Mitsubishi HS-M60V repair.

The venerable workshop VHS machine has finally given up after many years of sterling service. It doesn't make it into play or record. It'll be the loading belt, a small, rubber drive belt that's used to move the components of the mechanism into it's various modes. Why do I want to repair it? Because:
1) I can.
2) It's better repaired than in landfill. I hate to see stuff thrown out.
3) Because this machine is very good at recording and playing back system A video.

Thankfully a new belt kit is obtained from the lovely people at www.notobsolete.co.uk

As per usual, unplug the machine from the mains. Don't argue, do it.

There are two screws on the left and right sides of the top cover. Undo them and remove the top.

Grasp the joggly shuttler thing, and pull it off.

There.... put it safely to one side.
Undo the plastic catches holding the front panel on. Three on the top, two on the sides....








... and three across the bottom....
With a bit of jiggling, the front panel will now come off.

Put the machine on it's side.

Remove the three screws,. indicated with arrows stamped into the base plate, I've circled them in the photo.  Put the machine back on it's feet.

We now need to remove the deck. It's held in with 5 screws...

Two down the holes conveniently positioned on the cassette housing (the bit that loads the cassette onto the deck)

One to the rear right hand side of the deck...









One on the rear left...

... and one in the middle by the head amplifier.

While you're there, unplug the cable to the top of the head amplifier....

 ... and the one to the right of the cassette housing, which connects the end sensor..










You should now be able to remove the deck from the main board and case etc ... Just grab it by the cassette housing and gently pull...

Flip the deck over....

The big belt from the capstan motor to the reel drive gear is easy to get at, so replace it now, whilst we've got the machine apart.

The loading belt is located as shown by the arrow in the picture to the right. The shaft that the belt drives (the bit with the little fan on) is only held in position by the force of the belt and a small black plastic clip. Now we could disassemble the entire loading gear and gain easy access to the motor, belt and shaft, but then be faced with a re-alignment job... nah. Careful use of some small, needle nose pliers, and the belt can be worked out from around the shaft!

... and there's some access to the loading motor spindle from the top of the deck too...

The old loading belt had lost it's tension, and wasn't exactly round anymore....

Fit the new belt, Reassemble the machine, making sure to reconnect the end sensor cable, and supply to the head amp. When refitting the deck, check the large multipin connectors under the head amp have located and seated correctly. Test for correct operation. Have a cup of tea safe in the knowledge of a job well done, and another piece of kit saved from landfill! 

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!

Ugh... 

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!

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 ....

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 ....

Doz's Kitchen Workshop! - The Induction hob explained.

Simple TV using DG7-32 'scope tube

A while I go I set myself a challenge... to design an make a simple TV using only valves. Not a semiconductor in sight.

WARNING. Do NOT try this at home. This project calls for some high HT voltages. The main EHT rail is some 700 VDC. It will floor you or kill you if you come into contact with it. It will not warn you, blow a fuse, nor pull your safety trip. It may not give you a second chance. RESPECT IT'S AUTHORITY. 

I will not, under any circumstances, accept any liability if you decide to re-create this project for yourself.

I procured an electrostatically deflected DG7-32 oscilloscope tube from the usual sources, a base , a handful of very nice Russian 6N1P valves, a couple of EF91s and a ECL83 for the sound. An EZ81 provides (some) rectification. Enter some transformers from the scrap bin, some tag strips and a couple of scrap aluminium enclosures.

My first design was too complicated, used too many valves, and, most importantly didn't work.

 

You'll notice a few things:

1) The raster is all "in one corner", this is because I didn't employ a differential drive to the X &Y plates.
2) There's precious little video modulation. I gave most of it away to Miller capacitance.

All-in-all, rubbish.

It sat on a shelf for a year, sneering at me.

I took it to pieces, and completely re-vamped it. I'd subsequently robbed the mains transformers for another project, so I ended up with a nice transformer from an old audio amplifier to provide the +300V and 6.3V for the heaters. I also needed to develop the 700 V EHT. In the original design I had a separate transformer, with a nice high voltage winding and an isolated heater winding. No such luck this time. I had a small insolating transformer from an old bathroom shaver point. 240VAC in, 240VAC out. This would have to do.

Let's have a quick look at the schematic:-

Yeah, you're going to want to zoom in on that a bit...

