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Saturday, 19 May 2018

Synchronised Westerstrand Impulse clock driver.

A while ago, a friend, Alex, asked me how he could drive a synchronised clock he had procured.


It was a Westerstrand clock. The sort that you used to find in factories, offices and schools. It's an electo-mechanical driver, driving a conventional clock display. It doesn't posses any time keeping mechanism.

Now Westerstand are still alive, well and trading since 1906. Their website is here.

Anyway, according to some information gleaned from their website (they still make these things) the clock needs a pulse of 24 volts alternating in polarity to advance a minute. 






I quickly drew up a small circuit, with an arduino and a GPS receiver to drive Alex's clock.... he pronounced it too complicated, and removed his electro-mechnical movement and fitted a quartz clock movement .....

.... that's cheating.

I managed to procure the same clock movement in somewhat distressed condition, and managed to put it all back together.

So what about that proper driver? Good plan. I ditched the GPS receiver, as I have the GPS master clock, and added a 433 MHz receiver to receive the signals.

The clock will "free-run" after being set, using the 490Hz interrupt driven clock, as previously seen on the Arduino analogue clock.

Pulses are sent to the clock by using a small H-bridge.

Now the electronics has no method of knowing where the clock movement is, so before the synchronising signal from the GPS master clock is received, the clock needs to be set to 12 o'clock. I've added a minute and hour button to the PCB to allow the movement to be set.

The code also automatically adjusts for British summer time.




A board is designed...











 Modelled...
















... and turned into reality.

Code and eagle files can be found on my github page at https://github.com/andydoswell/Sync-clock-driver
















Here's a video of it in action...


Friday, 18 May 2018

More modifications to the Fellows A75 Laminator for toner transfer.

If you haven't seen the first bit of this ... click here.

Made quite a few boards with the laminator and toner transfer method recently, and results have generally ranged from acceptable to good.

There's the occasional fail, but I want to improve repeatability.

I've made some measurements of the temperature of the rollers, and it's temperature controlled at about 100 deg C, and sometimes, the board just doesn't get hot enough to melt the toner and cause it to transfer. We need to warm things up a bit....

I removed the PCB and reverse engineered enough of it to work out how the temperature control is done... IT'S IMPORTANT TO REMEMBER THIS CIRCUIT IS "LIVE" AT ALL TIMES, AS THERE'S NO TRANSFORMER. I used a mains isolation transformer whilst making measurements and working on this unit.

Mains comes in at JP1. Mains is dropped via a capacitive dropper C1 (R2 prevents the cap remaining charged up once the unit is unplugged, and removes a shock risk) R1 provides a bit of current limiting, D1 and D2 rectify the output, which is stabilised by ZD1, and C2 is used to smooth the supply. N3 is a 5 volt regulator, and C3 provides a reservoir for the 5v rail. The temp sensor looks like an ordinary 1N4148 diode.... it may well be. It's pressed up against the lower rubber roller. When it's at room temp, it's got about 4.2V across it. When it get up to temperature, there's about 2.4V across it. This is loaded by R32, and connected to the inverting input of N2 via R7, and back to ground (neural) via R8. A voltage reference is provided by the chain R9,R11,R12 and R13 (approx 2.6V) and applied to the non-inverting input, forming a comparator. Stability is provided by some feedback provided by R6. The output of the comparator is fed to the gate of a triac, T1... Now a note on D7... it connects to N1, which the manufacturer has very kindly filed the number off.... I imagine it's some sort of micro controller. It most likely provides some timing (the unit shuts the motor off after 30 mins) and some zero-crossing pulses to D7 so the triac can phase modulate the triac. I've not illustrated this part of the circuit, as it's not relevant, and difficult as I don't know exactly what's going on inside the mystery N1. 

OK, so we need to alter the voltage reference at pin 5 of N2. I remove R9, and place a 10K pot in it's place.

I adjust the pot until the temperature stabilises at 130 deg C.

The pot's then disconnected and measured, and it's 3.6K, which is rather convenient, as that's a preferred value! R9 is replaced with a fixed 3.6K part.


Tests prove transfer is definitely more "robust". The toner seems thicker, and more difficult to remove.

We're now operating this unit way outside of it's design spec (as if it wasn't bad enough before!), so don't leave it operating unattended!!

Wednesday, 16 May 2018

Well, knock me down with a feather....

Thanks to the lovely people at Feedspot , we have an accolade!

Apparently, this humble collection of electronic musings is now amongst the top 75 electronics blogs! 64th in fact!

I am truly honoured!


I'm not sure why they've sent me that , as I'm outside of 60 .... but not by much ;)

Thursday, 10 May 2018

The big hifi preamplifier project - The main preamplifier board.

