i want to repair my amp, but i don't have enough technical
information. 2 fuses blew and i am trying to figure out if the are
slow or fast-blow and what arrangement the fuses actually should be in
on the board. the fuse attachments are marked F1, F2, F3 and F4. all
of the fuses are 4A 250V, but 2 have BUSS AGC on them and the other 2
have 313 on them. the ones that blew are the BUSS AGC.

1. is there a difference between these 2 sets of fuses? (slow/fast)
2. if there is a difference, which fuses should be in which spot?
(F1,F2,F3,F4)
3. is there a service manual/schematic available?
4. could someone make this info available to me?

TIA
PizMD

HUMANS: the animal that killed the planet...

PizMD,

According to the manual, all 4 fuses are slo-blo or 'T' type. F1 and 2
are 1 amp and F3 and 4 are 4 amp. F1 and F2 power the regulated
supplies that feed the preamp and power amp, where as F3 and F4 are the
unregulated fuses for the rail that supply only the power amps.

If a second set of fuses blow then you have another problem. It is not
rare for an older NAD like this to have soldering problems and 'cooked'
small electrolytic capacitors that have dried out, especially near
sources of heat like the power supply regulator transistors.

Regards,
Tim Schwartz
Bristol Electronics


PizMD wrote:

i want to repair my amp, but i don't have enough technical
information. 2 fuses blew and i am trying to figure out if the are
slow or fast-blow and what arrangement the fuses actually should be in
on the board. the fuse attachments are marked F1, F2, F3 and F4. all
of the fuses are 4A 250V, but 2 have BUSS AGC on them and the other 2
have 313 on them. the ones that blew are the BUSS AGC.

1. is there a difference between these 2 sets of fuses? (slow/fast)
2. if there is a difference, which fuses should be in which spot?
(F1,F2,F3,F4)
3. is there a service manual/schematic available?
4. could someone make this info available to me?

TIA
PizMD

HUMANS: the animal that killed the planet...

Tim,

thanks a mill. based on what you have indicated, are the 1A fuses
250V as well? why would there be 4A fuses in F1 and F2?

the fuses blew because i did something foolish and had my otherwise
wonderful device hooked up to a large PA speaker. i do not know what
rating of the speaker was, but i had no business doing it. it
actually held up quite well under the circumstances. when i put the
amp back in my office/studio, i noticed an obvious degradation in
sound. i decided to fix it myself this time instead of putting out
$85 as i did the last time it went on the blink.

although i have a copy of the original user manual, i did not realize
until researching for this repair that this amp is only rated at 30W.
i have no idea how i missed that important fact. i have done a few
thing with this amp that should have killed it long ago. i have had
it for going on 10 years and it was used when i got it.

anyways, any further assistance you can offer is greatly appreciated.

TIA
On Mon, 19 Jul 2004 12:08:38 GMT, Tim Schwartz
wrote:

PizMD,

According to the manual, all 4 fuses are slo-blo or 'T' type. F1 and 2
are 1 amp and F3 and 4 are 4 amp. F1 and F2 power the regulated
supplies that feed the preamp and power amp, where as F3 and F4 are the
unregulated fuses for the rail that supply only the power amps.

If a second set of fuses blow then you have another problem. It is not
rare for an older NAD like this to have soldering problems and 'cooked'
small electrolytic capacitors that have dried out, especially near
sources of heat like the power supply regulator transistors.

Regards,
Tim Schwartz
Bristol Electronics


PizMD wrote:

i want to repair my amp, but i don't have enough technical
information. 2 fuses blew and i am trying to figure out if the are
slow or fast-blow and what arrangement the fuses actually should be in
on the board. the fuse attachments are marked F1, F2, F3 and F4. all
of the fuses are 4A 250V, but 2 have BUSS AGC on them and the other 2
have 313 on them. the ones that blew are the BUSS AGC.

1. is there a difference between these 2 sets of fuses? (slow/fast)
2. if there is a difference, which fuses should be in which spot?
(F1,F2,F3,F4)
3. is there a service manual/schematic available?
4. could someone make this info available to me?

