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Sansui Au-717 - repair help: recap of Board F2721 and protection issue

S

special_20

Active Member
I have a Sansui AU-717. Due to age and concerns over glue, I have undertaken a recap. Prior to recap, the unit worked. It did take awhile to power on (~10 sec) but was otherwise in good working condition.

I started the recap with board F2721 and this is the only board I have touched so far. Following the Leestereo thread I pulled parts, cleaned glue, and replaced parts (see list below). Post recap of board F2721, the unit goes into and stays in protection upon power up.

I brought unit up initially on a dim bulb. This test registered no shorts. I switched to mains power. I have been able to bring the DC bias into spec (~20mv DC). However the unit has stayed in protection. When I disconnect power to board F2721, by removing grey and yellow wires at pins 4 and 12, the unit comes out of protection. This indicates there is something wrong with the F2721 board.

Per Comment 5 of this thread, I measured from the positive test point of F2721 to case ground. Per thread, this is an approximation of DC offset voltage. I have 52.2 VDC (yes volts!). Voltage is unresponse to VR01 adjustments.

What's my next step here, audiokarma gang?

I've reviewed my parts selections, orientation against photos, and reflowed solder.

I am limited in tools to a multimeter, ESR meter, and a component tester.





Per Leestereo's write up on recapping a Sansui AU-717 I have replaced the caps, and diodes and variable resistors. In addition, I have replaced several components that had contact with the Sansui corrosive glue.

position / original / replacement
C02 / 33uf, 25v / 100uf, 35v
C04 / 33uf, 25v/ 100uf, 35v
C05 / 33pf, 100v / 33pf 100v COG
C11 / 470uf 63v / 1000uf, 63 v
C12 / 1uf 50v / 1uf, 100v
C14 / 470 uf, 63v/ 1000uf, 63v
VR01 / 100ohm / 100ohms bournes 25 turn sealed
VR02/ 2.2 k ohm / 2.4kohms bournes 25 turn sealed
VR03 / 1kohm / 1kohm bournes 25 turn sealed
ZD01 /eqb01-22 / BZX85C22
ZD02 / eqb01-22 / BZX85C22
TR03 / 2sa750 / KSA992
TR04 / 2sa750 / KSA992
R15 / 10kohm / 10 kohm 1/2 watt
R07 / 820 ohm / 820 ohm 1/4 watt
R11 / 1.2kohm / 1.2 kohm 1/2 watt
R23 / 150k ohm fusible / 150kohm 1/2 watt Correction 150 ohm
R25 / 150k ohm fusible / 150kohm 1/2 watt Correction 150 ohm
 
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Well considering it worked prior it has to be something you did. Just glancing at your parts list don't see anything that won't work - Having done many of these and having the same problem it's always been my mistake - double check the + and - of the electro caps, correct orientation of your (2) 22v Zener's and ECB of your transistors and the plug in wires going back in right spot. Just an fyi the 2 fuse resistors are 150 ohm 1/4 watt not 150K
 
Danny Miller said:
I had a similar problem due to flux residue causing protection issues..dbt looked normal..clean any residue left around solder joints..fixed my issue
Click to expand...
Any suggested techniques for defluxing board? I've tried some isopropyl alchohol, but I'm getting some white splotchy sticky residues.
 
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Hot water usually does a reasonable job removing flux. Very hot water. If there are areas of encrusted, hardened flux, it helps to mechanically remove as much residue as possible before washing.
 
what % are you using? 99-100% or nothing..I alternate between a toddlers toothbrush which is the softest & pipe cleaners or q-tips then wash any residue left with a alcohol bath then blow it dry with a computer duster ..a regular hair dryer will help dry it completely or let it sit..until the part you cleaned is not sticky at all..startup with dbt first..
 
special_20 said:
Any suggested techniques for defluxing board? I've tried some isopropyl alchohol, but I'm getting some white splotchy sticky residues.
Click to expand...
I use full strength acetone from Menards or Lowes. With a stiff bristled 1" paint brush.Cleans the boards up very well and rapidly drys.Been using this method for years.
 
Danny Miller said:
what % are you using? 99-100% or nothing..I alternate between a toddlers toothbrush which is the softest & pipe cleaners or q-tips then wash any residue left with a alcohol bath then blow it dry with a computer duster ..a regular hair dryer will help dry it completely or let it sit..until the part you cleaned is not sticky at all..startup with dbt first..
Click to expand...
I was using 91%. May need to dump and get some fresh IPA. I have given the board a good inspection for solder bridges. I do not see anything. I have given the board a rinsed in hot water and a scrub with a toothbrush to remove flux residue. Going to let it dry overnight and try again tomorrow AM. Keeping finger crossed it is just a flux bridge.
 
avionic said:
I use full strength acetone from Menards or Lowes. With a stiff bristled 1" paint brush.Cleans the boards up very well and rapidly drys.Been using this method for years.
Click to expand...
Thanks Avionic. IIRC nailpolisher remover = acetone? If so, the wife may have some.
 
if I understood you correctly you say you have DC offset of 52Volts? and it is unresponsive to VR01 ? Question: are you sure your VR02 is in a center position and not to the left or to the right ?
 
TupolevFan said:
if I understood you correctly you say you have DC offset of 52Volts? and it is unresponsive to VR01 ? Question: are you sure your VR02 is in a center position and not to the left or to the right ?
Click to expand...

I measured the old trim pots when I removed and reset the new ones to a matching value. I then tried to adjust. After that, I did "recenter" to ~13 turns into the travel of the sealed trim pot on subsequent attempts.
 
special_20 said:
I measured the old trim pots when I removed and reset the new ones to a matching value. I then tried to adjust. After that, I did "recenter" to ~13 turns into the travel of the sealed trim pot on subsequent attempts.
Click to expand...
Just to confirm we are talking about VR02 not VR01 remember there are 2 of them the V02 for fine tuning and it also should be centered ("resistance" wise not turn "wise"). I hope your dual FET did not get damaged in a process. Also ones you resolve this I would still recommend to remove output transistors and clean the old paste re apply new + new mica film. Also did you re apply the thermal paste to the driver transistors? it is possible to short the collector and the heatsink with the bolt there(probably not in this case).

Well I would connect the module and measure all Power Supply Voltages., Voltages across zenners. and Voltages on C11 and C14 (could be measured between the ground and one of the pins of R31 and R28.
 
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