SX-780 - help!

[ajcoates]

Hey all! I just finished a full recap of my SX-780, including an upgrade to the discrete output packs that @rcs16 came up with. I also replaced the three heatsinked power supply transistors to modern lower-heat equivalents. It sounded great and tested perfectly last night, but this morning, the DC offset is giving me headaches - the right channel is perfect, but the left keeps falling further and further negative and won’t stabilize. Any idea what could be responsible? Likely an output issue? Thanks!

 

[Colleton]

I think it can sometimes be the differential pair at the input of the amplifier board. You can make your own differential pair with a pair of ksa992 or ksc1845 transistors, depending on pnp or npn. At any rate, this is what I do when I have a problem with dc offset. Just a WAG on my part at this point. GL!

 

[ajcoates]

Hey all! Two problems - they may or may not be connected.

First off, I can hear a faint buzzing coming from the general area of the relay. This was happening prior to recapping the unit as well - the relay will occasionally take 30 seconds or so to click, while other times it’s a much shorter delay. I’ve additionally been curious whether the buzzing could be a chip or transistor oscillating, as I can’t isolate it to the relay, just the general area.

Second, upon hooking the unit up to speakers, I was disappointed to hear some nasty buzzing, along with some faint radio signal at high volumes. The buzzing does correspond to the volume level, and the audio signal seems unaffected, so I’m fairly certain it’s in the power supply.

I’m at a loss - any ideas as to how to hone in on a solution? I’m a 24-year-old and this is my first restoration project. Hoping I didn’t bite off more than I could chew! Thanks all!

 

[Ian C.]

is there an RF cage for the pre-section that's not installed?- some of mine gets a lot noisier without the cage on the pre stage/metalwork on. Also, with the above - wire routing sometimes will pick up a buzz for me.

 

[ajcoates]

Hi all! I've had a rollercoaster ride of a time restoring my SX-780 - I'm so close to being all wrapped up. Voltages look good, DC offset and bias are holding nice and steady, and the sound quality is fantastic.

The only problem now is that I'm getting a fair deal of buzz through the speakers, dependent on output volume. It's almost certainly a grounding issue, as it all but disappears when plugged into my phone via AUX. It's at its loudest without anything plugged into the inputs. What course of action would you folks suggest? Should I go through the schematic and reflow all joints along the grounding line? Does it sound like a cold joint somewhere?

I'm so frustratingly close to the finish line - just want to get a game plan for how to wrap this project up! Thanks in advance for your help!