There's a result then ...

A

Arfa Daily

Guest
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in either
mode. When playing back a known good test tape, there was input to two pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance to
ground at every pin on the IC. Two pins, exactly opposite one another, and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board, they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa
 
On Dec 23, 7:36 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in either
mode. When playing back a known good test tape, there was input to two pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device ....

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance to
ground at every pin on the IC. Two pins, exactly opposite one another, and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board, they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ...   :)

Arfa
Click to expand...
Mery Christmas!
 
"hr(bob) hofmann@att.net" <hrhofmann@att.net> wrote in message
news:d25a4c09-78a3-4141-9a3b-eb48c2ca105b@b2g2000yqi.googlegroups.com...
On Dec 23, 7:36 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come
up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in
either
mode. When playing back a known good test tape, there was input to two
pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device
...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance
to
ground at every pin on the IC. Two pins, exactly opposite one another, and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board,
they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were
replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa

Mery Christmas!
Click to expand...

Agreed. Merry Christmas to all the regulars here and it looks like we have
nearly made it through another year of this. As frustrating as it can be, we
wouldn't be doing it if we didn 't love it.

Mark Z.
 
Arfa Daily wrote:
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in either
mode. When playing back a known good test tape, there was input to two pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years,
Click to expand...
These capacitors are in fact the little mischievous elves that normally
assist in the distribution of power thoughout the product, but at
Christmas particulary they sit around doing nothing after a year or so
of either being exhausted, drinking too much, or just sitting there
cross-legged and idle.

It's the cross-legged stance that causes the short circuit. At least you
don't have the mess of the ones that drank too much, got bloated and
threw up electrolyte all over the board ...

--
Adrian C
 
On 12/23/2009 8:36 PM, Arfa Daily wrote:
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in either
mode. When playing back a known good test tape, there was input to two pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance to
ground at every pin on the IC. Two pins, exactly opposite one another, and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board, they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa


Arfa,
Click to expand...

Glad you found it, and sorry I missed your original post, as I've
likely got that manual packed away somewhere.

I would also like to echo Mark's wishes to all for w great holiday
season and New Year.

Tim Schwartz
Bristol Electronics
 
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:KAzYm.104267$II.69134@newsfe22.ams2...
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come
up with any diagrams, I decided today to fling it back up on the bench,
and have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in
either mode. When playing back a known good test tape, there was input to
two pins of the Sony Dolby processor IC, but no signs of anything coming
out on any other pins. I couldn't even find the correct data sheet for
this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance
to ground at every pin on the IC. Two pins, exactly opposite one another,
and with similar looking print traces, going off in the same direction,
both read pretty close to zero, When I followed the traces round the
board, they both arrived at 220uF 10v caps, sitting side by side. When
these were removed, both read short circuit. How odd is that ? When they
were replaced with 16v types, all record / play and metering functions
were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa
Click to expand...

Glad to hear that you solved your problem.

Merry Christmas and happy whatever to all.

Arfa, and others, you might want to check out some of the tech forums where
hundreds of techs share resources and tips. Two of the best are TechAssist
and Tech-Data. Both have subscription fees in the $50-60 range, but there
are hundreds of gigabytes of manuals between the two, thousands of tips, and
hundreds of techs with access and experience on most brands. You also won't
find the spam and nonsense that are so pervasive on Usenet. Only qualified
techs in the repair business are allowed.

Leonard
 
On 24 dic, 02:36, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in either
mode. When playing back a known good test tape, there was input to two pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device ....

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance to
ground at every pin on the IC. Two pins, exactly opposite one another, and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board, they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ...   :)

Arfa
Click to expand...
well done! it's great when that happens.
thanks also for posting up the solution here - might help someone else
in future. maybe post a link to this solution from the original
thread, for reference.
-B
 
"Adrian C" <email@here.invalid> wrote in message
news:7ph3rgFvdbU1@mid.individual.net...
Arfa Daily wrote:
Following on from my request earlier in the week for a schematic set for
a NAD cassete deck that was an urgent repair for Christmas, having not
come up with any diagrams, I decided today to fling it back up on the
bench, and have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in
either mode. When playing back a known good test tape, there was input to
two pins of the Sony Dolby processor IC, but no signs of anything coming
out on any other pins. I couldn't even find the correct data sheet for
this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years,

These capacitors are in fact the little mischievous elves that normally
assist in the distribution of power thoughout the product, but at
Christmas particulary they sit around doing nothing after a year or so of
either being exhausted, drinking too much, or just sitting there
cross-legged and idle.

