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

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]

rubber cement?

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]

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.

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.

[philpem]

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.

It seems OK, actually. From below it's terrible, but very good from above or straight on. It's at eye level on my test rack anyway, along with its slightly newer mate (which was apparently bought by the MoD, calibrated and shoved in a crate for a few years before being sold on).

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.

I'm using Wurth Elektronik 151051BS04000 LEDs - http://uk.farnell.com/wurth-elektronik/151051bs04000/led-5mm-blue-500mcd-470nm/dp/2322135
They're rated at 470nm, 500mcd brightness, 40 degree viewing angle and 30mA forward current. The blue diffused case helps spread the light out a bit, I guess. At the standard 7150Plus LED current, they're about as bright as the display on my Agilent U1251A, or perhaps marginally brighter.

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.

Sadly I didn't have a breaker 7150Plus, nor did anyone I could find online. I found one person who had a box of spare front panel assemblies, but they were all in a storage unit a fair distance away from both of us

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.

I actually quite like blue-background LCD panels, though not as much as I like the old Planar electroluminescent displays   I've hoarded a bunch of ex-Sun StorEdge touchscreen EL displays I found on eBay and reverse engineered - the amber glow is to die for

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.

I've wondered about the possibility of swapping out the entire display -- though it'd be a big job. You'd have to tap into the control signals going to the LCD controller, then somehow rework them into commands for (say) one of the EA DOGM graphics displays. You could probably read in the commands with a PIC's Parallel Master Port, or failing that a CPLD or FPGA and a micro.

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.

The hotspots are a little annoying, but in my opinion not annoying enough to rip the meter apart again

[philpem]

Right, I'll finish off with one final post

The newly refurbished meter is back on my testbench and working a treat.

The trick to cleaning up the rough, porous surface of the SLS-printed keypad is acrylic model paint - Tamiya X-2 gloss white works nicely. Paint on a few thin coats and it'll look like a nice, shiny, original plastic button.

Here's a rough outline of what I did to refurbish both meters:

  * Check the manufacturing date on the Schaffner AC inlet filter. If it's 1995 or earlier, swap it out. These things age badly and make a real mess when they blow. Mouser and RS stock them. Check the voltage selector tumbler in the new one against the one in the old line filter - if they're not the same, use the one from the old line filter. In my case, the new filter came with an identical voltage selector tumbler. Also swap the fuses from the old filter (or fit new ones).

  * Install new LEDs in the front panel. The LCD is socketed. Lever it up gently from each corner with a plastic tool. Try and equalise the force - lift it a little on one corner, then match up the other corners. Repeat until the LCD pops out of the socket. If you break this, you are *screwed*. Clip the old LEDs off at the PCB, remove the plastic reflector and desolder the wire stubs. Install new LEDs (check the polarity!), re-fit the reflector and solder in place.

  * If you like, replace the 4mm banana jacks on the front panel. Hirschmann 97235510x series are a perfect fit if you drill out the back of the existing holes to the same size as the outer diameter. You want one red (Volts Hi), one black (Lo), one brown (Current Hi) and one green (Guard). Farnell stock codes are 101-1411, 101-1407, 185-4709 and 101-1408 (you need one of each). You can fit them without Hirschmann's special tool; use needle-nose pliers to engage the notches on the nut. It's fiddly but, in a pinch, it works. The spring washer goes between the back of the panel and the nut to stop the nut from loosening in service.

  * Run through the test and alignment procedure in the service manual (Initial Setup). Pay close attention to Calibration Balance - if that's screwed up, your measurements will be asymmetric (same voltage applied with inverse polarity will result in a different reading). You don't need a calibrated meter to do this, just one that's repeatable.

  * If you adjusted the Cal Balance, recalibrate the meter. It's probably easier to send it out to a calibration house unless you have a good calibration source. If you don't care about perfect accuracy, you can bodge DC voltage calibration with a calibrated meter and a power supply. Enter the reading from the other meter and bob's your uncle.

Cheers,
Phil.

[Dawe]

If anyone needs to build replacement keypad for this Solartron 7150 plus as well, I attached PCB files that can be used for the purpose. It's exported gerber, pdfs and Eagle board files packed in one. Also engraving G-code exported from pcb-Gcode should you want to manufacture the board too.
Video for those who might wanna see how it looks.

https://youtu.be/ISUEsC89gFg

[Dawe]

I'll add some more pictures of the keypad itself which can be helpfull for a repair of broken off buttons.

That keyboard is made of individual plastic buttons bundled together into the white keypad molded part. You can see it at  2:26 of my video. Each button is loosely joined only by a small strap of plastics and it's very easy to break off one during handling - it is a fragile part.  In order to disassemble for siimple repair it is enough to remove the front panel which is 0.5mm thick aluminum sheet metal. After you remove it you get hold of the keypad itself. Either you have got the missing button or you might use 3D print to make one. This button can be glued back into the keypad e.g. by superglue w/ baking soda (last picture). I had to glue back in several buttons too as they are really fragile as hell and I broke off them during the repair. Good thing is that the stroke of the buttons in the assembly is really small and such a repair can endure regular usage of the meter.
On last picture it's glued from the back but it'll do to glue it from the front too - it would be the accessible side if you remove the panel only.

sidenote: yes, the studs on the back of the buttons are cut shorter to stack up with the smd tactile switches. And yes, that was the point I broke them.