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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.....




Friday, 15 December 2017

Simple, low power Arduino countdown timer.

I have a requirement for a simple, battery operated countdown timer.

It could have a variety of uses, in this use, it just needs to count down from a preset number of minutes to zero, and sound a bleeper.

I could go out and buy something for little money, but where's the fun in that?

I'm also interested in Arduino sleep modes, so it'll be interesting to play about with that too, to increase battery life.

The sketch can be found at https://github.com/andydoswell/Timer
I've also included the library for driving the TM1637 display, which is by Bram Harmsen
and can also be downloaded from  https://github.com/bremme/arduino-tm1637

Let's have a quick look at the hardware...


A momentary push button is connected to D2. A BC337 (or almost any general purpose NPN transistor)  is connected to A3, which is used to switch power on by grounding the GND pin of the TM1637 "light tube" 7-segment LED display. DIO and CLK of the TM1637 are connected to pins 10 and 11. The output of the pot is connected to A0, and the "hot" end connected to D13. Power by way of 3 AA cells, is supplied to the VCC pin of a pro-mini. A piezo beeper is connected to D3.

How the sketch works...

In normal use, the microcontroller is asleep. It's woken up from it's slumber by an interrupt going low on D2. Power is then switched on to the display, by taking A3 high, which switches on T1, and connects the display GND pin. D13 is then taken high, which switches on power to the pot. The pot position is sampled by A0, and used to define the number of minutes the timer is  required to run. After a few seconds, D13 goes low, saving us a few precious uA! The timer begins to count down, displaying the value in remaining minutes and seconds on the display. Once the timer gets to zero seconds remaining, the display flashes and the beeper sounds to alert the user the time has expired. Once this is complete, A3 goes low, disconnecting the display, and the micro goes back to sleep.

Power consumption is about 45mA when it's running... and a super-low 200uA when asleep. The batteries should have a good life.

I did remove the voltage regulator on the pro-mini, as initially I was going to use a 3.3V unit, and the regulator burns a few mAs even when the micro is asleep, but it's unnecessary. There's no difference in the microcontroller, so why bother? Just feed our 4.5V into VCC... Despite the light-tube stating it needs 5v, it seems happy right down to 3.3.

At the moment this project meets my requirements for use. I'm going to build a new ultra-violet light exposure box shortly, and I'll modify this to drive a relay to automatically control exposure time.

Thursday, 14 December 2017

The re-occuring Copland CSA515 fault.

Not the greatest of success stories, this one...

Many moons ago Gerald rang (his name's not really Gerald, but the names have been changed to protect the innocent)

"I've got this Copland Hybrid amp, it sounds thin and distorted on one channel, mainly in the mid-range, can you take a look?"

Yeah, why not? (in hindsight, there are now a good dozen reasons I can think of why not...)

It's superbly made in Denmark, and weighs a lot!













... the weight mainly due to the huge toroidial transformer, and gargantuan heatsinks!

Look at all that smoothing.








The problem quickly showed itself.... something in the back of my mind said "this sounds valvey to me" .... but swapping the two 12AU7's (that's ECC82 to you and me) made no difference. I eventually traced it down to a thermally poor bias transistor on the right channel... or so I thought.

The amplifier behaved faultlessly on test with my Rotel pre-amp driving it, so I sent it back....

About a week later the phone rang again ... it's Gerald

"It's doing it again" ...

Send it back....

It arrived. I put it on the bench, and listened to it all night, as I was working on other things... It performed faultlessly.

I removed my power amp from my hifi to give it a damn good test .... months it sat there, performing beautifully.

.... then ... it did it...

"I've got you now!" ....

Put it back on the bench, and it was fine! No amount of tapping or banging would get it to fault....

I removed the PCB (not an easy task in itself) and checked every soldered joint. Not one was poor.

I was about to reassemble it, and thought I'd just check the bias again... and sure enough the right channel had drifted a bit. I adjusted it, and put it back on test.... then about 10 mins later, it faulted again!!! I changed every cap in the right hand bias circuit.

Working again...

After a week of perfect operation, it was returned to Gerald...

... where it ran for a couple of days ... and came bouncing back once more....

On the bench, and the fault was immediately obvious. Bias on the right hand channel was miles out.

As I probed my way through the circuit, the fault was progressively putting itself right.... the DC bias to the bases of the output pairs was decreasing. I traced it all the way back to the drivers, when the fault just wasn't there anymore.... I switched the amp off and went for a nice cup of tea....

