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 but a replacement transformer is obsolete :(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.

Nakamichi Dragon.

Nate called.

"Can you give my Dragon a service?"

Yeah ... why not?

I though I'd share a few photos.

This must be the Rolls-royce of cassette decks. Adjustable everything.

There's a lot going on inside...

It's auto reverse, but in standard form, with two pinch rollers, rather than the "flip the tape round" of the Nakamichi RX series

... here's what makes it special...











... see those 4 gear wheels? They are used to alter the azmuth alignment of the head on the fly.

A truly remarkable piece of engineering, and the pinnacle of cassette reproduction!








I've heard some rumours on various forums that the playback of pre-recorded and tapes recorded on other machines is not so good on these decks. I must say that's not my experience at all. Playback is superb.

Nakamichi RX-202E

This arrived in the workshop.

It's a Nakamichi RX-202E Unidirectional Auto-reverse cassette recorder. A thing of rare beauty and quality. I must say the quality of reproduction on this unit, knocks the Pioneer CT-F1250 we looked at earlier in the year into a cocked hat, but it is substantially younger!

It unidirectional, because the tape only ever travels in one direction, so no need for two capstans, two pinch rollers, and a complicated and compromised head which will play both sides of the cassette or a complicated mechanism to rotate the head.

Oh no, this uses a complicated mechanism to remove the cassette from the transport and turn it over!

A video is worth a thousand words....



All this machine wanted was a service, and a repair to the position sensor pot.

Pioneer CT-F1250 Cassette deck repairs.

"Frequent flyer" John called..

"Can you look at my late father's cassette deck? It's not working, there's no reel drive and it's chewing up cassettes..."

Yeah... why not.

So this turns up...

It's a large deck, dating from the late 70's, it's a three head machine (one record/play, one erase and one play only), two motor machine, and built like a brick outhouse. A check of the heads show little signs of wear.

Disassembly of the machine is fairly straight forward. My immediate attention is drawn to the rewind and take-up mechanism, as this will have undoubtedly suffered the ravages of time. In usual fashion, there is a set of idlers and clutches arranged to provide drive, and these usually suffer with age-related issues, as they're made from rubber. They all appear in good condition, however.

Stripping the front panel off allows better access to the deck mechanism, and careful cleaning of the rubber tyres on the idlers proves no improvement at all. There's just no drive to the reels.









Further investigation proves one of the motors is no longer running. Giving the motor a sharp tap with a screwdriver handle and it temporarily struggles into life. It's short-lived. It's spinning freely enough, just not when it's powered up. I wonder if it has a stuck brush or similar?

I see if there's any chance of getting a new motor. Nope. None. It's very obsolete.

Removal of the deck from the machine is required to get the motor out. Not a particularly easy task, so I took plenty of pictures, which I promptly managed to delete from my new camera by accident! Agh!

So, with the motor in my hand, I can see that it's been spot welded together. No chance really of repair. With nothing to lose, I drop the motor into the ultra-sonic cleaning tank, and leave it fizzing away for an hour. Plenty of nasty looking black gunge floats to the surface of the tank.

After removing the motor from the tank, I blow some compressed air through it to dry it and leave it somewhere warm overnight to thoroughly dry out.

I'm a bit worried about having washed all the lubrication out of the motor, so I squirt a bit of silicone grease through the mounting holes, in an attempt to re-lubricate it.

Refitting the motor, and.....



.... it runs! I give the unit a long soak test, checking all modes, play, rewind, fast forward etc. It seems to be a reliable repair!










Here's a quick video of the unit running, and a demo of the bias calibration function.