It's the cross-legged stance that causes the short circuit. At least you
don't have the mess of the ones that drank too much, got bloated and threw
up electrolyte all over the board ...

--
Adrian C
Click to expand...
LOL ! Have a good 'un everyone, and catch y'all in the new year ! d;~}

Arfa
 
On Thu, 24 Dec 2009 01:36:41 -0000, "Arfa Daily"
<arfa.daily@ntlworld.com>wrote:

Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in either
mode. When playing back a known good test tape, there was input to two pins
of the Sony Dolby processor IC, but no signs of anything coming out on any
other pins. I couldn't even find the correct data sheet for this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance to
ground at every pin on the IC. Two pins, exactly opposite one another, and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board, they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa
Click to expand...
Good deal. I was always thrust into a position of a so called
emergency repair without a Sams manual or the luxury of the internet
(which Al Gore hadn't invented yet) back in the 70's-80's.

However, the real tough dogs being intermittent faults can stifle even
the most persistent, experienced tech. Like this Singer-Kearfott
resolver driver/processor I'm working on. Nothing on the net to give
me service errata and an intermittent error displayed on the op
terminal as a PG error relates to the resolver, Funny thing is you can
get the error anywhere from immediately to several days down the road.
And the error can shut things down immediately several times in a row
then work for a week.

As much experience as I have it doesn't substitute for a diagram
showing operation values for voltages, wave forms and digital pulses.
 
Meat Plow <meat@petitmorte.net> wrote in message
news:3csm2c.7nl.17.9@news.alt.net...
On Thu, 24 Dec 2009 01:36:41 -0000, "Arfa Daily"
arfa.daily@ntlworld.com>wrote:

Following on from my request earlier in the week for a schematic set for
a
NAD cassete deck that was an urgent repair for Christmas, having not come
up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in
either
mode. When playing back a known good test tape, there was input to two
pins
of the Sony Dolby processor IC, but no signs of anything coming out on
any
other pins. I couldn't even find the correct data sheet for this device
....

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance
to
ground at every pin on the IC. Two pins, exactly opposite one another,
and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board,
they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were
replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa


Good deal. I was always thrust into a position of a so called
emergency repair without a Sams manual or the luxury of the internet
(which Al Gore hadn't invented yet) back in the 70's-80's.

However, the real tough dogs being intermittent faults can stifle even
the most persistent, experienced tech. Like this Singer-Kearfott
resolver driver/processor I'm working on. Nothing on the net to give
me service errata and an intermittent error displayed on the op
terminal as a PG error relates to the resolver, Funny thing is you can
get the error anywhere from immediately to several days down the road.
And the error can shut things down immediately several times in a row
then work for a week.

As much experience as I have it doesn't substitute for a diagram
showing operation values for voltages, wave forms and digital pulses.
Click to expand...

Just going by the general appearance/style/era of that cnc board, I would
suspect a via somewhere, from dealing with similar in the past, re-solder
both sides of all vias


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
 
On Thu, 24 Dec 2009 16:30:33 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

Meat Plow <meat@petitmorte.net> wrote in message
news:3csm2c.7nl.17.9@news.alt.net...
On Thu, 24 Dec 2009 01:36:41 -0000, "Arfa Daily"
arfa.daily@ntlworld.com>wrote:

Following on from my request earlier in the week for a schematic set for
a
NAD cassete deck that was an urgent repair for Christmas, having not come
up
with any diagrams, I decided today to fling it back up on the bench, and
have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in
either
mode. When playing back a known good test tape, there was input to two
pins
of the Sony Dolby processor IC, but no signs of anything coming out on
any
other pins. I couldn't even find the correct data sheet for this device
...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance
to
ground at every pin on the IC. Two pins, exactly opposite one another,
and
with similar looking print traces, going off in the same direction, both
read pretty close to zero, When I followed the traces round the board,
they
both arrived at 220uF 10v caps, sitting side by side. When these were
removed, both read short circuit. How odd is that ? When they were
replaced
with 16v types, all record / play and metering functions were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa


Good deal. I was always thrust into a position of a so called
emergency repair without a Sams manual or the luxury of the internet
(which Al Gore hadn't invented yet) back in the 70's-80's.

However, the real tough dogs being intermittent faults can stifle even
the most persistent, experienced tech. Like this Singer-Kearfott
resolver driver/processor I'm working on. Nothing on the net to give
me service errata and an intermittent error displayed on the op
terminal as a PG error relates to the resolver, Funny thing is you can
get the error anywhere from immediately to several days down the road.
And the error can shut things down immediately several times in a row
then work for a week.

As much experience as I have it doesn't substitute for a diagram
showing operation values for voltages, wave forms and digital pulses.


Just going by the general appearance/style/era of that cnc board, I would
suspect a via somewhere, from dealing with similar in the past, re-solder
both sides of all vias
Click to expand...
Yeah I was thinking of that. Some of the topside connections aren't
the best soldered I've ever seen. One or the two 8 bit multiplying D/A
converters has been replaced as have a couple of the quad I/O gated
flip-floppers.

I have enough generic info and understanding now to take it to the
second level, dynamic tests. I have the specs on the resolvers and
what they need as far as an input from the resolver amp. But I must
confess I'm not the sharpest knife in the drawer when it comes to old
TTL and A/D - D/A stuff being more of an audio and RF tech in the
days. There are no LSI devices in this thing besides the 8086 CPUs on
the logic and servo CPU boards. Also some embedded software on
EEPROMS. Not being formally trained on TTL has hampered my
troubleshooting but I am finally starting to 'get it'. It's hard to
adapt to a primitive TTL aggregation that was replaced with LSI less
than a decade later. I was just getting into digital control, PLL,
etc.. when LSI started being used more and more making it much easier
to verify the operation of the circuit by data I/O on the pins of an
LSI rather than to understand the mathematics of data flow between
dozens of seperate TTL devices.
 
Meat Plow <meat@petitmorte.net> wrote in message
news:3cva81.7cr.17.3@news.alt.net...
On Thu, 24 Dec 2009 16:30:33 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

Meat Plow <meat@petitmorte.net> wrote in message
news:3csm2c.7nl.17.9@news.alt.net...
On Thu, 24 Dec 2009 01:36:41 -0000, "Arfa Daily"
arfa.daily@ntlworld.com>wrote:


I have enough generic info and understanding now to take it to the
second level, dynamic tests. I have the specs on the resolvers and
what they need as far as an input from the resolver amp. But I must
confess I'm not the sharpest knife in the drawer when it comes to old
TTL and A/D - D/A stuff being more of an audio and RF tech in the
days. There are no LSI devices in this thing besides the 8086 CPUs on
the logic and servo CPU boards. Also some embedded software on
EEPROMS. Not being formally trained on TTL has hampered my
troubleshooting but I am finally starting to 'get it'. It's hard to
adapt to a primitive TTL aggregation that was replaced with LSI less
than a decade later. I was just getting into digital control, PLL,
etc.. when LSI started being used more and more making it much easier
to verify the operation of the circuit by data I/O on the pins of an
LSI rather than to understand the mathematics of data flow between
dozens of seperate TTL devices.
Click to expand...

If one clock only then a trick I used once for tracing a digital fault in
similar circumstances.
I managed to change the clock crystal to 100KHz or maybe was a glass cased
10KHz one, can't remember, long ago. Clock worked and slowed everything down
so much that it highlighted, by little happening, on a suspect ,actually
dead TTL device.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
 
Leonard Caillouet wrote:
Merry Christmas and happy whatever to all.
Click to expand...