TIA
PizMD

HUMANS: the animal that killed the planet...

PizMD

HUMANS: the animal that killed the planet...

PizMD,

First, please use a valid e-mail address in you postings.

The info that I gave you about F1 and F2 being 1A fuses is from the
service manual. It is possible that the fuses in you amp were changed
at some point with the wrong fuses. Less likely is that the factory
changed the spec, or an error was made during manufacture.

The voltage rating on fuses is only a rating to stop arcing in the case
of a blown fuse, so it is always safe to use a 250V fuse in place of a
125V fuse, but not the other way around. on the assumption that you
live in a 125V line voltage area, 125V fuses would be fine, as would
250V ones.

Regards,
Tim Schwartz
Bristol Electronics


PizMD wrote:

Tim,

thanks a mill. based on what you have indicated, are the 1A fuses
250V as well? why would there be 4A fuses in F1 and F2?

the fuses blew because i did something foolish and had my otherwise
wonderful device hooked up to a large PA speaker. i do not know what
rating of the speaker was, but i had no business doing it. it
actually held up quite well under the circumstances. when i put the
amp back in my office/studio, i noticed an obvious degradation in
sound. i decided to fix it myself this time instead of putting out
$85 as i did the last time it went on the blink.

although i have a copy of the original user manual, i did not realize
until researching for this repair that this amp is only rated at 30W.
i have no idea how i missed that important fact. i have done a few
thing with this amp that should have killed it long ago. i have had
it for going on 10 years and it was used when i got it.

anyways, any further assistance you can offer is greatly appreciated.

TIA
On Mon, 19 Jul 2004 12:08:38 GMT, Tim Schwartz
wrote:

PizMD,

According to the manual, all 4 fuses are slo-blo or 'T' type. F1 and 2
are 1 amp and F3 and 4 are 4 amp. F1 and F2 power the regulated
supplies that feed the preamp and power amp, where as F3 and F4 are the
unregulated fuses for the rail that supply only the power amps.

If a second set of fuses blow then you have another problem. It is not
rare for an older NAD like this to have soldering problems and 'cooked'
small electrolytic capacitors that have dried out, especially near
sources of heat like the power supply regulator transistors.

Regards,
Tim Schwartz
Bristol Electronics


PizMD wrote:

i want to repair my amp, but i don't have enough technical
information. 2 fuses blew and i am trying to figure out if the are
slow or fast-blow and what arrangement the fuses actually should be in
on the board. the fuse attachments are marked F1, F2, F3 and F4. all
of the fuses are 4A 250V, but 2 have BUSS AGC on them and the other 2
have 313 on them. the ones that blew are the BUSS AGC.

1. is there a difference between these 2 sets of fuses? (slow/fast)
2. if there is a difference, which fuses should be in which spot?
(F1,F2,F3,F4)
3. is there a service manual/schematic available?
4. could someone make this info available to me?

TIA
PizMD

HUMANS: the animal that killed the planet...

PizMD

HUMANS: the animal that killed the planet...

Tim,

thanks again. i don't use my real email for privacy/security reasons.
i will email you assuming that your email is a valid one.

now i can make my trip to Radio Shack...


On Tue, 20 Jul 2004 12:04:12 GMT, Tim Schwartz
wrote:

PizMD,

First, please use a valid e-mail address in you postings.

The info that I gave you about F1 and F2 being 1A fuses is from the
service manual. It is possible that the fuses in you amp were changed
at some point with the wrong fuses. Less likely is that the factory
changed the spec, or an error was made during manufacture.

The voltage rating on fuses is only a rating to stop arcing in the case
of a blown fuse, so it is always safe to use a 250V fuse in place of a
125V fuse, but not the other way around. on the assumption that you
live in a 125V line voltage area, 125V fuses would be fine, as would
250V ones.

