• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sansui AU-7700 motorboating at max treble setting

Question for some of the expert techs here: do the tone and switch board as well as the volume pot and loudness switch boards use the chassis (metal chassis front plate) for ground? I ask because when I test the process with the boards UNmounted to the front chassis plate it will hum when I touch the volume pot or the tone switches. When I remount them to the chassis plate: no hum.

Thanks.
 
Last edited:
Zekeman said:
when I test the process with the boards UNmounted to the front chassis plate it will hum when I touch the volume pot or the tone switches. When I remount them to the chassis plate: no hum.
Click to expand...
Some models of Sansui are sensitive to this - namely, they rely on having the metal cans of the controls being in contact with the (grounded) front panel/chassis. For each board there is usually another means (wiring) of acquiring a ground path, but not always. Have you ever noticed that plastic shaft couplers have a shorting wire connecting the metal parts of the shaft? - this is part of the same noise/hum mitigation & reduction strategy. ;)
 
Hyperion said:
Some models of Sansui are sensitive to this - namely, they rely on having the metal cans of the controls being in contact with the (grounded) front panel/chassis. For each board there is usually another means (wiring) of acquiring a ground path, but not always. Have you ever noticed that plastic shaft couplers have a shorting wire connecting the metal parts of the shaft? - this is part of the same noise/hum mitigation & reduction strategy. ;)
Click to expand...
It appears this amp is sensitive to that. Last night I cleaned off lots of flux around points on the tone pot board and cleaned with denatured alcohol. That seemed to help but I’ll reserve judgement for now until further testing.

Yes I have noticed the shaft couplers.. They have their own circuit board identifiers.

Thanks.
 
Issue still exists. IF I touch either of the two resistors (circled for each channel) or the base of either transistor, the hum disappears. The hum is present with the tone and filter switch in the tone and filter down position…and The high filter is in the “out”
position. if I move the high filter to either the 7 or 12 kHz position the hum disappears.
 

Attachments

  • C4FCD552-2CB6-42A3-8EDD-88A2F59F14F3.jpeg
    C4FCD552-2CB6-42A3-8EDD-88A2F59F14F3.jpeg
    133.5 KB · Views: 19
Last edited:
Another question for the professionally-trained techs here: give the fact that each of the few voltages on these two boards (f-2095 and 2096 ) testOK, what then does the troubleshooting process look like? I have been pulling and measuring components in the treble circuit and so far all ok.

Any path to a solution would be greatly appreciated.
 
I think the issue is related to the replacement transistors and their different curves and hfe gain. Try to match them with the originals (if you still have them).
 
The replacements are common: A726 replaced by A992's and the C1313's replaced by C1845's. I have restored many similar amps with same. I do plan to pull each and retest though as I am making no progress.
 
I also used the same substitutes many times, but I noticed they don't measure quite the same. Maybe there is something about that circuit that makes it an issue.
 
Well I determined this fact tonight: I happen to have an all-original F-2095 board in my circuit board bin so i swapped it out…result=SAME problem!

So the issue must be in the F-2096 board.
 
Are these bad readings present only when the high filter toggle is in the defeat position? Or are they there in all HF positions?
 
Same result.

Also, when the tone controls are engaged the output transistors get white hot within minutes!

I measure all the others and they are posted. Problem is-I have no specs!

2976FCAB-3FED-463B-BAA1-2F2219B61F42.jpeg
 
Last edited:
Closer.

I tested with tone and volume boards mounted and started probing with the scope. I did NOT have the internal metal plate mounted to the bottom of where the eq/secondary power supply board is mounted. In order to get to the eq board I swung out the mother/amp power supply board. All tone controls worked free of distortion. As I moved the amp board back into position (again, without the internal separator plate in there) the distortion came back, at an increasing rate as the foil sides of the two boards got closer. I mounted the internal plate, powered it up and swung the mother board into place and it functions with no distortion.

EADC83B3-2EA3-42C7-905F-3BE51CDB85B0.jpeg
 
Had the same issue with an au7700. Heavy plops when switching tone & defeat. So have that amp switch off with a light bulb in series.
The cause in my au7700 was c11 and c12of the F-2096 board. Due to bad capacitor it fed back a decent dc in the defeat position. Dc was/is fed by R09 and R10. Change into film cap for a good dc decoupling.
 
I'm working on another 7700 and had the exact same treble hum issue. I installed the internal metal plate between the amp board and the input board and the problem is resolved.
 
You must log in or register to reply here.
Back
Top Bottom