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 but a replacement transformer is obsolete :(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.

Paddock battery charging.

Paddock battery charging. That is charging up our racing car battery whilst it's stood in the paddock awaiting it's next run. Both Julian (with the Mini) and Matt (with the Chevette) have expressed an interest.... right...

So we've started the car, ticked over in the cue lane for a few minutes, done our timed run, and driven back to the paddock. We can now charge up the battery waiting for our next run.

The car is fitted with an acid mat battery, and we have a spare, normal lead-acid battery to charge from in the paddock... great, just connect the two together right? Well, no. The fully charged voltage of a lead-acid battery is 12.6 V (at 20 degrees C) ... the best we can hope for is the two batteries, once connected, is to equalise, and it won't happen quickly. So we need to raise the voltage of the paddock battery to 14.4 V, which will charge the car battery without causing it to gas. We'll also limit the current to 5 amps or so.

Now, we could build ourselves a small boost circuit to do this for us, but thankfully, we can purchase a module from eBay for very little money.

It has adjustable voltage output and adjustable current limiting, and rated at 90W.












I'd also like to be able to measure the output current and voltage, and the input voltage...

Voltages are easy to monitor. I don't want the paddock battery to become discharged below 11.6V to avoid it becoming damaged.

Output could be measured by looking at the voltage drop across a small resistor in the negative lead to the battery under charge (in fact, that's exactly how  the constant current mode on the boost module is done), however, I'll use another module....

....enter the ACS712 current motoring IC.
It measures the current by looking at the magnetic field created around a heavy piece of wire which is moulded into the IC. There are 3 versions of the IC, a 5A version, a 20A version and a 30A version. I have chosen the 20A version, so I've got a bit of headroom.












There are two relay modules, which are used to switch the charger on, and the other to enter "bypass mode", whereby if the battery to be charged requires more than 5A at the minimum voltage, the booster will be bypassed, until the charge current falls below 5A, at which point the booster can safely take over.

This is the monitoring circuit. Power is supplied to the ATMEGA328P and accessories via a 7805 regulator.  The uC reads the voltages of input and output using ADC 2 & 3 respectively, via dividers formed by R2 & R3, and R4 & R5. Current is read (as a voltage of  100mV / A) by the ACS712 connected to ADC0. A relay output is provided, which will connect the output from the boost converter once the micro is up and running, and the battery to be charged is connected. If the current exceeds 5A, the boost converter is bypassed, so as to protect it until current is stable. Data is displayed on a 20x4 LCD, connected via an I2C interface. In the event of the input voltage falling below 11.6V, the beeper will sound to alert the user the input battery is becoming seriously discharged. I could stop charging at this point, using the relay, but it's more important that the car battery is charged than the input battery being flat (although at 11.6V, the input battery is entering deep discharge, and permanent damage/loss of capacity may be being caused)

A small PCB is etched ... 

Here's a great website for creating a 3D view. It only works with small boards (unless you want to pay for it) ... but's its a fun thing... just drag and drop your .brd file from eagle into it! You can zoom around and look at any angle, even from below ... Website can be found at 3dbrdviewer.com. (Sadly, since writing this, the site has closed.)

It's a pity it doesn't look quite so good in real life ;)









And the unit is assembled and tested...

Shown here charging a small SLA.












Code (and eagle files) can be found on my github page https://github.com/andydoswell/paddock-charger

Heed Orbit Series 2 power supply repair.

Colin dropped in to bring me an LP or two, and dropped off a mysterious black box.

"It's a Heed power supply, you plug it into your LP12, and it allows you to switch speeds without swapping the belt over, I think it's been got at. Care to take a look?"

Yeah ... why not...

It takes mains in, and spews mains out ....












The little switch on the front alters the output frequency between 50Hz and 67.5 Hz, thus changing the speed of the LP12's motor.

Mains comes in and is supplied to the larger transformer, where it is rectified into +/- 32V supplies. There is a crystal divided oscillator (with 2 crystals, one for each frequency), and this oscillator, after some signal shaping, drives a push-pull amplifier, which drives the primary of the output transformer, which turns the waveform back into 230V AC, at the frequency of the oscillator.



This unit had no output, and everything was getting hot. The heatsink, both transformers. Something was very wrong.

It didn't take long to find the negative rail was being heavily loaded, and the BD242C output transistor was short circuit.

I replaced this, and the other half of the pair, a BD241C just to be safe...







Replacement of the transistors provided the cure. I left the unit on, running into a small load to soak test it.

Although the 67.5Hz is a shade high...

Grundig GMU3 microphone mixer.

John got in touch ...

"Any room for a little 'un - this little 'un?
It's a Grundig GMU3 mic mixer from the late '50s. I was thinking "looks pretty, but maybe doesn't sound much good..." and it was possibly in line for e-Bay. Then I plugged it in. Plenty of crackle, gain not what it should be, but the sound quality coming though from just a bog standard dynamic mic had a real presence that made me think it has real possibilities and definitely not something I should overlook.
Might this be something you could take a look at?
Hopefully see you when I'm down soon."

