"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