Adcom GFA555-II oscillation (long & technical) - advise needed - tlenthe

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Adcom GFA555-II oscillation (long & technical) - advise needed

Hello! I've moved this query over here from the tweaks section as it gets pretty technical.
Background - I've successfully repaired my blown driver and pre-driver transistors on one channel and everything is back to normal (see an earlier post here on that...) Then I made up a long planned pair of CAT5 speaker cables (Gomer twist "recipe") which are now causing the amp go to into oscillation (worse on the repaired side which has higher frequency ON Semi MJ1500x drivers). I first noticed that the distortion indicator lights were slightly lit, a distinct lack of bass and a little power line "buzzing" through the speakers. I now deduce that the oscillation was significantly straining the power supply resulting in high amount of ripple/switching noise bleeding through the amp as well as reducing the available power/current for bass (4 ohm KEF-104 speakers).
Problem - Adding outboard Zobel networks doesn't help (resorting to the service manual, the schematic shows that there is an internal R/C network on the output.)
Breaking out a 'scope, I see that, even with the original, well behaved cables, both channels are showing a very low level (about 10mV p-p) oscillation at about 20Mhz (50ns cycle). It's much worse (larger amplitude) with the CAT-5 cables.
Has anyone else observed this type of response? Will adding a shunt cap in the feedback help solve this? The feedback network is 22K from the output to the inverted side of the input stage, followed with a 1K to ground. I don't see any obvious gain reducing caps in the schematic. Can/should I shunt the 22K with a cap to reduce the gain to unity above some frequency -OR- is there an alternative or better solution?
I was also curious to notice significant spikes (also on the amp output terminals) of high frequency noise at the 60/120Hz input power rate which I assume to be noise from the diode bridge switching. This noise had a similar 20Mhz frequency, but seemed to damp out fairly quickly (5-10 cycles) I have future plans to replace the conventional bridge with HEXFREDs, but I'm hoping to resolve this problem first. Is this likely to the fundamental source or just an aggravating contributor?
Thanks for your thoughts and suggestions! - Ted

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Topic - Adcom GFA555-II oscillation (long & technical) - advise needed - tlenthe 08:12:48 03/27/05 (10)

Solved!! (Adcom GFA555-II oscillation) - tlenthe 16:13:29 03/29/05 (1)

Hmmmm - chris_w 16:22:45 03/30/05 (0)

Fusible resistor blown? (long) - Charlie S in Maryland 16:00:00 03/29/05 (0)

More observations and another theory? - tlenthe 05:20:04 03/28/05 (0)

Why are you bothering? - Bare 18:16:29 03/27/05 (2)

:Why are you bothering... - Neil49 19:34:00 03/27/05 (1)

Re: :Why are you bothering...Theamp must be in the shop and have nothing better to do than putting down other inmate's - dimitri_P 23:53:06 03/27/05 (0)

My guess of what's going on - andy_c 12:13:28 03/27/05 (2)

Re: Description of circuit topology (Adcom GFA555-II) - tlenthe 16:48:15 03/27/05 (1)

Re: Description of circuit topology (Adcom GFA555-II) - andy_c 17:59:27 03/27/05 (0)

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