Hi all,
I have a GK800rb (schematic/service manual
here, preamp board rev E, power amp and power supply board are rev C) which has obviously seen heavy use. It is currently damaged on the 300w side and I'm having trouble pinpointing the exact problem.
Symptoms:
- DBT (75w bulb, no load attached) glows as I turn up the mains with a variac
- DBT does NOT glow if I disconnect the +/- 85VDC wires from the power supply board (all power supply rails are correct)
- DBT also does not glow if I have the +/- 85VDC wires from the PS board connected and I disconnect the +/- 15VDC P2 header from the power amp board
What I've tried:
- Replaced the obviously burnt components on the power supply and power amp board (most of the output transistors and some of the A06/A56s, a few resistors)
- Rechecked the output transistors for shorts/opens
I would like to probe voltages on the 300W section, but I don't want to leave the amp powered up with a bright bulb. My next thought was to replace U1 (even though the 100W side of U1 seems to be ok). Is there a way to check this JFET in circuit? Any other tests I can do without powering up the 300W side?
Thanks for any and all help!
