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Friday, 9 February 2018

The big hifi preamplifier project - input selection.

Amongst the ever changing line up of hifi kit at home, I feel the need for more homemade bits of it...

I mean there are some...

This lash up, for example. Built many, many moons ago, and largely undocumented...


It's a Systemdek with a unipivot arm, and a nasty otofon Red cartridge, home made all valve pre-amp with posh stepped attenuator, RIAA and EM84 indicator tubes, and an amp, made from junque parts at the time, featuring a GZ34 rectifier, an ECC82, and two something-or-other pentodes in SE output. A staggering 3 watts on a good day...

The current line up is a bit "not home made enough" ...  The BSR (here) is turntable of the minute, although there are more than a few turntables here! The amp is good, a Rotel 6 channel thing, with two channels bridged (and two unused), and the pre-amp is a Rotel. There's an Arcam CD which I ought to get round to putting a new optical ass'y in, as it's as weak as dishwater...





The Rotel pre-amp is OK, but it's MC phono input is as noisy as a hen with one chicken.










So, after watching a  Techmoan video on youtube (Here) I was somewhat inspired. Yes, I would like more inputs, especially two turntable inputs. It needs to have a phono stage for MM (Moving magnet) as well as MC (Moving coil). I'd like some subtle tone controls. I'd like remote control. I'd like some VU meters or similar. I'd like a nice colour display with touch screen. I like an option to use the new Korg Nutube . I'd like it all to fit in a 2U case.... not much to ask is it?

So I set about sketching some ideas down. Power supply board, main pre-amp board, input and output switching board, microcontroller board, and finally, but my no means least, a decent phono stage.

I'm going to tackle this in a few posts, as it's a big project.

First off... Input selection.

A couple of options come to mind to deal with input selection.
First though was some nice analogue switches. A slack handful of MAX4066 switches would work well, in a similar fashion to those I fitted to the Quad 44 here.  Reasonably easy to implement, but they do add some distortion and noise.
What about a bunch of small signal relays? Essentially noise-free, switching not quite as easy, and expensive ... but I don't want to be doing this twice...
Inputs?
Two phono. 1 CD (1dBV adjustable) , and then 4 line level (-10dBV)
Outputs? Recording output (although I don't really have any recording devices, the option to connect a laptop or something would be nice), and the main output to the power amplifier.

Right .....


From the top... 5V and the control signals come into JP1. K1 (the phono switching relay) is switched by Q1. All the other relays are driven by a ULN2003 darlington array. K2 is a "mute" relay and grounds the output. K1 switches between the turntable inputs. This is output to JP4, which will feed the phono stage. The phone stage will then feed the signal back to JP7, and via relay K3 to the output JP5 which will feed the main pre-amp board. The main pre-amp board then feeds the signal back to JP2 and JP3, and is routed through to the output sockets (via the mute relay in the case of the main output). K3 is the CD player input. Notice it has two attenuators, R2 & R3 to reduce the level a bit to match the other sources. K4 through K8 are the other normal line level inputs. Notice the different grounds, GND is the +5V ground, and used by the relays and associated switching. AGND is the audio ground. GNDA is the phono ground. I've included a link at JP6 should we wish to ground them here. I think it's going to be best to make a start point of grounds at the power supply, but I've included this here for a bit of flexibility.

Want more inputs? Add more relays another ULN2003 and some more sockets! (Don't worry about the number of selector inputs, we can use a shift register if you get seriously carried away)

Got that? Right ....

It should all fit nicely on a single sided board.

 
The red trace is a wire link.

It's duly created using toner-transfer and etched...

... silk screened ...












... and finally assembled.






Relays are Omron G6A-274P (available everywhere, but shop around, prices vary. I managed to get these for £1.79 each from Farnell) and the double phono sockets are PSG01550.

Coming next, the main pre-amp PCB ...

... and just what does CoaSt stand for?

Thursday, 8 February 2018

The Korg Nutube 6P1...

It is with much excitement that I opened a box from RS Components....

