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919 DRIVER AMP BOARD Rebuild...

ghamilton

ghamilton

Super Member
I'm going to rebuild my AU-919 driver amp curcuit board. The first thing is to find out which parts need to be replaced on this board. (I'm doing this to save those output transistors EW warned me about.)
I don't think I'll have a problem with picking out good electrolytic caps. Replacing those (so called) "black flag"caps is my main concern.
I'm thinking that they (the black flags) are all pf value caps. Is this correct?
Should I replace every cap of every kind on this board?
What other parts on this board should I replace?
Thanks Guys!
 
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EW knows a lot of the AU 919(and he knows a lot of other equipment), ask him, but I'm sure he advices you to replace all the "black flags" for silver mica caps.:yes:
 
ghamilton,

hello again! With this board, I do not believe it necessary to replace every cap of every kind. There are plenty of film caps on this board, and those can remain. The black flag caps are indeed pF values and are very low, such as 9 pF, 22pF etc. justradios.com has a good selection of values for good prices for these specialty caps.

When I did this, what really helped was to replace the trim pots with Bourns sealed 25-turn pots. See digikey for these, and be sure to get the orientation to match the originals. These are somewhat expensive, but so much more reliable and easy to adjust than the existing single-turn pots.

A lot of people find that the pair of large resistors at the bottom of the board are out of tolerance. These are located under the board's heatsinks, right by the output coils. so check these.

According to the SM, there is also a pair of electrolytic caps in the signal path that could be replaced with Blackgate or other high-quality electrolytic. These are C33, C34, located around TR23, TR24. Who knows what's on your board, but my manual says 1 uF, 50V.

The other thing, and this is important, is to clean off all the brown glue that Sansui used at the large capacitors. It has proved to be corrosive on those components that it touches. Just do a search here on "sansui glue" for more info. so if you see the brown gunk on a resistor lead, for example, its worth it to replace that part or at least clean it off really well, and scrape the stuff off of the board while you're in there. I forget what EW said he used to get all the gunk off...acetone perhaps. Others on AK know far more about this and other issues. This part of the rebuild is a fair amount of work, but will make later performance more reliable.

I know that sounds like a lot, but this board is pretty important to the amp.

Collin
 
Those output caps were 1Uf 80V on the unit that I just did. The service manual is woefully inaccurate on cap voltages and values. Use what is actually in the unit.
 
Thanks NT!
Below is a e-mail response from stereo surgeons that may be helpful. This is what they had to say back in october of 08:...

"While it is true that all electrolytic capacitors will eventually fail it is impractical, and very expensive, to replace all electrolytic capacitors, especially for a problem which 95 % of the time is decades away from occurring. What is more imminent, and will cause extreme damage, are the heat stressed connections which develop on both audio driver boards in addition to the mechanical collector connections used on the audio driver transistors. These connections are already bad and getting worse each time your item heats up and cools down. In a direct coupled design, which your amplifier is, just one intermittent solder connection can send your item into an extreme bias condition causing catastrophic failure of the audio output stage(s)."

Sincerely,

Ken ########
President, Stereo Surgeons, Inc.
http://########################
 
Quoting Ken won't win you many brownie-points here at AK.

On top of that, he's just flat wrong on most of the points he chooses to address (but I agree the connections on the drivers needs to be refreshed...new solder on the collector bolt hole and a good tightening of the scew & nut).
 
EchoWars said:
Quoting Ken won't win you many brownie-points here at AK.

On top of that, he's just flat wrong on most of the points he chooses to address (but I agree the connections on the drivers needs to be refreshed...new solder on the collector bolt hole and a good tightening of the scew & nut).
Click to expand...

Thanks EW. We'll take your advice!
I have now got the board out.
There are eight 3pf black flags that I can't find exact (3pf) matches for in silver mica at Digikey. What do you suggest in those places? (C23-24-25-26-53-54-55-56)
 
noveltone said:
ghamilton,

hello again! With this board, I do not believe it necessary to replace every cap of every kind. There are plenty of film caps on this board, and those can remain. The black flag caps are indeed pF values and are very low, such as 9 pF, 22pF etc. justradios.com has a good selection of values for good prices for these specialty caps.

When I did this, what really helped was to replace the trim pots with Bourns sealed 25-turn pots. See digikey for these, and be sure to get the orientation to match the originals. These are somewhat expensive, but so much more reliable and easy to adjust than the existing single-turn pots.

A lot of people find that the pair of large resistors at the bottom of the board are out of tolerance. These are located under the board's heatsinks, right by the output coils. so check these.

