Had a quick look today. There are two stereo effects amps which appear the
same - both with accessible connectors to their PCBs. Unplug the one
marked rear and the amp stays on. Plug it back in and unplug the other and
it trips.

--
*Letting a cat out of the bag is easier than putting it back in *

Dave Plowman London SW
To e-mail, change noise into sound.

In article , Dave Plowman (News)
wrote:
Had a quick look today. There are two stereo effects amps which appear
the same - both with accessible connectors to their PCBs. Unplug the one
marked rear and the amp stays on. Plug it back in and unplug the other
and it trips.

Well that may offer two easy 'solutions'.

One is to simply use it with the duff board out of circuit. :-)

The other is to compare voltages, etc, between the two to help find the
fault - if you can keep the thing working for long enough without the
protection making this impossible! :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article ,
Jim Lesurf wrote:
In article , Dave Plowman (News)
wrote:
Had a quick look today. There are two stereo effects amps which appear
the same - both with accessible connectors to their PCBs. Unplug the
one marked rear and the amp stays on. Plug it back in and unplug the
other and it trips.

Well that may offer two easy 'solutions'.

One is to simply use it with the duff board out of circuit. :-)

That is one way. ;-)

The other is to compare voltages, etc, between the two to help find the
fault - if you can keep the thing working for long enough without the
protection making this impossible! :-)

I've downloaded a PDF of the service manual for $8. The offending power
amps are strange (to me) in that they have 3 HT rails. +/- 35v for most
and +60v for the output pair.

But at least no surface mount stuff and common components so I'll have a
go at fixing it.

Slainte,

Jim

--
*If you're not part of the solution, you're part of the precipitate *

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 17 Nov 2009 14:51:30 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Jim Lesurf wrote:
In article , Dave Plowman (News)
wrote:
Had a quick look today. There are two stereo effects amps which appear
the same - both with accessible connectors to their PCBs. Unplug the
one marked rear and the amp stays on. Plug it back in and unplug the
other and it trips.

Well that may offer two easy 'solutions'.

One is to simply use it with the duff board out of circuit. :-)

That is one way. ;-)

The other is to compare voltages, etc, between the two to help find the
fault - if you can keep the thing working for long enough without the
protection making this impossible! :-)

I've downloaded a PDF of the service manual for $8. The offending power
amps are strange (to me) in that they have 3 HT rails. +/- 35v for most
and +60v for the output pair.


Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets added
on when the power requirement is higher.

If you want to post the schematic somewhere I dare say that some (at
least) of us here can help you fault-find.

d

In article ,
Don Pearce wrote:
I've downloaded a PDF of the service manual for $8. The offending power
amps are strange (to me) in that they have 3 HT rails. +/- 35v for most
and +60v for the output pair.


Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets added
on when the power requirement is higher.

Right - but if you removed the 60 volts you'd get no output?

If you want to post the schematic somewhere I dare say that some (at
least) of us here can help you fault-find.

Thanks Don. When I get stuck I'll do just that. But four identical amps
gives even me a fighting chance of finding the fault. Assuming I've found
the true cause, that is.

--
*Errors have been made. Others will be blamed.

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 17 Nov 2009 15:27:33 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Don Pearce wrote:
I've downloaded a PDF of the service manual for $8. The offending power
amps are strange (to me) in that they have 3 HT rails. +/- 35v for most
and +60v for the output pair.


Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets added
on when the power requirement is higher.

Right - but if you removed the 60 volts you'd get no output?

So the lower voltage is only for the earlier stages? OK, not class H
then.

If you want to post the schematic somewhere I dare say that some (at
least) of us here can help you fault-find.

Thanks Don. When I get stuck I'll do just that. But four identical amps
gives even me a fighting chance of finding the fault. Assuming I've found
the true cause, that is.

Audio amps are real buggers because of the global feedback. A dozen
different faults can lead to the single symptom of the output being
stuck at one of the rails. Luckily you haven't got that problem so you
have a fighting chance.

d

In article , Don Pearce
wrote:
On Tue, 17 Nov 2009 15:27:33 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article , Don Pearce
wrote:
I've downloaded a PDF of the service manual for $8. The offending
power amps are strange (to me) in that they have 3 HT rails. +/- 35v
for most and +60v for the output pair.


Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets
added on when the power requirement is higher.

Right - but if you removed the 60 volts you'd get no output?

So the lower voltage is only for the earlier stages? OK, not class H
then.

Hard to guess without the diagrams, etc. But two things came to mind here.
One was that the output stage has voltage gain. e.g. bipolars with
collectors to the output and emitters to the rails. The other was FET
devices with an aburdly high resistance or minimum source-drain
requirement. However I'm also wondering... does the above mean *only* +60
for the output and not +/-60?... Curious.


Audio amps are real buggers because of the global feedback. A dozen
different faults can lead to the single symptom of the output being
stuck at one of the rails. Luckily you haven't got that problem so you
have a fighting chance.

Yes. I can recall the bafflement of people I worked with who were used to
ac coupled stages when first confronted with dc throughout and overall
feedback. 8-] The trick, of course, is to look for where the voltages on a
component or stage are clearly inconsistent and so only explainable by a
specific fault at that point. Can be a poser if you don't know the circuit
well - particular if you have 'protection' providing other influences and
interfering! Glad not to have to fix such things. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

In article ,
Jim Lesurf wrote:
Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets
added on when the power requirement is higher.

Right - but if you removed the 60 volts you'd get no output?

So the lower voltage is only for the earlier stages? OK, not class H
then.

Hard to guess without the diagrams, etc. But two things came to mind here.
One was that the output stage has voltage gain. e.g. bipolars with
collectors to the output and emitters to the rails. The other was FET
devices with an aburdly high resistance or minimum source-drain
requirement. However I'm also wondering... does the above mean *only* +60
for the output and not +/-60?... Curious.

Here's the diagram of the offending amp.
http://s139.photobucket.com/albums/q...kies/A2070.jpg

--
*If a mute swears, does his mother wash his hands with soap?

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 17 Nov 2009 19:21:10 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Jim Lesurf wrote:
Could be class H. This is a power saving scheme that uses two power
rails - one low for normal listening, and a higher one that gets
added on when the power requirement is higher.

Right - but if you removed the 60 volts you'd get no output?

So the lower voltage is only for the earlier stages? OK, not class H
then.

Hard to guess without the diagrams, etc. But two things came to mind here.
One was that the output stage has voltage gain. e.g. bipolars with
collectors to the output and emitters to the rails. The other was FET
devices with an aburdly high resistance or minimum source-drain
requirement. However I'm also wondering... does the above mean *only* +60
for the output and not +/-60?... Curious.

Here's the diagram of the offending amp.
http://s139.photobucket.com/albums/q...kies/A2070.jpg

Ah, they have followed the application note in the data sheet

http://www.datasheetcatalog.org/datasheet/nec/UPC1225H.pdf'

pretty well. They have dropped some of the good stuff, though, that
could well do with going back in. The output stability choke and
resistor plus Zobel network, for example, which you will find on page
10. Anyway, there are not too many bits to check, the bases and
collectors of all those transistors will do, and compare them with a
good channel. Most likely it is the IC that has keeled over.

d

In article ,
Don Pearce wrote:
Here's the diagram of the offending amp.
http://s139.photobucket.com/albums/q...kies/A2070.jpg

Ah, they have followed the application note in the data sheet

http://www.datasheetcatalog.org/datasheet/nec/UPC1225H.pdf'

Right. ;-) They are only the surround effects amps - the main ones are
much more beefy MOSFETS.

pretty well. They have dropped some of the good stuff, though, that
could well do with going back in. The output stability choke and
resistor plus Zobel network, for example, which you will find on page
10. Anyway, there are not too many bits to check, the bases and
collectors of all those transistors will do, and compare them with a
good channel. Most likely it is the IC that has keeled over.

Wonder if they could be hooting if there's no Zobel network? That could
cause the protection to operate. I'll have to dig out the scope.

--
*Being healthy is merely the slowest possible rate at which one can die.

Dave Plowman London SW
To e-mail, change noise into sound.