It's here!
Possibly the only new thermionic device made this century! Oh the excitement!!

It's a tiny double triode which promises "Real vacuum tube sound. The real triode structure produces a warm, unique vacuum tube sound, delivering excellent linearity."

It's built using vacuum florescent display technology, and is tiny. 

Marketing guff can be found here ... http://korgnutube.com/en/

and real data can be found here:









It already is appearing in guitar pedals all over the internet, and it's going to feature in a project near here very soon.

Tuesday, 6 February 2018

Tip for drilling homemade PCBs.

Here's a quick tip .... if you're as myopic as I am, try this out...

Here's my PCB drill in it's stand. It a foul thing. It was dirt cheap when I bought it 15 years ago ... The rubber coating on it has gone all sticky ... yuck...


Now look at the bottom...

I've mounted one of those small 3w LED's there, left over from another project...










Makes alignment of the drill bit with the pad on your PCB a breeze.....


You'll notice it's slightly off-centre, this is avoid drilling through it ....

.... yeah, you know how I found that out!


Saturday, 3 February 2018

Modifying the Fellows A75 A4 laminator for making PCBs. Toner transfer the posh way!

Now, if I'm making something up, it's often easy to make a simple single-sided PCB up quickly, and professionally by using the toner-transfer method of PCB manufacturing. I've been doing it for years, but the results are always a bit hit-and-miss. Use google if you haven't heard of it before, there are many guides on the web (although no two the same!) , and I don't intend to duplicate those here.

I have tried many different types of transfer paper over the years, from the proper stuff, to plain old A4 paper, the best I found for years was a copy of the New Scientist magazine! Sadly they changed to thicker, shiney paper a few years ago, which doesn't work well at all. The best at the time of writing seems to be "The Economist", which is a shame, as it's nowhere near as good a read as "New Scientist" before it get's recycled into PCB making materials!!

... anyway ...

The last board I did, I tried an experiment, by borrowing Mrs Doz' laminator from her office. I idea was to see if it would transfer the toner from the paper to the board, whilst keeping a constant and steady pressure on it. Well, results were mixed... it really pushed the board through too fast for proper transfer to take place. I ended up transferring the toner with the conventional clothes iron, and then passing the board through the laminator, which resulted in an improved board after a couple of goes. It did, however give me an idea ... make the motor run slower....

So, rather than incur the wife's wrath and modify her laminator, I picked up a Fellows A75 A4 laminator from Argos for £16....




I initially tested it would take the thickness of the board by passing a blank piece of PCB stock through it... yep, works a treat.

So, to disassembly... There are 4 pozi screws underneath (great, not even some torx or bizarre security screws to bother me!)

So (from left to right) we have an essential mug of tea, the motor, drive rollers and heaters, and finally a PCB on the right....







A couple of things to notice about the PCB... 

There's some sort of timer in there to shut the unit down after it's been on for 30 mins (just in case you leave it running). It's unlikely to bother us much....

The low voltage electronics (the timer, and LED's etc) are driven from a power supply formed by a wattless capacitor dropper... so a word of warning... all of these voltages on this board will appear to be referenced to the mains. This would present us with a shock hazard and a problem if we want to interface to them...

On to the motor....


It's something I used to refer to as a "Le Cruzet" motor. A synchronous motor with built in gear box. As we can see from it's label, it rotates at 4 RPM (4.8 RPM if you live on that funny bit of the planet that uses 60Hz mains). That's a bit of a pain. I was hoping for a small DC motor I could drive with some PWM to get the speed nice and regulated (although if I was building a cheap laminator, I'd be using exactly this motor too!) 

So, how can we control it's speed?

Well, it's synchronous, so we'll need to lower the AC frequency driving it .... Ugh ... don't fancy this idea much.. generating a few tens of hertz at 240 V RMS is not going to be a cheap fix....

What about phase modulation? Daft idea. It may vary the speed, but will destroy the motor's torque, and won't vary the frequency. May cause overheating and failure in the motor's windings because of the resultant unpleasant waveform.

