one after it failed spectacularly, one pre-emptively to avoid failure) and are showing their age - the display backlight LEDs are tired and dim, and the standard banana plug jacks won't take shrouded test leads.
I started refurbishing one a few months ago -- I successfully replaced the LEDs, then drilled out the panel to take some new Hirschmann shrouded 4mm sockets. Sadly my drill bit got stuck on the plastic, the G-clamp slipped and the panel went spinning. This broke (more like utterly destroyed) the Mylar flexprint cable which ran from the keypad to the front panel PCB... I tried to get a new keypad or front panel assembly, but they're like hen's teeth. You can't get them for love nor money. So I decided to do something different... I made a new keypad.
The original keypad is an AMP-made thing consisting of a black plastic base with four locating pins (held in the frame by a locking washer), a Mylar sheet with silver ink printed on it to form the conductive pattern and cable, and a white frame (made of a plastic which tends to weaken and yellow over time) which covers the front and provides the keycaps. The whole assembly is about 3mm thick, with the front white plastic accounting for most of this. My plan was to reuse the white plastic but replace the black support and Mylar section with a PCB and tactile switches.
I knew extremely low profile tactile switches existed -- I'd seen them in cheap digital cameras (like many engineers, I occasionally tear down broken-beyond-repair hardware for fun). After a few hours of searching, I found the E-Switch TL3315NF160Q in the Farnell catalogue (P/N 2103642). This is a 4.5mm square switch with a thickness of only 0.55mm and a switch travel of 0.2mm. Perfect. I also ordered a sheet of single-sided 0.8mm photoresist coated FR4 PCB material to produce the back PCB. This meant the total height of the finished unit would be comparable to the existing part.
I spent a few hours measuring the white plastic piece with my digital callipers, then created a scale drawing in LibreCAD (the open-source fork of QCAD). Once I had a CAD drawing of the keypad, I measured the positions of the actuator pins and laid out a PCB in EAGLE. I etched it and assembled it, but the actuation pins on the back of the keycaps were too large and fouled on the switches. After about an hour of trying to alter the pins to work, I ended up breaking the plastic frame... time for drastic measures.
I spent another few days designing a drop-in replacement keypad in OpenSCAD, then sent the resulting STL file out to 3DPrint-UK to have it 3D printed. What I got back was a perfect replica of the original keypad, but in SLS-printed nylon instead of the original mystery plastic (probably ABS), and with the texture of a Trebor Extra Strong mint. (I'm still trying to figure out how to add a hard-wearing glossy finish).
I combined the keypad and the PCB, soldered in a ribbon cable to attach it to the front panel board and now I can control the meter from the front panel again :D
The front panel, by the way, is glued on - removing it involves levering it up along the edge and using a wide putty knife to pull it away. Before you put it back on, you'll need to remove the old glue and replace it. The best way I've found of doing this is a couple of household scouring pads -- cover the front panel in masking tape to protect it, then scrub the old glue off. If it won't come off, score a crosshatch pattern in the glue with an X-acto knife.
Now I need to find a suitable glue to reattach it -- I'm thinking E6000 (aka Goop) or Evo-Stik Impact might be the best choice. The problem is, if the glue is too hard to remove, the keypad becomes irreparable (which may be necessary if the wire breaks away from the solder joints).
Nearly there, though! :)
firehopper:
rubber cement?
philpem:
--- Quote from: firehopper on September 22, 2013, 02:22:02 pm ---rubber cement?
--- End quote ---
I found some E6000 in the drawer - I spread it on the plastic part of the panel with a plastic glue-spreader, then attached the metal front panel and held it in place with a few spring-loaded plastic clamps (the ones that look like plastic alligator clips).
For anyone who's interested, the AMP label on the old keypad reads:
0-0923998-0
AMP
CP 39 87
And the S7150plus looks much nicer with a blue backlit LCD (instead of the barely-visible orange) and the shrouded connectors :)
JoeD:
What's the angle of view like with blue LEDs? I changed the LEDs in one of my 7150+ to white and found that the already narrow angle of view, inherent in early LCD displays, was made even more restrictive. Presumably this is due to the filter wavelength which is designed to work with the orange LEDs. What current are you running the LEDs at? I found the old orange LEDs were running at around 30mA which was way too high for modern high brightness devices and turned them down to 5mA and they are still much brighter than the old ones.
Regarding the keyboards they are are a pain if they get damaged, like you I damaged mine whilst doing the LED mod but luckily I had the totally dead carcass of a 7150 that I keep for spares and could salvage the keyboard from that.
JoeD
soren:
--- Quote from: JoeD on September 24, 2013, 10:41:25 am ---What's the angle of view like with blue LEDs? I changed the LEDs in one of my 7150+ to white and found that the already narrow angle of view, inherent in early LCD displays, was made even more restrictive. Presumably this is due to the filter wavelength which is designed to work with the orange LEDs. What current are you running the LEDs at? I found the old orange LEDs were running at around 30mA which was way too high for modern high brightness devices and turned them down to 5mA and they are still much brighter than the old ones.
--- End quote ---
I can't stand blue LED's, but I put some random "Super Bright White LED 10k mcd 3-4v 5mm" from eBay in mine and they work well with the stock 270ohm resistors.
As for the viewing angles, try playing with the contrast trimmer pot at the top edge of the display board. With mine set to my preference, the contrast is pretty good from slightly below horizontal to seen from almost above it. Not sure if it can be adjusted to work well seen from below such as when sitting on a shelf.
The only thing I might try to change about the display is to block the gap between the left/right edges of the LCD and the front panel, as some light from the LED's leaks around the ends and onto the front of the LCD. Not a big deal though.
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