Regards,
Tim Schwartz
Bristol Electronics


PizMD wrote:

Tim,

thanks a mill. based on what you have indicated, are the 1A fuses
250V as well? why would there be 4A fuses in F1 and F2?

the fuses blew because i did something foolish and had my otherwise
wonderful device hooked up to a large PA speaker. i do not know what
rating of the speaker was, but i had no business doing it. it
actually held up quite well under the circumstances. when i put the
amp back in my office/studio, i noticed an obvious degradation in
sound. i decided to fix it myself this time instead of putting out
$85 as i did the last time it went on the blink.

although i have a copy of the original user manual, i did not realize
until researching for this repair that this amp is only rated at 30W.
i have no idea how i missed that important fact. i have done a few
thing with this amp that should have killed it long ago. i have had
it for going on 10 years and it was used when i got it.

anyways, any further assistance you can offer is greatly appreciated.

TIA
On Mon, 19 Jul 2004 12:08:38 GMT, Tim Schwartz
wrote:

PizMD,

According to the manual, all 4 fuses are slo-blo or 'T' type. F1 and 2
are 1 amp and F3 and 4 are 4 amp. F1 and F2 power the regulated
supplies that feed the preamp and power amp, where as F3 and F4 are the
unregulated fuses for the rail that supply only the power amps.

If a second set of fuses blow then you have another problem. It is not
rare for an older NAD like this to have soldering problems and 'cooked'
small electrolytic capacitors that have dried out, especially near
sources of heat like the power supply regulator transistors.

Regards,
Tim Schwartz
Bristol Electronics


PizMD wrote:

i want to repair my amp, but i don't have enough technical
information. 2 fuses blew and i am trying to figure out if the are
slow or fast-blow and what arrangement the fuses actually should be in
on the board. the fuse attachments are marked F1, F2, F3 and F4. all
of the fuses are 4A 250V, but 2 have BUSS AGC on them and the other 2
have 313 on them. the ones that blew are the BUSS AGC.

1. is there a difference between these 2 sets of fuses? (slow/fast)
2. if there is a difference, which fuses should be in which spot?
(F1,F2,F3,F4)
3. is there a service manual/schematic available?
4. could someone make this info available to me?

TIA
PizMD

HUMANS: the animal that killed the planet...

PizMD

HUMANS: the animal that killed the planet...

PizMD

HUMANS: the animal that killed the planet...

[Pabs]

Hi Tim and anyone else reading this!

My name is Paul and I own a Nad 3130 amplifier. I have a problem with it that I am trying to rectify, and the above information sounds very intrieging.

I picked up my Nad 3130 second hand as faulty, in the attempt to get it fixed. I cleaned everything up (very dirty) and it now looks brand new
However, it still doesnt work properly.

I get a sound out of it (both channels) but it is VERY quiet and VERY distorted. My first thoughts were the output transistors.

However, after reading the above, I am a bit confused. My amp has 4 fuse sockets, but only 2 fuses are in the amp, F1 and F2. F3 and F4 are missing.

Do i need all 4 fuses? I actually thought at first I only need 2 for the 240v system we have in the UK - the others were for the USA. But reading the above makes me think I need all 4.

(I am a bit of a novice so please excuse me if I am being thick here!)

Anyway, I need a circuit diagram, service manual and or owners manual. Everyone I ask is in the US and not willing to ship to the UK.

Can anyone help me?
My email address is . Please help as I would love to get this amp working again!
Cheers
Paul

[Pabs]

Oh, and thanks in advance to anyone who can help!
Even if it is just a link!

Pabs wrote:
Hi Tim and anyone else reading this!

My name is Paul and I own a Nad 3130 amplifier. I have a problem with
it that I am trying to rectify, and the above information sounds very
intrieging.

I picked up my Nad 3130 second hand as faulty, in the attempt to get it
fixed. I cleaned everything up (very dirty) and it now looks brand new

However, it still doesnt work properly.

I get a sound out of it (both channels) but it is VERY quiet and VERY
distorted. My first thoughts were the output transistors.

However, after reading the above, I am a bit confused. My amp has 4
fuse sockets, but only 2 fuses are in the amp, F1 and F2. F3 and F4 are
missing.

Do i need all 4 fuses? I actually thought at first I only need 2 for
the 240v system we have in the UK - the others were for the USA. But
reading the above makes me think I need all 4.

