Sansui AU-7700 problems.

Trentdarby

Trentdarby

Active Member
My AU-7700 is having a few problems. When I first turn it on, it's fine. After 20 or 30 mins of warming up the left channel starts to produce some fizzing with the occasional popping, regardless of volume level, or input signal. It makes the woofer on the speaker go in and out like crazy. It only seems to be when the 'tone and filter' switch is on. Here's a video of it on YouTube.

https://www.youtube.com/watch?v=yFtuOfXuUBI

I've done a bit of Deoxing, and it does seem to have gotten better. It's not as loud and it only happens once the amp has warmed up.

I would love to get this amp going reliably, because when it's going it sounds great. Any help would be greatly appreciated!

Thanks,
Christian.
 

Attachments

  • DSC_1096.jpg
    DSC_1096.jpg
    98 KB · Views: 59
  • DSC_1097.jpg
    DSC_1097.jpg
    110.9 KB · Views: 55
  • DSC_1099.jpg
    DSC_1099.jpg
    102.2 KB · Views: 52
  • DSC_1100.jpg
    DSC_1100.jpg
    111.1 KB · Views: 50
  • DSC_1101.jpg
    DSC_1101.jpg
    85.2 KB · Views: 46
Last edited:
sounds like it could be a bad transistor in the tone amp .
your video is set to private by the way
 
petehall347 said:
sounds like it could be a bad transistor in the tone amp .
your video is set to private by the way
Click to expand...

I thought it must be something somewhere in the pre-amp. I tried it separated, using a Pioneer SA-9500II as a power amp, but the problem was still there.

I'm pretty useless at this stuff. I might have to get someone to look at it.

The video is public now!:yes:
 
4 fuses on the power board

Hey i have an AU-7700 apart right now and it does not have the 4 fuses on the power board like the ones in your pictures, I have looked at other photos that have them gone also, was that something they did in the early ones and found it was not required?
 
norm1001 said:
Hey i have an AU-7700 apart right now and it does not have the 4 fuses on the power board like the ones in your pictures, I have looked at other photos that have them gone also, was that something they did in the early ones and found it was not required?
Click to expand...

Interesting...I don't know. I'm in Japan, so maybe that has something to do with it?
 
Hi,

Accessing to the servic manual, the 7700 I'd full of 2SA726 and 2SC1313 transistors in the equaliser and tone control circuits. These are known trouble makers for causing hiss, crackle etc. ( and as they warm up the noise can get worse)

Cheers
John
 
skippy124 said:
Hi,

Accessing to the servic manual, the 7700 I'd full of 2SA726 and 2SC1313 transistors in the equaliser and tone control circuits. These are known trouble makers for causing hiss, crackle etc. ( and as they warm up the noise can get worse)

Cheers
John
Click to expand...

Thanks! That sounds exactly like my problem! Do you know where I can get a service manual from?
 
skippy124 said:
Hi,

Accessing to the servic manual, the 7700 I'd full of 2SA726 and 2SC1313 transistors in the equaliser and tone control circuits. These are known trouble makers for causing hiss, crackle etc. ( and as they warm up the noise can get worse)

Cheers
John
Click to expand...

Looking at your signature you'd be the right guy to ask about Sansui!
 
I know this thread is 4 years old but I have a similar problem... Au7700 with a left channel hiss and crackle when it warms up. I have it apart on my bench now and getting ready to order parts. Should I go ahead and replace all instances of the 2SA726 and 2SC1313 transistors in the entire amplifier? or just the tone controls? Any thoughts?
 