Merry Christmas, you old buzzard! ;-)

Merry Christmas to everyone. :)


--
Offworld checks no longer accepted!
 
On Fri, 25 Dec 2009 16:17:01 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

Meat Plow <meat@petitmorte.net> wrote in message
news:3cva81.7cr.17.3@news.alt.net...
On Thu, 24 Dec 2009 16:30:33 -0000, "N_Cook" <diverse@tcp.co.uk>wrote:

Meat Plow <meat@petitmorte.net> wrote in message
news:3csm2c.7nl.17.9@news.alt.net...
On Thu, 24 Dec 2009 01:36:41 -0000, "Arfa Daily"
arfa.daily@ntlworld.com>wrote:


I have enough generic info and understanding now to take it to the
second level, dynamic tests. I have the specs on the resolvers and
what they need as far as an input from the resolver amp. But I must
confess I'm not the sharpest knife in the drawer when it comes to old
TTL and A/D - D/A stuff being more of an audio and RF tech in the
days. There are no LSI devices in this thing besides the 8086 CPUs on
the logic and servo CPU boards. Also some embedded software on
EEPROMS. Not being formally trained on TTL has hampered my
troubleshooting but I am finally starting to 'get it'. It's hard to
adapt to a primitive TTL aggregation that was replaced with LSI less
than a decade later. I was just getting into digital control, PLL,
etc.. when LSI started being used more and more making it much easier
to verify the operation of the circuit by data I/O on the pins of an
LSI rather than to understand the mathematics of data flow between
dozens of seperate TTL devices.


If one clock only then a trick I used once for tracing a digital fault in
similar circumstances.
I managed to change the clock crystal to 100KHz or maybe was a glass cased
10KHz one, can't remember, long ago. Clock worked and slowed everything down
so much that it highlighted, by little happening, on a suspect ,actually
dead TTL device.
Click to expand...
The amp board is dual function. It provides the resolvers with an AC
current at 10khz at a specific voltage. The resolvers then send
reference back to the amp board where the phase angles of the resolver
feedback are analyzed, compared and then converted to digital going
thru a serial to parallel data path and off to the servo cpu to
interface with the logic cpu and off to the servo packs to control the
pulse-width modulate 48kz control fed to an SCR pack that ends up as
power for the servomotor. <whew!>

This allows each axis of motion to be precisely controllable down to
the 10,000th of an inch. Also allows for a precision infinite control
of the motor speed.

One thing remains the same in that a resolver does the same thing no
matter who built the electronics. Eventually I will be able to find
the proper points for measurements, there are three trimmer pots on
the board I can rock back and forth and see what areas on the board
are influenced signal-wise.

Here is a page that describes resolver transducers if you're
interested.

http://www.admotec.com/TT02.pdf
 
Just goes to show, with a bit of perseverence, and a lot of
years' experience, just occasionally, you *do* get lucky... :)
Click to expand...
That is not luck.

Given the problem -- no recording and no playback -- I assume you started
with something common to both processes, the power supply, and found that
the voltages were more or less correct.

Where to from there? It's easier to check playback than recording, so you
inserted a test tape and found the signal stopped at the Dolby chip. That
pointed to either the chip or the surrounding components. The latter are the
easiest to check, so...

No, Arfa, it was not luck. It was good troubleshooting techniques all the
way.
 
No, Arfa, it was not luck. It was good troubleshooting techniques
all the way.

Weeelll, I guess that's sort of true, but sometimes with problems like
this
and no service info, you can go round in circles for ever. Such situations
are doubly difficult when you have your arse in a sling over timing
(remember it was needed in a hurry) and when not just one, but two
components are faulty, as was the (extremely rare) case with this one. The
problem could have been down to the Dolby processor chip itself, or at
least
tests without a schematic might have made it look so, in which case I
would
have been stuffed without a replacement to try. Likewise, it could have
been
a resistor which had gone high, or a little ceramic decoupler that had
gone
leaky, both of which would have been difficult to locate by in circuit
resistance checks, given that I would not have been able to find such
problems by voltage checks, without service info for the unit, or the
correct data sheet for the chip. In the timescale I had, and given the
value
of the unit, it would not really have been commercially viable to start
pulling and refitting or replacing components willy-nilly, in the fond
hope
of finding the bad one.

