Hi,

I'm hoping someone might be able to help. I stupidly tried to solve an
intermittent channel fading/distorting problem on my old beloved Nad 304. I
know nothing about circuit boards but thought it may be a dusty pot or dry
solder joint.

I opened the case (power off!) and sprayed Servisol switch cleaner in the
pots. There are four plastic 'screw adjusters' on the board(named vr302 and
vr303). I also sprayed these and screwed them open and closed a few times to
ensure fluid starts cleaning.I think this has caused the problem.

I then left the amp for a couple of days to 'dry out'. I then connected up
and switched on. Everything seemed perfect, no more crackling/fading. But
after about 1 minute, I noticed that 2 larger components(covered in
plastic?), were cooking, and stinking! I think it was caused by adjusting
the plastic screw adjusters, whatever they are, and not turning them back to
their original position.

I quickly turned off, and re-tuned these plastic screws to the position I
thought they were in originally. Again I turned on and again, the amp was
sounding great.No cooking components. I thought I had solved it, but after a
few hours use, 2 fuses blew on the circuit board.

Obviously something still isn't quite right.
Can anybody tell me what the 4 white plastic screw adjusters are, and how
would I ensure the correct positioning of them? Also the plastic covering on
the larger components that overheated, is slightly but noticeably 'melted'.
What are these and should/can they be replaced?

If I took the amp to be repaired, and gave them this information, could they
diagnose the problem without long, expensive testing?

Sorry about the long and 'novice' message, but I really don't want to lose
this amp if possible.

Thanks for your time,

Len

Hi,

I'm hoping someone might be able to help. I stupidly tried to solve an
intermittent channel fading/distorting problem on my old beloved Nad 304. I
know nothing about circuit boards

No comment seems necessary...

but thought it may be a dusty pot or dry
solder joint.

I opened the case (power off!) and sprayed Servisol switch cleaner in the
pots. There are four plastic 'screw adjusters' on the board(named vr302 and
vr303). I also sprayed these and screwed them open and closed a few times to
ensure fluid starts cleaning.I think this has caused the problem.

Ummm... more than likely.

I then left the amp for a couple of days to 'dry out'. I then connected up
and switched on. Everything seemed perfect, no more crackling/fading. But
after about 1 minute, I noticed that 2 larger components(covered in
plastic?), were cooking, and stinking! I think it was caused by adjusting
the plastic screw adjusters, whatever they are, and not turning them back to
their original position.

I quickly turned off, and re-tuned these plastic screws to the position I
thought they were in originally. Again I turned on and again, the amp was
sounding great.No cooking components. I thought I had solved it, but after a
few hours use, 2 fuses blew on the circuit board.

Obviously something still isn't quite right.
Can anybody tell me what the 4 white plastic screw adjusters are, and how
would I ensure the correct positioning of them?

I do not know that specific product. However, from your description,
I'd guess that these may very well be the "bias" and "offset"
adjustments for the amplifier output stage. The "bias" control
adjusts the amount of "bias" or "idle" current which flows through the
output transistors when there's no music playing. If the bias current
is set too low, then there's an excessive amount of distortion when
playing low-level signals. If the bias is set too high, the output
transistors dissipate more power than is necessary, can overheat, and
can even go into a nasty condition known as "thermal runaway" which
can result in letting all of the Magic Blue Smoke out of the
transistors.

The "offset" pots, if present, are used to balance out small errors or
inequalities in the circuit's symmetry, thus ensuring that the average
voltage coming out of the output transistors (the "DC offset") is
close to zero. If it's misadjusted, DC current can flow through your
woofers (which may make result in some audible problems under some
circumstances) and there may also be excessive current flow through
part of the output stage.

Now, my guess is that you probably tweaked things in a way which left
the amp with *much* too high an idle current setting - too much bias.
You may also have deranged the offset setting, if the amp has one.
The excessive bias would have resulted in too much current flowing
through the output transistors, overheating them.

Basically, by misadjusting the bias, you injected your amp with a massive
dose of "speed", and it went meth-freak on you :-)

Your amp may have individual transistors, or more likely has a larger
"power module" containing both the output transistors, a driver stage,
and other components. This module (perhaps one per channel) would
probably be fastened to the heatsink. I suspect that this is the
"larger components, covered in plastic" that you mention.

Also the plastic covering on
the larger components that overheated, is slightly but noticeably 'melted'.
What are these and should/can they be replaced?

They're probably the power module. If there's an SK part number on
them, that's likely what they are.

If I took the amp to be repaired, and gave them this information, could they
diagnose the problem without long, expensive testing?

I suspect so.

At the very least, the amp will need to have its bias and balance
controls readjusted to meet the manufacturer's specifications... and,
of course, the blown fuses will need to be replaced.

