Akai Casette PSU voltages too high.

[steenamaroo]

Hi everyone.
I'm hoping someone might be able to help me troubleshoot an issue.
I have an Akai multitrack cassette recorder (same motor as GX52 etc).

The machine intermittently (almost always) runs at top speed and the speed selector/pitch knobs appear to do nothing.
It was stored faulty last year. I just pulled it out of storage and it worked perfectly for about half an hour, after which the fault returned.

If I manually impede the capstan, the speed selector and pitch knob do have effect so I doubt we're looking at switches/pots/sensors.

I've found out that pretty much all the voltages on the PSU board read too high.
There's meant to be +/-14V DC at the motor board, but it reads upwards of +/-18.
There's a hall effect terminal that should read 0.3V - 0.8V depending on speed. It always reads 0.8V-0.9V.
This rises if I manually impede the capstan.
The FG board should have 2.8V, but it has 3.2V

Also, the transformer secondaries should be dual 14VAC.
They read 18VAC with fuses removed, and 17VAC during normal use.

I'm a bit green here, but I think this is some kind of a PSU issue.

I wonder, would you expect that the transformer is faulty? I was tempted to source a spare dual 14VAC transformer to test with. Worth a go?
A friend suggested checking zener diodes. I don't doubt him, but don't understand why.

Any thoughts at all would be really helpful. I can provide photos or more info if you need it.
Many thanks in advance,
Paul.

I can't find a service manual, but the GX52 manual covers the same motor block, and possibly parts of the PSU - http://sportsbil.com/other/Akai/AKAI_GX52-sm.pdf

 

[JohnRoberts]

That is a pretty unusual fault condition.

Any change you have a 100V (Japanese) unit being operated at 120V?

Just a wild a__ guess..

JR

 

[steenamaroo]

It is strange, huh?

Thanks for the suggestion, but unfortunately that's not it.
The transformer has a switch for 110/120/220/240.
I'm on UK 240v and have it set accordingly.

 

[JohnRoberts]

Is there some usually heavy load on the PS missing? Some lamps burned out?

Whole thing may be a red herring and not the fault, just a side effect of some fault.

JR

 

[steenamaroo]

No missing load that I'm aware of.
Excepting this fault, everything about this machine is flawless.
All lights present and correct, preamps work beautifully, not even a scratchy pot or crack in the faux leather.

I unplugged all connections to the motor board and control panel and verified that the voltages are still high.
There are some hardwired cables from the PSU disappearing into the main mixer section, but I don't think I need find/unplug those.
All mixing functions and sound quality are tip top.

 

[JohnRoberts]

Is VOM accurate? Low battery could give high readings.

JR

 

[steenamaroo]

Yup. Battery is low. I'm off to bang my head against a wall for a while.

Transformer voltages are fine.
FG voltages are fine.

Motor voltages read +15.6V and -16.7V : Should be +/-14v
Hall effect reads 1.0v : Should be 0.3-0.8V.

Thank you; I'm suitably embarrassed.

 

[steenamaroo]

Just to clarify, there is still a fault with the machine...

 

[steenamaroo]

Here's a note of some of the voltages I'm reading.
Black text is original schematic - Red text is my readings.

The red 1.0V reading at TR14 seems to control the capstan speed.

 

[JohnRoberts]

http://www.alldatasheet.net/datasheet-pdf/pdf/161632/NEC/UPC1043C.html

Here is a data sheet... some of those nodes with dc voltages in schematic are actually AC waveforms.

Do you have a scope?

Data sheet can identify what pins do what...

JR

 

[steenamaroo]

Hi John,
Thanks for coming back. I don't have a scope.
I had looked at that data sheet before, but to be honest I'm out of my depth with it; I don't know what I'm looking for.

The voltages around TR14 concern and baffle me in equal measure.

 

[JohnRoberts]

Well I can't do a vulcan mind meld and zap you the knoweledge.

Hint... It's a servo speed control.  While you chopped off the schematic the two pins labelled FB are probably feedback from the motor.  They go into pins 1 and 2 preamp input.

the signal gets crunched, tweaked and shaped.  Your red numbers look like they correctly reflect the motor running tooo fast (voltages are too high. (that was easy).

But the output of the speed control has figured out that the speed is too high so has turned off the drive. Pin 10 to  .65v instead of 1.1. (So controller is saying whoaa slow down)

However the buffer drive transistor TR 14  shows 1.7V on its base instead of correct speed 1V and 1V on it's output instead of .4fv for correct speed... (ummm that's why the motor is running fast). .

SOOO You have sniffed out the problem area....  The voltages you measure are inconsistent with a properly operating TR14 transistor... Instead of small current going into the base, you show current coming out of the base roughly 200 uA, and the the transistor is still turned on and sourcing current to make the motor run fast.

Either the transistor is bad (leaking base collector junction) or less likely some alternate source of current is leaking into the base in the circuit board...

I would check that nothing is touching that TR14 base, that shouldn't be (smutz on PCB) and if that area looks clean, replace TR14 with a new device.  It looks like a general purpose NPN should work but replace with the same device part number if you can get one.

OK I now know more than I care to about AKAI cassette decks... and your voltmeter battery's state of charge.

JR

 

[steenamaroo]

Thank you very much.
My suspicion was TR14 but the difference between us is that you can explain why.
I appreciate you doing that. I'll swap that component and see how we go.

I think that FB is actually FG - Frequency generator.
That pic is just an enlarged section of the posted schemo in post 1.

 

[steenamaroo]

I replaced TR14 and she's working a charm.
I'll give it some time to see if that was a symptom, or the root cause.

I owe you, JR. Thank you for your patience.