I guess that experience and years of solid fault-finding work, probably
*did* point at it being a little blue electro at fault, and yes, good
fault-finding practice did lead me to the Dolby chip in exactly the way
you
surmise, but for all that, it still *felt* lucky to have dropped on a
short
circuit cap (and its mate next door) that was measurable in circuit !
Click to expand...
You're confusing luck in the problem being due to a readily replaceable
part, and luck in troubleshooting it. "Yes" to the former, "no" to the
latter.
 
"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:hh54o5$jhg$1@news.eternal-september.org...
Just goes to show, with a bit of perseverence, and a lot of
years' experience, just occasionally, you *do* get lucky... :)

That is not luck.

Given the problem -- no recording and no playback -- I assume you started
with something common to both processes, the power supply, and found that
the voltages were more or less correct.

Where to from there? It's easier to check playback than recording, so you
inserted a test tape and found the signal stopped at the Dolby chip. That
pointed to either the chip or the surrounding components. The latter are
the
easiest to check, so...

No, Arfa, it was not luck. It was good troubleshooting techniques all the
way.
Click to expand...
Weeelll, I guess that's sort of true, but sometimes with problems like this
and no service info, you can go round in circles for ever. Such situations
are doubly difficult when you have your arse in a sling over timing
(remember it was needed in a hurry) and when not just one, but two
components are faulty, as was the (extremely rare) case with this one. The
problem could have been down to the Dolby processor chip itself, or at least
tests without a schematic might have made it look so, in which case I would
have been stuffed without a replacement to try. Likewise, it could have been
a resistor which had gone high, or a little ceramic decoupler that had gone
leaky, both of which would have been difficult to locate by in circuit
resistance checks, given that I would not have been able to find such
problems by voltage checks, without service info for the unit, or the
correct data sheet for the chip. In the timescale I had, and given the value
of the unit, it would not really have been commercially viable to start
pulling and refitting or replacing components willy-nilly, in the fond hope
of finding the bad one.

I guess that experience and years of solid fault-finding work, probably
*did* point at it being a little blue electro at fault, and yes, good
fault-finding practice did lead me to the Dolby chip in exactly the way you
surmise, but for all that, it still *felt* lucky to have dropped on a short
circuit cap (and its mate next door) that was measurable in circuit !

Arfa
 
<snip>

it still *felt* lucky to have dropped on a
short
circuit cap (and its mate next door) that was measurable in circuit !

You're confusing luck in the problem being due to a readily replaceable
part, and luck in troubleshooting it. "Yes" to the former, "
Click to expand...

Well, whichever way you look at it, that brings us right back around to the
start. Sometimes you gets lucky. I got lucky ... :)

Arfa
 
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:KAzYm.104267$II.69134@newsfe22.ams2...
Following on from my request earlier in the week for a schematic set for a
NAD cassete deck that was an urgent repair for Christmas, having not come
up with any diagrams, I decided today to fling it back up on the bench,
and have a go at the fault 'blind'.

The basic problem was no record, no playback, no meter indications in
either mode. When playing back a known good test tape, there was input to
two pins of the Sony Dolby processor IC, but no signs of anything coming
out on any other pins. I couldn't even find the correct data sheet for
this device ...

Based on the fact that the chip was surrounded by little pale blue
electrolytics, which I have had give trouble on many different items of
electronic equipment over the years, I set about measuring the resistance
to ground at every pin on the IC. Two pins, exactly opposite one another,
and with similar looking print traces, going off in the same direction,
both read pretty close to zero, When I followed the traces round the
board, they both arrived at 220uF 10v caps, sitting side by side. When
these were removed, both read short circuit. How odd is that ? When they
were replaced with 16v types, all record / play and metering functions
were restored.

Just goes to show, with a bit of perseverence, and a lot of years'
experience, just occasionally, you *do* get lucky ... :)

Arfa
Click to expand...
test
 
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