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Wednesday, 5 December 2018

Philips Digital Compact Cassette recorder repair DCC 951

I missed out on the whole DCC thing. I was a big fan of minidisc...

Anyway, this found it's way into my possession....


It's a DCC 951, dating from 1994. It doesn't work. It's totally dead.


Off with the lid ...












Poking around with a multimeter proves there's AC on the output of the transformer. Time to examine those "wickman" fuses on the rear PCB...











Which involves removal of the back panel, which is easy enough... a few torx screws, and bending these little tabs out of the way ...











One on each side...



















.. and lifting the panel off. The mounting screws can now be removed from the PCB, and it can be hinged up without disconnecting anything...










Sadly the wickman fuses are all intact ... but the regulators on the heatsink are horribly dry-jointed, and a quick solder up on these, and the mains input socket and switch (which are also looking poor) , solves the problem!











Great front loading mech... really smooth!

A conventional analogue tape can be used (analogue only, sadly), and goes in with the exposed side forward. Sadly I don't posses a DCC casette to see what the digital bit's like.

I clean up the tape path, and change the loading belt (it was OK, but I had a new one to hand!)
DON'T DEMAGNETISE THE HEADS!




Here's a few other shots...




















... now to find a cassette ... anyone?

Sunday, 2 December 2018

It's (another) Arduino Christmas.

Here's a super simple persistance-of-vision display.

Drive the neighbours mad with 8 static leds, that when they move their head, or drive past, delivers a festive message!

Ingredients.

1 Arduino board (I used a 3.3V 8MHz mini, but anything will do)
8 LED's (if using a 3.3V board, make sure you have enough voltage to light the LEDs!)
8 current limiting resistors

Connect each LED to pins 0-7 (noting 0 and 1 are RX and TX) via a current limiting resistor, and ground.

Load the code from https://github.com/andydoswell/christmas-POV

and enjoy the festive season!



And here's Stuart W's version!


Friday, 30 November 2018

Building Frank C's Hedghog 625 PAL to 405 System A standards converter.

It's time to big up someone else's project.

Enter Frank C's Hedghog converter...

Details of the project an be found here.. http://electronics.frankcuffe.ovh/hedghog

It's a stunning project, and features a few useful functions not found on my other standards converter, the Aurora SCRF405 (which for those of you who don't fancy the DIY approach, can be found here http://www.tech-retro.com/Aurora_Design/Single_Converter.html)

It consists of a digital video converter (The TVP5150) which converts PAL to an 8-bit ITU−R BT.656 format. These one's and zero's are fed to an EP2C5 FPGA development board, which mounts to the top of the PCB. The magic happens in here, and the resultant 405 line system A video output emerges from the resistor ladder DAC. This is then passed to a MC44BS373 modulator IC, which is tunable on all Band I and Band III channels. The tuning of the modulator is also taken care of within the FPGA, and is set with a hex switch. There's a second MC44BS373 which is modulating audio at the required 3.5MHZ spacing below the vision carrier.

The unit produces aspect ratios of both 4:3 and 5:4 (5:4 being in use up to 1950)
There's a Pedestal function which lifts black level 50 mV above blanking level. This was abandoned on transmitters post-war, as it reduced the efficiency of the transmitters. It may help with early sets that suffer from flyback lines.
You can select between normal broad pulses or broad pulses with equalising pulses. Equalising pulses were never in the spec, although it may help interlace on some sets (ironically it upsets interlace on some, and causes the top of the picture on some Bush sets to "hook")
The three-line interpolator has three settings: soft, medium and sharp interpolation apertures.
Switchable 1KHz or 400Hz test tones.
There's also a PM5544'esque test pattern, and stair-steps.

These functions are all available via switches on the front panel.

PCBs were ordered from the most-excellent pcbway.com, and arrived within 7 days.

The FPGA board came from eBay.
The MC44BS373's I had to source from AliExpress, as it's now classed as obsolete, however, at the time of writing this, there appears to be plenty of stock from "grey" suppliers. I dislike doing this, as many times I have been caught out with fake parts! Caveat Emptor. I used the AliExpress supplier YT Electronics components co.,ltd


The phono sockets also came from AliExpress (link) and fit with a minor modification.

Assembly is straight forward, don't let the surface mount put you off. Get a decent quality gel flux. If you've seen my videos, you may have realised I suffer with a benign tremor (nothing to worry about, I've had it since I was 14), and I can mount this stuff with ease. I do have the advantage of a microscope.

A bit of blue-tac helps hold the board in place. First off I fit the semi's.

Then the passives...

Programming the FPGA is straight-forward in windows, using the Quartus II (13.0 sp1) software, and the "USB-blaster" supplied with the FPGA board. I failed to get the software to work under Ubuntu (although it would "see" the programmer hardware, the program option remained unavailable). When correctly programmed, there's a binary counter running on three LEDs on the board.
 Sockets and switches are mounted.

And the FPGA board fitted.

... and it's switched on!











However, some fault finding was required, as although the test patterns and test tones were OK, there was no converted video. With the help of the designer, Frank and the VRAT forum (here), it was apparent something was wrong with my FPGA board. After some fault-finding, a tiny solder bridge was found on the FPGA board, shorting out two of the lines. Once this was removed, the converter worked faultlessly.


Moire pattern is being caused by the camera, the actual picture quality is superb.

I like the additional functions available from the front panel. Picture quality appears to be on a par with the aurora converter.

