Sansui 5000a DC issues

[RuairiMutt]

I've been doing work on a Sansui 5000a recently on and off.

The issue it's had since the day it ended up on my bench is 38 volts DC on both speaker channels.

I tested the transistors on both drivers and none test as shorted, and I went ahead and replaced all 4 output transistors with ON Semi's MJ15024G. A bit overkill for the application, but I got a bunch on a good deal. None of the original outputs test as shorted in anyway.

Yes. Before the question is asked, this 5000a was lucky and has the updated F-6013 drivers.

I'm sure as old as this unit is, it's cap coupled. So I figured I would post here and see what others thought.

I'm wondering if, since both channels are doing it, that the main filter caps are probably long gone? At least that's my hope. The post is here for exactly; What else should I be looking at besides a recap of the drivers, replacement of all the big filter caps, and a recap of the power supply?

Sansui uses higher values then Marantz and Pioneer which is what I primarily fix. So, I don't have any filter caps on-hand. I have to order them all out. Two are 1500uf at 80V the other two are 2200uf at 90V. Everything I own is 63V or less.

 

[Robisme]

You need an 8 ohm (or something close) load connected to the speaker terminals to check for DC on a cap coupled amp.

Rob

 

[RuairiMutt]

You need an 8 ohm (or something close) load connected to the speaker terminals to check for DC on a cap coupled amp.

Rob

I'm certainly not going to be plugging speakers in to it with my volt meter telling me both channels have a solid 38VDC going directly to the speaker plugs.

 

[Robisme]

First off, I don't believe I said speakers. A resistor would be fine.

That said, do a little searching. Cap coupled amps have DC on the speaker outputs with no load.

Just about any low value resistor will work. Just keep the volume at minimum with no source playing.

Perhaps someone else will chime in with different advice, but I doubt it.

Rob

 

[two.dogs]

On a more modern receiver you'd have both a positive and a negative 40V power supply and the transistors would be a complimentary pair. This results in zero DC volts at the speaker so no need for an output cap. The 5000 is an older receiver with an 80V power supply and identical (non-complimentary) outputs transistors. The speakers connect right in the middle of the outputs - halfway between zero and 80V. The output caps allow AC (sound) to pass, but prevent DC current from flowing into the speakers. You'll still measure 40V at the speaker terminals.

 

[Robisme]

With a load, that 40 volts of DC will not be there.

Rob

 

[two.dogs]

True. One speaker terminal is grounded, and the other goes to the output cap. When we connect an 8ohm resistance between the terminals then one leg is grounded and the other is an open circuit as far as DC is concerned. Because no DC current can flow, then ohms law means that there is no voltage developed across the resistor and both terminals read 0V. If we disconnect the load, then that allows the output cap to charge up to 40V. Then if we connect a speaker we'll get a thump as the cap discharges and both terminals return to 0V dc.