Autotransformer tap question

[paggerman]

Hi all, I hope everyone is well. Just got a question to which I'm sure I'll get a sensible answer. I've picked up an Akai M7 R2R and I have started work on one of the chassis to make a little practice amp. In the meantime, I need to get the power supply sorted. As many of you will know, the M7 has two discrete chassis. Each of these chassis has a PT with a 100v/117v primary. Within the cabinet there is also what would appear to be an autotransformer with a voltage selector. The taps on the autotransformer are 0v, 100v, 117v, ....230v, 240v. There is also a ground connection to the frame.

The end game is 230v in, 100v out. I do not need a voltage selector.

I have tried to trace the wires in and out, but they have left me a little confused. From the mains lead, the neutral conductor goes through the fuse and voltage selector to the 230v tap. The live conductor goes into the connector that goes to the mechanism and chassis. There is also a single connector on the 0v tap and 100v tap which go the chassis. The fourth conductor is earth.

If I get rid of the voltage selector and connect the autotransformer output to the PT, what taps should I be using? If I was starting from scratch and having referred to multiple images from Dr Google I would have thought that mains live would go to fuse to 230v tap. Neutral would go to 0v tap. I would then take 0v and 100 v to the Chassis PT. But right now, there is only one connection on 0v. And one of the mains conductors goes direct the chassis.

Appreciate your help everyone. Thanks.
Jeff

[Chuck H]

Trying to follow that popped a vein in my head. How about a schematic?

[Jazz P Bass]

My head hit a wall on that one!

[g1]

Unfortunately service manual is a 46 meg file:
AKAI M7 SM Service Manual free download, schematics, eeprom, repair info for electronics
If anyone could extract the schematic and post it?

[Chuck H]

I never subscribe to that sort of thing. They force you into their program and it always has shizzle attached in the way of offers, links, peripheral programs that link advertisers to your computer, etc. Sometimes changing default programs and browser settings. It's nothing but adware!

[Leo_Gnardo]

That wiring sounds all bass-ackwards. So let's attack the problem in a similar way. "The chassis is connected to one side of the AC line." Whaaaat? Is your AKAI a "hot chassis" machine? If so, that's big-time dangerous. I really hope that is not the case.

Let's for the moment assume the venerable tape deck is not a "hot chassis" device. Then AC ground should attach to the deck metal chassis. AC neutral attaches to autoformer "0 V" and on to the deck's AC supply, the side that does not have the 100/117V switch. I take it you're in a 230VAC supply area, so your AC "hot" goes to the autoformer 230V terminal. You need not remove or bypass the 100/117V switch on the deck. Select one of the voltages 100 or 117, and connect it to the matching output of the autoformer. Is there an AC fuse in the deck? Let's say there is, and it's 2 amps. My ballpark guess for how much would be enough to run an ordinary consumer tape deck. I would recommend also having a fuse on the hot side of the autoformer, and the 230V fuse value would be half the deck's fuse, so 1 amp if my guess is correct.

IF however your deck really is a "hot chassis" then you will need a transformer, not autoformer, to power it safely. In order to safely connect it electrically to any other equipment, the chassis may be at ground, or floating. Definitely NOT connected to one side of the AC line - that would make for shocks, major hum, or worse.

[Jazz P Bass]

I never subscribe to that sort of thing. They force you into their program and it always has shizzle attached in the way of offers, links, peripheral programs that link advertisers to your computer, etc. Sometimes changing default programs and browser settings. It's nothing but adware!

That is not how Electrotanya operates.
Down below the preview, you have to wait until "needed progs" changes to "Get Manual".

[g1]

I never subscribe to that sort of thing. They force you into their program and it always has shizzle attached in the way of offers, links, peripheral programs that link advertisers to your computer, etc. Sometimes changing default programs and browser settings. It's nothing but adware!

That is not how Electrotanya operates.

I thought he was referring to pdf editing programs?

[Jazz P Bass]

The power transformers (left & right channels) are clearly made for 100/117 Vac operation.
Simple hook up.
Two wires.
High tap & the Zero tap.

The auto transformer will have to be sussed out to achieve the desired 100/ 117 Vac.

[Jazz P Bass]

I use Cute PDF Writer & Bullzip PDF Printer.
No adware.

[Chuck H]

I was. Thank you. Meant no offence. Any time I have to download a file and I'm forced to upload some non standard editing program I know it's a trick. They DO give you the file, but it's never as free as you think it will be.

[Tom Phillips]

I see the auto transformer and the power transformers on the schematic. I feel like I could describe how they should be electrically connected to use the 230V line and the 100V PT primary as Paggerman requested. However, the Akai unit seems to have a 100V / 117V PT primary selector switch and a switch or a rotating plug that selects the taps on the auto transformer. The part that is missing is operator instructions for setting those switches. One can't tell from the schematic where they are located or how they are manipulated. It would seem that an operating manual with some pictures or some photos of the actual unit is what is needed for us to understand how to configure the line input options.

[Leo_Gnardo]

Have we established whether the AKAI is really a hot chassis item or not o schematic readers?

[Enzo]

Unless the schematic is wrong, there is no mains connection to chassis. Even in hot chassis amps, we don't usually see a direct connection to chassis, that is usually done with a cap.

This is two amp chassis with power transformers, then an autoformer to feed them. No different from plugging an amp into your variac.


I missed the reason we need to get rid of the voltage selector. If it is intact and works, why remove it?

[Chuck H]

I was thinking the same thing. Don't fix what isn't broken!!! The changes from normal operation may cause confusion and problems. Just fix it!!! If the owner needs a manual, get him one.