Tascam 424 MKIII UK replacement power supply

[kjnilsson]

Hi,

Not 100% sure this is the right sub-forum so let me know and I'll move if necessary.

Someone recently lent me a Tascam 424 MKIII portastudio without a power supply. It appears due to the awkward design it is nearly impossible to find in the UK although I have seen a few US replicas. Does anyone know if there are any options for finding a UK replica or perhaps even have it made? It would be a shame if this machine now had to become a huge blue paper weight.

Cheers

[Wonks]

There was this thread a few years back on the subject.

https://www.soundonsound.com/forum/view ... 50#p310215

The OP for that hasn't been on here for a year or so, but if you send them a PM, they might see it and respond and tell you if they managed to get one made up.

[kjnilsson]

Ok thanks, I have but given it is over 8 years ago I'm not holding much hope

[Hugh Robjohns]

The original PSU used a simple transformer delivering 12-0-12 AC...



...but I have seen it suggested that the two 12V AC supplies were coupled via diodes inside the 424 machine simply to provide sufficient current (1.25 Amps), and a single 12V AC supply of sufficient rating can be used instead. I can't confirm this as fact, but it seems plausible given the video evidence, and the fact that all the related machines employ a single 12V AC supply.

There are some further details available in a video here: https://www.youtube.com/watch?v=KJHnCeztynY&t=89s

So if you can't find a UK PSU -- and I can't find any after a quick google -- your best bet is probably to modify a standard beefy 12V AC adapter as suggested in that video, or have a custom 12-0-12 supply made for you.

A decent tech should be able to figure out the correct wiring if they disassemble the machine.

John Godsland at Orchid Electronics will be able to make a bespoke PSU for you without too much trouble.

[ef37a]

The post that Wonks showed states that the supply is 2 X 11.5V AC and used to derive 15-0-15V from the onboard regulators. That seems wakky to me since 11.5V would only produce a smell over 16V DC peak unloaded. To get 15V regulated would need a 15V AC input.

I suspect the internal DC is 12-0-12 volts? The first step would be to find a service manual for the deck to establish exactly what the rails should be. There is a very helpful and knowledgeable guy called Sweetbeats over at homerecording.com an American base forum but still a very civilized one!

Ah. Well done Hugh. I stand by my idea that the DC rails are 12V. Were it I, I would just put a 30VA or so 12-0-12 toroid in a tin but of course mains connections are involved....

Dave.

[kjnilsson]

Thank you all, I have contacted Orchid Electronics to see if they can make something up for me. Fingers crossed.

[Tim Gillett]

Here is a link to what appears to be the Service Manual:

https://tetrakansupermonobloc.home.blog ... ce-manual/

The external PS appears to supply a DC-DC converter inside the deck.

[Hugh Robjohns]

Well found!

https://tetrakansupermonoblochome.files.wordpress.com/2020/04/tascam-424-mkiii-service-manual.pdf

So here is the wiring of the 6-pin DIN:

Pins 1/2 carry one 12V supply, pins 3/5 carry the other, and pins 4/6 carry the centre tap/ground.

But the two supplies are summed through diodes to provide a single 12V AC supply internally, so a single 12V AC supply would actually work as well, if beefy enough.

[James Perrett]

But the two supplies are summed through diodes to provide a single 12V AC supply internally, so a single 12V AC supply would actually work as well, if beefy enough.

I've not read the manual but your circuit diagram shows 4 outputs to me - one of which is -16V-ish while the others are +16V-ish (assuming that there are smoothing capacitors on the wires that go off page.

[Tim Gillett]

But the two supplies are summed through diodes to provide a single 12V AC supply internally, so a single 12V AC supply would actually work as well, if beefy enough.

I've not read the manual but your circuit diagram shows 4 outputs to me - one of which is -16V-ish while the others are +16V-ish (assuming that there are smoothing capacitors on the wires that go off page.

I can see only three connections for two outputs, as Hugh said. I guess the paralleled terminals on the 6 pin connector are to double current handling capacity.

[James Perrett]

I can see only three connections for two outputs, as Hugh said. I guess the paralleled terminals on the 6 pin connector are to double current handling capacity.

There are two inputs and a ground on the input connector but definitely 4 outputs going somewhere else on Hugh's diagram.

A quick look at the full manual shows that one goes to the +9V regulator, another goes to the -9V regulator, another to the +12V line while the last output is some kind of control signal.

[Hugh Robjohns]

The full service manual is on the link I posted above and the PSU is shown on page 23 (or 25). Here's the full page:

Of the two lines going up the page from my cut-out picture, one carries a lumpy 12V to feed an IC regulator generating +9V, and the other goes to a discrete regulator circuit producing -9V. The two lines extending left pass 12V to another board and to another 5V regulator, as well as to a DC/DC converter that generates 27V and feeds a power-down sensing circuit.

[Hugh Robjohns]

John Godsland of [url=www.http://orchid-electronics.co.uk/]Orchid Electronics[/url] has just written to me to say that he was able to help the OP with his missing power supply problem and has very generously provided an accurate diagram of the connector wiring details:

[The OP] did contact me about making a suitable replacement PSU for his Tascam 424MKIII tape machine - and after finding much conflicting information on the web, some erroneous - and some with drawings NOT showing DIN plug pin numbers, I can confirm that the attached PDF drawing is accurate and does work correctly with the machine.

Most suppliers stock a suitable toroidal transformer - a 30VA model with 0-12, 0-12 windings - which will be fine.

Can you please be so kind as to share this diagram so that anyone can find accurate information, now and in the future.

[Wonks]

That's very nice of them. Big thumbs up for Orchid!