What I'd really like is a centre-cut out function for dealing with a 16:9 input.... Frank?

Monday, 1 October 2018

The best laid plans of mice and man... Fellows A75 laminator continued ...

If you've been following this for a while, you'll know I modified a Fellows A75 laminator to get a more reliable way to make toner transfer PCB's. (You can find the original articles here and here.)

As I had predicted, we were now using the item way outside of it's specification, and that's it's life expectancy was certainly going to be, erm, compromised....

Well, tonight it happened. It stopped getting warm. Now I've got a commercial prototype to get out, and I need it. Let's see what's happened.....


So, the poor thing is once again disassembled. Those two red wires you can see running from the other side of the PCB are connected to the element.... A quick check with the meter, and ... it's open circuit. Damn.

Now, in the last modification I decided to raise the temperature by modifying the controller. So what if there's one of those non-resetable thermal fuse things on the element somewhere....





And there is! Mounted against the lower heating element...

It's duly removed, the two wires soldered together, and insulated with a piece of heatshrink...









Back in business!

Now, we've removed an essential safety device, so I'll re-iterate my previous warning... We're putting paper in here, which is being exposed to higher temperatures for more time than it would be in normal use.... If it should jam, there's a risk it could smoulder and catch fire, and ruin your day... DON'T USE IT UNATTENDED!

... you nkow what , I may just build a combined speed/ temperature controller for it, and have done ...

Sunday, 2 September 2018

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....


Thursday, 30 August 2018

Tascam Portastudio 488 MKII Power Transformer replacement and the dreaded gear "c".

It's not often that Mr Often calls...

"Got this Tascam 488, it's gone dead, can you take a look?"

Yeah, why not....

What a feat of engineering ... 8-tracks on a cassette tape!

This one's a Japanese model, designed for 100V mains operation ... the trouble is it had been run on a 115V step down transformer.... What's 15V between friends? ...

Well, on inspection the mains transformer was open circuit across the primary.











A quick look at the circuit diagram shows there's a thermal fuse in the primary... I may be able to pick it apart and replace it, but to prove the transformer is OK, I temporarily short it out....

The transformer is NOT OK! :( It's got shorted turns somewhere, thankfully I ran it up slowly using a variac whilst monitoring the current.... it draws a few amps at just a few volts of input... game over ....



I call Tascam... but a replacement transformer is obsolete :(

So what about building a replacement supply?

Looking again at the circuit diagram, I think we can replace the single, multiple winding transformer with three separate transformers. We're going to be stuck for space, and there may be a magnetic shielding issue, so it'll have to be outboard, and in a separate enclosure...

The top most winding, appears to be about 12-0-12 V, so that's easy. The next winding is a bit tricky... we'll come back to that. The bottom one wants to be about 50V. It supplies some negative voltage rails and the +48V phantom power. I'll try a 24-0-24 as it's easily obtainable.

Now that centre winding .... it's centre-tap is fed with -25V DC, and the two phase outputs go off via a separate plug to the control PCB... Ah-ha, this is the filament voltage for the vacuum fluorescent display! No Idea what the actual voltage is though...

Some transformers for the top most and bottom most windings are sourced, and lashed up... A small  6-0-6 transformer is used for the filaments.... (The 12-0-12 transformer is under the PCB in the photo)

The top and bottom transformers are powered up, as I'm pretty sure I've got that right, and all the relevant voltage rails appear on the power supply PCB. So far so good. Now I slowly increase the input voltage to the  6-0-6 transformer via the variac, until the display is evenly illuminated. It wants about 100V (across it's 220v primary) .... I switch the small transformer's primary to 110V operation, and repeat the process. It seems happy being fed with the ~48V from the bottom transformer! Great!

Here's the schematic..


The pin numbers from JP1 correspond to the pin numbers for P2 (the red connector on the Tascam Power PCB). Note the transformers TRI and TR2 have the primaries wired for 240V operation. If you're in that funny bit of the world that uses 110V, adjust accordingly. TR3 is wired for 120V operation. You can adjust the filament voltage slightly by using a 5W resistor (a few 10's of ohms should do it, but you'll need to experiment) in series with the primary of TR3. You could also switch to a 5-0-5 transformer. If you can see the filaments in the display glowing, the voltage is too high (note here, if it's way to high, the filament will fail, and that's game over). If the display is not evenly illuminated, your voltage is too low.

The whole thing is tidied up, and mounted in a nice enclosure. Connection is made by a 9-pin D connector to the main unit.


9-pin D mounted on the unit ... 



.... and wired to the PCB .... 


... and tested... great, another repair done... 


... except it's never that easy, is it... During testing the tape mech proves to be faulty... It's removed for inspection.

"It's probably belts" ... nah, is that a broken gear I can see??? 


Off with the loading motor plate , two screws at the back ... 

... and one on the top ... 

... oh that gear's broken alright ... It's known as Gear "C".


Thankfully, there's a guy called Sam Palermo, in the USA, who can supply a newly manufactured gear (the originals are unobtainium). It's quite expensive, but it's that or landfill.


After removing the gear chain, clean off the sticky grease with a cotton wool bud and some IPA.

I also gave the mode select switch a birthday whilst I had it apart.


It's then time to reassemble the gear chain, with the new gear. 

The deck is then reassembled, re-fitted and passes testing with a clean bill of health (phew) 

The guilty parties....

A lot of work, but another saved from landfill.