What about PWM? Nope, DC only ..... or is it????
How about really, really slow PWM? Really really slow.... 
I'm talking about switching the AC to the motor on for ~.5 second, off for a second, and so on ......

It'll inch the board forward, cook it for a bit, and inch it forward again ....

It's just stupid enough to work....

So what do I need? 

A 555 timer, a slack handful of passives, a transistor and a relay...
or an arduino (or similar, running a modifed blink sketch!),a transistor and a relay...?

I'll go with the latter, as it'll be easy to adjust by fudging the software, and most of the hardware is built for us.

Enter the relay module, and the weird and wonderful Digistump Tiny85 module... just the ticket, and small enough I should be able to squeeze them into the case.

I've used the normally closed relay contacts, so if I do want to use the laminator without the speed control, it will work fine with the speed control switched off. The relay contacts are connected in series with the motor, and the relay control pin to P0 of the digistump.



A design is laser printed onto a piece of paper from "The Economist", and a test piece of board stock is cleaned up (I know they're a different size, but it's only a test)
















The paper is tightly folded round the board so it doesn't move, and stuffed into the mouth of the hungry laminator! The "printed" side needs to be facing the bottom roller, as that appears to be the roller that's heated.

I left the speed controller off (so the unit was running at normal speed) whilst it reached temperature.

I ran a couple of tests, trying different speeds.




and into the sink!













I finally settled on 4 seconds off time, and 0.5s on time.... which produced nice, repeatable results ....




and left us with the digistump code looking like this....

void setup() {
  pinMode(0, OUTPUT);
}

void loop() {
  digitalWrite(0, HIGH); //Turn the motor off
  delay(4000);
  digitalWrite(0, LOW); //Turn the motor on
  delay(500);
}

So the electronics is all boxed up, and driven from a redundant mobile phone charger...



A few words of warning....

We're putting paper in here, which is being exposed to high temperatures for more time than it would be in normal use.... If it should jam, there's a risk it could smoulder and catch fire, and ruin your day... DON'T USE IT UNATTENDED!

It clearly states "Up to 80 Micron" on the box. We're really taking the P*$$ here, as my boards are 1.6mm thick, plus the paper (folded) so don't expect it to last ... but while it does .... get making!









... oh, and you can use the same idea to print a "slik screen"...


Thursday, 18 January 2018

Arduino Shakespearean Insult Generator

Hither is a little excit'ment thing I've hath built, to fig thy friends in the words of the bard ...

There are plenty of Shakespearean insult generators out there as webpages, but here's an implementation you can keep in you pocket.... (but someone came along and invented the smartphone, which sort of renders it all academic ... never mind... carry on!)

It's basically an exercise in cramming lots of text into the PROGMEM of an Arduino Uno.

There are 3 words stored, each having 53 variants, giving a total of 148,877 different insults....

Compiling for Uno, shows us the efficient use of memory ..

Sketch uses 4658 bytes (14%) of program storage space. Maximum is 32256 bytes.
Global variables use 85 bytes (4%) of dynamic memory, leaving 1963 bytes for local variables. Maximum is 2048 bytes

Code can be found on my Github ... https://github.com/andydoswell/Shakespearean-Insult-Generator

Have fun !


Sunday, 7 January 2018

National AE-670 Basket case repairs and restoration.

A while ago I offered to restore my friend Richard's late fathers radio.

It's a National AE-670 radio, dating from around 1960. It's got medium wave, and two short-wave bands, and a "gram" input.


It's also a basket-case! It's been dropped at some stage in it's life, the glass dial is broken, and case front is cracked, and the woodwork's in a bad way... let's see what we can do....

At some stage, someone has added a two pin connector on the rear .... I hope that hasn't got mains on it, as it's pins are exposed....
..and it is! Connected to come on with the radio! I wonder if it was used to drive a table lamp or similar. Some TV's and radio-grams had similar sockets, although not exposed. I disconnect the socket, as it's not safe...

