Loading ...

Translate

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 but a replacement transformer is obsolete :(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.