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.

Tri-amping

For the New Lab, I thought it would be a good time to improve the sound system a bit. I have a pair of Kef Concerto speakers from the 70s, I have the power amp boards from the project a couple of years ago, I have a donor home cinema amp or two so some bits come together.

I was sorting out someone's lousy sounding hi-fi on Hayling last year or so, where they said the sound was muffled. It turned out that their speakers were bi-amp capable, and they were only driving the bass part. I'd wired them in properly, and that thought was in the back of my mind. I liked the sound of those bi-amped speakers, and it stuck with me. I did some research online, found that there were people who did recommend tri-amping as well as bi-amping for smaller bookshelf-y sized speakers, so I was ready for it.

Then an offer from Seeed studios dropped in the mail box for 5 10cm square boards for about a tenner including postage, so I went for it.

This is the Linkwitz filter - two required, one for each channel. The idea is that it is fed from a regular 700mV kind of level output from a pre-amp or mixer, has some fairly steep active filters that provide the low end, the mid-band and the high end signals to discrete power amplifiers, which are then wired directly to the speaker drive units (maybe with an inline fuse in a vain attempt to save the life of a speaker cone if the amp decides to fail in dramatic fashion).

KiCad .. !

Well, kind of getting used to it. Thought I would design something and do a layout .. so here is the schematic. This is a good ol' PTH (that's Pin Thru Hole for those of you who are not suffering from the effects of age on the eyes, and can use SMT devices without a stereo travelling microscope) design of an MSF Receiver. MSF is the time signal broadcast from the National Physics Lab. now from somewhere in Cumbria at 60KHz. So a simple receiver with an external ferrite rod and MW coil feeds into a simple circuit to recover a digital signal.

.. and here the layout!

It's really good. Functionality is good - of course it is very different to Eagle, and finding ones way around the menus while adopting a new workflow ..  but it performs well, has some really useful features and quite a few bits I like a lot.

Now there are a couple of things I don't like about KiCad, but getting used to them.

1) No curved traces! Really, guys. My signature curvy traces no more?
2) No embedded autorouter. Well, to be honest I can route well enough and quickly enough not to worry me. The push/shove router works very well, and you can buy plug-in autorouters, but I am talking myself out of them at the mo.

I like the decoupling of the component from the footprint used. It makes the schematic more of an abstract representation of the function of the circuit rather than a particular instance - which helps the mind to keep open.

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 ..

Class B DIY

I thought I would finally get around to building the amps from the bits that I bought a long time ago.

I came across the pages at Elliott Sound Productions a long time ago, and since I am no good at designing analogue electronics beyond an emitter follower or a long-tailed pair, I bought the components. I filed them safely in a box, and forgot about them.

More recently, I thought it would be fun to exercise some layout skills and engage my friends at Itead Studios in creating a small PCB.

So I am sure that Mr Elliott won't mind me showing the results of this small bit of labour. It's his design, just I didn't buy one of his PCBs - I thought it would be fun to try my own. I am going to build five of them, after all - two for the Studio, where the reference monitor is currently running on an ageing Cambridge P60; one as a test amp for the laboratory, and two in bridge mode for a subwoofer project, to drive a Peerless 850140 10 inch driver I bought 17 years ago from Wilmslow Audio. I've been buying little bits and pieces from them for more than 40 years.

Now if only I could lay my hands on the heatsink I put aside for this project when I was young ..

  

The first prototype board. I couldn't find any BC546A in the lab, but I did find some MPSA42. They're high voltage NPN transistors so they will be fine. The pinout is opposite to BC546A so they are in backwards compared to the silk screen. 

PS: I have a handful of the blank boards left over; if anyone would like one or two, please drop me a line via Google, and we can arrange for them to be sent.