There's surely a procedure for this in the service manual. If there
was no actual damage to the power modules, then it probably wouldn't
require more than 15 minutes to half an hour to do the job.

The bad news is that there's some possibility that the overdriving has
damaged the power module. One or both of the modules may have shorted
out, internally. Or, the damage may be less obvious than that... the
transistors may still be partially OK, but may have drifted out of
spec and/or become unstable. It's possible that the amp would work
for a while, if re-biased properly, but might then "run away" and blow
its fuses again.

The service shop would probably replace the fuses, re-do the
bias/offset adjustments per the manufacturer's spec, and then operate
the unit into a "dummy load" at partial and/or full power for an hour
or so to make sure that it remains stable. If it doesn't operate
properly and reliably after adjustment, the power modules would have
to be replaced and the adjustments re-done.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Len wrote:

Hi,

I'm hoping someone might be able to help. I stupidly tried to solve an
intermittent channel fading/distorting problem on my old beloved Nad 304. I
know nothing about circuit boards but thought it may be a dusty pot or dry
solder joint.

I opened the case (power off!) and sprayed Servisol switch cleaner in the
pots.

Very bad idea. It's not 'pot cleaner' and doing this usually just results in the
*needed* lubrication in the pot being 'washed out'.


There are four plastic 'screw adjusters' on the board(named vr302 and
vr303). I also sprayed these and screwed them open and closed a few times to
ensure fluid starts cleaning.I think this has caused the problem.

You bet - they are 'presets' that are adjusted at manufacture to set up bias
current in the output stage typically.

By 'opening and closing them' you ruined the manufacturers setting.


I then left the amp for a couple of days to 'dry out'. I then connected up
and switched on. Everything seemed perfect, no more crackling/fading. But
after about 1 minute, I noticed that 2 larger components(covered in
plastic?), were cooking, and stinking!

The output transistors.

I think it was caused by adjusting
the plastic screw adjusters, whatever they are, and not turning them back to
their original position.

Very likely.

I quickly turned off, and re-tuned these plastic screws to the position I
thought they were in originally. Again I turned on and again, the amp was
sounding great.No cooking components. I thought I had solved it, but after a
few hours use, 2 fuses blew on the circuit board.

Goes to show you didn't remember right.


Obviously something still isn't quite right.

Uhuh.

Can anybody tell me what the 4 white plastic screw adjusters are, and how
would I ensure the correct positioning of them?

They are presets that you shouldn't touch, VR means variable resistor.

You'll need a service manual and test equipment to get them back in the correct
position.

Also the plastic covering on
the larger components that overheated, is slightly but noticeably 'melted'.
What are these and should/can they be replaced?

Probably just insulating covers.

They can probably be replaced if they are a common type.

If I took the amp to be repaired, and gave them this information, could they
diagnose the problem without long, expensive testing?

Yes.

You'll give them a good laugh too no doubt.


Sorry about the long and 'novice' message, but I really don't want to lose
this amp if possible.

You're lucky it didn't fry totally.

Your original problem is most likely a bad solder joint that no amount of
Servisol would fix.


Graham

Dave Platt wrote:

The bad news is that there's some possibility that the overdriving has
damaged the power module.

I don't hink that NAD has ever used those nasty modules.

I suspect that the plastic covers will be 'diamond shaped' to cover some TO-3
transistors.

One or both of the modules may have shorted
out, internally. Or, the damage may be less obvious than that... the
transistors may still be partially OK, but may have drifted out of
spec and/or become unstable.

No, that doesn't happen - transistors either fail or are OK after abuse IME.
Transistors don't fail 'unstable'.

It's possible that the amp would work
for a while, if re-biased properly, but might then "run away" and blow
its fuses again.

I don't think so - see above comment


Graham

In article ,
Pooh Bear wrote:

I don't hink that NAD has ever used those nasty modules.

You could be correct there.

I suspect that the plastic covers will be 'diamond shaped' to cover some TO-3
transistors.

Also quite likely.

No, that doesn't happen - transistors either fail or are OK after abuse IME.
Transistors don't fail 'unstable'.

I was thinking more along the lines of a power output module, which
contains driver transistors, power output transistors, bias regulation
and temperature-compensation components, and various other passive
components as well. Overheating of such a module might have bad
effects on the passives (causing resistors to shift values, causing
'lytics to leak), or bad mechanical effects (e.g. damaging the
mounting of a thermal-compensation diode, so that its temperature no
longer accurately tracks that of the transistor case or heat-sink).

It's possible that the amp would work
for a while, if re-biased properly, but might then "run away" and blow
its fuses again.

