Need Sansui 7070 help..speaker killer

[svelto916]

I need some help with my Sansui 7070. I am not very techy as my explanation of the problem will demonstrate.

My Sansui 7070 had been working great, nice CD player connected + Airport Express/airplay and B&W 601's..life was good.

I recently went to use the 7070 and my left speaker was toast. *&(*(^!!@!! At first I thought the wires came loose with dusting, so I moved my right speaker over (to the left) and it worked fine, moved my left speaker over to the right and there is almost nothing coming out. I always turn the volume all the way down when I turn the unit off, so when i refer to turning the unit on..the volume is always at the lowest setting.

I notice that when I turn on the 7070, after about 2 seconds I hear a click and the left VU meter jumps about 1/2 way then goes back to zero and I can get nice sound out of the 7070. That is the current situation...however, a time or two the left channel VU meter pinned itself to the right (full) for a second or so on startup before I turned it off.

This is all I have done so far to try and trace the problem:

I replaced the 3 fuses. Nothing inside the unit seems odd, overly hot/warm or burned with the exception on board F-2626, R13 and R14 at the top (small white rectangle boxes) get almost too hot to touch. Everything else is fine, lights, fuses, treble, mid-range, bass control, AM/FM, filters etc.

If I remove board f-2626 and turn it on, I get pop from the left speaker and a slight hum. If I remove both f2626 & F2624, and start the 7070 I get a nice, normal sounding click after two seconds with no pops or movement from the VU meters.

Does anybody know, based on this description, what is going on and how to fix it? Any help would be greatly appreciated. Even if it is a repair shop recommendation (near the northern part of IL)..because last time I had a problem with it (two years ago) the repairman in my area said it was not worth fixing...I was very thankful I found this forum. I will also add that we've had some bad storms, and this could be power surge related...but have not had problems with any other electronics in the home.

Thanks

-Chris

 

[slimecity]

It could be excess DC on the out put of the amplifier - however I would be surprised if it was this as your protect circuit should work to prevent the output from engaging in the event of excess DC.

Get a meter and test your speakers to see what happened - on the working one you should be getting (ie) 8 ohms - do you get a short circuit on the "blown" speaker?

On the unit - once the meter clicks "on" measure the DC offset for each channel. Set the meter to DC mV, and put the red probe to positive speaker out and black to the negative.

Let us know what you get.

If it isnt excess DC then I would suspect oscillation in the output blowing your drivers.

 

[Tom B]

I think your problem may be the driver board 2624 or possibly shorted outputs.
Resistors R33-R36, and R40 to R50 are all fusable resistors that fail on the driver board 2624. They drift way out of value or they can open. This can cause many different symptoms, including high DC on the speaker outs and relay shutting off on protect.
The hum you have also is a bit of a concern and could be shorted output transistors. (they could have failed themselves or as a result of an open resistor on the driver board.
First of all, don't connect your speakers until the unit is working
get a multimeter and test for DC voiltage on the output speaker terminals. The reading on both channels should be 0 volts or very close to 0 Volts . Of course the relay has to click in to measure for voltage.
If you are getting 0 volts and or can't get relay to engage, power off the unit, unplug it, and using the OHMS function, check for shorts on the output transistors ( typically when they short it is emitter to collector short.)
If the outputs look good, remove the driver board, and either test all of the above resistors on the ohms function, or just replace them all (Methodically as to not make any mistakes)
Its best to use a dim bulb tester when testing after repair.

 

[svelto916]

Thanks, I appreciate the help...I think this one is over my head, took it to Track One Vintage Stereo in Antioch...has a big line in front of me, but says about 3 weeks. I figure there might be other lurking issues that he can jump on as well. BTW the Bowers speaker still makes sound just very distorted "scratchy" and when i press on the woofer it sounds a little better. Here is a video if the popping issue.