Akai MG614 tape speed fault.

[steenamaroo]

Hi all,
I've come here really hoping that someone can help as i can't find anyone to work on my AKAI in the UK.

I don't know a lot about this unit, it only recently came into my possession and in mint boxed new condition with the exception of one fault.

Regardless of where the tape speed switch is set, the tape plays back at one speed.

If i play a tape, it seems to start off at a speed and very gradually reduce speed over a reasonably long period...say 30 seconds?
It doesn't grind to a halt or anything; There's just a notable pitch drop.

The pitch control knob on the unit has no effect either.

Another symptom of what i imagine to be one golbal fault is that if i play a tape back from the start, the capstan will feed tape through faster than the take up can receive it, so it gets chewed.

If i FF a bit and play again, it'll be fine as the diameter of the tape on the take up now great enough that take up speed is sufficient.

The physical switches and potsare fine, this fault goes deeper than that.


I've guessed that tape speed and pitch are both voltage controlled and perhaps that something in both their paths has shorted out allowing full voltage to the motor at all times. Does this sound reasonable?


There are markings on some internal connectors that say 9.5 and 4.75. Again, i guess that these are voltage outputs for the two speeds.
If i even knew what the voltages are meant to be i could trace the fault a little farther.

Anything at all is helpful on this guys.


Many thanks in advance.

[barrymagrec]

I don`t know anything about this particular machine but the markings 9.5 and 4.75 are likely to refer to tape speed, 3.75 and 1.875 inches per second respectively. It certainly sounds as if there is no control on the capstan motor. Check for any loose wire or connectors in the vicinity of the motor.

Barry

[steenamaroo]

Thanks for the advice Barry. I can see all the wires that disappear off to the motor, but getting at the back of the motor itself is looking tricky.

I might have to bite the bullet and just do that.

[steenamaroo]

Ok, i took a look.
The motor is getting 2.9v on two wires at all times.

There is a top board with all the controls, a lower board beside the psu and the motor/cassette assembly.

It seems that the top board is sending 15.8v and -16.6v out to the lower board.
These voltages swap places if I toggle the speed switch.

a range of 1.3v to 2.2v is sent out on different wires and this range corresponds to pitch control position.


These voltages all arrive at the lower board, but the output from that board which goes straight to the motor is just a constant 2.9

Would it be safe to assume i'm chasing a fault on the lower board?

[barrymagrec]

Originally Posted by steenamaroo

Ok, i took a look.
Would it be safe to assume i'm chasing a fault on the lower board?

That would seem to be the most likely area to look at.

Barry

[steenamaroo]

Pics of that board are at
www.steenaudio.com/1.png
www.steenaudio.com/2.png

The capstan speed voltage comes out of the top left connecter, at the pins labelled FG (5th and 6th from left)
The chip below that connector is UPC1043C.

I've flipped image 1 so that solder side and component side are visually comparable.

[oshifis]

Any loosy belts inside?

[steenamaroo]

No, it's all new/tight.

Like i say, voltage to motor doesn't change when speed switch is changed.

I'll label the pics later when i figure out some more.

[barrymagrec]

Presumably the UPC1043 feeds the tabbed power chip. You may be able to measure whether the voltage out of the UPC1043 is being controlled and therefore deduce whether the power chip is O.K.

[steenamaroo]

Ok, i'm not really sure how to do that, But...

voltages at chip (viewed from above) are
Low speed
2.9........4.9
2.8........2.9
3.00........0
2.8........5.4
2.8........4.2
2.9........1.5
10.8........0.8
0...........5.8

High speed
2.9........4.9
2.8........2.9
3.00........0
2.8........5.0
2.8........4.1
2.9........1.5
10.8........0.8
0...........5.8

The terminals for the motor (FG) read 2.9v each, but if motor is disconnected they read 2.9 and 0.5.

It seems motor speed is controlled by whatever returns to FG right? so what would make that change?