Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?

Both channels seem to match ok before this happens.

--
*I almost had a psychic girlfriend but she left me before we met *

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

"Dave Plowman (News)" wrote in message
...
Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?

Both channels seem to match ok before this happens.

**First off: The "Mains hum" is not mains hum. It is 100Hz hum. It is the
sound made by an amplifier when so much current is consumed from the power
supply, that ripple appears on the outputs.

Second off: You need to ascertain if the output devices get hot with no load
connected. If they still get hot, then look for a fault in the bias section,
or further down the chain. Something is causing the output devices to
conduct too much current.

If the output devices run cool when no load is connected, then it is likely
you have a large DC Voltage at the output. If so, the fault could lie
anywhere in the amp.

Since the fault is 'triggered' by some external issue, it's gonna be tricky.

If you have a link to the schematic, it might be helpful.


--
Trevor Wilson
www.rageaudio.com.au

Trevor Wilson wrote:

Got an 'interesting' fault on an ancient Linsey Hood amp.
Switch on a
light etc, and sometimes *one* channel bursts into loud
mains hum - but
carries on working. And the output transistors get very
hot. It's not
normally susceptible to mains born interference - so any
guesses before I
rip it apart?

Both channels seem to match ok before this happens.

**First off: The "Mains hum" is not mains hum. It is 100Hz
hum. It is the sound made by an amplifier when so much
current is consumed from the power supply, that ripple
appears on the outputs.

Is it usual...or was it...to have two separate power
supplies in an SS amp? Note the hum is only on one channel.

Second off: You need to ascertain if the output devices
get hot with no load connected. If they still get hot,
then look for a fault in the bias section, or further down
the chain. Something is causing the output devices to
conduct too much current.

Oscillation, perhaps?

If the output devices run cool when no load is connected,
then it is likely you have a large DC Voltage at the
output. If so, the fault could lie anywhere in the amp.

Since the fault is 'triggered' by some external issue,
it's gonna be tricky.

Except the triggering might be a good clue?

If you have a link to the schematic, it might be helpful.

If the problem could be oscillation, what kind of fault
could narrow the stability margin such that it can be set
off by a spike on the mains?

The schematic of an early JLH amp should be easy to find if
the model is known. Were they commercially produced, or is
it home made? "Improvements" are probably common.

Ian

"Dave Plowman (News)"

Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?


** Pretty much guaranteed to be supersonic oscillation of the output stage -
easy to see on any CRO or " scope " as the Yanks insist on calling them.

Lotsa possible causes, but I would start with a careful examination of the
zobel (ie RC) network at the speaker output part of the circuit. The cap
and or resistor may be open circuit or have bad soldering.



..... Phil

"Ian Iveson" wrote in message
...
Trevor Wilson wrote:

Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before
I
rip it apart?

Both channels seem to match ok before this happens.

**First off: The "Mains hum" is not mains hum. It is 100Hz hum. It is the
sound made by an amplifier when so much current is consumed from the
power supply, that ripple appears on the outputs.

Is it usual...or was it...to have two separate power supplies in an SS
amp? Note the hum is only on one channel.

**Oops. Missed that one. An important point.


Second off: You need to ascertain if the output devices get hot with no
load connected. If they still get hot, then look for a fault in the bias
section, or further down the chain. Something is causing the output
devices to conduct too much current.

Oscillation, perhaps?

**Indeed. Good chance.


If the output devices run cool when no load is connected, then it is
likely you have a large DC Voltage at the output. If so, the fault could
lie anywhere in the amp.

Since the fault is 'triggered' by some external issue, it's gonna be
tricky.

Except the triggering might be a good clue?

If you have a link to the schematic, it might be helpful.

If the problem could be oscillation, what kind of fault could narrow the
stability margin such that it can be set off by a spike on the mains?

The schematic of an early JLH amp should be easy to find if the model is
known. Were they commercially produced, or is it home made? "Improvements"
are probably common.


**All good points. The oscillation one is a good candidate. As Phil has
stated, a CRO will likely be required.


--
Trevor Wilson
www.rageaudio.com.au

"Ian Iveson" wrote in message
...
Trevor Wilson wrote:


The schematic of an early JLH amp should be easy to find if the model is
known. Were they commercially produced, or is it home made? "Improvements"
are probably common.


IMO "JLH" amps were generally home built (though often from kits).

David.

Dave Plowman (News) wrote:
Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?

Both channels seem to match ok before this happens.

Which JLH amp?
His book on electronics is still available in many public libraries
with details of the 75 watt class AB and the 15? watt class A amps,
and of course the internet has plenty of circuit diagrams.
The 75w amp needs regular servicing to adjust the bias current and output offset voltage,
and that unbiased electrolytic at the bottom of the feedback ladder could be a problem.

Other than that, I have no idea. You could try a Russ Andrews mains cable. :-)

--
Eiron.

On Tue, 19 May 2009 00:50:13 +0100, "Dave Plowman (News)"
wrote:

Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?

Both channels seem to match ok before this happens.
Can't remember what the biasing arrangement is, but if the quiescent
current is set with a preset variable resistor, in some circuit
arrangements if the preset goes O.C. the quiescent current goes up
resulting in very hot O.P. transistors and a lot of ripple on the
P.S.rails.

Bill

In article , Dave Plowman (News)
wrote:
Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before
I rip it apart?

Both channels seem to match ok before this happens.

My immediate suspiscion is as others have commented. That the channel may
be oscillating at HF. That can explain the 'sometimes', etc.

Check the bias (when not showing the problem), dressing of the wiring loom,
output zobel, etc.

Slainte,

Jim

--
Change 'noise' to 'jcgl' 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 , Trevor Wilson
scribeth thus

"Dave Plowman (News)" wrote in message
...
Got an 'interesting' fault on an ancient Linsey Hood amp. Switch on a
light etc, and sometimes *one* channel bursts into loud mains hum - but
carries on working. And the output transistors get very hot. It's not
normally susceptible to mains born interference - so any guesses before I
rip it apart?

Both channels seem to match ok before this happens.

**First off: The "Mains hum" is not mains hum. It is 100Hz hum. It is the
sound made by an amplifier when so much current is consumed from the power
supply, that ripple appears on the outputs.

Second off: You need to ascertain if the output devices get hot with no load
connected. If they still get hot, then look for a fault in the bias section,
or further down the chain. Something is causing the output devices to
conduct too much current.

If the output devices run cool when no load is connected, then it is likely
you have a large DC Voltage at the output. If so, the fault could lie
anywhere in the amp.

Since the fault is 'triggered' by some external issue, it's gonna be tricky.

If you have a link to the schematic, it might be helpful.



Hang a scope on it .. I bet its 'oscillatin at RF somewhere..
--
Tony Sayer