Sansui AU-717 HUM problems after restoration

[dedero]

Hi Guys!,

I've made my AU-717 a full restoration following the amazing Leestero's post but I added some additional restoration steps like full disassembly and cleaning of all switches (tone, sub/high filters, loudness, muting, speakers and input selectors) all potentiometers (tone controls, balance which was a bit noisy) but the volume one, I removed the screws but I couldn't find a way to remove the kind of washer-spring from the back so I will leave the cleaning as soon as it becomes noisy or the grease stuck it.

AU-717 behavior before restoration:
- Permanent blinking power LED, never turned the relay on
*After PSU restoration due to massive glue destruction*:
- Considerable hum noise starting from 1/4 volume on Tuner and AUX inputs, with no input connections, speakers only
- Considerable hum AND hiss noise starting from 1/4 volume on both phono inputs, with no input connections, speakers only
- Signal "mixing" when playing something in AUX input
- Balance pot noisy and sometimes losing 1 channel

AU-717 behavior after restoration:
- After 5 secs of power LED blinking, relay is activated, altough I can see 1 additional LED blinking even after the relay is activated
- Still hum noise starting from 1/2 volume this time on tuner and aux inputs, with no input connections, speakers only, the noise intensity was reduced about 50%
- Still hum AND hiss noise starting from 1/2 volume this time on both phono inputs, with no input connections, speakers only, the noise intensity was reduced about 50%
- Signal "mixing" when playing something in AUX input still present but reduced its intensity to about 50%
- Balance pot working perfectly
- DC Offset and BIAS very stable at 0mv and 20mv respectively

Additional information and notes:

- I've replaced everything as shown on Leestereo's post except:
- I didn't replace C01 & C02 on F-2723
- I didn't replace C15 & C16 on F-2720 (I forgot to order them!)
- I replaced TR05,TR06,TR09,TR10 with KSC1845FTA due to glue damage
- During F-2663 I've removed one transformer and after the re-assembly a very small "HUM" is coming from it afterwards even though I made sure twice it's properly tight
- Checked all board voltages and all are between the service manual numbers

Would you guys have any hint what can be wrong here?

Thank you!!

Cheers,
Bruno

 

[Hyperion]

AU-717 behavior before restoration:
- Permanent blinking power LED, never turned the relay on
*After PSU restoration due to massive glue destruction*:
- Considerable hum noise starting from 1/4 volume on Tuner and AUX inputs, with no input connections, speakers only
- Considerable hum AND hiss noise starting from 1/4 volume on both phono inputs, with no input connections, speakers only
- Signal "mixing" when playing something in AUX input
- Balance pot noisy and sometimes losing 1 channel

There was a point here where you should have dealt with some of the issues you describe before proceeding with the rest of the restore. It is true that some of the issues you note could possibly have been dealt with simply by performing the restoration activities. If we had been aware of that list - we would have told you not to proceed and fix at least the 'signal mixing' (=crosstalk), and 'hiss & hum' issues first.

However, now you have a host of problems, and you have been everywhere in the amp changing components and disassembling things, all of which makes helping you now 100 times harder than before.

Referring to your 'After Restoration' list of issues, it does sound like there are some pretty significant grounding issues in play, but exactly what/where they are, I find it hard to say. If you feel you don't have the skillset to trace and correct the issues yourself (even with help from here), because you have the vital - in this case - 'eyes on' view of the gear, it would be best to take it to a technician who from his experience should be able to resolve these issues quite quickly.

PS: Well done for starting a new thread for your issues, the right thing to do.

 

[ConradH]

Hum with open inputs isn't uncommon and I'd only worry about what you get with shorted inputs. IMO, you're going to need a scope and some good skills to fix this. You have to break the problem down into manageable bite-size pieces, that is, determine the performance of each stage in terms of hum and noise. The cross coupling sounds like a bad ground somewhere, but again, you have to short unused inputs. Sansui didn't always do a good job of isolation, but I don't remember much about the one 717 I did years ago. Murphy's Law says the problem will always be in the parts you didn't replace, but I don't have a schematic handy to see what those caps are.

 

[Hyperion]

Additional information and notes:

- I've replaced everything as shown on Leestereo's post except:
- I didn't replace C01 & C02 on F-2723

Input capacitors for phono stage - no problem

- I didn't replace C15 & C16 on F-2720 (I forgot to order them!)

