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