It's been a while. This project is taking up a lot of time (and money!) ... here's the latest instalment...

So, we have our sources selected (see here) and the signal needs to be amplified.

I'd like everything to be remote controlled. I'd like a simple and subtle bass and treble controls, and balance.

I'm going to put power supply regulation on the board, so we'll need to supply the board with +/-24V of well filtered DC.

I'm going to incorporate a "side chain" with the Korg Nutube in it, and make it switchable in or out.

I'll use NE5532 op-amps, because they're cheap and cheerful, and a damn fine performer.

So... after some thoughts about remote control, I'm left with 2 options.

Motorised pots: Damned expensive, and I'll need 4 stereo ones. Mechanical mounting issues.
Digital pots: I've used these with great effect on previous commercial projects. Very cheap compared with motorised pots, and cheap compared with normal mechanical pots, considering the spec. Enter the Microchip MCP41100.  Just the job. 100K, 256 positions, simple SPI interface,linear. I'll put some resistance across the output to simulate log, and if that's not good enough we can tweak the response in the software.


OK, so our power arrives in at JP4, there's some filtration by C9,10,12 and 13 just in case any pickup has occured from the rectifier and filter unit. The +5v will be regulated externally, and has a separate ground. The +/-24V is regulated by an LM317 and LM337 respectively to +/- 17V  (I know it says +/-15V on the diagram!) Each IC has it's own 0.1uF ceramic decoupling cap. There are 3 leds to show the supplies are present. Control signals arrive at JP1 from the microcontroller board. The SPI signals are taken to each digital pot, and each digital pot's CS (active low) lines for each IC are also available. There's a bit of bunkum on the web that digital pots are only capable of working within their supply voltage, of +5V and GND. This is wrong. It's digital part, yes. The resistance output, no. It's just that, and behaves like any normal pot, so will be quite happy with our audio signal. 
Audio arrives at JP2, and is given some gain by IC2. this is fed to a buffer amp, IC1, which then returns the audio to the REC OUT socket, for recording purposes. Audio is also fed from IC2 to the tone control, formed by two digital pots (per channel) and IC10. Bass is controlled by IC3 (and IC4) and the treble by IC7 (and IC8). Maximum boost and cut is limited to about +/- 3dB, centred on 20Hz and 20KHz. Audio leaves IC10 and passes to IC11 (and IC12) which are the volume controls. I did, originally add a balance control in, and then realise this was utterly pointless, as I could have independent control over both the left and right volume controls, and do the balance in software, and not run the risk of compromising channel separation. Audio leaves the volume controls, passes K1 into the final buffer. IC13 , and out via JP5. If we want more (or less gain) we can alter the value of R59 (R60) to suit. K1 is used to switch in the Nutube valve. The Nutube cannot operate differentially, so the audio is DC isolated by C29 (and C40), and bias is applied from the 3.3V regulator (IC5) via R30 (R48). Some experimentation will be required to set the correct bias. Anode is supplied from our 17V rail, with a bit of filtering courtesy of R17 and C11/15. The audio is developed across the anode load resistor R32 (R45), and passed to a simple FET buffer Q2 (Q1). Now. I think I've dropped a whatsit here, I've passed the audio via a 100K pot R58 (R61) which is going to present a rather high impedance to the output. I'll take some measurements, and see how it performs. Sadly I didn't notice this schoolboy error before I sent the board for manufacture..  The required 1.7V for the filament is supplied from the 3.3V reg via R22/R23. As the tube is directly heated, the filament is also the cathode, so it's AC coupled to ground via C19/22 (C21/23).
Audio is also passed from the output to the meter drive circuit, buffered by IC14 (IC15), and rectified by two germainium diodes D5/6 (D7/8). This output is used to drive the output transistor T2 (T3) which drives the meter connected at JP6 (JP7). R68 (R69) is used to control the calibration of the meter. It's responce should be more PPM-like than VU (which, as every recording engineer knows, stands for "virtually useless"). It's not going to be that accurate, but will give us something to look at ;) You may need to alter the values of C57 (C58) and R82 (R83) to suit your meter. If you can't find germainium diodes, try a schottky diode. The relay for the Nutube is driven from the microcontroller board via T1. The 5v ground and audio ground are kept separate, to avoid clicks and pops when the relays switch, and to avoid noise being picked up from the uP.

Board layout.... 
Single sided board is going to be a problem. There's quite a lot going on, and I want to keep the form factor reasonably small. Double sided it is, and whilst I can do this at home using UV film resist, it's not easy to get a decent etch. I could purchase board readily prepared with UV resist, and these usually etch very well, but they are damned expensive here in the UK now. It's a shame that the spray on UV etch resist is no longer available. So, I'll get the board professionally made. I used to use a company called Olimex in Hungary, but they are at capacity with their own work now, so are not currently taking on small batches, which is a pity as pricing was good, and quality excellent. My friend Ben had some small boards made in China by www.smart-prototyping.com. The quality was great, so after a brief and helpful conversation on-line with Minnie Yu, I sent over the gerbers. Within 10 days, 5 boards arrived. Quality is at least as good as Olimex. 

