Sansui AU-717 Restoration/Reparation Advice

D

dedero

Member
Hi All!,

I'm starting the planing the restoration of a just bought AU-717, I could read the very useful information around this forum regarding this amazing amplifier, as this will be my 3rd amp restoration (after a successful AU-217 and AU-317 ones), I feel enthusiastic enough to go forward with this amp.

I bought it for 350eur aesthetically mint condition but unfortunately with a protection problem which I would like to detail:

- Turning the amp on, the protection LED blinks permanently and the relay is never engaged
- While the LED is blinking I tested with my headphones and the good news is that both channels are working perfectly
- After some minutes (around 10) I started to feel some heat coming from the protection board F-2663 so I could realized that the transistors TR01-TR02-TR03-TR04 were quite warm, not really untouchable hot but getting warmer with the time
- BIAS current was around 25mV for both channels (20mV is the correct one as per service manual)
- DC Offset didn't measure anything as the relay is never engaged (just an assumption)

NOTE: It seems this amp was working quite hot for some time as several caps alongside the protection board F-2663 have their cover a bit shrank (I hope this is the correct expression) and the board itself it's a little bit bended, in addition to that, the deadly caps glue is really dry/roasted alongside the whole amplifier

As I could see, the protection board is not really easy to take out in order to test the voltages shown in the service manual right away, I'm thinking 2 options:

- Restore every single board: replace caps, diodes, fuse resistors, trimpots, relay, pots cleaning, check cold/broken solder joints, etc etc etc and then search and resolve the problem
- Repair the actual issue, make sure everything is working as it should and the restore it afterwards (I'm considering this option just in case there's a problem with one or several transistors)

What do you think could be the issue with the protection and what do you think which direction should I take?

Any advice will be really appreciated!

Cheers,
Bruno.
 
Last edited:
dedero said:
What do you think could be the issue with the protection and what do you think which direction should I take?
Any advice will be really appreciated!
Click to expand...
Hard to say a this point, but IMHO rebuild that board is advised F-2663, look for leestereo's 717 restoration documentation about it. a lot of good stuff there.... any doubts, just ask.
 
dedero said:
- Repair the actual issue, make sure everything is working as it should and the restore it afterwards (I'm considering this option just in case there's a problem with one or several transistors)
Click to expand...
^^^ This ^^^

Remember, restoration or recapping is NOT a fault finding technique.

Right now we are reasonably sure that most if not all of the amp is untouched, but has a fault somewhere which we might be able to deduce the cause of with a bit of well placed direction. If you restore/recap it you will have 'touched' it everywhere - so not even we could help you.

Having said that, the transistors you mention overheating!? - that is very odd.
 
Last edited:
Hi,

Agree with John's (Hyperion) advice above.

A suggestion re offset measurement when the amp is in protection - you can do a "rough" DC offset measurement at either of the bias test points (this will give you a value that is within 20mv of the actual offset voltage, and is prior to the speaker protect relay).

Cheers
John
 
Hi Guys,

Thanks a lot for your replies!

Makes a lot of sense starting with the repair first before the restoration.. but in this case it seems the glue did some damage already as seen on this diode:

Deadly glue

Deadly glue


Unfortunately this is not the only component affected, there is actually 2 more diodes fully covered in this kind of oxide/corrosion plus several other diodes and several resistors, basically everything close to the caps is affected, in addition the C605 is leaking...

I guess I should replace at least the affected corrosion affected components before measuring anything, does it makes sense?

Thanks and cheers,
Bruno.
 
skippy124 said:
Looks like the power supply board might need a rebuild first !!
Click to expand...
Yeap - now I see it, that doesn't look good, in which case you will be forced to rebuild that board (only). Hoping that doing so flushes out the problems noticed, so you can test functionality before you proceed. ;)
 
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yep, would definatly remove the glue, and replace those parts oxidized by the glue. the glue is partially conductive as it ages....

do that first then go from there.
 
Hi All, thanks a lot for your hints, I started yesterday counting the components affected so the result = 39, including diodes, resistors, caps and 1 transistor (TR10=2SC1708 as schematics and C170882F printed in the component, I need to find out whether it's still available and the correct model number!)

