Roland JC-40 - DC on outputs

[thetubefairy]

hey folks! i'm a tube amp tech, first time posting here, excited to learn some more about solid state circuits!

i've got a Roland JC-40 on the bench with DC on the outputs, just a few tenths of a volt on one channel and it still sorta works, but the other has pretty much the full rail across it. power transformer still puts out 15-0-15 unloaded, speakers both measure 4.2DCR, no popped caps or signs of overheating.

haven't done any testing yet beyond that, but a search shows that this may be a fairly common issue for this amp, so i wanted to post and ask if anyone is familiar with typical failure states for this amp and has any suggestions for what to check or upgrade.

my general plan is to get all the output transistor voltages and power supply node voltages and go from there, but i'm also not very knowledgeable about SS diagnostics beyond that. i do have the schematic from a previous post on this site. wondering if there are any general guides to SS amp diagnostics you all would recommend?

also wondering what your general approach would be to an SMD amp like this... would you likely actually get into replacing passives on the component level? or would one typically only replace power devices and e-caps, and if there's a more complex failure, is it a better idea to just get an authorized service center to order board replacements?

thanks in advance!

[DrGonz78]

Do you have a schematic for the unit? I don't think the output transistor is completely shorted but who knows? I say this as the fuse is working and powers up without blowing. Either way it doesn't have an output transformer so no need to have a speaker or load connected. Remove the load or speaker and then take voltages. Post schematic to make sense of the issue at hand. I imagine a smaller transistor could fail and the unit would work but with issues.

[Jazz P Bass]

I would not be too concerned about "a few tenths of a volt" offset.
Here is the SM if you want to dive in.

[DrGonz78]

As far as surface mount goes... my eyes hurt just looking at board layout on the schematic. I am a moron when it comes to SMD work but I somehow get it right. I even use an old vintage 10 watt pencil and always use chip qwik. I have yet to get into hot air station territory. But seriously a tiny transistor is a bit hard not to F up the pads. I find chip qwik gets the device off the board fastest. It cost money but it's a god send. Try it out on crap boards that don't matter. It's not that hard to do. Oh check out reverse tweezers for soldering in a tiny component. Takes a bit of practice.

[g1]

Verify that both sides of preamp are ok by taking the L and R line outs to another amp.
If they are both ok, I would move straight to the output IC's.  Check the supply voltages at each IC and whether signal is getting to the IC inputs.  If that is all good replace the defective output IC(s).
Finding real TDA2050L-TB5-T may be difficult.