OK, I have fixed the final(?) issue with my long-suffering Nakamichi
CR-7 cassette deck.

A fusible resistor in the flourescent display circuit had blown,
causing the display to go dark.

Well, the replacement fusible resistor finally arrived from England,
and when I installed it, the flourescent display was as good as new.
It looks and works great.

BUT...........................

I am still measuring just under 7v AC going to the flourescent
display. According to the service manual, this should be only 3.25v
AC.

I took measurements at the flourescent display terminals AND at the
actual terminals of the transformer secondary, and the readings were
identical... 6.84v AC, give or take a tenth of a volt.

I checked the two .1uF buffer capacitors and they are both OK. And of
course, the fusible resistors are now both OK.

This is a really simple circuit, consisting of only the transformer,
the flourescent display, the fusible resistors, and the .1uF
capacitors, so there isn't a heck of a lot to diagnose.

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?

"EADGBE" wrote in message
ups.com...
OK, I have fixed the final(?) issue with my long-suffering Nakamichi
CR-7 cassette deck.

A fusible resistor in the flourescent display circuit had blown,
causing the display to go dark.

Well, the replacement fusible resistor finally arrived from England,
and when I installed it, the flourescent display was as good as new.
It looks and works great.

BUT...........................

I am still measuring just under 7v AC going to the flourescent
display. According to the service manual, this should be only 3.25v
AC.

I took measurements at the flourescent display terminals AND at the
actual terminals of the transformer secondary, and the readings were
identical... 6.84v AC, give or take a tenth of a volt.

I checked the two .1uF buffer capacitors and they are both OK. And of
course, the fusible resistors are now both OK.

This is a really simple circuit, consisting of only the transformer,
the flourescent display, the fusible resistors, and the .1uF
capacitors, so there isn't a heck of a lot to diagnose.

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?


Is this a mains frequency transformer, or a high frequency inverter? If the
latter, a standard multimeter will read incorrectly, you need a true RMS
meter to measure it.

"EADGBE" wrote ...
OK, I have fixed the final(?) issue with my long-suffering Nakamichi
CR-7 cassette deck.

A fusible resistor in the flourescent display circuit had blown,
causing the display to go dark.

Well, the replacement fusible resistor finally arrived from England,
and when I installed it, the flourescent display was as good as new.
It looks and works great.

BUT...........................

I am still measuring just under 7v AC going to the flourescent
display. According to the service manual, this should be only 3.25v
AC.

I took measurements at the flourescent display terminals AND at the
actual terminals of the transformer secondary, and the readings were
identical... 6.84v AC, give or take a tenth of a volt.

I checked the two .1uF buffer capacitors and they are both OK. And of
course, the fusible resistors are now both OK.

This is a really simple circuit, consisting of only the transformer,
the flourescent display, the fusible resistors, and the .1uF
capacitors, so there isn't a heck of a lot to diagnose.

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?

Yikes! I wouldn't call that "slightly off-spec". It is more than
DOUBLE what the service manual specifies. I would be concerend
about the life expectancy of the display unit. It could also be the
reason the original fusible resistor blew, etc.

I can think of several questions...

1) Is the manual correct?
Are you sure that the service manual is for the model and
serial number you are working on? For Sony stuff (with
which I am more familiar), they sometimes had a specified
range of serial numbers which the service manual applied
to. Because they would make significant changes to the
product in mid-stream and had to publish a different manual
when they made the change, etc.

2) Is the transformer and mains wiring OK?
Are the other voltages coming out of the transformer closer
to what the manual specifies?

3) Is it the same display as in the manual?
Can you compare the full part number to confirm that the
display in your unit is the same one the manual thinks it is?

4) Is the measurement from a center-tap?
Is there a grounded center-tap on the filament winding?
Could the manual measurements be from ground to
each side? I'm pretty sure that the circuit requires ground
reference somewhere for the circuit to work properly,
This is frequently done by grounding a center tap on
the transformer winding, etc. Manuals frequently show
voltage readings referenced to ground, etc.