Pre driver power amp coupling capacitors - no problem

- I replaced TR05,TR06,TR09,TR10 with KSC1845FTA due to glue damage

Location - F-2663 assumed, regulator circuit transistors - no problem, especially as you say the voltages are all correct.

 

[dedero]

Thank you so much for your input guys!

John, totally agree with your statement on how to handle the restoration process... the thing is this amp had a non-working PSU and when I fixed it I didn't check the ground issues, my mistake there... but this will be taken as lessons learned

Actually I was testing the ground issues without the whole shields, another mistake here... after I put everything together the ground issues now are almost gone, both phono's are still a little bit noisy and the crosstalk it's still present (thanks for the correction John) but after 3/4 volume position and the AUX and Tunner inputs are completely silent even at top volume, I assume this is not still normal and makes probably the things even more complicated and tricky to diagnose

I've checked the whole cable connections, I followed all cables around the amp and nothing seems to be wrong so I will still trying to do some troubleshooting and will search some info here in the forum.

I'll keep you posted with the results! (if there's any)

Thanks and cheers,
Bruno.

 

[Hyperion]

OK, so I'll go through some of these things and see if I can help at all... I am glad that you are making progress...

but after 3/4 volume position and the AUX and Tunner inputs are completely silent even at top volume,

So does this mean these inputs are not silent before 3/4 position?- do you hear any noise at all?

both phono's are still a little bit noisy

Have you changed any transistors in the phono amplifiers? - because there are some potentially noisy ones in there - TR3 & TR4 - 2SA726 might be causing some of the noise - but understand that even in good health there will be a little hum and hiss coming from the phono inputs. Note that some people remove and replace every 2SA726 that they find - replacing them with KSA992, 'gain matched' if replacing differential pairs.

When you are checking for hum (and hiss) do you have all the internal covers on - or not?

 

[dedero]

Thanks a lot for your help John!!

So does this mean these inputs are not silent before 3/4 position?- do you hear any noise at all?

Sorry I wasn't clear here!, actually the AUX and Tuner inputs are fully slient, in all volume positions, here I'm totally happy!

Have you changed any transistors in the phono amplifiers? - because there are some potentially noisy ones in there - TR3 & TR4 - 2SA726 might be causing some of the noise - but understand that even in good health there will be a little hum and hiss coming from the phono inputs. Note that some people remove and replace every 2SA726 that they find - replacing them with KSA992, 'gain matched' if replacing differential pairs.

Good point here!, I didn't change any transistors in the phono amps... I've read about that some could be noisy but I didn't change them... so that could be a reason, absolutely... because now from both phono's is coming no noise at all until 3/4 volume, afterwards hiss comes up, no hum anymore!, so may be the transistors as you're saying... and probably no ground issues, I still need to troubleshoot!
By the way, crosstalk is still there but only at full volume and very quiet, I can hear what's playing on AUX from Tuner, Phono1/2

When you are checking for hum (and hiss) do you have all the internal covers on - or not?

I used to check the hum and hiss without any covers on, now I'm doing the checks with all covers on

Cheers!
Bruno.

 

[Hyperion]

Ok, that's good, actually it sounds like the phono inputs are OK - pretty normal noise output from your description, remember they are very sensitive inputs. One thing you mentioned earlier that was a bit of a worry, you said you had another light blinking as well as the protection light? - has that gone away since connection of all the grounds?

Regarding the crosstalk, that seems normal as well! - especially if you can only hear it at 3/4 volume or above. You could change those transistors if you want to, but I think you may be out of trouble now. - maybe time to listen to some music and relax.

 

[dedero]

Hi John,
Thanks a lot for your always useful input!
Great to hear regarding the crosstalk and the phone inputs!! great news!

Sorry I wasn't clear with the blinking light, actually there's no additional light blinking during the protection check, it's the very same power LED that makes an additional blinking right after the relay "clicks" but I cannot recall whether this situation was present before the restoration...

I'll keep the transistors in mind, I'm travelling for 2 weeks so I won't be able to work on it until June... so I will think about changing the transistors, otherwise in the meantime I'll follow your suggestion: listen and enjoy!

Thanks a lot for everything!
Cheers,
Bruno.