(I am a bit of a novice so please excuse me if I am being thick here!)

Anyway, I need a circuit diagram, service manual and or owners manual.
Everyone I ask is in the US and not willing to ship to the UK.

Can anyone help me?
My email address is . Please help as I would
love to get this amp working again!
Cheers
Paul



Paul,

All 4 fuses are required. F1 and F2 are 1 amp slo blo type and are for
the low level supplies. F3 and F4 are 4 amp slo blo and run the power
amps. If the fuses blow at turn on, you have a problem of a load or
short, do not feed the unit fuses, find the problem.

Regards,
Tim Schwartz
Bristol Electronics

[Pabs]

Thanks for the reply tim,

When I purchased the unit I purchased it as faulty - owner couldnt get anything to happen.
I have since managed to get some noise out (cleaned some pots and connections) but then realised 2 fuses were missing. The previous owner didnt remove them, (but then he never had it working) so I can only assume the owner before that removed them for a reason.

However, seeing as I dont know what that reason is, I need to feed it at least one set of fuses to see what happens.

I actually found a sewing pin stick in the circuit board, in such a way that it was shorting two wire links together.... I am kind of hoping that this was the problem and 2 new fuses would now give me a working unit.
However, i doubt that is the case (I can only wish) so I need to see if the fuses blow straight away, in which case I will then fault find further. (Possibly output transistors.)

Does this make sense?

Pabs wrote:
Thanks for the reply tim,

When I purchased the unit I purchased it as faulty - owner couldnt get
anything to happen.
I have since managed to get some noise out (cleaned some pots and
connections) but then realised 2 fuses were missing. The previous owner
didnt remove them, (but then he never had it working) so I can only
assume the owner before that removed them for a reason.

However, seeing as I dont know what that reason is, I need to feed it
at least one set of fuses to see what happens.

I actually found a sewing pin stick in the circuit board, in such a way
that it was shorting two wire links together.... I am kind of hoping
that this was the problem and 2 new fuses would now give me a working
unit.
However, i doubt that is the case (I can only wish) so I need to see if
the fuses blow straight away, in which case I will then fault find
further. (Possibly output transistors.)

Does this make sense?


Yes, try one set of fuses. If it is the 4A pair, you could use 2 amp
slo blo instead, as they should be OK with no speakers connected, or at
low volume. This might prevent further damage if there is a problem. If
it then comes up, measure the DC voltage at the speaker terminals. It
should be less than 0.1VDC.

--Tim

[Pabs]

Thanks tim

I have had problems getting some slo-blo fuses, but the 4A's should be on their way to me now. Will arrive soon I hope.

So I will put the fuses in and switch on - if they don't blow then I will check the voltages across the speaker terminals. If it is 0.1VDC then i will get some speakers plugged in and test. If it isnt then obviously there is a problem.

I can get new output transistors quite easily, but am not sure on how to set the bias on them - I have found the two potentiometers but without a circuit diagram, service manual or even user manual its impossible to guess.

Do you know where i might obtain a circuit diagram? I have tried NAD, their distributor in the UK and ebay etc, but nobody has them.. I dont really want to spend £15+ on a service manual if the amp isnt worth that much money - trying to get an amp on a budget, and this was only really for messing around with to see what I could do!

In article ,
Tim Schwartz wrote:
Pabs wrote:
Thanks for the reply tim,

When I purchased the unit I purchased it as faulty - owner couldnt get

Yes, try one set of fuses. If it is the 4A pair, you could use 2 amp
slo blo instead, as they should be OK with no speakers connected, or at
low volume. This might prevent further damage if there is a problem. If
it then comes up, measure the DC voltage at the speaker terminals. It
should be less than 0.1VDC.

An old trick is to wire a light bulb - low power, 20 - 40 watts @110VAC - in
series with the amplifier. Many shops have such a box used on the bench; I
owned a Hickock tube tester - "Cardamatic" - which had a bulb inside the unit
which would light if there was dead short in the tube under test.

If the unit has a serious problem the light will start to glow; if it glows
as if directly connected then the amp is a dead short.