I came back, and switched the amp on. Excellent fault present for a few minutes, then the bias was decreasing to a sensible level once again. Out with some "freezer spray on the cheap". Output was connected to a dummy load, and monitored on the scope. Finally some progress. Freezing and cooling a transistor in the voltage amp would cause the fault to come and go...




Gotcha....


















... I hope

               ... and so does Gerald...

Sharp Optonica SM-5100 integrated amplifier repair.

Colin wondered in ....

"Got this old Sharp amp, smoked last time it was switched on. Can you take a look?"

Yeah, why not....

It's from the unfairly overlooked range of Sharp Optonica hifi, an SM-5100. Optonica was a sub-division of Sharp, primarily to complete with other manufacturers high-end brands such as Panasonic's Technics.


Nice slender looking thing.... dating from the early 80's.


Removal of the top panel (not as easy as you'd think, it's a very tight fit!) reveals the culprit, the dreaded Rifa X class filter capacitor... in this case it's 0.0047uF 275VAC and X2 rated....









... which is soon evicted and replaced with a nice Vishay one.. again X2 rated. The X2 rating is crucial in this application.










Powering up and there's no smoke. There's also no output from the left hand channel. Pots and switches are all cleaned up to no avail...










After a little bit of head-scratching, and in fear of one of the STK-0050II output modules having failed (replacements are still available, but many are turning out to be faked, and either don't work or fail rapidly) RLY501, the speaker protection relay is found to have failed. A suitable replacement is sourced and fitted.



There's a good range of input and output sockets, two tape loops and a pre-amp to power amp bridge.

Two sets of speakers are catered for.

Look at that nicely shielded toroidial transformer.




The mains lead is replaced, at someone had cut it short at some stage, and fitted one of those daft "Masterplug" things with the tiny round pins.

It performs very well. Outputting a very reasonable 35W in 8 ohms. The phono stage is very quiet.

The amp is soak tested with "Upstairs at Eric's" by Yazoo, an album I never get tired of :)

Wednesday, 15 November 2017

Tannoy SFX5.1 Powered subwoofer repair.

Paul popped by.

"I've got this Tannoy subwoofer, it crackles and pops. Can you take a look?"

Yeah - why not......


It's a smallish cube, mounted on little cones, with a downward facing speaker...

I powered it up, and all was quiet. 

Just as I was about to connect the signal generator, I heard thunder in the distance .... and it got louder and louder! The speaker was popping and crackling alright. I quickly powered down.

I removed the screws around the outside of the rear panel, to reveal the speaker. The electronics and amplifier are mounted on the rear panel itself....

The main amplifier, a TDA7293 is heatsinked to the rear panel.










Removal of the board showed signs of distress....

Q151 and Q152 and there associated components had been running rather warm! The colour of the cases of the transistor should be black, you can see in the photo that they've changed to a sort of powdery brown.

Q151 is a 2S1815, NPN, and Q152 is it's complementary PNP, a 2SA1015.











Static testing on the transistor tester showed both to be fine. But the fault seemed to exist only when the thing was drawing current, so I threw caution to the wind and replaced both transistors. The two diodes, the two resistors and the caps all tested within tolerance.







Switch on again, and it's cured!

In the above picture, you can see two wire links across F152 and F153, where there was place for a couple of fuses. The wire links are original, so the fuses were obviously deemed unnecessary, relying on the non-replaceable thermal fuse in the transformer...

Not sure I like that idea much....



*** STOP PRESS *** 12th May 2020.

Many, many people visit this site to look at this page, and, sadly can't find the correct transistors. 

The SCA1015 is obsolete, a KSA1015YTA is still available..  https://uk.rs-online.com/web/p/bjt-bipolar-transistors/8064403. You can try a BC556, but you need to fit it backwards, and reverse pins 1 & 2 (Check the datasheets)
The 2SC1815 is still available from mouser, or you can fit a BC546, again reversing the leads. (Check the datasheets).

These transistors are pushed hard in this application. When (if) I ever see another one, I'll look for a better solution. 

*** STOP PRESS *** 13th January 2021.

PeterCB has been working on his subwoofer, and replaced the 2SA1015 with a TIP32, and the 2SC2818 with a TIP31.

"So I went ahead and replaced the transistors with TIP31C/32C in a TO220 package. Been soak testing for about six hours now, they don't even get warm. The new transistors fit perfectly in place of the originals, just facing the opposite way, see picture"


This has got to be the best solution. An easily available transistor, that's not being pushed beyond it's limits. 

Thanks PeterCB! 

*** STOP PRESS 15th March 2021 ***

Avid reader Springknees has kindly created a schematic of the offending power supply section. 

It can be found at https://www.chantrybarn.co.uk/woofer.php and is reproduced here with permission. 