The chassis looks complete and largely un-molested.





Removal of the front panel knobs, and four screws underneath the cabinet allow easy withdrawal of the chassis. I also remove the speaker board, and the damaged dial glass. A Piece of veneer on the front comes away when I remove the surround. I'll glue this back later.







On this inside of the cabinet, there's a circuit diagram, layout and re-stringing information. Something looks to have eaten it a bit!

The valve line up is 6X4 rectifier, 6AR5 output, 6BE6 frequency changer, 6BA6 IF amp, and a 6AV6 AGC, detector and 1st audio amp. There;s also a 6DA5 indicator tube.






The circuit will come in handy later...












With the chassis removed, we can start on the electronics....












The chassis has quite a few waxy capacitors, which will need replacement. There's also a 3uF 300V electrolytic which is probably past it's prime!









The main smoothing can is something else... look at all those sections!



















After a few cursory checks to the mains and output transformers to check they're not open-circuit, I change all the waxy capacitors for modern equivalents.















There are a couple of wax caps I can't change. They are C50, a 0.003uF and C29, a 0.05uF.

These need to be modern safety components.

C29 must be X2 rated, as it's connected across the mains. C50 must be Y rated as it's between the mains and chassis. If C50 were to fail short-circuit, our chassis would become live. Not good.

I've ordered the X2 and Y rated caps, but for now the set will function without, so the wax capacitors are simply snipped out.

You can also see the chassis has a mains transformer, so is an isolated design, unlike many British sets of the time.



There's also some rubber insulated wire that's perished. It drives the dial lamps on the front panel. It's replaced.











I gently bring the mains up via a variac and isolation transformer, monitoring the HT as I gently wind up the voltage. This allows the electrolytic capacitors to reform.








The main multi-section electolytic shows no rise in temperature, and is pronounced fit. The small 3uF cap is not in such good condition, and is replaced.


The set performs well, although the indicator is a bit dim... they never seem to be very bright thesedays...











Here's a long exposure pic!











I leave the set on soak test once I'm happy with it, and set about the cabinet.













The dial back, escutcheon and knobs get a good wash in the sink...










Where the front panel is damaged and de-laminating it's glued back together with PVA glue. The missing piece of lamination is made up with pieces of paper and glue, a bit like decoupage, until it's level. If I don't do this, the replacement speaker cloth will never sit straight.








The centre rib has been pushed in. It's repaired and glued. The croc clips will hold it all in place until the glue dries.

The speaker cone is ripped. It's been glued in the past, but has really had it. I'll get a replacement.













And on to the case...

The years have not been kind...
































The piece of veneer from the front moulding is glued back...











.... and the deeper scratches filled with some fine wood filler.













Now to don a dust mask, and rub the thing down....

Looks good eh? Now to stain the wood again...

First off hoover up all the dust left from the sanding, and wipe the cabinet over with a tack-wipe.




Now, I must admit I bought this stuff ~10 years ago to paint a front door with... came out nicely too. There's still a good 3/4 of a tin left...

Now...

Step 1. Read the instructions on the tin.

Step 2. Ignore the instructions on the tin!



It does say on the tin "Apply evenly by brush along the grain" ... which is great if you're doing a front door... Find yourself a good lint-free cloth, dab it in the woodstain, and wipe it on, along the grain. It's really easy to get a nice, thin even coat like this. Not only that, but it will have dried by the time you've finished reading this paragraph!






Now that's looking good. If you want to go darker, just add another coat or two, until you get to the shade you're happy with. As it is, I think just the one coat is about right. You'll notice it's not too shiny. The woodstain gives us a satin finish, I intend to add a gloss varnish when I'm done...







Now we need to re-create the black front, with the fade towards then centre. we can do this with some black aerosol paint. The end result is the given a coat of lacquer.









I'm rather pleased...












During all the cabinet work, I had a thought... Gram input. Be nice if it was bluetooth....