Another method is to use a Variac/Powerstat to start with very low input AC
and then bring it up slowly with a voltmeter watching the power supply
voltage.

Mike Squires
--

Mike Squires (mikes at cs.indiana.edu) 317 233 9456 (w) 812 333 6564 (h)
mikes at siralan.org 546 N Park Ridge Rd., Bloomington, IN 47408

[Pabs]

ok thanks for the help guys.

Hoping that the fuses will arrive soon, and then I shall pop a couple in, and see what happens.

I really should source a circuit diagram, but I want to try the fuses first.

[Pabs]

Hi guys

Right, fuses arrived. Put both 4A fuses in, and just plugged the unit into the mains power supply. No speakers, no input.

When I switch the unit on, I got a very large spark and pop sound coming from the rear of the amp circuit, looks like its was on the soft clipping part of the board or close to the top output transistors.

I inspected the fuses and neither looks damaged. I cannot see any damage to the area.

I dont have a multi-meter nearby to check anything out, which is a shame.

What I wanted to try is to only put in 1 of the fuses and see if the other channel (front of the amp) works fine, but I dont know which fuse (F3/F4) powers which amp. Without a circuit diagram I cant really tell.

Does anyone have any ideas? I dont want to put fuses back in and take something else out.Cheers all

Pabs wrote:


Hi guys

Right, fuses arrived. Put both 4A fuses in, and just plugged the unit
into the mains power supply. No speakers, no input.

When I switch the unit on, I got a very large spark and pop sound
coming from the rear of the amp circuit, looks like its was on the
soft
clipping part of the board or close to the top output transistors.

I inspected the fuses and neither looks damaged. I cannot see any
damage to the area.

I dont have a multi-meter nearby to check anything out, which is a
shame.

What I wanted to try is to only put in 1 of the fuses and see if the
other channel (front of the amp) works fine, but I dont know which
fuse
(F3/F4) powers which amp. Without a circuit diagram I cant really
tell.

Does anyone have any ideas? I dont want to put fuses back in and take
something else out.Cheers all



You need to stop before you completely ruin that amp. (what's left of
it) On the outside chance you do see "where the smoke is coming from"
how do you expect to repair it without any test equipment?

In article ,
Pabs wrote:

Hi guys

Right, fuses arrived. Put both 4A fuses in, and just plugged the unit
into the mains power supply. No speakers, no input.

When I switch the unit on, I got a very large spark and pop sound
coming from the rear of the amp circuit, looks like its was on the soft
clipping part of the board or close to the top output transistors.

I inspected the fuses and neither looks damaged. I cannot see any
damage to the area.

I had a problem in a NAD receiver which I purchased used, some years
ago. Sound on one channel was intermittent.

When I tapped the PC board, I saw some sparking from one area.

It turned out to be an artifact of the then- (and perhaps still-
popular) "Japanese" style of construction, in which many small
components such as resistors and diodes are mounted vertically, with
one very-short lead soldered right into the PCB and the other bend
down alongside the component and soldered into an adjacent hole in the
PCB.

A couple of these vertically-mounted resistors had been bent or leaned
over, and the bare-wire leads were making intermittent contact and
creating a short circuit.

Bent 'em both back into vertical position, the sparking stopped, the
intermittent audio outages went away, and I've had no further problems
with it.

I'd suggest checking for any such bent-over components or other
semi-mechanical faults which could cause short circuits e.g. retaining
screws or nuts out of position, too long, etc. Look for any wires,
contacts, and metal surfaces which show any signs of pitting or
burning... these might be the points of short-circuiting. Check all
wire leads to make sure that there isn't a loose strand of wire
sneaking out of a push-on or ring connector and making an improper
contact.

You should probably also check the output transistors with a DMM, and
make sure that none of them is internally shorted.

--
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!

[Pabs]

Thanks again for the replies guys.

Please dont misunderstand me, I only put the fuses in to simply test and see what would happen. I would in NO WAY keep doing this or do silly things to the amp.

How am I supposed to test the amp if I have no fuses? I had to put them in - for all I knew it could work fine! Alas this is not the case.

