SANSUI AU-G90X DC ON OUTPUT

[Subchannel]

I just bought a very cheap AU-G90X with noise on RIGHT output. To remove as a possible problem, all dust on the inside are removed and all pot´s and switches are cleaned.
The Left channel is no proble adjusting to +/-5mV on output, but right channel is minimum 1.2VDC, and when turning the volume knob, the DC can go as high as 13VDC. If volume is turned back and forth fast, the amp goes into Protection. The DC will go back to 1.2V when I stop moving the volume pot, no matter where.
Anyone here familiar with this amp/ problem?
Thank you in advance!

 

[fojica]

AU-G90X is very similar with any other Sansui Alpha or X-Balanced amplifier - the topology is the same. Enough details on "Sansui Alpha" around here, do a good search. But - very important - NOT like "normal" amps, from Sansui or other companies.
You can easily find the service manual for it - if you look at the schematic, you'll understand why it's not like "normal" amps.
All Sansui Alpha / X-Balanced series are DC-coupled - what this means - that if flat or control (pre)amp has DC on the output, then that DC offset will be amplified by the following power-amp and then found on the speaker output - like in your case.
So you must:
- check that there is no DC offset (less than 1 mV) at the flat or control (pre)amp output. If it is, fix that first
- check the bias on the output stage - there are 2 bias trimmers for each channel, not only 1 !
- check or better replace trimmers on power-amp board(s) with multi-turn 3296 series. Get out the old ones, measure them, adjust the new ones to the same values and then solder them on the board
- if still not good, then input double JFET on the power-amp section could be leaking. uPA68H were sometimes found leaking and causing DC offset problems
- last option, less likely though - a possible leaking electrolytic cap, as the amp has ~40 years age. Check for any bulging cap, which should be obviously appear to be "worse" on the R-channel than it's brother on the L-channel

@moefuzz - no thermo resistors there, as far as I'm aware of. Also no thermal compensation with diodes

Hope this help

 

[Subchannel]

Thanks for very good info Fojica, and excuse my english, but what do you mean by Flat Or Control Preamp? If there´s DC from the preamp part? I have studioed the schematics, but that doesn´t really tell me where the fault is, I´m not that good on (Alpha) transistor amp´s.

 

[fojica]

Board F-4510 in the service manual - called "Control". That is the "control preamp". Without tone controls engaged, is essentially just a "flat amp" - or simply a preamp if you like more this way.
Unfortunately there is no "magic bullet" to simply say "the fault is <there>, component <x>". Schematics only help to understand where and what to check. A bit of troubleshooting and knowledge is required to pin-point where the problem is exactly. Could be equally well not only a single "big" problem, but a compound of more small problems here and there. It's a chain - potential problems on each stage do sum-up and get reflected at the end.
Control-amp or flat-amp has also a differential input stage based on a dual JFET, like the power-stage. That one could play a part, though 2SK389 are not known (not by me at least) to have a bad habit of leaking. But could be. Or just a bad DC-adjust trimmer over there. Or simply requiring adjustment.
Checking a DC-offset on control amp or flat amp requires no special "Alpha" skills - the only "NOT like normal" stage is the power amp stage + power supply. Everything else is "normal", there are no balanced inputs on this model.

 

[Subchannel]

Thanks again Fojica, I´l check up on the schematics and follow your advise.

 

[Hyperion]

Because these amplifiers are 'not like normal' if you are unfamiliar with them it is a steep learning curve to get up to speed with fault finding the more complex circuitry. There are actually two power amplifiers for each channel internally connected in a bridged arrangement - this is the feature of the design that presents difficulties for a relative novice to grasp its operation.

They have 2 bias trimmers, and 2 DC offset trimmers per power amplifier channel, the DC offset trimmers are for 2 different DC offset adjustments. There is a further DC offset adjustment in each channel of the Control Amplifier.

Another one of the 'not normal' aspects of these amplifiers, and an important one is that none of the black speaker terminals is at ground potential. They should both be considered as 'live' so if you connect a grounded test cable to a black speaker terminal you will seriously damage the amplifier.

If the volume control affects the DC offset as much as you say I'd be looking closely at the Control Amplifier, but make sure you eliminate any effect from the Phono or 'EQ' input, by selecting the AUX input.

On the 'up' side it doesn't appear to be a serious fault.

Good luck!

 

[Subchannel]

I´ve done repair on AU-717, and a lot of recapping, but this may be over my head. And regarding the negative speaker terminals, that I did not know, so thanks for the heads up!

 

[Hyperion]

Just another much smaller 'heads up' your avatar appears as a blank box for me using Chrome browser - possibly because Chrome does not recognise the image file type, needs to be a common type like *.jpg.

 

[Subchannel]

Hm, should be jpeg but I will check. Thanks for the tip!

 

[Hyperion]

Hm, should be jpeg but I will check. Thanks for the tip!

I can see it now.

 

[AmpRa]

I’ve repaired many Sansuis. They’re very sensitive. You should definitely re-cap before anything.

 

[Hyperion]

I’ve repaired many Sansuis. They’re very sensitive. You should definitely re-cap before anything.

Well actually, it is best in most cases to find and resolve any faults first. However, I accept that at the age of some Sansui's they may be so burdened with issues that a replacement of at least some of the electrolytic capacitors may be needed to get the unit into a 'something like' working condition before proceeding.