Friday, 31 October 2014

Meade EXT105 Telescope repair.

A while ago a friend contacted me with regards to a Meade telescope that belonged to a colleague of his. Time elapsed, and eventually it made it's way into the workshop.

This is some bit of kit. Now I'm no star-gazer (although I've always fancied the idea) but this thing looks fantastic. Once calibrated, you can search it's database and set it to automatically point in the right direction of the celestial body you wish to view. Brilliant..... except for this one, which threw a fit when asked to point to something, and smoke bilged out from the motor that makes it move up and down (I would say it's an EL or elevation motor, but apparently when dealing with telescopes, it's a DEC or declination motor, lord knows why).

Beast of a thing. First challenge was to get the optics out and placed to one side, out of harm's way... Two cap screws this side, and two the other hold the complete optics onto the base. It takes some gentle persuasion to extract them. 
There's some sort of electronics attached to the top of the optics ( not really sure of their purpose! Let me know! ) . This board and it's cover are held in place by a single cap screw. There's a 4 pin connector to undo to release it.

Once the optics are out we can disassemble the stand. Removing the cover off the motor for the DEC drive, reveals some damage!

Unidentifiable burned-out IC's ...

We must never forget that electronics is driven by high-pressure smoke. You can see here where the failure has been caused by the smoke escaping from the IC's.

Emailing Meade and a few of their dealers gets me nowhere. Not even a reply. Thanks guys.

It does, however dawn on me, that the circuit that drives the Azimuth motor may be the same...

..and on closer inspection, it is certainly similar. The components are in a different position, but the circuit appears the same. 

By a process of elimination, the bits we need are a SI4947ADY & SI4936CDY. eBay is our friend. 

Each of these IC's house two MOSFETs. The SI4947ADY is P-channel and the SI4936CDY N-channel.

Having identified the parts, we give ourselves another problem! Which is which....

Having a careful look at the remains under a lupe, one IC gives us a clue, the SI4936CDY is on the left, the SI4947ADY in the middle (It's a 7805 voltage regulator on the right)

The old components are removed, and the board cleaned up. There's some damage to the print under the SI4947ADY in the middle. I have to add a tiny piece of wire here to make up the pad, and attempt to fit the IC on the top....

New IC's are fitted. I check for shorts & open circuit connections. It all checks out OK. The tiny wire under pin 1 is good to go!

 I decide to add a 0.01uF capacitor across the motor terminals to try and protect the electronics a little more from the spikes coming from the motor. I wouldn't mind betting this is what damaged the electronics in the first place.

... and then the daunting task of re-assembly. Nothing really to note here, other than be careful to route the wiring through the DEC motor board which connects to "the mystery electronics" on the top of the optics!

And then , bullet firmly between teeth.... Power-up!


WEM Watkins Copicat, repairs, renovations and electrical safety upgrades!

Minding my own business, when the telephone goes, and it's a friend with a wobbly Watkins that no longer works.

Cabinet's OK. Shabby chic. Hinges need replacing...
Obviously heavily gigged in smokey pubs! Tape path is dirty, and the heads are worn, but they'll go for a while yet.

Most of the caps in the supply are in poor condition. These two were replced, the originals being high ESR and low capacity. There were also two caps decoupling the +/- 12 volts rails. One was short circuit.
One of the main problems with the copicat, is it's lack-luster electrical safety. It was made back-in-the-day when standards were different. There's a nice metal box, the chassis of which is used to ground the audio incoming and outgoing. Now here's the issue. It's not grounded to a safety electrical earth. There's only a 2 core mains lead fitted. Now in this day and age , this isn't good enough. Not unless special precautions are taken (double insulated). This certainly isn't double insulated, and, although a remote possibility, if a live wire were to come off the motor or mains transformer, and connect with the metal chassis, the ground of your guitar would now be live. It's going to hurt, or kill. Not good. It HAS happened.