When I laid the board out, I tied to keep the digital signals away or perpendicular to the analogue stuff. A friend of mine always used to say "Keep the ones and noughts out of the rig" ! 
There's an analogue ground plane each side of the board. 


A most exciting delivery from China...



Parts were ordered, and the board assembled...


One thing worth noting is the output coupling capacitors (C1,C2,C51 & C52)... I left plenty of room on the board, as I originally intended to use some poly's.. but Mr Self has written about non-linearities in some, so I though I'd try some multi-layer ceramic's... and they are TINY! This is a 2.2uF 50V example!....



I was going to carry on in this post, with control electronics and firmware, but, quite frankly, it's taking some time, and it's bloated into a project in it's own right...  So far it consists of a Teensy 3.2 microcontroller, and a few logic gates to expand the IO, I'll show more in the next exciting episode!

CoaST?

Monday, 30 April 2018

Numark iCDMIX2 repair

Lee put out an SOS on Facebook. The CD player he uses to teach disadvantaged individuals to DJ on had packed up. It just so happened I had a spare in the loft he could have ...

When he came to collect it, he dropped the broken one off...

"Care to have a look?"

Yeah, why not....


It's a Numark iCDMIX2 , and the left hand CD isn't reading discs... 

On testing it, I noticed the left CD wasn't engaging the disc, and you can hear the tray belt slipping. Lee seemed to think someone had been a bit heavy handed with it, and it's mis-aligned. I recon it's the tray belt slipping....

Three screws at the top of the front panel, two on the left, two on the right, and four countersunk ones underneath are removed to gain access to the CD transports...









and I can't *quite* get enough access.... off with the top.... four screws each side, and four across the back...












Gently lift the lid, careful not to unplug anything... and prop it up with a screwdriver!













With the tray in the eject position, pull the small lever to the left of the tray outwards, this will free up this side of the tray.... There's a similar lever on the other side, and you will now be able to extract the tray, to gain access to the belt. Lee was right, the belt was blameless in this case, and the unit just needed re-aligning. The large cam which engages the tray is moved round by hand unit the "gap" in the teeth is to the right hand side of the mechanism. Now the tray is inserted, and the unit tested...


Bingo. Another saved from landfill....

Monday, 9 April 2018

Dansette Junior Deluxe repairs & modifications.

Will turned up for an emergency curry. There was something ominous lurking in the boot of his car ....


It's a small Dansette Junior Deluxe record player...

"Can you take a look?"

Yeah, why not ....

It's in good cosmetic condition, and dates from around 1961.

Further inspection reveals some issues...

The valve (yes, just one!) is rattling around in the bottom, but is thankfully undamaged. The motor suspension has a few bits missing and the platter won't seat. These are really quite minor issues and are soon sorted.
Once the thing is spinning round reasonably, We try playback .... It's awful. Distorted and really low in volume...

The amplifier is simplicity itself. It has a nice mains transformer so it's reasonably safe, even has a 3 core mains lead :)

The output from the transformer is rectifier by a metal rectifier, and smoothed by the 16 and 32uF capacitor block, along with a 47K resistor. The smoothing cap is in really good condition!

The cartridge is a BSR TC8 medium output crystal cartridge,  producing an output of a few hundred mV or so. This feeds the top of the volume control, and straight onto the grid of the EL84 pentode. There's no cathode bypass capacitor fitted to this unit, nor an indicator bulb, but this is the nearest schematic I had ...


The distortion is partially caused by the 0.05uF tone control capacitor (a dreaded hunts), and is changed ...

The rest of the distortion, and the woefully low output is being caused by a poor cartridge. It's terminals have got chronic verdigris, and this means the crystal inside will have turned to goo. It's a very common failure. (It's not all bad though, as this mono heavyweight cartridge will have chewed Will's records to pieces.)
So we dig around in the box of phono carts to find a suitable donor. The big issue here is, we have nothing to mount to, except the original "flip over" mount...

There's a couple of Acos M7 moving magnet carts that may well do ...











... and after a bit of filing, it fits !

Tracking weight is set for about 2.5g, and it manages to track the test record.... but we can't hear it ....
The output from the Acos M7 is around 1mV .... not enough to drive the EL84's grid much at all... we will need to make an amplifier. Also the output from the crystal cartridge approximately followed the  RIAA equalisation curve, so no correction in the amplifier was needed. We'll have to add this, otherwise the output will sound very thin and tinny.


