Gallien Krueger 800rb repair - No power

[beedoola]

Removing the 100w amp power rails removes the light bulb short issue so it's on that part of the circuit.

Do I need to bias each amp section with the amp unloaded - no load.

[beedoola]

Can someone shed some light on this.

The issue seems to be on the 100w amp side - when I disconnect the power rails for the 60+/- that stops that component shorting. But on the schematic it says to adjust R-38 for bias but R-38 on the schematic seems to be associated with the 300w amp, and R14 with the 100w amp despite the schematic seeming to indicate otherwise.

But with the 300w still hooked up I don't get any component burning issues on R-38/Q-14 which is what happens when the 60+/- is hooked up.

I'm sure the power rails from the power supply board going to the transistor board are correct.


Not sure why the photos uploaded displaying horizontally - sorry!

[shortcircuit]

Just for simplicity

[beedoola]

Anyone?

[g1]

Can you post some pictures of the amp board and wiring?

[beedoola]

[g1]

These are current pictures? I'm seeing burnt and incorrect parts on the power amp board.
Have you checked that the MJE15030 & 31's (Q9,Q7,Q2) tabs are isolated from the heatsink?

[g1]

pwr amp board layout: https://music-electronics-forum.com/...4&d=1571939847

[beedoola]

The burned components have been replaced - I haven't fired the amp up. I also replaced Q-13 with a new A06 in case that was the issue.

the tabs on Q7, q9 and q2 are all isolated from the heatsink. I did a continuity test on them all.

[g1]

All those 5W resistors on the heatsink need to be 0.33R, not 5R. You will not be able to get the bias right unless they are all .33ohm.
Check for any low resistance across the 4 supply rails. (you can check across the filter caps)

[beedoola]

Good catch!

I replaced one and based it off R27 or R26/25 which I see are - as the schematic shows - .33r not 5R.

Thanks. I'll report back once I change them.

[beedoola]

I put the .33R resistors in there and still getting a short on the light bulb limiter.

[g1]

You have no load on the output but bulb is lit?
With power off and HI amp still connected to supply, do resistance checks across the filter caps for the + and - 60V rails.

[beedoola]

C7/c8: 1.9m

c5/c6: 3.1m

[g1]

No obvious shorts there. Also check resistance across speaker jacks for shorts.
I'll ask again, when the bulb lights, you have no load connected?