Sunday, 5 November 2017

Classic Mini 1275GT Electronic GPS Speedometer and multifunction display revisited.

A few months ago (was it really that long?) I created an electronic solution to the classic Mini speedo (here)

When I posted the article up on a couple of sites, there were a few comments from people that they didn't like the movement of the speedo needle. I can see why, as it only moves in discreet "clicks" every 1 mph. They would like it smoother.

"Easy" I thought.... oh how wrong I was!

I initially started by changing the speed value from an integer to a float, and dumping the map function. It was an improvement, but had a tendency to jitter about. Nope. Not good enough.

Back to integer speed, and an attempt to control the update rate by creating an array and loading the array with the car's speed every time it updated (every 250mS). I then got the microcontroller to detect the slope of the changes in speed and move the needle quicker when the car was accelerating (or decelerating) hard, and slowly when the acceleration wasn't so hard.

Well, it didn't work very well. I could see the sketch was working, but the amount of data needed to sample, and the time it took to calculate the amount of delay to add to the pointer caused the update rate to slow to an unacceptable amount, and the speedo to appear "laggy". I attempted to improve the speed of the sketch, and revised the code a number of times, and even forced the compiler to optimise for speed over code size, all to little avail. The idea of delaying the speed of the pointer seemed sound though.

I drank some tea .....

A few weeks passed, and I had another idea. Don't attempt to measure the acceleration of the car. Control the acceleration of the needle with regards to it's position. The sketch now sampled the speed, and set the needle motor target position to that value. Now change the motor position, one step at a time, towards the target position. As the pointer approaches the target, increase the delay between each step. A few tweaks to the amount of delay applied, and it works! In effect I've created a PID controller for  the pointer position (actually, there's no D).

I've also changed the code to the multifunction display, which now displays a warning for ice, no GPS, and no charge (ICE, GPS or CHG are displayed)

The revised code (including the required libraries) can be found here :  https://github.com/andydoswell/GPS_Speedo_Smooth

Just click on "Clone or download" and download the zip. Unzip into your Arduino directory. Place the libraries in the library directory.


Wednesday, 18 October 2017

Metcal MX-PS5000 repair.

Been very busy, in a commercial sense, building some transmitting equipment ...

... and just as I'm about to solder the last two wires on the front panel of said equipment, the iron goes cold ...

The soldering iron in question is the very, very wonderful Metcal MX-PS5000 system. I used to think Weller made the rolls-royce of soldering irons, but these things are just in a different league!

They heat the element by RF induction heating. 13.560 MHz to be precise. It's got all manner of error detecting, it'll put itself to sleep when not in use, and also has outputs for two irons. It's also expensive.

So I think, it's probably gone to sleep ... nope.
Perhaps the tip has failed? No... (takes two seconds to change too!!)

Damn ... the emergency iron is flagged into service, and the job finished...

A new MX-PS5200 is ordered (at about £500!.. ouch). We simply can't be without it.

But let's see if we can't do anything about the old one...


Firstly, remove the retaining nuts from each of the F-connectors.


















Flip the unit over, and remove the 4 security torx screws, one in each corner.

















Right, that's got it out. To remove the board there are several small screws to undo, and remove the screws from the heat sinked (heat sunk?) devices, noting which are insulated from the heatsink, and which are not. The wires connecting the power supply in the back and the main board are quite short.















First thing was to check the presence of the +48V  and  +24V supplies from the switched mode mounted in the rear third of the enclosure.. all present and correct... damn.. out goes any ideas of a blown fuse and a quick-fix! There's no hope of getting a schematic, so it's down to the poke-and-hope method of fault-finding....

Checking a few parts, shows the main output FET to be dead short ... It is (or rather was) an IXFH12N50P, which is no longer available. An IXFH16N50P looks like a good candidate, and is pressed into service.... still not warming the iron... damn. It's gate is driven by a drive amplifier, which is in turn driven by an oscillator formed by a crystal, and a 74HC04 hex inverter... the hex inverter is duff, and is replaced, and so is the supply to the drive amplifier...














It's an LM2576-ADJ (not that you'd know from the photo) .... and replacing that restores the supply to the second stage of the drive amp, but not all of it .... damn.














The final piece of the jigsaw is D12 in the small switched mode buck converter .... it too was dead short ... lord knows what the original is, there's no circuit anywhere on the web, so I selected a 1N5819HW for the job, based on the datasheet for U3. It's physically smaller than the diode it replaced, but it seems to work! I've got a hot iron!













Excellent ! We now have a spare in the workshop.

Not sure I like Metcal's new logo much ...