Ram Audio S6000 (aka LD Systems SP 6K) problems

[KhronX]

https://khronscave.blogspot.com/2019/05/44-ram-audio-s6000-teardown-repair.html

In the link above is a bit of a "preamble" along with some internal photos of this beastie.

After what's there, the owner (a buddy and neighbour) said the built-in limiter kicked in way too early, along with considerable distortion.

Some further digging revealed that R187 (page 36 of the service manual) had failed open-circuit. Replacing that solved the too-early-limiting, which happened due to half of the channel A output section not actually working.

Trouble is that now, once the output (on BOTH channels, oddly enough) goes above around 3Vpp, the top of the sinewave breaks into some sort of high-frequency oscillation. Which is REALLY strange, if you ask me.

I'd welcome any input any of you brighter-minds-than-i might have   

Attached - a scope shot of the amp output (T7A / T8A) below the oscillation threshold

[KhronX]

And here's a scope shot (same settings, 10:1 probe, same test point) with the volume turned up just a smidge.

Following an idea from another forum, i replaced the twelve 10u/100V "bypass" caps (measuring around 8-9uF and 2-2.5 ohm ESR) on the amp board with some low-ESR Rubycon YXJ 47u/63V ones, but to no avail. Note that the main power supply output caps are also 63V units, so going down from the 100V originals shouldn't be an issue.

I'm stumped... 

[cozza]

The replacement IGBTs are not the same as the ones that came out. They do not have the inbuilt soft recovery diode, indicated by the "D" on the end of the part number.

[KhronX]

I went with what the service manual said (ie. IRG4PC50U).

In case you didn't read that section in my blog post, and weren't made suspicious about the funny-looking markings, i have my doubts the ones i took out were genuine, in the first place. 
This amplifier HAD been serviced once or twice before (but not by me).

Regardless, the power supply board has external freewheeling diodes anyway (4A 600V).

And either way, how would or could that manifest in oscillation in the amplifiers, starting from quite low levels, and only on the positive half of the signal?

The replacement IGBTs are not the same as the ones that came out. They do not have the inbuilt soft recovery diode, indicated by the "D" on the end of the part number.

[cozza]

Is the positive output rail of the power supply clean under loaded conditions?

[KhronX]

If i turn up the volume enough for the oscillation to appear on the output, there's some "fuzz" visible on the positive rail (AC-coupled scope probing), with the same "spacing" as on the output.
Which, i guess, is somewhat to be expected(?).

Is the positive output rail of the power supply clean under loaded conditions?

[cozza]

Good, that you replaced the devices with the correct parts. If the old ones were fakes, they wouldn't have lasted 5 minutes in operation under load.

It's a matter of narrowing it down I suppose.

It could be the channel just repaired is oscillating, and affecting the other channel, or power supply noise common to both channels, or a fault in the line level input processing circuitry common to both channels.

I haven't had a look at the manual yet, but I assume the output zoebel networks are intact?

[KhronX]

I ordered the new IGBT's from Mouser, after managing to poke around with my meter probes without disassembling anything, and found one was a dead short.
Once the spares arrived, i took out the old ones and discovered that interesting milling(?) pattern in that marking window, which "got my spider senses tingling"...

Before finding that failed-open resistor on channel A, i had a temporary issue on channel B, which turned out to be a wonky connection for the feedback taken from the output connector.
The weird thing is, at that point, i had no oscillation visible on channel B, despite a larger output signal level.

And yes, the zobel resistors both look fine, and measure as they should (10 ohms).

Good, that you replaced the devices with the correct parts. If the old ones were fakes, they wouldn't have lasted 5 minutes in operation under load.

It's a matter of narrowing it down I suppose.

It could be the channel just repaired is oscillating, and affecting the other channel, or power supply noise common to both channels, or a fault in the line level input processing circuitry common to both channels.

I haven't had a look at the manual yet, but I assume the output zoebel networks are intact?