I don't think so - see above comment

Well, any time a component has failed or been seriously overstressed,
I think it's still a good idea to make sure that the total system
works properly out to the limits of its expected behavior. A quickie
"Yeah, it powers up and plays music OK" test might not catch some
sorts of residual faults which had been caused by the earlier

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Thanks Dave and Graham,
That's a big help.I can now take it to an engineer and actually give him the
names of the troublesome components. That should help.

A couple more questions if you don't mind:

I always thought 'Switch cleaner' was the safest to use for dirty pots etc,
could you give me a name of a dedicated 'pot cleaner'?

Also, what was actually frying when the output transistors gave off the foul
stench and smoke? There are actually 4, 2 smaller ones, and 2 larger ones.It
was the smaller ones that cooked.Would it have been the transparent 'glue'
at the base, or the coating?

Thanks again

Len

"Pooh Bear" wrote in message
...
Len wrote:

Hi,

I'm hoping someone might be able to help. I stupidly tried to solve an
intermittent channel fading/distorting problem on my old beloved Nad
304. I
know nothing about circuit boards but thought it may be a dusty pot or
dry
solder joint.

I opened the case (power off!) and sprayed Servisol switch cleaner in
the
pots.

Very bad idea. It's not 'pot cleaner' and doing this usually just results
in the
*needed* lubrication in the pot being 'washed out'.


There are four plastic 'screw adjusters' on the board(named vr302 and
vr303). I also sprayed these and screwed them open and closed a few
times to
ensure fluid starts cleaning.I think this has caused the problem.

You bet - they are 'presets' that are adjusted at manufacture to set up
bias
current in the output stage typically.

By 'opening and closing them' you ruined the manufacturers setting.


I then left the amp for a couple of days to 'dry out'. I then connected
up
and switched on. Everything seemed perfect, no more crackling/fading.
But
after about 1 minute, I noticed that 2 larger components(covered in
plastic?), were cooking, and stinking!

The output transistors.

I think it was caused by adjusting
the plastic screw adjusters, whatever they are, and not turning them
back to
their original position.

Very likely.

I quickly turned off, and re-tuned these plastic screws to the position
I
thought they were in originally. Again I turned on and again, the amp
was
sounding great.No cooking components. I thought I had solved it, but
after a
few hours use, 2 fuses blew on the circuit board.

Goes to show you didn't remember right.


Obviously something still isn't quite right.

Uhuh.

Can anybody tell me what the 4 white plastic screw adjusters are, and
how
would I ensure the correct positioning of them?

They are presets that you shouldn't touch, VR means variable resistor.

You'll need a service manual and test equipment to get them back in the
correct
position.

Also the plastic covering on
the larger components that overheated, is slightly but noticeably
'melted'.
What are these and should/can they be replaced?

Probably just insulating covers.

They can probably be replaced if they are a common type.

If I took the amp to be repaired, and gave them this information, could
they
diagnose the problem without long, expensive testing?

Yes.

You'll give them a good laugh too no doubt.


Sorry about the long and 'novice' message, but I really don't want to
lose
this amp if possible.

You're lucky it didn't fry totally.

Your original problem is most likely a bad solder joint that no amount of
Servisol would fix.


Graham

"Len" wrote in message
...
Thanks Dave and Graham,
That's a big help.I can now take it to an engineer and actually give him
the
names of the troublesome components. That should help.

A couple more questions if you don't mind:

I always thought 'Switch cleaner' was the safest to use for dirty pots
etc,
could you give me a name of a dedicated 'pot cleaner'?

Also, what was actually frying when the output transistors gave off the
foul
stench and smoke? There are actually 4, 2 smaller ones, and 2 larger
ones.It
was the smaller ones that cooked.Would it have been the transparent 'glue'
at the base, or the coating?

Thanks again

Len

That would be the plastic packaging that encases the transistor. The
transistor was getting so hot that the case it's in was becoming crispy
critters.

Len wrote:

Thanks Dave and Graham,
That's a big help.I can now take it to an engineer and actually give him the
names of the troublesome components. That should help.

A couple more questions if you don't mind:

I always thought 'Switch cleaner' was the safest to use for dirty pots etc,
could you give me a name of a dedicated 'pot cleaner'?

There is no reliable 'pot cleaner'. Any solvent based product will remove the
important lubrication grease. This also true in many switches.

If you really knew what you were doing, you could disassemble the pot and clean
the 'track' and 'wiper' individually - do you have watchmaker-like skills ? If
the track has been damaged due to years of wear, this won't help though.


Also, what was actually frying when the output transistors gave off the foul
stench and smoke? There are actually 4, 2 smaller ones, and 2 larger ones.It
was the smaller ones that cooked.Would it have been the transparent 'glue'
at the base, or the coating?

Without knowing the exact details, my best guess is that you overheated the
plastic insulating covers on the power transistors.

I don't know what 'glue' you mean - but glue would smell nasty when hot too.


Graham