Keithley 2701 - New Project!

Keithley 2701 Digital Multimeter.

I had so much fun with the Tektronix oscilloscope I bought a broken Keithly 2701 off ebay! Here is how it looked in the advert. They removed the sticker on top before they sent it to me,

As you can see the top looks like it took a knock and the plastic front panel ripped off.

The 2701 is intended for system testing. It has two card slots where you can plug in cards capable of measuring from up to 30 channels, can do a variety of mathematical functions and triggering. It has ethernet capability as well as GPIB and serial. The unit is also not too shabby as a bench DMM as it has 6 1/2 digits accuracy with very low drift.(specs and user manual including mechanical drawings is available here)

To be honest I think this isn't a great unit for home (even though it was pretty cheap). Having a VFD display is attractive as it is easy to read and the accuracy and ethernet logging is nice but I don't really need the card interface. Also it lacks a diode function which would be useful. My thinking was that I would fix it and sell it (hopefully for some profit but really just for the fun of fixing it). I did want to sell it for what I paid for it plus the cost of repairs and I chose this unit as it was much cheaper than the price of similar units in working condition.

Inspection

I pulled the unit apart and carefully checked over the board in case it had been damaged but it looked ok. It did smell a bit suspicious but there were no burn marks, damaged insulation etc and the fuse was ok.

The main board is quite easy to remove. I removed all the connectors and then there are a series of screws around the side, a set of pins screw the DB9 etc connectors to the back. Then the whole board slides across and lifts out.

I decided it all looked Ok, re-inserted the board and re-connected all the connectors.I removed the voltage selector from the rear and configured it for 240V (the local supply voltage) and with the front panel sitting there I powered it on.

It came up and went through it's boot sequence with no errors. The wires going to the front panel probe sockets had all been pulled out. The current wire seemed to go to some sort of metal fitting inside the part where the current plug goes. I turned it off, figured out which wire went where, re-connected them and powered it on. I connected it to my bench power supply (with the current limit set low!), I connected my other bench DMM so I could compare and turned on the supply.

Amusingly it not only worked but appeared to be very close to dead on. My other bench DMM is a HP3478A but while it has lots of digits it is out of calibration. The first 4 or so digits on the HP match my calibrated U1242B. The first four or five digits on the Keithley matched the HP meter so that's great! I swept the voltage through the range of my PSU and it all worked fine. I found some resistors, checked the ohms range, 4 wire ohms range and then connected it to the PSU again with another meter in series and (again being careful with the current limited) checked the current ranges.

It all works! Well on the bright side this means it was a cheap buy but on the down side - where is the fun in that!

Front panel PCB

Ok not so fast - All of a sudden the lights went out! I nudged the screen a bit and then it all came back. I turned it off, turned the front panel over and the ribbon cable came off entirely! Turns out it was soldered into the front panel board and had fatigued off (probably from hanging off the front of the unit for a while).

I have to say I'm pretty unimpressed that the ribbon was soldered to the PCB rather than being attached with some sort of connector but anyway...

Front Panel Ribbon was a bit special in that each wire was a single (non stranded) core. I dug around on the internet and the Element 14 catalog but this is pretty hard to search for (not stranded, single core etc didn't produce any results).

By luck I found an old floppy drive cable and attacked it with side-cutters to find it too contained a single strand per wire.

Before I could solder it in, I first needed to remove the remains of the original ribbon. I snipped the pins off close, added a little solder to each and then used my Rhino ZD-985 to suck out each remaining lead. This went pretty well and left me with a set of clean holes ready for a new ribbon.

The next part was not so easy and that was the task of stripping the ribbon. The length of the stripped wires had to be very even and the wires are thin so its easy to break them as you pull the insulation off. I couldn't use my wire stripped as it would just break the wires. I found the way to do it is you cut the insulation with a scalpel first on both sides (using a ruler you cut a line), Then you use the scalper to cut between the wires in the bit you are stripping. Finally you use pliers to carefully pull the insulation off. This video explains it better than I can https://www.youtube.com/watch?v=rDF6ur9FIhM

Stuffing the conductors into the board was like picking a lock. You gently push down and nudge the one with the most pressure on it into place first and then the one with the next most pressure and so on. I soldered one in place and then the rest was easy!

I put a new connector on the ribbon but to run the new cable back under the transformer I had to remove the main board, and undo the bolts on the transformer. I put it all back together and tested it again and it all worked.

Parts

For the last couple of months I've been getting parts for my 40 year old Tektronix 475 so I figured getting parts for this would be easy.

The Keithley user manual specified part numbers for the front panel and the website said to contact a service centre for repairs and spare parts. I contacted the Singapore service centre which was the closest one for me. They said I had to contact Scientific Devices Australia as they are the local distributor. I emailed them and specified what I was after. Some time passed and I emailed again but no response.

Actually there is a bit of background here worth mentioning - before I bought the 2701 I contacted Keithley about spares for a damaged 2000. The 2000 had a bent and damaged chassis and was in far worse condition. Before the auction for the 2000 finished I contacted Keithley who directed me to Scientific Devices Australia. SDA said it was not economical to repair the 2000 as the cost of the parts would exceed the cost of a new unit ($AUD1895 !!)

I didn't hold out a lot of hope but this time I only needed a single plastic part. I contacted SDA again (using the contact from before).

In addition to the front plastic panel I needed a new front foot. It turns out there is an accessory called a 2000-BENCHKIT that is used to replace the feet and handle if you want to turn a rack mounted unit back into a bench unit. I found one of these for sale at various places in the US for around $30USD

Some time passed and I got no response from SDA so I emailed again and then more time and once again. I then emailed the 'sales' email address at SDA and the general manager responded and said he'd look into it.