5) Does it look like the filament wires in the display are
being overdriven? IME, they should be glowing a very
dull reddish color when viewed in a totaly dark room with
your eyes accustomed to the darkness. If you can see
them glowing under normal room lighting, it could mean
that they are being overdriven. If you aren't sure what I
mean by the filament wires, see the 2nd photo here....
http://en.wikipedia.org/wiki/Vacuum_fluorescent_display

On Oct 9, 3:47 pm, "James Sweet" wrote:

Is this a mains frequency transformer, or a high frequency inverter? If the
latter, a standard multimeter will read incorrectly, you need a true RMS
meter to measure it.

I don't know. I was assuming that the lines in question were just one
of the transformer secondaries.

The power transformer in this cassette deck has four (4) different
secondary outputs:

18v AC
15v AC
5v AC
3.5v AC --- This measures 6.84v AC - but my multimeter is NOT true
RMS.

"James Sweet" wrote ...
"EADGBE" wrote ...
OK, I have fixed the final(?) issue with my long-suffering Nakamichi
CR-7 cassette deck.

A fusible resistor in the flourescent display circuit had blown,
causing the display to go dark.

Well, the replacement fusible resistor finally arrived from England,
and when I installed it, the flourescent display was as good as new.
It looks and works great.

BUT...........................

I am still measuring just under 7v AC going to the flourescent
display. According to the service manual, this should be only 3.25v
AC.

I took measurements at the flourescent display terminals AND at the
actual terminals of the transformer secondary, and the readings were
identical... 6.84v AC, give or take a tenth of a volt.

I checked the two .1uF buffer capacitors and they are both OK. And of
course, the fusible resistors are now both OK.

This is a really simple circuit, consisting of only the transformer,
the flourescent display, the fusible resistors, and the .1uF
capacitors, so there isn't a heck of a lot to diagnose.

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?


Is this a mains frequency transformer, or a high frequency inverter? If
the latter, a standard multimeter will read incorrectly, you need a true
RMS meter to measure it.

Indeed. In my response, I had just assumed that the filament
was being driven from a tap on the mains power transformer.
If it is from a high-frequency inverter, your (unspecified) meter
may not be reading properly.

"EADGBE" wrote ...
"James Sweet" wrote:
Is this a mains frequency transformer, or a high frequency inverter? If
the
latter, a standard multimeter will read incorrectly, you need a true RMS
meter to measure it.

I don't know. I was assuming that the lines in question were just one
of the transformer secondaries.

The power transformer in this cassette deck has four (4) different
secondary outputs:

18v AC
15v AC
5v AC
3.5v AC --- This measures 6.84v AC - but my multimeter is NOT true
RMS.

Is it a big "heavy iron" transformer with the primary connected
to the mains power cord? Or is it a smaller, lighter ferrite-core
transformer driven from a power transistor (or two) Or perhaps
a combination of those, i.e. a switching power supply?

On Oct 9, 4:08 pm, "Richard Crowley" wrote:

Is it a big "heavy iron" transformer with the primary connected
to the mains power cord? Or is it a smaller, lighter ferrite-core
transformer driven from a power transistor (or two) Or perhaps
a combination of those, i.e. a switching power supply?

It's a big transformer, with the primary connected directly to the
mains power cord via the on/off power switch.

The service manual I am using is for this exact model. According to
the service manual, there were NO revisions at all in the power supply
(I double checked). The schematic and diagram apply to ALL Nakamichi
CR-7 cassette decks.

"EADGBE" wrote ...
"Richard Crowley" wrote:
Is it a big "heavy iron" transformer with the primary connected
to the mains power cord? Or is it a smaller, lighter ferrite-core
transformer driven from a power transistor (or two) Or perhaps
a combination of those, i.e. a switching power supply?

It's a big transformer, with the primary connected directly to the
mains power cord via the on/off power switch.

Then your meter is most likely reading correctly (within its
limits). Unless you are measureing across the windings
when the manual is specifying from the center tap to each
side, I would be VERY CONCERNED about feeding double
the voltage into the filament of the display.

The service manual I am using is for this exact model. According to
the service manual, there were NO revisions at all in the power supply
(I double checked). The schematic and diagram apply to ALL Nakamichi
CR-7 cassette decks.

Does the manual show that that winding is center-tapped?
Do the filament wires in the display look like they burning?

..

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?