The Power Supply

Mains comes in and is fused by F1. TR2 and TR3 are actually one transformer, with centre-tapped heaters and HT winding. R1 and R2 provide a little peak current limiting to keep our EZ81 rectifier happy. The cathode of the rectifier is connected to our main smoothing capacitor C1. This must be no larger than 50uF, so 22uF is fine here, and we're not pulling much current. R3 and C2 provide additional filtering, and R4 and C3 provide a lower voltage supply for the screens of our video amplifiers. TR1 is the shaver transformer, and provides and quick and dirty 350V supply via D1 and C4. This 350V supply is not referenced to earth, but our 300V HT supply, giving around 650 VDC (More like 700VDC off-load)

I'll repeat my warning at the start of the article at this point. This power supply is more than capable of supplying enough current to kill you. Yes. Really. This means you. Don't do it. 

Whilst I was developing this gadget, I kept a voltmeter permanently attached between GND and 700V. Also, I kept a 10W 10K resistor connected to GND, and to an insulated lead, which was connected to the +700 VDC rail whilst I was working on the circuit. I've also installed 2x220K resistors in series directly across C1, C2 and C4 to bleed away any charge within a few minutes. These resistors aren't shown on the diagram. 

The original version also had separate fuses for each HT supply, and, stupidly, for the EHT supply. I used the ordinary type of 20mm fuse. During development, I inadvertently shorted out the EHT supply. The 20mm fuse blew violently, vapourising the wire against the inside of the glass, which carried on conducting! The resulting burned mess of fuseholder resulted in a lesson learned. Fuses have voltage ratings. 20mm fuses are rated to 250V. 

The Signal Stages.

Video comes in on X1, and is loaded by a 75 ohm resistor. Video is passed via C1 to the grid of our first video amplifier VT2, an EF91 (6AM6) pentode. This is further amplified by VT3 and passed to the first grid of the CRT. 

Video is also taken from VT2 to VT4a, the sync amplifier. Here some of the video information is lost (mainly through miller capacitance), but the line and frame sync pulses are retained. The sync pulses are passed from the anode load to VT4b, the frame sync separator, whereby the higher frequency line sync pulses are filtered out by C13 and R19, and the remaining frame sync pulses are used to sync the fantastron oscilator formed by VT5a & VT5b. This gives us a nice sawtooth waveform, locked to our video frames at 50Hz (in the UK). Although I haven't tried it, I'm sure it will work just fine with NTSC at 60Hz.

The frame sawtooth waveform is then passed to VT9A & VT10B (what happened there? It should read VT9B!)  which forms a differential amplifier and drives the Y-plates of the CRT. The plates are coupled to the anodes of the differential amplifier by 10nF, 1.6KV capacitors, as the deflection plates are referenced to the EHT by R46 and R47.

The sync pluses are also fed from VT4a via a high pass filter formed by C16-19 and R23-26, to pass the line sync pulses, to another fantastron amplifier, this time synced to the line pulses at 15.625 KHZ (again, I've not tried it, but it should be OK with NTSC too).  Once again this syncs another fantastron oscillator, and provides us with our sawtooth waveform to drive the X-plates via differential amplifier VT7a and VT8b (Again? This should read VT7B!)

The CRT is provided with a focus voltage via R50, and the cathode is supplied by R51 to slightly elevate it above 0V, to allow our negative going video to modulate the grid. 

Audio is provided by a very simple ECL83 amplifier. It's not going to win any awards for design or performance, but it functions. The output transformer is simply a 9V mains transformer. I know all about the fact that the transformer is totally unsuitable due to DC on the windings etc etc, but I had it to hand, and it works! 

Construction is fugly to say the least. 

I intended it to be a sort of homage to the home constructor of the 1940's / 1950's. 

There it is... exposed HT wiring an' all...

Performace isn't fantastic. The circuit is simple, the tube was never designed to show a raster scanned image... but it's definitely Dr. Sheldon Cooper!

Some retrace (flyback) lines are visible, as there's no supression. 

Feeling cheated? Yes, me too. I promised a design with no semiconductors, and yet the EHT rectifier is clearly a grain of sand. As I said originally, the EHT was originally supplied by a separate transformer, and this allowed me to use a valve rectifier (I actually used an EZ80), and I elevated it's heater winding to 350VDC, so as not to compromise the heater/cathode insulation. I did try putting the EZ80 into this circuit (if you look carefully you can still see it's Noval base below the speaker magnet) but the heater cathode insulation was just not good enough to hold off the ~700 volts and flashed over :(