Soundstream REF604 - dc offset

[durwood]

I recently repaired a 604 for a friend. Dreaded switches took out the output channel.

-Replaced the switches
-replaced all the outputs on the bad channel
-replaced the bad current sharing resistors (didn't have enough to replace all 6 but 2 were ok)
-replaced one MPSA14 transistor that blew up on that channel

Repair last time

Last time I had it the power supply had died and one of the FEB boards smoked the 50 ohm resistor. This was on the same pair of channels but not the same channel that died this time around. I replaced that FEB with another one I had that had also smoked that one resistor. It was in better shape and I had replaced that resistor. I have a brand new FEB ready to go if I need it.

-also replaced the outputs on that channel
-replaced the current sharing resistors
-replaced the high power/high current switches (really wished the switches used in these amps were of better quality).

There are one set of channels that has never needed touched. So with all the history out of the way, the untouched channels show 33mV dc offest. The repaired channels show ~100mV, is this anything to dig deeper into or would it be considered acceptable given what has transpired and let it fly?

[Perry Babin]

Replacing the differential amplifier transistor may help. I think this amp uses the 2SC3067 which can be purchased from jandrelectronix on eBay.

You have to determine what's acceptable. 100mV on most speakers is safe but could cause turn-on and turn-off pops. I try to get most amps to under 50mV. Others consider 25mV to be the maximum acceptable offset.

[durwood]

Well update, it was something to look further in to. On that pair of channels one of the channels was inverted and then I found the left channel was not even working with only the left RCA jack plugged in. Popped the new FEB board in and the dc offset is 3mV but channel only works when the RCA is plugged into the right jack. Right channel still has DC offset of 100mV. I think you were right-points to a faulty differential input perhaps. I think I will tell him to spring for another FEB since I will probably have to take it out anyway, unless anyone has any other thoughts.

[Perry Babin]

You can replace the differential amplifier transistor. It's easier and less expensive than replacing the board.

[durwood]

Grrr...it was the brand new switch. Moved it back and forth a couple times powered back on and now no inverted channel and it works fine. I really don't trust these switches. I'll deox them and pray and hope it holds up. Not much else I can do unless I hardwire it in a certain mode.

I will probably order some of those differential amps so I have them but there are no turn on pops currently.

[Perry Babin]

What do you mean by 'inverted' channel?

[durwood]

Speakers were connected with correct polarity but when testing the polarity was reversed on one channel only, what most people confuse with 180 out of phase. IIRC with these soundstreams, moving the switch from stereo to mono inverts one of the preamp channels and disconnects input from the unused input, I think this is what I had seen from a similar schematic once. It would explain why I was not getting signal output with only one RCA connected to the left channel, mono operation uses right input only on these IIRC. It seems to point to an intermittant switch problem, and I just put new switches in this thing.