Milli ohm meter getting warmer ...

So the aforementioned PCB arrived back, have assembled. 

I had to play with resistor values and so on,  but now I have a circuit that will pump 500mA through the device under test. There is a x100 amplifier which boosts up the voltage developed across the DUT to a suitable voltage for an Arduino 10 bit ADC to convert. 

So, a 100milli ohm resistor will develop a full scale reading. I'm going to have to add a range switch to lower the current if I want to measure bigger resistors than that. But then we have multimeters that can go down to a few hundred milli ohms in general. 

In general, quite pleased so far. I am going to assemble this into a bench power supply which happens to have one of my old boards that can read switches, write data to a flourescent display (which is only 30 years old). I am also going to float the power supply ground so we don't liberate magic smoke from things when I am using it. 

More Kicad stuff ...

 Playing more with KiCAD these days. I designed a milli-ohm meter (it's not original - it's another of Rod Elliots designs, but I have done a schematic and laid it out. Quite pleased with the results. KiCAD is slowly getting easier to find my way around - clearly a lot of hot key work needed to make it slick. 

Fixed the Cambridge P500

Another one off the list of things to sort out .. turned out to be one of the darlington output transistors. They have integrated emitter resistors, and one had gone from 0.22 ohm to something in the hundreds of ohms, so it got replaced. While I was there I checked all the caps (no problems) and I replaced all of the heat sink compound, as there was pretty much none on the transistor I changed. Actually I changed both on that channel to make sure I had a matching pair. Applied power, and success! Sound in both.

There's quite a lot of discussion on t'Internet about the bias current, but I found in the spec for the trannys themselves that they recommend 40mA. So I put some meters on, looking for 17.6mV across the two 0.22 ohm resistors. They seemed to want to settle at 20.4mV which is close enough.

This is a budget amp, so I wasn't expecting too much, but the heatsink is a bit pissy and it feels cheap. The whole amp is not worth saving, really but it might do for a bench amp at a push. At least it is now off my bench so I can move on down the list of things to be repaired.

Fixed Scope!

The power supply issue on the HP 54502A eventually succumbed to my repair type ministrations.

I had found a bunch of capacitors in the ageing power supply that needed replacement. 12 in all, all 2200uf 35v that I bought a bulk of Panasonic replacements for. That, and a test load brought the power supply back to something that worked a bit, for a while.

The thing with SMPS is that they are a nest of feedback loops. Especially early ones like this, using discrete componentry. After having reminded myself of what a UJT oscillator looks like, I found that the feedback for the +5v from secondary to primary was way out of whack.

That took me to a circuit in the secondary that compared the +5v and the -5v rail in a long tailed pair. -5V being generated by a linear regulator from the -12v, itself a linear regulator off a separate secondary.

Trouble is, with all this feedback going on, the secondary outputs all look wrong, so how to find out what was working? The trick in the end turned out to be switch on, run down all of the outputs to see if they start off ok or not. And guess what? The -12v that fed the -5v that was compared to the +5v to drive the feedback to the oscillator never got bigger than -6v.

So I drove the -12v regulator from an external 15v power supply, to see if it worked by itself. Still faulty! Getting warmer .. but it is only a few components. Swapped all the capacitors. No change. Measured resistors (by probing them near the body from the top) no problems. Hmm ...

Then noticed some burning or black stuff on one leg of two resistors - near the big capacitors on the 5v I had replaced. I hadn't noticed that one of the caps had leaked. I removed the resistors, used a Dremel to remove the black stuff and clean up the PCB as much as possible, replaced the resistors ..

Nothing happened. But all of the rails look like they are at the correct voltages?
And then noticed I had removed the connection to the CRT while working on it, and hadn't put it back. Tried again ..

Making progress again ..

The Dell machine arrived. After much faffing around trying to get Windows 98SE running on it (it's a pig, it can't find all of the device drivers for the network etc), I decided to go for a simpler solution. I found this FreeDOS thing, which at first sight seemed to be ideal.

So, I downloaded FreeDOS onto a bootable CD, booted the machine with that and created a 2GB (cuz that's how big a normal DOS partition could be) partition.

I then installed Linux on the beast, running dual boot so I could get the machine on the network, and copied over the files from the Fireball server on to the DOS partition. (Linux can write FAT file systems, you know).

I then ran FreeDOS up, so I could try to run LIFUTIL - and was shocked to find it works. Now all I had to do was to find a 720K diskette, format it in LIF format, and then put the right driver for the 1GSa/s card on. I went through all the files, and eventually found that the SYSTEM_0.13 file was the correct one (by a process of hex editing the files and looking for the description string), put that on the disk and moved it over to the HP 16500A front drive.

Wow. We have a working scope.

Running LIFUTIL

So the laptop has died that I use to run LIFUTIL. It started with the diskette drive failing, but quickly became beyond repair when the LCD decided to go dark as well.

