Gallien-Krueger 800RB amp, 1987

[N_Cook]

Excessive hiss above 1/3 vol or so. Change U1 first opamp at input,
LF353, to something low noise or some other known cause. When trying it
out, the hiss peaked for a while, then dropped back, not returning on
twizzling. Not tried heating/cooling yet

 

[Sjouke Burry]

On 09.02.15 13:41, N_Cook wrote:

Excessive hiss above 1/3 vol or so. Change U1 first opamp at input,
LF353, to something low noise or some other known cause. When trying it
out, the hiss peaked for a while, then dropped back, not returning on
twizzling. Not tried heating/cooling yet

Get a scope and check for oscillation.
Your symptoms describe just what you heard/saw.

 

[N_Cook]

On 09/02/2015 22:27, Sjouke Burry wrote:

On 09.02.15 13:41, N_Cook wrote:
Excessive hiss above 1/3 vol or so. Change U1 first opamp at input,
LF353, to something low noise or some other known cause. When trying it
out, the hiss peaked for a while, then dropped back, not returning on
twizzling. Not tried heating/cooling yet

Get a scope and check for oscillation.
Your symptoms describe just what you heard/saw.

Nasty short bursts of high power did not show any spurious ringing or
unwanted oscillation of the Hi-amp anyway. Crude visual pk-pk of noise
on scope was about 80mV on 8R load on hi-amp output with shorted input
and main volume at max and Hi-Master volume at max, other controls at
mid. Warming U1 ic,localised, to about 80-100 deg C made the noise level
jump to about 200mV . Squirting freezer spray could make a minimum of
50mV but would then jump up and down abruptly between about 80 and
150mV, between discrete levels, which is probably what was most
intrusive in use proper, the hiss varying in level.
Localised heating of the other ICs nothing observable on scope. So out
with U1.
What did we use before freezer spray?, I suppose alcohol on a cotton
ball and blowing breath through a straw.