Are you measuring the filamant voltage across the filament i.e from
one terminal to the other and not one terminal to ground. The
filament voltage is a differential voltage terminal to terminal.
There is also a boas voltage above ground. If you want to correctly
measure the FILAMENT voltage, you should place your meter probes on
the two filamaent terminals, with NO connection to chassis ground.

Is that what you are doing?

Mark

On Oct 9, 5:11 pm, "Richard Crowley" wrote:

Then your meter is most likely reading correctly (within its
limits). Unless you are measureing across the windings
when the manual is specifying from the center tap to each
side, I would be VERY CONCERNED about feeding double
the voltage into the filament of the display.

Does the manual show that that winding is center-tapped?
Do the filament wires in the display look like they burning?

RICHARD:

I don't know if the service manual shows that the winding in question
is
center-tapped. I have copied part of the schematic for the power
supply, and
put it online for you to look at.

Check it out: http://i240.photobucket.com/albums/f...RANSFORMER.jpg

The filament wires do NOT look like they are burning. Nothing at all
is glowing
except the display elements that should be glowing.

WHAT ABOUT THIS: Perhaps I am taking the voltage readings
incorrectly? I put
the meter's probes across BOTH of the blue wires coming from the
transformer, thinking that the circuit had its
own floating ground. That's when I get the reading that is more than
double
3.25v AC. BUT, when I put just ONE probe on ONE blue wire and the
other probe
on chassis ground, I get a reading of 3.96 volts AC...much,MUCH closer
to what I
want to read.

What do you think?

"EADGBE" wrote in message
oups.com...
On Oct 9, 5:11 pm, "Richard Crowley" wrote:

Then your meter is most likely reading correctly (within its
limits). Unless you are measureing across the windings
when the manual is specifying from the center tap to each
side, I would be VERY CONCERNED about feeding double
the voltage into the filament of the display.

Does the manual show that that winding is center-tapped?
Do the filament wires in the display look like they burning?

RICHARD:

I don't know if the service manual shows that the winding in question
is
center-tapped. I have copied part of the schematic for the power
supply, and
put it online for you to look at.

Check it out:
http://i240.photobucket.com/albums/f...RANSFORMER.jpg

The filament wires do NOT look like they are burning. Nothing at all
is glowing
except the display elements that should be glowing.

WHAT ABOUT THIS: Perhaps I am taking the voltage readings
incorrectly? I put
the meter's probes across BOTH of the blue wires coming from the
transformer, thinking that the circuit had its
own floating ground. That's when I get the reading that is more than
double
3.25v AC. BUT, when I put just ONE probe on ONE blue wire and the
other probe
on chassis ground, I get a reading of 3.96 volts AC...much,MUCH closer
to what I
want to read.

What do you think?


If the voltage were really that far out of spec the filaments would be
glowing like a lightbulb. If it looks ok then you're fine, leave it alone.

On Oct 9, 8:33 pm, EADGBE wrote:
OK, I have fixed the final(?) issue with my long-suffering Nakamichi
CR-7 cassette deck.

A fusible resistor in the flourescent display circuit had blown,
causing the display to go dark.

thats not an 'issue' thats a problem! ;-)

seriously, your only real chance is to experiment with series
resistors if nothing else yields a result.
ben

On Tue, 09 Oct 2007 14:39:00 -0700, EADGBE
wrote:

Check it out: http://i240.photobucket.com/albums/f...RANSFORMER.jpg

This clearly shows 3.5 volts either side of center tap.
They add to seven across both hots. You're fine.


All good fortune,

Chris Hornbeck

"EADGBE" wrote ...
I don't know if the service manual shows that the winding
in question is center-tapped. I have copied part of the
schematic for the power supply, and put it online for you
to look at.

Check it out:
http://i240.photobucket.com/albums/f...RANSFORMER.jpg

The schematic diagram shows that there should be 3.5 VAC
on either side of the center tap. The center tap is the line that
goes up to the emitter of Q413. Sounds like everything is OK.

On Oct 9, 8:03 pm, "Richard Crowley" wrote:

The schematic diagram shows that there should be 3.5 VAC
on either side of the center tap. The center tap is the line that
goes up to the emitter of Q413. Sounds like everything is OK.