I have an older machine in the office, and I found an old diskette drive in the museum box. (that pile of stuff I can't bear to part with). The older machine is a Dell Optiplex 780 desktop, and it has a hole in the front for a diskette drive, so I dug around for a cable, eventually found a power supply cable for it as well, and went inside the Dell.

Couldn't find where to plug the 34-pin cable. Ah, found the site for the cable on the motherboard, but it was not populated in manufacture. Ah.

So, I remember I have yet another machine in the shed - a Lian Li case I bought off eBay with an older motherboard in it. Lian Li cases are lovely - all aluminium, all soft edges that don't cut the fingers and can carry multiple fans, huge power supplies, multiple hard disks and a huge variety of motherboards. Go get it, clean it off, vacuum it out, leave it to warm up and dry out properly before installing diskette drive and applying power.

And nothing happened.

Replaced power supply (temporarily - it doesn't fit, but it does plug in kind of), and something happens. It boots! But, it is booting Windows 7 which won't help with running LIFUTIL, so I create a supposedly bootable Win 98SE CD to put in it. (at this stage I must have done something wrong since the CD didn't turn out to be bootable later)

DVD drive won't eject. It's stuck with an old CD in it. Eventually free it without damaging the drive. The CD, not so much. However, the new CD with Win98 on it won't boot. Hmmm .. however I have the IDE drive from the laptop with Win 98SE already on it .. I could put that on one of those converter things that allow you to plug a laptop drive onto a 40-pin IDE connector. I have one in the museum box. Woo hoo.

Boot machine. It spends an awful lot of time changing its configuration and installing drivers for motherboard chip sets, bridges, IDE controllers .. and then asks me to put the CD in to read something else it needs. Put CD in drive. Wonder why clicking on the continue ... means nothing happens. Click here and there. Nothing. Reboot machine. Nothing. The motherboard has failed.

I go to re-seat the memory DIMM and find one memory chip at a hugely high temperature. I find this with my thumb, which is still stinging. Relegate this beautiful case to the list of Things To Do for the future - buy a new power supply, disks, motherboard, memory and decent display card for a new Office Machine (Hackintosh compatible). Fireball will be relegated to be just the workhorse server she is now, but won't be tampered with by me trying to use it for desktop things any more.

Have now bought another Dell Optiplex GX620 from eBay. It's only 21 quid. Has been tested, has DVD drive, a network port and a diskette drive .. hopefully this will be the last subroutine call before we start working back up the call stack. When it arrives, I will only have to install Windows 98SE, get it on the network, download LIFUTIL and the drivers for the 1G sample/sec scope, create the diskette and off we go.

New board for the HP 16500A

I need a scope to debug the old SM4100 amplifier I have. Noisy on one channel, so it is likely a capacitor has gone a bit south. However, the scope's power supply has failed. It is a bit of a complex thing to fix without a scope. This is an attempt at recursion which I can't support. Physician, heal thyself.

So to satisfy the need for a scope to debug the power supply for the scope, I was lucky enough to find a cheap 1Gs/sec plug in for the HP16500A I have.

And, it arrived from the US! Much excitement from Wifey since she thinks I am buying stuff from the US for Christmas for her.  And I plugged it in - but it is not recognised by the mainframe. That's because the file for the board is not on the boot diskette .. so I need to get the file on to the diskette.

I have the file downloaded from Keysight Technologies (who says there is no such thing as Silicon Heaven?), and I have an old laptop with the LIFUTIL program that writes data in the strange 77-track format that HP needs .. and guess what? The diskette drive in the laptop has ceased to function.

So, I have an amp in need of repair, a dead scope, another scope that needs a software update, and a file to update it with, but now I need to find either a replacement diskette drive for the laptop, or I need to install a diskette drive into one of the desktop machines, fire up Windows 95 or 98 on it, download the files, run LIFUTIL ..

Lots of nested calls here with very few returns. 

.. And in the meantime I thought it a good idea to buy a little runaround ..

So I am working on the big girl. I have it in mind to do a bob/rat/chop on her .. she is mechanically very sound (probably needs the clutch sorting out along with the brakes needing a rebuild, new air filter, oil filter, new tyres, seized fuel tap, seized choke, seized throttle cables, switchgear needs sorting out, new sprockets and chain, rusty frame needs sorting out and the seat needs recovering and repadding. Front headlight is rusted out on the rim, and the front mudguard is cracked. The battery is long dead. The front handlebar bar ends are rusted out to where I am not sure it will be possible to get new grips on her).

My vision statement for her is something like this ..

