The Air Ministry Power Unit Type 87. Ref no. 10K/201.

Just a short video about this wonderful piece of history ...

Here it is in action....

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?

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 ...

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:

https://docs-emea.rs-online.com/webdocs/15db/0900766b815db1b5.pdf

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

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.....