According to the SM, there is also a pair of electrolytic caps in the signal path that could be replaced with Blackgate or other high-quality electrolytic. These are C33, C34, located around TR23, TR24. Who knows what's on your board, but my manual says 1 uF, 50V.

The other thing, and this is important, is to clean off all the brown glue that Sansui used at the large capacitors. It has proved to be corrosive on those components that it touches. Just do a search here on "sansui glue" for more info. so if you see the brown gunk on a resistor lead, for example, its worth it to replace that part or at least clean it off really well, and scrape the stuff off of the board while you're in there. I forget what EW said he used to get all the gunk off...acetone perhaps. Others on AK know far more about this and other issues. This part of the rebuild is a fair amount of work, but will make later performance more reliable.

I know that sounds like a lot, but this board is pretty important to the amp.

Collin
Click to expand...

Hello EW. Are you in agreement with NT on this? My trim pots work fine when adjusting the bios & DC offset. I'll keep em. Now those resistors,...I'm not sure. And "a pair of electrolytic caps in the signal path"? I thought this was a DC amp.
 
Stock trimpots are crap. Replace them.

The 1µf 50V cap is not strictly a signal path cap...it is an AC bypass across the bias servo transistor to prevent the audio signal from modulating the bias current. You can replace with an electrolytic, or a 1µf 50V Panasonic stacked film cap.
 
Dude,

You don't have to rely on myself, EchoWars, and the other folks who have contributed this thread alone for advice. Do a search for this amp and you'll find a ton of material and past issues/work people have done to their various units. From neat little tricks to solve inherently annoying problems all the way to must-do asap stuff, and a bunch of stuff in between. I am not just making up work for its own sake, but trying to save you from having to go back in there and swap out a troublesome trim pot later, for example. Lots of people swear by these Bourns parts, which is why I put them in. Watch that karmic wheel turn. By all means if you think yours are fine, keep them, but since you asked, I gave you one guy's version of the whole story, that worked.

You'll find as many opinions as there are AK members. That's the beauty of it. EchoWars is tremendously knowledgeable around here and AK would not be the same without him. Other guys are also useful, and I'm not necessarily one of them. The great thing about AK is that unlike diyaudio or head-fi people usually don't jump all over you when you say something a shade ill-informed or have a newbie question. Also people are usually less strung up on their high horse about various personal opinions.

Anyway, food for thought. :)

Collin
 
ghamilton,

yes, I have been too busy to catch up lately. Those parts look good from what I can remember. I know the 3296P is the right part, (25-turn, sealed) and those orientations are correct for the driver board. When you take out the old ones, be sure the new ones have the same value range.. ie 100 ohm for 100 ohm. Somewhere in this amp the pots were different than the SM says. Can't remember where, but just look on the side of them to see what the existing pots are. You can also measure them with a DMM, turning the pot up and down to find its max value.

If you have the SM, the bias and offset adjustment procedures are in there. using your DMM it might be wise to set the offset pots to 1/2 their value (50 ohms if above is correct). The bias pots are more tricky. When you take them out, measure them with the DMM to see where they're set currently and set their replacements to the same value as a starting point. That way when you turn the amp on, the bias won't be sky-high. I find the bias and offset to affect each other, so you have to go back and forth anyway until they're both satisfied.

One last note from me: after you solder in the new pots, be sure you have a clear path for your screwdriver down through all the components to the bias pots. If you don't ,just twist it in place until you do. No biggie.

I'm sure others will have their own tricks. There is also a sticky on the Sansui forum regarding DC offset and bias. it's super long, but EW has a nice intro post. I'm sure you've already found it by now. Best of luck.

Collin
 
Thank you NT. I'm sorry if I ever said anything stupid to upset you. Your help is very valuable to myself and others. This is what makes audiokarma the place to be!
Greg :yes:
 
OK, I got it back together and I really feel good about it! With these new pots I am having trouble getting the relays to kick in because I don't know where to set them for start-up. I went by the SM but these are different pots. Now what? :tears:
 
I replaced all the trimpots in my AU-919 with the bournes 25-turn pots. Makes dialing in values a breeze.

I also did the 'whole enchilada' restore and replaced every capacitor whether it be electroltyic, mylar, or mica. With the exception of the ripple filters that is! I use the amp daily in my bedroom with a pair of Dynaco A-25's and AKG-701 headphones for night listening while the wife sleeps. It sounds great. Sometimes I just find myself sitting on the edge of the bed in a trance staring at it!
 
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