SWR LA15 DC on output and buzzing

[Nyqusit5]

Hi all,

I have an SWR LA15 here with a severe case of "DC outputting". I have 55.6V on the main output (+ rail is 57V). After reading all previous posts on the LA15 subject and checking all active components and changing a few suspect caps, I have had no success whatsoever. So I decided to change all transistors out of desperation, as well as the bridge rectifier and D4 (the schematic is elsewhere on this forum).
The bias pot has no effect and C8 has blown up due to a too high voltage on its pins (that has already happened to someone in one previous post)
The - rail is only 46V when measured with a DMM and it looks like this on the scope:

Again, the ouptut is at 55.6V, and it also does look a lot like what you see on the scope screen.

Any help would be greatly appreciated!

[Enzo]

Scope your power rails. In fact, set your scope to DC coupling and turn down the gain so you can still see the 57 volts on screen. Are both rails smooth as silk, or is one all bumpy like that?

DO us a favor, if you know the schematic is elsewhere, grab the link and post it here, so we don't have to go find it.

[Nyqusit5]

No problem, here it is:swr_la12_la15_power_amp_sch.pdf
The picture of the scope shows the -46V rail (which should be -56V). In DC coupling mode with gain set low, the - rail is still bumpy with a slightly different shape. The +57V rail is smooth. The power amp output is as bumpy as the - rail, only the bumps are a bit more "curvy".
Also, when powering off, the +rail stays at 57V, while the minus rail goes straight to 0V (although I cannot find a short circuit anywhere).
Thanks for your help!

[Steve Conner]

What is the volts per division on the scope shot you showed? Is the flat line at the top 0V?

If so, it looks like you have some bad connection somewhere that's causing the negative supply filter cap to do nothing. Or the cap itself is dead. Looks like one of the transformer windings may be burnt or disconnected too, as you're only getting half-wave rectification, even though you replaced the bridge rectifier.

[Nyqusit5]

Yes, that is what I had in mind too when I got the amp in. So I replaced the original bridge rectifier with a new one and checked the filter cap. And the problem remains...

[Steve Conner]

Well then, check the transformer windings for continuity and check the PCB for cracked traces and cold solder joints.

[Enzo]

I don't know how you might have checked the cap, but it doesn't look like it works. Check continuity from the negative terminal of the main V- cap and the rectifiers and the V- rail to the power amp stage. And don't overlook the ground end of the cap.

[Nyqusit5]

Enzo and Steve, thank you for pointing at the obvious which should have been more obvious to me. Of course the cap was faulty
I had checked it for ESR but didn't imagine it would just cease functionning. What should have been a simple repair has taken me far too long!
Again, thank you. As far as i am concerned, i'll just go and slap myself around the head in shame.

[Steve Conner]

Good but it still concerns me that you saw a half-wave rectified waveform instead of full-wave.