Yamaha A-S701 amplifier grounding

[Audiorepair]

Hi, first post here, so apologies if some ground rules broken.

I have a Yamaha S-701 hi-fi amp that had one power amp blown.  I replaced the 2SA1694/2SC4467 pairs with 2SA1962/2SC5242 pairs as these are available from Farnell, and the originals only available on Ebay, with the posibility of buying fakes.
Anyway I got the amp working and testable, can always sort out the transistors later.

BUT, both power amps have 500mV DC on the outputs - turns out this is actually 500mV DC on the speaker grounds.
This trips the amp protection if the output is raised to more than about 10v RMS, load or no load, either side independently, but will power speakers up to that point.

Now, the speaker grounds are also the power amp power supply grounds, but not connected to anything else.  They are not connected to chassis.
They only connect to the power amp transformer secondary centre tap, so floating in effect.  (There are separate windings for other circuits)
All the input sockets are connected to chassis, and these signal grounds give a ground reference to the power amp at the input stage, though this is not the (floating) speaker output ground.

So the signal ground is only connected to the Speaker ground by components in the input stage of the Power Amp, but the Speaker ground is 500mV higher than signal ground.
The schematic will show this, might make more sense than me.

Connecting a load on the power amp outputs at 500mV  draws a current consistant with the DC offset.  Connecting the Speaker grounds to chassis passes no current to chassis, the offset goes, and the amps work properly with no protection cutout.

The 240v mains input IEC socket has no ground connection to chassis or anywhere else, the chassis appears to be grounded only by the connected equipment.


This has me confused, I don't really just want to connect the speaker ground to chassis, as this appears to be a deliberate design decision not to do this.

But why the 500mV offset?

[SpecialK]

Hmmm.  I looked at the manual from Electrotanya, and on page 58 of 77 it shows the speaker (-) connections connected to a chassis star grounding.

[Audiorepair]

Yes, but the Star Grounding goes nowhere other than the centre tap of the mains transformer.

This is the issue here.


Note you said "chassis star grounding", this is not correct.

[SpecialK]

Is it possible that the impedance selector is dodgy? One of the rails might be cutting out causing an imbalance.

Also, what is the idling current?  You might need to clip R139 and R142 or alternately re-install them.

[duak]

Audiorepair, this is generally how things are done in integrated amplifiers if the power amp board is separate from the preamp board(s).  It has a lot to do with the power transformer inducing an AC voltage in the signal ground/circuit common/power supply return that manifests as hum.  I know this because I designed and built my own integrated amplifier in the mid 70's and it took me a little while to figure out how to wire it so it wouldn't have hum.

Anyway, does it use some sort of plug in flex cable between the boards? Or just wires?  I suggest checking the contnuity between J3 pin 2 (?) and J3 pin 8 (E) and the corresponding pins on the Main board.  Likewise CBL01 pins 2 & 4, and all pins in CBL06 to wherever they go.  I'll bet the resistance is higher than it should be.

Cheers,

[Audiorepair]

Ah,

I hadn't noticed the Star Grounding going to that connector!

Starting to make more sense now.


Cheers.