Again some time passed and I emailed again. The manager replied saying he was still looking into it but that the cost of the parts would be in the hundreds of dollars. I said I only needed the plastic part not the electronics and checked he understood this. Here is the email:

Not as Yet.

By the way what are you expecting to Pay for the parts ?.                                                                                                                                    

I do now want to disappoint you but I am guessing it will be in the  hundred’s of dollars  due to minimum order charges & freight  etc.

Best Regards

Hamish Clark
General Manager (XXX)

I didn't get any response to this. So now I can't get any response out of the local distributor at all so I emailed the Singaporean service centre again explaining the situation. A couple of days pass and no response. This time I email the info@keithley.com address, explain the situation.

Keithley responds, confirms the part I wanted and then in the final email said:

Hi Tom,

Now the part comes as a front panel assembly and the part number to order is RPT-FP2701. This part include front panel (2701-310-front panel, 2700-313A-overlay and 2701-311A-Display lens) You can order this through Scientific Devices. If I am not wrong the price is around USD200 you can confirm this with our distributor. Let me know if you need any more details.

Ok $200 is a bit steep but not insane. This is a precision instrument and they probably don't sell many and probably don't stock many parts.

Amusingly the local distributor comes back the next day with a quote (what a coincidence!) and said

Tom,

Our apologies for the delay.  The parts just meet the minimum order requirement from that Keithley Department.

This part include front panel (2701-310-front panel, 2700-313A-overlay and 2701-311A-Display lens), unfortunately they do not sell the individual parts.  Also we cannot match the US Price list as we use the ASEAN (International) Price list.

Model 2000-benchkit $49.00 + GST

Model RPT-FP2701 $396.00 + GST

Freight is $130.00 + GST/order.

If you require any further assistance please do not hesitate to contact us.

Best regards

Chris Dawes

State Manager

Scientific Devices Aust

Ph: XXX

www.scientific-devices.com.au

Seriously! $400 for the plastic front panel! They pretty much doubled the retail price on the benchkit too. And then $130 shipping! The parts would be lucky to weigh 200g! The whole unit cost me $80 to ship from the Philippines.

So they want over $500 for two plastic parts!

Ok So I'll Fix it then

So unless I can find a part some other way the only option left was to try and fix it Essentially the front plastic panel is attached with four tabs to the outside of the metal chassis. These tabs have holes in them that snap over round metal protrusions from the chassis.

In my case when the unit was hit the front panel was ripped off and the tabs broke off. One tab was still intact but was bent out of shape.  A bit of heat from my surface mount solder blower sorted it out.

I thought if I could glue on some replacement tabs this might hold. After all they are just there to hold the front panel on. I asked on EEVBlog Forum about the best plastic to use and frankly I was overwhelmed with the response! One guy offered to give me a front panel from another model!

Another guy suggested a product called Devcon Plastic Welder. This isn't a normal hardware store item here in Australia so I had to order it but it was here in a few days.

I found an old drive bay cover from a PC case that had a similar feeling plastic. The plastic was the right thickness too at around 2mm.

I used a hacksaw to cut tabs out of this material that were 15mm wide. I cut off the broken tabs so they were flush with the adjacent plastic. My thinking was that they are thicker at the bottom and I would get a better glue bond with the thicker cross section.

I use the plastic welder to glue on the tabs I cut and I glued some small off-cuts to the outside for support.

 I let this all dry overnight and carefully measure the unbroken tab and the pins they lock onto. The pin diameter I measure to be 6.5mm but in the end I used 7mm to get a good fit. The tabs are 15mm wide and 30mm apart, The holes are dead centre and 28mm up from the ledge in the plastic.

The front foot support bolts over the top of these so you won't seem them when they are done. Unfortunately there wasn't enough clearance to leave the support material in place so I carefully dremeled it off.

I measured and marked up the hole locations, drilled a 1mm pilot hole as accurately as I could and then drilled the 7mm hole.  I test fitted a couple of times and had to ream a fraction here and there. I did mark the current probe tower with the drill bit but didn't cut through it.

After some monkeying around the front panel went on quite well and holds quite firmly.

I'd call that a success!

I carefully re-attached all the wires, straightened out the bent power push rod with a little heat and re-assembled it all. It's tricky to pass get the switch rods, wires and everything all back in at once but with a little patience it all snaps together. I put the lid back on and the back cover.

I powered it on and ran through some tests and it works! And looks Ok!

More Photos

Before I put it all back together I snapped a few more photos of the board. I haven't had much of a chance to really look at it much but it seems pretty nicely made. Some fancy precision resistors plus a gas discharge tube in the background. Not sure about the big pile of high wattage surface mount resistors in the foreground. 

Precision voltage reference plus some bridge rectifiers that are likely part of the RMS converter.

Xilinx Spartan FPGA plus associated RAM. Is that a programming header? Main processor is just above and it is a Coldfire MCF5407 Is that a programming header?

Net+ARM handles the Ethernet interface. Note the routing between the digital and analog parts of the board.

Part of the board is under that Aluminium shield. Note the big resistors with their own bit of exposed copper for (presumably) heat sinking.

Lots of PSU circuit on the digital side. Only a small amount on the analog side. The front end analog switching relays are *really* loud.

The End

So now I am left waiting for the benchkit so I can replace the broken front foot. At that point it is ready to go.

I really should sell it. No really. Ok maybe I can play with it while I am waiting for the bench kit.