So, what can we do about it? Easy... fit a three core lead, and ground the chassis. Brilliant. Guitarist now safe. Except for the thing now hums like billy-o, because we now have an earth loop. More here.
OK, so what can we do? Remember the isolation transformer we fitted to the Ekco radio? Just the job. This will prevent any earth currents flowing , and prevent the thing humming whilst maintaining electrical safety. 

Here, I've fitted the isolation transformer , and a nice new 3-core mains lead. The Earth is secured to the chassis, using a soldered ring terminal, and a nut, bolt and star washer to ensure good contact
The tape-tensioning mechanism is stripped, cleaned and lubricated.

All cleaned up and ready for the next 30 years of service!

Wednesday, 22 October 2014

Ekco U319 Radio Restoration.

I have one of these radios myself, and it sits in my hallway, minding it's own business. It's VHF doesn't work, and the speaker cloth is ghastly. But a friend of mine passed comment on it... "I've got one of those! Can you get it going for me?" Of course I can ....

Arrival. A few issues are immediately obvious. The case is cracked, the dial glass is broken, there's no pointer in the dial...

 Not much we can do about the crack, unless we fill it using car body filler, flat it and paint it up to look like bakelite.

Electronically, things are looking better...

 Chassis is removed and examined.
The on/off volume control is a replacement, but has worked lose, and has wrenched all it's wiring off, and the suppressor cap has long since exploded!.....

... Here's it's other end!

Plenty of horrible hunts. These have little cardboard sleeves, wrapped round little black caps, which interestingly have the value printed directly on the cap's body too!

The more traditional wax cap. They will all have to go!...

... as well as re-stuffing the electrolytic's where possible.

The mains smoother has been replaced in the past, with this expensive component! .... we'll see how it reforms....

... And after a few hours on the "Dreadnaught" capacitor reformer, it passes with flying colours, exhibiting correct capacity and acceptable ESR & leakage.

Case is removed....
 ... and given a bath
Chassis and dial is cleaned. Tuning is re-strung...
 ... and a dial pointer fabricated...
... capacitors changed, and electrically tested, medium wave and long wave working well, but no VHF reception....

VHF tuner is removed (a tedious job)...
 ... and a small decoupling capacitor is found to be electrically leaky (no surprise, it's a Hunts!) ... and VHF operation is restored...

 This radio also has a "gram" setting. This allowed connection of a record player (gramophone) back in the day. My friend wished to connect it to his iPod. The gram input was originally electrically isolated from the live mains chassis by some (now replaced) capacitors. Now in this day and age, I'm not happy with one side of the chassis being connected to neutral, and using this method to isolate a hand held device.

So I've fitted a small audio isolation transformer, which is eventually fitted to a bracket by the IF transformer. This is checked with a megger tester to confirm isolation from the mains.

Now all that remains is to re-assemble and sit back and listen!

Caroline ... on 319!!!

.... now I really should look at mine ....

Thursday, 9 October 2014

Thank You.

Thank to all the visitors to my website. There's been over 800 visits since it started, and I think that's nothing short of miraculous.

It amazes me the diverse countries that have visited. Everywhere from USA, UK, France, Germany, Russia, Argentina, Australia, Iraq, Thailand, Brazil, Canada, Indonesia, Romania. This list is constantly growing. I am humbled!

Coming up in the next few weeks:

1957 Ekco U341 repairs and restoration, and iPod modification.
WEM (Watkins) Copycat repairs and restoration.

I'm also planning on doing a bit about my 1978 Mini 1275GT, since the MOT (annual inspection) failed! Expect lots of pictures of corrosion here!

There's also a plan to add temperature monitoring to the pond pump controller. I'm slightly concerned that the pump may try to run if the water is frozen, and either damage the pump, or attempt to pump water over a frozen waterfall, and empty the pond.

There's a lot to do, and a lot to share, so pop back soon.

840 visits ... who'd have thought it ....