Crown K2 overlap adjustment problem.

[Craig Skeet]

Hi, I just repaired a Crown K2 amplifier ( multiple issues including blown output transistors on one channel, bad caps and blown comparators on the bad channel) and am having a problem adjusting the overlap as per the service manual, but the adjustment pot on both channels doesn't seem to change the overlap on the scope. It seems Ok, maybe a tap too much, but is unaffected by the adjustment of R25 pot on both channels. Also the power draw from the mains is around 160 Watts and should be just less than 100 Watts. The pot does reduce the current draw a bit, but not much.
I'm using TP 53 attached to my scope and the metal case as ground as the service manual does not specify the correct ground to use and I can't find the specific ground for the output boards.
I turned the pots fully one way to get the lowest current draw from mains then I got the magic smoke from the main power relay after it was all working perfectly before.
Surprisingly the relay still tests good as do all the other components on the power in /soft start board.
However, I now have a major short somewhere (dim bulb does not dim on switch on)
I'm going to change the Main relay and bridge rectifier on the soft start mains input board trace the short and repair the amp again, but I'm not sure why the overlapping adjustment didn't work before.
This amp has a complex grounding system with two separate grounds.
Has anyone had experience servicing these amps?
I would be really grateful if any knows where I should ground the scope when checking the overlap.
Am I doing something stupidly wrong here?
Was I using the wrong ground reference for the scope which made the scope read wrong?
I suspect I overlapped it too much and this was not registered by the scope traceand cause a power surge.
Any help her would be greatly appreciated.

 

[tadawson]

Assuming a bipolar power supply, I would always take measurements using the center of that as a referencen and pretty much never chassis ground.
That would appear to be connector p3, pins 5, 6, 7, 17, 18, and 19.(the lines coming from the transfomer center tap, in case you have a different variant). This appears to be the reference for all supplies, and seems to only have a cap between it and chassis ground, no direct parh.

 

[Craig Skeet]

Assuming a bipolar power supply, I would always take measurements using the center of that as a referencen and pretty much never chassis ground.
That would appear to be connector p3, pins 5, 6, 7, 17, 18, and 19.(the lines coming from the transfomer center tap, in case you have a different variant). This appears to be the reference for all supplies, and seems to only have a cap between it and chassis ground, no direct parh.

Many thanks for your reply. That sounds good.
Looks like I have a short output MOSFET (MPT16N25E) on one channel. Can't find any more damage on first inspection.
It is one of the two MOSFETs that were blown when I repaired it initially, interestingly. These replacement did match the specs and withstand voltage of the original MOSFETs, so I don't think they're fakes.
Anyway, I have to wait for the new main relay to arrive, before I turn on again as I don't trust the one that smoked.
Those MOSFETs are very small for such a powerful amp with no fan.