• Please note that Audiokarma will be offline briefly for updates early on Monday morning, September 28th.

Tracking a short in an AU-666

TheThreat

TheThreat

New Member
I'm trying to track down a fault in an AU-666 I bought recently and I'm looking for some ideas. I think I've narrowed it down to one of the Driver Blocks but I can't seem to pin down the problem. I'm going to refer to it as the "bad" board until I can find another cause. With both driver boards out I was able to run the Pre-Out to another amplifier and it sounded just fine.

I'm fairly new to this game so I'm hoping I've missed something obvious.


Link to the service manual.

Link to the schematic.

Symptoms:
  • Instantly blowing F003 or F004, depending on where the bad board is.
  • No audio whatsoever from either speaker channel with the boards in.
  • Can't adjust current bias on either channel (0mA)
Things I've tried or noticed:
  • Swapped the position of the driver boards. Blew the opposite fuse.
  • Examined the board for bad traces, anything burnt and can't find anything.
  • Compared the resistance between all the pin outs between both driver boards. Almost everything went to overload. Some traces charged caps. Nothing was shorted.
  • Measured all the resistors in circuit on the "bad" board. Nothing out of the ordinary.
  • Tested the output transistors and seem fine, though I have replacements. Tested using a generic amazon Cap/transistor tester.
  • Removed all the transistors, diodes and caps from the "bad" board and tested.
  • Nothing tested as shorted, and almost everything seemed to test reasonably. One electrolytic cap was high (reading 123uF, nominal was 100uF), and a tiny ceramic (15pF) wouldn't read on the tester, but on my DMM seemed okay (it charged)
  • When I go full CCW on the 1k current bias pot, I can't get it close to 0ohms. The lowest I can manage is ~170ohms/1100ohms, and it stays about that until about 10-15% of the sweep when it starts acting as I'd expect. The last 10-15% of the sweep has the same behaviour.
I'm committed to getting this thing back in running order as its cosmetically really nice and clean. If anyone can share some wisdom I'd be extremely grateful.
 
Register to hide this ad
First, check all the voltages in your power supply board with both amp boards removed and compare them with schematic values. If all looks good then move to amp board.
 
Replace the trimmers and all the transistors on the bad driver board.
And check the bias transistor.

The input pair should always be replaced anyways, and the flying saucers need to go, so really you're just beefing up the driver transistors for no reason. I had one and dang near replaced every single part in it I liked it so much. The bridge rectifier had toasted.
 
First attempt at embedding a photo. Sorry in advance for doing it wrong.

On the Plus ripple block (F-1231), thing seem around where they should be. Expected a bigger drop. Expected 39.5 in and 38 out, I had 40.8 and 40.3VDC.

On the Minus (F-1277) it was even less of a drop (rise?). Expected -39.5 in and -32 out. I had -41.1 in and -40.5 out.
16124875534768289663792418726665.jpg
 
ivandezande said:
Replace the trimmers and all the transistors on the bad driver board.
And check the bias transistor.

The input pair should always be replaced anyways, and the flying saucers need to go, so really you're just beefing up the driver transistors for no reason. I had one and dang near replaced every single part in it I liked it so much. The bridge rectifier had toasted.
Click to expand...

I'm guessing the one circled is the Bias Transistor?
upload_2021-2-4_21-26-21.png
When I tested it these were the results:
HFE : 309 (Nominal is 60-320)
Vf : 841 mV

Which are the input pair you're referring to?

I'm considering just stripping both driver boards and redoing with fresh bits. It would probably be less work that this is.
 
petehall347 said:
i take it you tried the good board on its own ?
Click to expand...
Good board in by itself doesn't pop the fuse. Haven't had a chance to hook speakers and a source to it yet.

I've got a cart started at Mouser for some new TR and I'll probably end up with some caps in there too. Sadly the two most popular replacements for the "troublesome" 2SC871s are backordered until October.

Should I bother ordering replacement caps for the driver board?
 
Last edited:
TheThreat said:
Good board in by itself doesn't pop the fuse. Haven't had a chance to hook speakers and a source to it yet.

I've got a cart started at Mouser for some new TR and I'll probably end up with some caps in there too. Sadly the two most popular replacements for the "troublesome" 2SC871s are backordered until October.
Click to expand...
Can you set the bias and dc offset on the good amp? The Schematic doesn't show any 2SC871 but you can use KSC1815 if you can't have ksc1845.
 
TheThreat said:
I'm guessing the one circled is the Bias Transistor?
View attachment 2120980
When I tested it these were the results:
HFE : 309 (Nominal is 60-320)
Vf : 841 mV

Which are the input pair you're referring to?

