JBL model 6290 power amp (oscillating)

Hi all,

does anybody have a scematic diagram for this amp? It´s a two channel power amp for pro use. It uses 5 sanken 2SA1494 and 2SC3858 output transistors per channel.

One channel is oscillating at a frequency too high for my scope to trigger :( and at about 4V peak to peak.

Input signals are amplified correctly (plus the added oscillation)

Does anybody have an idea where to look for the source of oscillation?

william
 
You can check for bypass capacitors. try right where the power leads enter the channel cards. Select one that matches the one on the card and hook it to a set of probes and then connect it in parallel with the cap you want to check. Observe polarity and make of capacitor. The A75 had a slight oscillation that was cured by a 150mf bypass on the incoming power to the card. Also if you have a working channel, try reading resistance comparisons from good to bad with the power off. Or voltage comparisons from good to bad with power on, though be careful and keep one hand in your pocket and away from ground. Good luck