Thanks for setting me straight. I was laboring under the assumption
that the 3.5v AC circuit was not tied to chassis ground but instead
was its own circuit; therefore, I kept measuring both of the 3.5v
lines at the same time. An embarrassing mistake, but one that I"m
happy to finally recognize as indeed being a mistake!

EADGBE wrote:

I am still measuring just under 7v AC going to the flourescent
display. According to the service manual, this should be only 3.25v
AC.

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?

It's not unknown for the specs of such items to change over the lifetime of the
product and that info not to make it into the service manual.

If you could see the filament glowing red hot I'd be concerned but don't forget,
this equipment has worked for years with that voltage being applied to it.

Graham

EADGBE wrote:

I have copied part of the schematic for the power
supply, and put it online for you to look at.

Check it out: http://i240.photobucket.com/albums/f...RANSFORMER.jpg

It's 3.5V - 0 - 3.5V. That's 7V centre tapped (with the centre tap being connected
as it should be to a negative voltage).

It's meant to measure that way.

Graham

"EADGBE" wrote ...
"Richard Crowley" wrote:

The schematic diagram shows that there should be 3.5 VAC
on either side of the center tap. The center tap is the line that
goes up to the emitter of Q413. Sounds like everything is OK.

Thanks for setting me straight. I was laboring under the assumption
that the 3.5v AC circuit was not tied to chassis ground but instead
was its own circuit; therefore, I kept measuring both of the 3.5v
lines at the same time. An embarrassing mistake, but one that I"m
happy to finally recognize as indeed being a mistake!

In your case the center-tap is actually connected to the
-14 VDC supply (IIRC?) this is the voltage that makes
the selected (grounded) parts of the fluorescent display
glow. But you likely didn't want to get that far into the
circuit analysis. :-)

"EADGBE" wrote in message
oups.com...
On Oct 9, 8:03 pm, "Richard Crowley" wrote:

The schematic diagram shows that there should be 3.5 VAC
on either side of the center tap. The center tap is the line that
goes up to the emitter of Q413. Sounds like everything is OK.

Thanks for setting me straight. I was laboring under the assumption
that the 3.5v AC circuit was not tied to chassis ground but instead
was its own circuit; therefore, I kept measuring both of the 3.5v
lines at the same time. An embarrassing mistake, but one that I"m
happy to finally recognize as indeed being a mistake!


A mistake, yes, but not quite as you are imagining. For future reference,
you should try to get straight and understand the whys and wherefores of
this circuit.

As we pointed out to you when you first posted your question about this a
couple of weeks back, this supply is *not* tied to ground, but is floated on
a negative bias voltage equivalent to the display drive voltages, in order
to reduce the stress between the filament and the display elements. This is
usually done by just having a bleed resistor from the negative supply to one
side of the filament supply. In the case of this item, it is done in a
slightly different way, by having a centre tap on the filament winding, and
then connecting that tap to to the emitter of Q413, which supplies -14.5v.
So, you have an AC voltage, 'floating' on a negative DC voltage. This
explains the reason that the designer saw fit to put a fusible resistor in
each leg of the winding. The actual AC voltage across the display filament -
ie the actual voltage that the filament 'sees' and which is being used to
power the filament - is indeed the full 8v or whatever that your meter is
recording. That is correct. The schematic clearly shows that the indicated
voltage of 3.5v is from winding end to tap, as it is shown thus by the sides
of both winding ends. With your meter set to DC, if you measure from ground
to either end of the winding, you should see the negative bias voltage.
Anyway, glad you got it going, and the group was able to help you with the
diagnosis, and learning a bit more about how these things are done.

Arfa

"Eeyore" wrote in message
...


EADGBE wrote:

I am still measuring just under 7v AC going to the flourescent
display. According to the service manual, this should be only 3.25v
AC.

MY QUESTION: Should I worry about this slightly off-spec voltage? Or
should I just chalk it up to a variance in the transformer
manufacturer and let it slide...hoping that the flourescent display
can handle just under 7v AC?

It's not unknown for the specs of such items to change over the lifetime
of the
product and that info not to make it into the service manual.

If you could see the filament glowing red hot I'd be concerned but don't
forget,
this equipment has worked for years with that voltage being applied to it.