 

 She is a bit of a pig at the moment though, lots of bolts shearing and so on. I did get the exhaust off, but one stud snapped, so I will have to borrow / buy EasyOffs to get the stud out. I like the wrapped look as the picture above .. the wrap comes from China, and is not too expensive.  I like the idea of a matt or satin black frame where it needs repainting, satin black wheels and spokes, whitewall tyres, creamy white paint job on bobbed mudguards and tank, with an airbrushed orange edging. WS2812B LED strings around to pick out the lines of the bike at night - default colour orange again, but hey! what would be the point of WS2812 without a microcontroller doing some fancy lighting patterns.

So, it's going to take a little longer to sort out the old girl than I hoped. I might make some good progress in August - anyway, it gives a window of time where I am not driving a car, and I am now working across a few of the NHS Trusts in the South. The car parking is outrageous at hospitals, and it is no better for visiting staff. It's not much better for permanent staff - they might let you park in the park and ride for a fee (!). I'm also over on the other Island quite a bit, so I thought it would be a good idea to get a little runny around kind of bike in the meantime. She was cheap, so there are some bits to tidy up. The grips need replacing, the sprockets and chain were well beyond sell-by date, the rubbers on the pegs are worn. But she starts first time, which is one of the the major requirements of a commuter bike. 

 

I've got some new grips on order from China, and very pleased with the gold chain and replacement sprockets. 

Might have to replace those shocks and springs sometime - they look a little shabby. But then I am in danger of spending more time and money on the little runaround that keeps me on the road, when I should be spending money on the big girl.

Fooze that bike!

I've decided that this is the year to get my bike back on the road. It's in a bit of a sorry state, but nothing that a few tens of hours with sandpaper, primer and paint won't sort out. I need a bit of a something to get my teeth into that isn't just electronics.

The bike used to look like this when it was new 16 years ago:

I hope to to something a bit different with it, and get it on the road again this year. It would be fun to get over to the Island on it! and I should be spending a couple of days a week there this year for the project I am working on at St Mary's.

Bargain!

I can't help showing off a bargain from that auction site again. This time I found an HP 16500A logic analyser for very few pennies. It didn't have any software with it, but eventually I found that on a site linked from Agilent.

I later found a blogger who had put up details on how to write the software onto diskette, and it went something like;

1) find some 720K 3.5 inch diskettes
2) find a machine that can run DOS and has a diskette drive
3) run the LIFUTIL program
4) format the diskettes to a very strange HP-UX compatible format

5) write the system code to a diskette
6) write the correct code for the options you have on a second diskette
7) enjoy

720k diskettes? Pah! a mere bagatelle. Remember when we used to get boxes of diskettes in perspex holders? I do, and I have a couple of boxes still.

A machine that can run DOS? Well I have an old Toshiba Satellite 110CT, which boots into Windows 98. Close enough. And here it is. Unfortunately the little pigtail cable for the Xircom ethernet PCMCIA is missing. I have two other PCMCIA network adapters, but they are both cardbus and won't fit.

But we do have a USB diskette drive for the Linux machine, so I can download the software and move it over with diskettes! 3.9MB at 720K is five bites. Actually because of the way the files are split up, it takes seven.

And THEN, we get to the point of writing the diskettes with the correct format (77 tracks, not 80, 1024 byte sectors, no interleave).

Wow. Don't diskettes take a huge amount of time to write and read?

Ta da.                                 

Soldering pencil update #2

Aw, I like it!

Soldering pencil update

Ah, seem to have a bit of a problem with driving the Triac.

It seems that these logic triacs are a bit finicky when it comes to working sensibly. Even though I don't need the isolation of an opto-isolator with a diac, I have decided that is what I am going to do - stops all that tedious messing about. So the schematic now looks like this ..

.. and I should update this blog in a wee while with the results of this change.

.. ta da.

More soldering pencil fun ..

I was playing with the Hakked Weller soldering pencil, and decided that I really need to leave it alone and play on another prototyping board. You see, the thing is that there isn't a lot of room inside the Weller case for experimentation, so now it's working OK, I want to build another one for playing with.

I've got a few NEC VFD displays from a retail project I was working on, and I wanted to join that together with an Arduino base. So, I made up a nice PCB for it. Another nineteen dorrar down the drain.

Schematic ..

Just a regular 328P processor layout, with a 26-pin connector for the VFD; a filter with a gain of about 23 for the PTC element in the solder pencil, a pot to set the desired temperature and a driver for a logic triac. The whole thing runs off a 12-0-12 2A transformer, and I have included a switching PSU for the +5v, and a linear regulator for the -5v. Actually there is a -15v supply hiding near the triac driver, since the triac gate is driven by current flow, and the current flow needs to be between MT1 and the gate so the 'ground' for the driver needs to float about 15v below the voltage on the MT1 gate.

And now I am testing out the result .. so far, so good but the triac driver is causing some concerns. I mean it works, but it's messy. I have a real opto triac driver coming from China soon. I can't bring myself to paying UK prices, when they are for one off, twice the price of five coming from China. AND I have to add postage to the UK one.