I would suspect the TR01 2, 3, 4 on the driver amps (Differential amp 1st stage) They are 2SA726 and known to get noisy there. As a way to confirm, listen via headphones and measure the DC voltage at the speaker out connections without speakers hooked up. See if the DC starts to jump when the crackling starts. If so its usually those transistors. another way to verify is to pull the Pre out - main in jumpers, (or move switch to seperate if it doesn't have jumpers. Feed the pre outs to main in of another amp, or aux on another amp or receiver. listen to the sound to see if it is coming from a section before the power amp. If it is, then of course it could be tone amps control amps etc. Lastly if you vave another high end unit, feed the pre amp out of that unit to the main in of your unit. If the crackle starts up, it most probably is the differential amp transistors.
I have had a bunch of issues with 2SA 726 but not so much with the 2SC1313.
I usually just replace the "bad ones" but I just finished up a reel to reel that had at least (3) 2sc458 small signal transistors opening up while room temperature, but conducting and working fine when a little heat was applied, so not trusting any I replace all 14 in the unit.
Mouser has KSA992's was are good Fairchild replacements for the 2SA726, and KSC1845 's are a good replacement for the 2SC1313
Good luck
 
wow, thanks for the detailed information. a couple follow up questions..

1 - by differential amp 1st stage, you mean the tone control board? ie: where the tone control switches are? sorry, I've not done much work on stereo amps. most of my (limited) experience is in old tube guitar amps. This is the first old stereo amp i've done more than replace a power cord or lamps in.

2 - so I'm guessing it's safe to run this amp for a while with no load hooked up, just headphones? Again, coming from tube guitar amps - that's a no-no as far as I know. Just want to make sure I don't make a stupid mistake.

I do have another amp that can separate the pre and power (2000a) - but that's my main system and is in constant use.. so I'll have to try this first so I don't have to disconnect everything and hump it down to the workshop. :)

thanks again!
 
Did a little thinking and realized that I often listen to my 2000a (and this one when it was working correctly) with just phones & speakers turned off so I guess it's fine. :) Just wondering about the "differential amp 1st stage" and if that applies to the tone board. thanks!
 
SO... DC output on right channel starts out at 0 and gradually (over the course of 15 mins or so) drifts up (it's at ~9mV right now, climbing slowly).. The LEFT channel (where I'm getting the crackles) is around 30mV ...doesn't seem to climb higher than that, and when the crackling starts - its goes haywire. Down to 15, back up to 30 - and all points in between.. but seems to hover right around 30mV when not crackling.

None of the tone control levers seem to have any effect on either the crackling or the DC reading.

I got similar results on both A and B speaker outs. However now after warming up for 15 mins or so speaker the left channel on BOTH speaker outs (A & B) seems to hover closer to 15-20mV when it's not crackling.

Where should I be looking next? Thanks!
 
UPDATE:
I adjusted the offset - and got the left channel to stabilize around 0mV and the right to stabilize around 1mV - the LEFT (where my crackling is) seems to be staying stable - right gradually crept back up (up to 3mV over a few minutes. Man, those adjustment pots (variable resistors?) are TOUCHY. The tiniest movement would cause one to drop or raise drastically. Crackling still intermittently present in LEFT channel.

Gonna go pick up groceries and leave the amp on and check when I return unless this is a terrible idea?
 
Last edited:
After adjusting DC offset my RIGHT channel is staying stable at ~1mV so no problems there (yet).

Left channel is pretty stable at 8mV but goes fluctuates + or - 1.mV when the channel crackles. I hit each of the transistors in the front end with some freeze spray and think I may have found a culprit (2SA726). However, there are a couple of the small orange caps (50v, 2.2uF) on there that make a pretty obvious buzzing sound through my headphones if i touch my finger to the top. Only 2 of the caps do this, the others of the same value aren't affected by touching. Wondering if those could be a problem too? Cold solder joint? Guess the board is coming out and it's getting some new bits either way.
 
You must log in or register to reply here.

Similar threads

rasenthiran
Thoughts on Sansui AU 7700
2
Replies
24
Views
20K
Dr. Ears
Dr. Ears
bolero
  • Poll
POLL: Sansui 8080DB vs AU/TU-7700 ?
2
Replies
22
Views
3K
bolero
bolero
H
Sansui AU-7700 amp
Replies
6
Views
2K
Okeh
O
X
Sansui AU 7700 troubleshoot for SP - 2500s
Replies
0
Views
994
xpragmaticx
X
sienahounds
Sansui AU-7700
Replies
4
Views
738
BilboBaggins
BilboBaggins
Back
Top Bottom