There's precious little on MW and SW these days, and this would give the receiver a bit of a lift!!

Enter the BK8000L bluetooth module and adaptor board.




A simple 7805 power supply is built up on some perfboard. The keen eyed amongst you will notice there's only a single diode. The board uses half wave rectification. This is because the board is to be powered from the heater winding on the transformer. One side of this winding is referenced to chassis ground, so we can't use a bridge rectifier. The module only uses about 50mA when it's running, so there's not a lot of smoothing required either (220uF seems to work well).




The module's left and right output "positive" outputs are coupled together via two 0.1uF capacitors, and the signal fed into the gram socket.

It works really well :)

And the radio reception isn't affected by it either!






The speaker board gets a nice piece of blue cloth glued, clamped and clipped into place....











... groovy ....












The hole where the magic eye pokes through is given a coat of PVA glue. This will make it stiffer, and easier to cut. It'll also stop it from sagging after I've cut it.

It's left to dry.







Meanwhile, the X2 and Y class safety capacitors have arrived......










... and are duly fitted.














Now onto the dial glass... it's in a bad way ...












 in fact it's in three pieces...













The rubber grommets are removed from abound the holes for the control shafts, as they've gone hard and have shrunk. I'll find some new ones...

But what to do about the broken glass? I've had very little success over the years attempting to glue it. Superglue is just too brittle. Epoxy works, but has to applied thickly to get it to stick and the results are generally not nice to look at.



My mate Stephen showed me this glue the other day. "It'll stick anything" he says. It comes in a pen, and is cured by the UV LED mounted in the end....

Further research show people have been using it to repair broken and cracked mobile phone digitisers, as a sort of stop-gap repair.

What have I got to lose?











Having carefully cleaned the edges of the broken parts with isopropyl alcohol, I apply a line of the glue from the pen, and shine the UV LED at it...

It allegedly cures in 5 seconds...

I'm sceptical...








... but it works superbly!

Where the glue ran out of the join when I put the two halves together, there's a bit of a bead, which is very slightly soft. The packet says the stuff can be shaped, sanded, drilled, polished and painted... so I clean up the surface with a bit of wire wool.





I repaint the rear of the dial, where it's supposed to be opaque, with some black paint, as some of this was damaged when the glass was broken.

At this point, the dial breaks again, in the middle of the "V" ... there was a hairline crack there before... It again glues well...






Now onto the speaker. It's a 20cm x 12cm elliptial. It's shot. Not only is the cone torn, but the edges are all breaking up. It's also 3 ohms. Digging about in my box of spare speakers turns up a couple of similar sized speakers, but they're 8 ohms. I asked the usual sources and put out a couple of requests on various forums for something suitable. No good. eBay was not forthcoming either. Then, whilst googling I found this ....







I'm lucky enough to have a branch of Halfords locally, so I popped down, and picked one up...

It's a similar size, and 4 ohms... near enough. Notice on the label it states "Min. Input 20 watts" .. I've only got 4 or 5 .... I'm sure they don't mean that really ..... only one way to find out!





The speaker is mounted up onto the speaker board and fixed back into the case. The output transformer was originally mounted on a bracket mounted to the old speaker. I'll worry about that later ...








Now to refit the dial-glass. This was originally mounted by two rubber grommet type mounts, located on the four holes around the holes where the control shafts pass through, on each end of the chassis, as shown here .....








The rubber has long since turned into some kind of gunge...  I had a look in the box of rubber grommets, and selected a couple of these conical shaped things. They're designed for accepting wiring looms though the bulkhead of cars. I used a few on the mini...







A grommet is fitted to the hole in the chassis, and the conical grommet cut to size.











The output transformer is mounted to existing holes that fix the front bezel to the cabinet and speaker board. The dial lights are re-fitted, and the magic eye back into it's spring-loaded holder.

The bluetooth receiver is glued to the bottom of the cabinet.







... the back replaced,  and it's given a thorough testing. The "min 20 watts" label on the speaker proves to be meaningless, as I thought.....