Au222 right channel noise

[AnalogueBen]

Guys I've picked up nice little Sui but she's got issues in the right channel.

Pots and switches are clean and quiet. I've pulled the 2sc871 on the pre board and dropped in a 1845 to see if that fixed it... nope.

The drivers are 2sc815 and 2sa539.
These are commonly replaced with ksc815 and ksa539. I have read a post on here with the use ksc1815 and ksa1015, which I have on hand.
http://audiokarma.org/forums/index.php?threads/au-222-repair.577488/

Would it hurt to try them? They seem close in comparison. Otherwise how can I fault find the issue, I'm armed, but not very dangerous with my fluke dmm and a scope (not familiar with checking schematics).
Thanks for any advice

 

[richardpp]

I used the 2SC1815 and complement 2SC1015 as a replacements with no problems.

However, before physically changing out transistors, you might find the freeze spray method will make it easier to localise the problem. By spraying each transistor in turn, hopefully the culprit will stand out.

The 2SC871 is known to be noisy. It looks like there are 6 of these in the AU-222. They can be replaced with 2SC2240 or KSC1845.

The trim pots VR801, 802, 803, 804 on the main amp board might also be causing problems if they are original.

 

[AnalogueBen]

Ok thanks, the ksc 1815 and ksa 1015 aren't quite compatible it seems anyway. The ones you've suggested will be my go to.
This may sound wierd but I get a bit funny about dosing transistors with freeze spray (I'm still a novice), I hold some belief that it may upset the "form" of them and may not function as normal following this...yes wierd i know but if you think along the lines of "things are only original once" you may understand my cautiousness, I'd hate to ruin a part that was otherwise functioning normally.
Still looking for ways to test for faults using tools of the trade,,, help anyone??

I've changed out the bias pots for bourns type already, the left channel seems to fluctuate quite a lot, so I think there are at least a few issues going on with this amp....sad face...

 

[richardpp]

Verifying correct power supply voltage is always a good place to start. The schematic shows the voltage readings you should be getting.

If the supply voltages to each board are good, you could start probing other parts of the circuit, taking care not to create any shorts. Again, the voltages you should expect to see at specific points in the circuit are marked on the schematic. By comparing voltages readings with the schematic and between channels you may get closer to the problem.

Since you’ve replaced the trimmers VR801-4, I assume you’ve also carried out the amp alignment procedures.

Does the noise increase with volume? If so, the problem is likely to be on the Head Amp Board F-1064. If not, then more likely it is on the Control Board F-1083 or the Main Amp Board F1082.

The AU-222 is a little masterpiece, take your time with it. I'm sure you'll be rewarded!

 

[slimecity]

but she's got issues in the right channel.

What are the exact symptoms to the issues?

Noise, crackling?

The fluctuating bias in the left channel is a separate issue?

 

[AnalogueBen]

Okay great advice, I will have a probe around when I've got some time to spare. So when taking the voltage readings, does my negative lead of the meter just need to stay connected to common ground or is it not done like that?
Sorry should have been a bit more specific in the symptoms.. It's a static noise with some cracking. And yes the left bias seems a separate issue. I dont remember now how much it was affected by volume increases, but I don't think it was, so I'll follow the advice regarding which board the problem my be in.
Hopefully find a few hrs over the weekend to investigate.
Cheers

 

[richardpp]

I will have a probe around when I've got some time to spare. So when taking the voltage readings, does my negative lead of the meter just need to stay connected to common ground or is it not done like that?

Negative lead to common ground, a connection to the chassis is fine.

 

[quadmatrix82]

I used the 2SC1815 and complement 2SC1015 as a replacements with no problems.

However, before physically changing out transistors, you might find the freeze spray method will make it easier to localise the problem. By spraying each transistor in turn, hopefully the culprit will stand out.

The 2SC871 is known to be noisy. It looks like there are 6 of these in the AU-222. They can be replaced with 2SC2240 or KSC1845.

The trim pots VR801, 802, 803, 804 on the main amp board might also be causing problems if they are original.

Spot on advice my bet is the 2sc871 are the issue why not recap while your at it it’s a simple amp to work on and will come up very nice

 

[kevzep]

I hold some belief that it may upset the "form" of them and may not function as normal following this

Nonsense, use the freeze spray. Transistors are designed to have a wide operating temperature, as they age, this is not the case any more, using the freeze spray is a way of testing the operation/leakage of a transistor through different temp ranges.
Its very standard operating procedure.
Get rid of all those old transistors, put them in the round filing cabinet, get some new silicon in there and it will sound as good as new...

 

[AnalogueBen]

Okay so it's back up and running!
Oh how musical it is, sweet as..