There wasnt any SMOKE either, just a quick pop and a flash from around that area. I did indeed note MANY bent over resistors etc and I am sure some of them looked to be touching. All look ok now. I have powered the unit back up WITHOUT the two power transistor fuses and it all still works as it did before, so pre-amp is fine.

I will NOT be plugging anything else back in until I get home with test equipment, mainly multimeter etc.

I also have the NAD 3130 Service manual now - noticed the config page and adjustments as well...

What I think is best to do is to put 1 fuse in at a time, and see if I can get 1 channel up and running. I believe the channel closer to the front of the unit (right?) may work fine as nothing seemed to go wrong there, just the upper part of the circuit.

Just strange that the fuses didnt go - going to be weird trying to plug this unit in again - if it sparks a second time i wont really know what to do, unless I can identify closely where the spark is coming from.

In article ,
Pabs wrote:

What I think is best to do is to put 1 fuse in at a time, and see if I
can get 1 channel up and running.

Every time you put a fuse into a channel which may have a problem,
you're compounding the risk of doing further damage.

Rather than using a fuse in the output-stage rails, you might try
inserting a suitably-sized power resistor... one small enough to let
the amp's specified amount of idle-stage current pass through with
only a few volts of drop, but large enough to prevent a severe
overcurrent in case something's shorted. This could let you at least
identify the failing channel (by measuring the current through /
voltage across your limiting resistor) without running too much risk
of doing further damage.

Just strange that the fuses didnt go -

Transistors are noble, honorable devices. They're renowned for their
willingness to sacrifice themselves (throwing themselves upon a short
circuit) in order to save the lives of any fuses in their neighborhoods.

Some of 'em will even commit suicide if you look at 'em sternly.
Guilty consciences, perhaps.

--
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!

Pabs wrote:
Thanks again for the replies guys.

Please dont misunderstand me, I only put the fuses in to simply test
and see what would happen. I would in NO WAY keep doing this or do
silly things to the amp.

How am I supposed to test the amp if I have no fuses? I had to put them
in - for all I knew it could work fine! Alas this is not the case.

There wasnt any SMOKE either, just a quick pop and a flash from around
that area. I did indeed note MANY bent over resistors etc and I am sure
some of them looked to be touching. All look ok now. I have powered the
unit back up WITHOUT the two power transistor fuses and it all still
works as it did before, so pre-amp is fine.

I will NOT be plugging anything else back in until I get home with test
equipment, mainly multimeter etc.

I also have the NAD 3130 Service manual now - noticed the config page
and adjustments as well...

What I think is best to do is to put 1 fuse in at a time, and see if I
can get 1 channel up and running. I believe the channel closer to the
front of the unit (right?) may work fine as nothing seemed to go wrong
there, just the upper part of the circuit.

Just strange that the fuses didnt go - going to be weird trying to plug
this unit in again - if it sparks a second time i wont really know what
to do, unless I can identify closely where the spark is coming from.


These are NOT speaker fuses they are positive and negative power supply
rail fuses, and both fuses effect both channels. DO NOT put fuses back
in the amp. Get an ohm meter and find the shorted components, likely
output transistors, and by now driver transistors, open resistors,
shorted bridge rectifier etc.

Regards,
Tim Schwartz

[Pabs]

Hi all
thanks for the replies again.

Right, took everything onboard from the posts. No fuses have gone back in. Checked the output transistors using a scope, and found the 2 on the right channel to be totally kn*ckered.

Removed them from the board, put fuses in and got the meter across the left channel outputs and made sure they were sitting at between 0v +/- 30mV, and it was fine. Adjusted slightly to be as close to 0 as possible. Fuses are now fine and don't blow.

Now seems that the left channel is working fine. Connected a signal generator to an input and a speaker to the output, and all seemed fine.

*****************
SO, down to the right channel. Discovered one of the driver transistors (q410 i think - the one that shares the heatsink with the o/p transistors) wasnt actually sat ON the heatsink, rather the screw wasnt tightened enough to mate it with the heatsink.