I'm considering just stripping both driver boards and redoing with fresh bits. It would probably be less work that this is.
Click to expand...
Don’t pull transistors! Just test for shorts in circuit. Transistors work, or they don’t. Save the solder pads the wear.
The input pair is literally the first pair of transistors.
That’s exactly what I was suggesting, replacing all active components.

Newark may still have KSC1845.
 
Shorts can be a real PITA to track down, everyone finds a different cause. Good advice from sanjib saying to test good board first.

Correct, 2SC871 are on the tone and phono boards only. Your main boards input pair are PNP, so be mindful there, can use BC560,KSA992, ZTX795A, check the specs and pinout to decide for yourself.

Flying saucer sub's I go for Toshiba TTA004B and TTC004B.

Are you using a Dim bulb tester? Please do, you'll want to save your outputs..
 
AnalogueBen said:
Shorts can be a real PITA to track down, everyone finds a different cause. Good advice from sanjib saying to test good board first.

Correct, 2SC871 are on the tone and phono boards only. Your main boards input pair are PNP, so be mindful there, can use BC560,KSA992, ZTX795A, check the specs and pinout to decide for yourself.

Flying saucer sub's I go for Toshiba TTA004B and TTC004B.

Are you using a Dim bulb tester? Please do, you'll want to save your outputs..
Click to expand...

Looks like I'm a little late seeing I had a reply. I submitted an order to mouser late Sunday night for a while array of transistors and caps and trim pots to try and rebuild the board.

The dim bulb tester is something I just recently learned about so I'll be building one this week or weekend before I do any more live fire testing.

I'm hesitant to post what I'm using as replacements in case someone copies my misinformed decisions but I made a spreadsheet of all the caps, transistors, diodes, etc on the parts list so I might share that to help someone else in the future.
 
Been a month without an update but progress has been made. Using the DBT I made, i had one board lighting the bulb right up and the other other side caused basically nothing to happen.

Short story:
Some success. Swapped components and now DBT indicates no shorts, and I can set the DC bias but can't adjust current.

Long story:
I received my Mouser order and started swapping components out on the shorted driver board (or at least the one causing the DBT to go bright.) I swapped caps first, then diodes, trimmers, and finally transistors. I set the trimmers to about halfway as a starting point. I was testing on the DBT every once in a while but it wasn't until the very last transistor that the DBT went out. Success. DBT gave a little blink on power up and went right out in both channels.

Did all the same components on the second driver board and popped it in. DBT would blink and go out, then slowly come back on. I triple checked all my part numbers, orientations, etc. Then i remembered I hadn;t set the trimmers. Set those to about the middle and tried again. Blinked on then straight off. More success.

Tried setting the DC bias and no problem. New trimmers are way easier to adjust. I tried setting the current per the manual and I'm not seeing any current across the fuses. Anyone had this issue before? Or can think of some silly thing I'm not doing right?
 
TheThreat said:
Tried setting the DC bias and no problem. New trimmers are way easier to adjust. I tried setting the current per the manual and I'm not seeing any current across the fuses. Anyone had this issue before? Or can think of some silly thing I'm not doing right?
Click to expand...


Could be a problem with your meter, check the following;
.Set to mA DC
.Ensure connection are clean
.Check milliamp fuse in meter
If above is sll OK then keep keep on DBT initaly until you get a reading following service manual.
I have a link that shows pics of how to set up... I'll look now..
 
AnalogueBen said:
Could be a problem with your meter, check the following;
.Set to mA DC
.Ensure connection are clean
.Check milliamp fuse in meter
If above is sll OK then keep keep on DBT initaly until you get a reading following service manual.
I have a link that shows pics of how to set up... I'll look now..
Click to expand...

You are a genius. The fuse in my meter was popped. I must have blown it on my first test before I made the DBT. I set the voltage and current adjustments super easily. I hooked up my Pioneers and it sounded oh so sweet.

A huge thanks to everyone that contributed to this thread. I'll post the parts I used too now that I know they've worked.
 
No genius here, I've just seen so many simple things come up with working on electronics that could send you crazy for hours until they get noticed.
Quick example, and an important one too!
Serviced an amp over the weekend and checked over the power cord and saw this damage
Screenshot_20210316-225442_Gallery.jpg

Luckily I missed getting a jolt, but I always check the simple things such as power cords.

Yes! Please share your parts list and even photos if you're able :beerchug: The 666 is a wickedly good unit...but it is a Sansui after all :bowdown:
 
You must log in or register to reply here.
Back
Top Bottom