Sadly, as John was coming down, I was heading off to Spain. He dropped it in with a mutual friend. It's got an odd selection of knobs (the mixer, not my friend).

Back panel has 3 mic inputs , and a "channel" input, which is at line level.

Incidentally, this unit was probably intended for high output crystal mics, but is quiet enough for use with an ordinary dynamic.

Output, however, is at mic level, to feed a Grundig tape recorder of similar vintage.

Removing the case reveals some nasty electrolytics...











Which are long past their best...

and a pair of lovely Mullard ECC83's (12AX7)...

The electrolytics are evicted , and modern replacements held in place with a dab of RTV.

... and there's a lot to do on the underside, a myriad of nasty waxy's.. all replaced. The 25uF cathode bypass caps all measure very leaky except one, which was in tolerance, and had reasonable ESR too, but all are replaced anyway.

The switches, pots and connectors are all cleaned up....

... and it all works very well!

Most of these units seem to have the later, EM84 type of indicator tube.

This has the type which has two "wings" that close up, and is the brightest I've seen for ages.

The 13A fuse in the plug replaced for a more sensible 3A....











The some more suitable knobs found.

And it's pronounced fit to travel back to Scotland to John!

Despite Grundig being a German brand, this was made in the UK!

The guilty parties...

Samsung NQ50H5537KB microwave failure and a simple Arduino PWM fan controller.

First off, read the red disclaimer to the right. I'm about to go inside a microwave oven. Don't do it.

Saturday afternoon.

There's a rather unusual heatwave in the UK. The weather's great. My wife is cooking a jacket potato for lunch in our combination microwave. I'm sat in the lounge, watching a bit of telly..

I notice a loud 50 Hz hum coming from the kitchen. I go to investigate.

It's coming from the microwave oven. Something's heavily under load, and there's a smell of warm transformer.

It stops, and appears to carry on cooking. I go and sit back in front of the telly.

So, later on the wife has gone to work, and I go to make some beans on toast (living the dream, eh?)

Beans in the microwave... and they come out cold :(

Now every microwave oven has a label on it... "Microwave energy - do not remove cover" ... so don't remove the cover. This means you.

I remove the cover.

I check to ensure there's no remaining charge in the high voltage capacitor, and proceed to check the high voltage fuse. It's open circuit.

So, remembering the smell of warm transformer earlier, and the sound of a transformer under heavy load, I check the rectifier diode....

It measures dead short circuit. Now with these high voltage diodes, you can't check them with the normal diode setting on your meter, as it requires a few more volts to start conducting. Normally a silicon junction will forward bias at about 0.7V or so, but these high voltage types are actually a stack of diodes, so the 2-3 volts your meter provides will not be enough. You can use a bench supply to check them, but a short circuit is a short circuit, and this will show up on a normal test.


A new diode is ordered and fitted and normal operation is restored. Thankfully the transformer appears to have survived it's ordeal.





It does, however, bother me that it's failed. The unit is only a few months outside of it's guarantee, and it replaced a previous oven that was also short lived.. I wonder if it can't get rid of the heat generated quick enough. You can see the big "cage" fan in the unit, and there's also another fan that forces air through a duct across the fins of the magnetron.

The oven is a built-in type, and sits above a conventional oven, and there's a vent up the back of the cupboard. Perhaps it's not enough.

Let's add some forced air cooling...

I order 5 off 80mm fans from eBay, the sort you use to cool a PC, and a small switched mode supply, capable of 12V at 2A.














The fans are mounted on some aluminium angle, so they will sit over the vent at the top of the cupboard, sucking the warm air out.

Now, we don't want the fans on all the time, and we want to keep them as quiet as possible, so a circuit is designed, based somewhere between the Simple Thermostat project and the fan controller on the Arduino Audio Wattmeter

I don't need a display, or adjustable set-point, so this simplifies the design, and the required code.

Here's the schematic...

Power comes in to JP1, and is regulated by IC1 to provide 5v for the ATMEGA328. A DSB18B20 sensor connects to JP2. The micro determines the required fan speed, and outputs a PWM signal on pin5, which is used to control the fan via the STP55N60L logic-level MOSFET. The FET should run cool, no heatsinking is required.

A board is designed...

Modelled... 

.. and turned into reality.

The code, and eagle files, can be found on my GitHub page here.

The fans arrive, and I set about making a frame out of 10mm aluminium angle...

I've found an excellent eBay seller of metal profiles like this... they're called mwprofiles (click here to link to their eBay page)

And it's all set up and tested. Power is supplied by a plug-in 12V 2A supply.

Sadly, I can't get a shot of it installed, as there's not enough room to get the camera in!