Checked said transistor, and compared with the believed-to-be-working one on the left channel, and it doesnt look good. Looks blown/duff.
The circuit board has a red mark over this transistor too - looks like the last owner identified a problem here. Also another identical transistor has a crack in it - probably blown by me when I initially powered on with the fuses.

****************
Right, so questions. We went down to our local "Maplins" store and picked up 2 output transistors, 2N3055 and MJ2955. Fitted them to the board, but then checked them with a scope - the 2N3055 looks TOTALLY different to the left channel version - think either we have purchased a duff one or the specs vary.
This is the question - do the o/p trans. need to be identical to originals? Ie manufactured by Motorola? The ones we purchased are made by ST. Havent turned the amp on with them in.

Finally, where can we get hold of the 2SD669A driver transistors? Nobody seems to stock them, thoughts are to go direct to NAD...

Sorry for such a long post. Hope it makes sense. Chuffed to get this far, but still a lot of work to do. Thanks in advance!

In article ,
Pabs wrote:

Right, so questions. We went down to our local "Maplins" store and
picked up 2 output transistors, 2N3055 and MJ2955. Fitted them to the
board, but then checked them with a scope - the 2N3055 looks TOTALLY
different to the left channel version - think either we have purchased
a duff one or the specs vary.
This is the question - do the o/p trans. need to be identical to
originals? Ie manufactured by Motorola? The ones we purchased are made
by ST. Havent turned the amp on with them in.

2N3055 is an extremely generic sort of part. The specs for this part
number are so loose that you can find all sorts of different NPN power
transistors labeled with this number. Reportedly, it's common
practice among semiconductor manufacturers to test the transistors
they make, grade 'em accordingly, and call a lot of the parts which
don't make the grade for higher-performance part numbers "2N3055".

Net result is that you can end up with "2N3055" parts which vary a lot
in terms of their beta and f(t). It's likely that they'll all exceed
the formal 2N3055 specs, but it's also likely that some of them will
exceed the specs by quite a lot, in one or more parameters, in ways
that aren't terribly predictable.

If you have an amp designed for relatively old, slow power
transistors, it's possible that a "2N3055" of modern design might be a
lot faster, and might thus present some stability problems under
certain conditions.

--
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!

[Pabs]

Hi

Thanks for the reply.
I now have some driver transistors on the way to replace the blown ones I have.
Shall get them in and then get the scope/meter across it all again.
Cheers

[Pabs]

Hi Guys


Right. Driver transistors replaced. O/P transistors replaced. Checked all components I could think of, all seem to be ok.

Now, when we turn the unit on, without the F3/F4 fuses in, the Soft Clipping LED turns on with the Power LED.
This didnt happen before. Is this right, or is there something else I have missed?

After checking everything over with scope etc, we put a psu in place of the fuses, and left low voltage/current input, to see what would happen. It seems that the circuit draws 0mA to start with, then builds up from 1/2A very slowly, but then draws more and more quicker and quicker, in a logrithmic kinda way......

What would be doing this? The bridge rectifier seems to be ok, and without the dead channel components in the other channel seems to work fine. So its something on 1 channel (left i think)

What could it be? Any ideas?
Cheers

I have no idea what is wrong with your amp and this is also very late in
the game but I do have a suggestion for you. If you don't want to take
the chance on frying the device completely, connect it to the mains
through a Variac (variable AC power transformer). That will permit you
to insert all fuses, and gradually increase the AC voltage to the amp
while monitoring test points inside the amp.

You won't have any problems with shorts or blown parts causing further
damage if you follow the above advice because you will catch any
abnormal conditions before sufficient power is drawn to cause trouble.

[Pabs]

Thanks for the reply.

Kinda doing that at the moment, but just cannot find what component would be drawing the current - it seems to spiral out of control - the fuses dont blow straight away - takes 5-10secs for them to go. Capacitor or something perhaps.....

No fuses now - using PSU and slowly increasing output of PSU to see what happens in the circuit...


Anyone else?

[Pabs]

Still looking at it, but cant find anything... if anyone knows please let me know.

[Pabs]

It now actually looks like its the other 1/2 of the circuit/amp thats not working - although it was the opposite channel that was blowing the fuses originally.