I had not changed out the center voltage pots when I did the bias ones(late night), so with this change alone I was now able to get a stable bias on the left channel.
I gave the transistors a hit with the freeze spray as positively encouraged to. I found the 2sc871 that I'd left in (wasn't any issue last time of testing) was weak, and now cracking like the other I pulled and replaced with a 1845.
So now, the pre amp board that has x2 transistors are both 1845s inplace of 871's,
I do feel that the bass has suffered slightly, less of that thud I knew it to have, but I'm rewarded with a cleaner presentation.
I'll definitely try out a few alternative transistors in this position down the track.
Also got my meter hooked up and tested some voltages per the schematic, that was a first for me, and I plan to continue building that skill.

Still has some constant static in the right (little in the left too) at all times, but it doesn't get noticed once music is playing. New driver transistors shortly..

Thanks for the advice guys, Its great to have support from the AK community and then be rewarded by your own efforts. Peace!

 

[AnalogueBen]

Update with pics..
Instead of the ksc1845's , I've installed ztx694b's... big difference, full sound stage, dynamic, mucho bass.
I compared by having both a 1845 and ztx694b in place, then switch to mono and used the balance pot to go from left to right, there was to me a clear favorite.
But, they are a bit high in gain and the volume pot doesn't need much tweaking, I'll play with caution here guys...

 

[pete_mac]

Get rid of all those old transistors, put them in the round filing cabinet

Round filing cabinet... hehehe

 

[AnalogueBen]

Let me start off today by reiterating how incredible this small amp is, it's 22 watts are addictive to listen to. I put it back into rotation a few days ago, paired up to AR6 and also some Seas 3 way's.
No comes the problem
The hum in the right channel is back and its pretty ugly now. At first it was a small hum, not really much to worry about, then it just got worse, very noticeable. It is always there and not affected buy volume changes, input switches or the rest, it's just there!
I replaced the small signals on the board with ksc815 and ksa539, even poped in a new 1845 transistor on the pre just to cover all the small transistors. Crossed fingers and switched on, hum now a little louder (new transistors had bit more gain) and within a few seconds it blew the speaker protection 1.5A fuse. Before changing the transistors it didn't seem to blow the fuse, maybe due to lower output from original drivers.
On the main board right channel, I lifted the leads of 2 diodes to check for continuity, checked the 2 mica caps, and tested the bias 2sc281 transistor. Doesn't seem to be shorts on the outputs ( these are soldered onto board) and besides there is an output..justva violent one.
I'm really not sure what is up and maybe it's getting beyond my skills of repair, It took out another $10 fluke fuse too

As much as I love to dabble in this hobby, it takes a heck of time and money!! I've put the tools down for a few days while I have a rest and hopefully I can get shown some guidance to find the issue and create for myself some skills in tracking faults, after all I've got a scope and DMM, plus other necessary items for the fixes, just need the know how.

Cheers guys

 

[slimecity]

You want to build a DBT and power it up thru that whilst testing.

She needs a recap anyway - it could be that some of the caps arent blocking DC where they should be.

Have you measured DC offset on the noisy channel at the speaker terminal?

 

[AnalogueBen]

I should have used my DBT, probably might have saved the Fluke fuse.

I used my scope (I'm very new to it tho) to pick up the distortion that seemed to apper after the output cap, but couldn't find any noise before it. Forgot to mention that I pulled that cap and installed another hoping it was the issue, it wasn't..
So the cap went back.
DC offset? remember this is cap coupled and that might be a non issue reading.
I really don't want to go recap just yet, It was sounding so darn amazing before the hum..
Cheers

 

[AnalogueBen]

Quick update -
It's running on the DBT now, with the fues in the lighy goes straight on. Instead of a 52v supply I'm now getting around 8v.

Is this the recommendation for testing, while it's on the dbt? Because without the fuse in I get no power to the channel to take readings.

Away for a couple days now...thanks as always guys

 

[Hyperion]

It's running on the DBT now

What is the reason for running it on a DBT at the moment?

 

[AnalogueBen]

You want to build a DBT and power it up thru that whilst testing.

So just to clarify; the bulb is now ON whilst the right channel speaker fuse is IN.
The fuse will blow with a few seconds if not connected to the DBT, then only left channel works and unable to check voltages on the affected right.
Kind of in a hard place aren't I?? Maybe I shouldn't run even a low voltage while there is the issue?

When I was probing around with the scope I only could see the noise up until the - side of C818 (1000uf output cap) but not on the + side... im wondering now if a toggle switch could be shorted internaly and only the right channel contacts?

 

[tom_tm]

Hey @AnalogueBen! I just found this thread, and I thought I'd read them all! Obviously not. Great resource, and I've now bookmarked it.

BUT...

I guess you figured out what was wrong in the end? I'd love to know!

 

[AnalogueBen]

I guess you figured out what was wrong in the end? I'd love to know!

This little guy still very much crackles away and unfortunately I haven't been doing a lot of testing with it. The issue with blowing the fuses were the replacement drivers having different pinout, that frustrated me to know I did that I'd love to have it perform like new, it's power and sound are both deceiving and I really like its minimalistic layout.