So the adventure is just starting!, I will go ahead and try to do the corresponding upgrades shown in Leestereo's post

Cheers,
Bruno.

EDIT: regarding the transistor 2SC1708 I found in the awesome post regarding transistor substitutions https://audiokarma.org/forums/index.php?threads/once-more-sansui-transistor-substitutions.583009/ that the replacement is KSC1845 but in Mouser and Digikey the only available is KSC1845FTA, do you guys know whether this could be used instead of the KSC1845?
 
Last edited:
Hi,

The "KSC1845FTA" is a "full part number" of the KSC1845 - the F denotes the gain ranking, and the TA denotes (from memory) that they are supplied on tape rather than loose parts (bulk supply).

In other words the KSC1845FTA is a specific version of the generic KSC1845 part number.

Cheers

John
 
Hi All,

Thanks for the hint John!

I'm still collecting information and replacements for the components, so I'm coming back with another help request question ;)

- I need to replace diodes from D05 to D12 and D601-D602 which by service manual are 10D2, quite obsolete as I could search even the replacements in several posts are now obsolete... after checking datasheets for quite some time I found the STTH1R02QRL do you think it could fit as replacement?

- For D17-D18 shown in service manual 1S2473D I could see that 1N4148 is a good match, is that correct?

Thanks a lot!

Cheers,
Bruno.
 
dedero said:
Hi All,

Thanks for the hint John!

I'm still collecting information and replacements for the components, so I'm coming back with another help request question ;)

- I need to replace diodes from D05 to D12 and D601-D602 which by service manual are 10D2, quite obsolete as I could search even the replacements in several posts are now obsolete... after checking datasheets for quite some time I found the STTH1R02QRL do you think it could fit as replacement?

- For D17-D18 shown in service manual 1S2473D I could see that 1N4148 is a good match, is that correct?

Thanks a lot!

Cheers,
Bruno.
Click to expand...

@dedero you may want to bookmark this thread: http://audiokarma.org/forums/index....sansui-transistor-substitutions.583009/page-5
 
Hi all!,

I could finally restored/revived the board F-2663 and now looks awesome, witout this corrosive glue, in addition I did the measurments shown in Leestereo's post:

At PSU Board F-2663
Voltage at PSU pin 19 - +35.8
Voltage at PSU pin 25 - -34.8
Voltage at PSU pin 20 - +35.2
Voltage at PSU pin 26 - -33.8

Any clue if those voltages are between the corresponding range?. I couldn't find any information about the supposed voltajes for those pins in the service manual.

On the other hand, TR01, TR02 (2SD356), TR03 and TR04 (2SB526) are working a bit warm, I can touch the heat sink without problems and I can feel them warm but I can keep my finger touching (sorry I cannot give precise temprature!!) and I guess that's the reason why it has a heat sink but make sense they're warm without any speakers connected?

Thanks and cheers,
Bruno.
 
Thanks a lot John for your info!

Just another concern... before mount the F-2663 back to its position I had to unscrew one transformer otherwise I was not able to mount it properly with the screw, the thing is once I did the final test with everything mounted this transformer that I've moved is buzzing now :wtf: If I press it a bit with my hand the buzzing is being reduced, this might be due to the umount I've done? or could be some wrong wiring during the F-2663 mount? any ideas?? :(

Cheers,
Bruno.
 
dedero said:
Hi Guys,

Thanks a lot for your replies!

Makes a lot of sense starting with the repair first before the restoration.. but in this case it seems the glue did some damage already as seen on this diode:

Deadly glue

Deadly glue


Unfortunately this is not the only component affected, there is actually 2 more diodes fully covered in this kind of oxide/corrosion plus several other diodes and several resistors, basically everything close to the caps is affected, in addition the C605 is leaking...

I guess I should replace at least the affected corrosion affected components before measuring anything, does it makes sense?

Thanks and cheers,
Bruno.
Click to expand...

Is this an common or inherent problem with this amplifier?
 
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