Graham


Everyone is barking up the wrong tree here, and giving invalid advice, as
they would see if they looked at the OP's schematic - see my post above ...
d;~}

Arfa

ARFA:

You are truly THE MAN. I wish I had half the knowledge and experience
you have! Honestly, you have been a huge help to me with this issue.
Thank you!

I am not a professional technician (did you guess that?), just a Nak
enthusiast who doesn't have access to competent, reasonably-priced
service in my area. So when I have a problem, I turn to the
internet. It's great that there are folks like you who are willing
and able to help out. It's a lot more than what I have where I am!

NOW TO MY LATEST QUESTION. Last night, the OTHER fusible resistor in
this circuit blew. I have now replaced it and the display is OK once
again. BUT....if the fusible resistors continue to blow, should I
take a look at Q413's voltages (which is what I'm going to do
anyway)? By the way, I have checked the two ceramic disc 0.1uF
capacitors tied to the fusible resistors. They are both OK.

ANOTHER (RELATED) QUESTION: As I said before, this deck got hit with a
power surge, causing damage to several components in the power supply
(now fixed and stable). In order to explain why BOTH of the fusible
resistors in this circuit have managed to blow...is it possible that
both resistors were weakened by the power surge, and now that the deck
is being used regularly once again, they just couldn't keep up the
pace? Or...and I shudder to ask this....are they a symptom of a
deeper problem, possibly with the transformer itself?

OK, something is definitely whacko, because I am only getting -4.97v
DC at the emitter of Q413.

Looks like I have some more detective work to do.

EADGBE wrote:

NOW TO MY LATEST QUESTION. Last night, the OTHER fusible resistor in
this circuit blew.

Considering one blew, the other was probably weakened. I expect this is the last
problem you'll have with the display.

I have now replaced it and the display is OK once
again. BUT....if the fusible resistors continue to blow, should I
take a look at Q413's voltages

Won't affect it.

Graham

EADGBE wrote:

OK, something is definitely whacko, because I am only getting -4.97v
DC at the emitter of Q413.

Looks like I have some more detective work to do.

Spoke too soon !

Graham

EADGBE wrote:
OK, something is definitely whacko, because I am only getting -4.97v
DC at the emitter of Q413.

Sure looks to me like ZD406, R422 and Q413 regulate the -14.5V rail
down to -5V which is applied to the center tap of the 7V winding.

-- Adam

"Adam Goldman" wrote in message
...
EADGBE wrote:
OK, something is definitely whacko, because I am only getting -4.97v
DC at the emitter of Q413.

Sure looks to me like ZD406, R422 and Q413 regulate the -14.5V rail
down to -5V which is applied to the center tap of the 7V winding.

-- Adam

Yep, you're quite right Adam. I must admit I didn't look at the value of the
zener. I saw the label that said -14.5v and didn't notice the speck of
fly**** that was the arrow pointing at the emitter of Q412 ! So -5v it is,
and the
-4.97v that the OP is reading is correct.

As far as the resistors failing goes, I would suspect that they might have
suffered some strain as a result of whatever nastiness was done to to the
thing in the first place, or even just that the originals had aged and gone
up a bit in value, which is not at all uncommon with these fusibles. If the
value even doubles to 2 ohms, this can result in a significant increase in
the dissipation in them. I would feel inclined to just go ahead and replace
the one that's failed now, and then feel them both to see if you can detect
any heat. I would normally expect them to run stone cold. You could also try
measuring the AC voltage drop across each one, then work out the current and
the power dissipation in each one. With a 1 ohm resistor, the voltage
reading will give you the current direct (I=V/R) so if you read 0.4v you
would have 0.4 amps flowing. The power in the resistor will be the current
squared (P=IsqR) for a 1 ohm so would be 0.16 watts. This should give you a
good idea if anything else is actually wrong. It's hard to see what could
be, but an intermittent fault around Q412 is maybe a possibility. If your
meter has a peak hold facility, might be worth leaving it hanging on the
centre tap just to see if anything unexpected happens to the voltage from
Q412.

Arfa

"EADGBE" wrote in message
oups.com...
I don't know if the service manual shows that the winding in question
is
center-tapped. I have copied part of the schematic for the power
supply, and
put it online for you to look at.

Check it out:
http://i240.photobucket.com/albums/f...RANSFORMER.jpg

It's center-tapped. You're fine.

Peace,
Paul