OK, so the above was quickly dreamed up. It's a (very) basic phono stage, and has some simple EQ in the feedback loop, to approximate the RIAA curve. 
  
I've stolen some power from the 6.3V heater winding on the mains transformer inside the record player. You'll notice there's only a half wave rectifier here, as one side of our heater winding is connected to ground. It's imperative to get this the right was round or you'll short out the heater winding on the transformer, and it will burn out pretty quickly... 

C1, R2 and C3 form a filter for this simple power supply. The cartridge is connected to JP2, and it's loaded by R1. C1 couples the audio though, which is biased to half-rail by R3 and R5. This is then fed to the op-amp, which does it's amplifying and RIAA correcting duties, and then passes the audio out via C8 to our existing volume control. C4 provides DC stability, and a little sub-sonic filtration.

A board is etched...

And duly fitted....

You'll notice the cable coming from the cartridge is screened. It was originally a twisted pair, but the hum pick-up was ghastly, as the arm is all plastic and has no shielding, lives over the mains transformer, by the mains input!
... and, because nothing in life is every easy, the mains switch failed open circuit during testing ! It was stripped and repaired....

How's it sound? ... honestly? Pretty damn awful, but at least Will's records are safe (r) 

here's some more pictures, including the obligatory arty valve shot ;)


... the big pre-amp project is coming ...

Sunday, 11 March 2018

Kenwood BM250 repair.... and Doz' wholemeal pizza dough recipe

Friday night. Pizza night.

I like to make my own pizzas. I don't particularly like white bread bases, so a few years ago I took to making my own wholemeal pizza base. I use a Kenwood BM250 bread maker I bought about 10 years ago, on it's dough setting to make the dough, roll it out and bake the pizza. Excellent.

Well I'd set the machine going, and it was whirring away in the kitchen whilst I was working in the workshop. *CLUNK* and the house power tripped. The mains monitor text me! I went to investigate. The RCD had tripped, indicating an earth fault somewhere, and refused to reset. I unplugged the bread maker, and it reset... and I was stuck with some half mixed dough...


After ordering domino's... I decided to take a look...

First thing of note was the damaged mains cable ...

Tsk tsk! I should have noticed this before...









Much unscrewing followed, and the mains cable was replaced. Although requiring replacement, the cable's inner insulation was intact, and not the source of the problem...











Here you can see the motor drive for the stirrer thing, and the control PCB. I performed an insulation test on the motor at 1KV, and all was well....

The heater element inside the oven compartment, however, showed very poor insulation of just 10 Kilohms when cold. No good at all.










The element is removed. There's a small screw and a nut holding one end in....










... and a screw and nut holding this ceramic saddle clamp arrangement on the other.











A replacement element was found inexpensively on eBay (£6.39 delivered) and put the problem right. Another saved from landfill, and £70 cheaper than a new machine.



and if you want to try a healthier wholemeal pizza dough ... here's my recipe!

230ml Water
380g of wholemeal bread flour
~30ml Olive Oil ( a good glug! )
1 packet of bread machine yeast.
Some Italian herbs and spices!

Enjoy!

Saturday, 3 March 2018

Arduino programming tool. ISP and IDE.

The big hifi pre-amp is on hold for a bit, as I await some parts to arrive.

In the interim, I've wanted to make a programming tool for ATMEGA328 chips, that will ease programming changes when using them in stand alone units. I've looked about on the net, and there's some solutions which are nearly what I want... but nothing quite hits the mark.

The idea is to have a ZIF (zero insertion force) socket into which the blank chip to be programmed is inserted. I want it to be able to program chips with the bootloader, as well as straight from the IDE.


The fitted micro is loaded with the "Arduino as ISP" sketch. The switch, S1 & S2 (actually 1 four-gang switch, I just didn't have a library for it!) is used to switch between ISP programming for bootloader, and IDE Programmer. LEDs indicate various functions. D1 is ISP heartbeat, to show it's OK, D2 is Error in ISP programming, D3 is ISP programming activity, D4 indicates ISP mode, D5 indicates IDE mode (also lights when in ISP mode, as I forgot about the steering diode D5), and D6 indicates slave activity during bootloading (ICP1).

...a simple board is designed and etched....


The keen-eyed amongst you will notice the steering diode, D5 is missing from the layout. I mounted it underneath the board, as I'd missed it...


... and it works a treat !

Oh yeah, a quick note on R2 and R7... both 1 megohm across the crystal. I've picked up this bad habit from the original Arduino design. It doesn't need to be there. It was originally to provide some bias for gate-based oscillators. The ATMEGA328 has bias provided internally. It's a waste of 1 meg resistors! Don't bother to fit it, and I promise never to include it again!!

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?