Trouble with flat amp on AU-919

[lbcgav]

I'm getting some static sound from my speakers that does not vary with the volume control. This happens intermittently and only occurs when the flat amp is in use (goes away with the jump switch on). For now, I've been bypassing the flat amp, but I'd like to use the tone controls, so I was hoping someone had a suggestion concerning the origin of the problem.

This amp has been redone according to Leestereo's fantastic thread. None of the transistors or the dual FETs have been replaced on the flat amp. The sound is pretty loud and is a combination of static and crackling along with some intermittent pops. As stated above, it does not vary with volume and goes away entirely when the jump switch is engage.

Here is the flat amp after restoration:


If anyone has any ideas on what could be causing the static sound, I'd be very grateful.

 

[Leestereo]

TR01-04 are 2SC1313 types and are somewhat failure-prone, resulting exactly in the symptoms you describe.

 

[Hyperion]

Interestingly and topically I have just had a wrestle with an AU-919 Flat Amp, doing exactly the same thing (left channel only), and this one had also also been reconditioned. I changed the 4 x 2SC1313's and the dual diodes - and still something wasn't right. So I changed all the remaining T092 transistors - making 14 +2 dual diodes changed. Now the Flat Amp seems quiet, you could be pretty sure it would make the noise for a short period after power up - and intermittently at other times - a sound that I would liken to 'rustling papers'.

Time will tell whether I have solved the issue, the Dual FET's and TR05 & 06 are the only semiconductors not yet changed.

 

[JoseHH]

Could oxidization of the connectors also be causing some of these issues?. It had happened to my AU-D907 in the conector that brings the signal into the power amplifier. Admittedly, a different board alltogether but this issue could happen anywhere where dirty connectors may occur. Just a thought..

 

[Hyperion]

Just a thought..

The only contacts on the AU-919 Flat Amp that are not soldered are the power supply connection where (Flat Amp) L&R are common - all the signal connections are soldered.

 

[kevzep]

Interestingly and topically I have just had a wrestle with an AU-919 Flat Amp, doing exactly the same thing (left channel only), and this one had also also been reconditioned. I changed the 4 x 2SC1313's and the dual diodes - and still something wasn't right. So I changed all the remaining T092 transistors - making 14 +2 dual diodes changed. Now the Flat Amp seems quiet, you could be pretty sure it would make the noise for a short period after power up - and intermittently at other times - a sound that I would liken to 'rustling papers'.

Time will tell whether I have solved the issue, the Dual FET's and TR05 & 06 are the only semiconductors not yet changed.

These sorts of scenarios are becoming more common as these units age more. I am finding more and more components which we would never have suspected are now becoming suspect. I have had some good ones lately, involving ceramic and poly capacitors......
So we need to broaden our fault finding more now.

 

[slimecity]

Could oxidization of the connectors also be causing some of these issues?.

Second this - clean up those contact pins & connectors while you have this out.

Also check that the pins are soldered OK into the PCB still and that pulling the PCB in and out hasnt affected any of the points where the pins are soldered into the PCB. Reflow these to be sure.

It sounds like you may need to go down the road that Hyperion suggested also

 

[lbcgav]

Interestingly and topically I have just had a wrestle with an AU-919 Flat Amp, doing exactly the same thing (left channel only), and this one had also also been reconditioned. I changed the 4 x 2SC1313's and the dual diodes - and still something wasn't right. So I changed all the remaining T092 transistors - making 14 +2 dual diodes changed. Now the Flat Amp seems quiet, you could be pretty sure it would make the noise for a short period after power up - and intermittently at other times - a sound that I would liken to 'rustling papers'.

Time will tell whether I have solved the issue, the Dual FET's and TR05 & 06 are the only semiconductors not yet changed.

This sounds exactly like my problem. If you have time, would you by chance have a list of the replacement parts you used?

 

[Hyperion]

The 2SC1313's were replaced with KSC1845's - the 2SC1845's were replaced with KSC1845's and the 2SA992's were replaced with KSA992's

To summarize
2SC1313 ---> KSC1845
2SC1845 ---> KSC1845
2SA992 -----> KSA992

And the dual diodes were replace with 2 x 1N4148 in series.

I also matched the hfe of all the replacements so that both L&R had the same hfe in the same position, not essential, but I thought it would be a good thing to do. Incidentally I measured the hfe of the outgoing devices which were all over the place, ranging from as low as 340 to as much as 580 - and we know that devices with very high hfe for the type can be suspect.

As a health check I very closely monitored the DC offset from that board after power on, paying attention to the warm up drift, and the time for each channel to settle. It was only after changing all the T092 transistors that I got rid of an odd 'kink' in the DC offset warm up characteristic for the left (faulty) channel.

Other techniques like the use of freeze spray, tapping the board, and gentle 'nudging' of transistors were of no use here. (I did use some harsh language - it didn't help, but it made me feel better )

 

[Multimode]

The reconditioned unit just repaired by John (Hyperion) is mine. I received it back this morning and am listening currently to Dvorak's 'New World Symphony' at extended volume. As good as it gets musically... and with no unwanted noise!

Earlier I played Bill Frissell's 'Pipe Down' from his 'East/West' album, this being one of my favourite test tracks for evaluating system performance. Lots of other stuff inbetween, from the aggressive 'Steam Train' album by Hi-Lo (not my sort of music, but the title interested me as a steam engine nut) to the lovely singing voice of Cyrille Aimee.

All were rendered perfectly and I'm very grateful indeed to John for the dedication and expertise he put into identifying the possible causes of the noise and likewise in correcting it.

 

[lbcgav]

The 2SC1313's were replaced with KSC1845's - the 2SC1845's were replaced with KSC1845's and the 2SA992's were replaced with KSA992's

To summarize
2SC1313 ---> KSC1845
2SC1845 ---> KSC1845
2SA992 -----> KSA992

And the dual diodes were replace with 2 x 1N4148 in series.

I also matched the hfe of all the replacements so that both L&R had the same hfe in the same position, not essential, but I thought it would be a good thing to do.

As a health check I very closely monitored the DC offset from that board after power on, paying attention to the warm up drift, and the time for each channel to settle. It was only after changing all the T092 transistors that I got rid of a odd 'kink' in the DC offset warm up for the left (faulty) channel.

Other techniques like the use of freeze spray, tapping the board, and gentle 'nudging' of transistors were of no use here. (I did use some harsh language - it didn't help, but it made me feel better )

Thanks John! Your help is always appreciated.

When you say the double diode, you mean the MV-12, correct? Any need to replace D01 and D02?

 

[kevzep]

Thanks John! Your help is always appreciated.

When you say the double diode, you mean the MV-12, correct? Any need to replace D01 and D02?

Yes MV12 is 2 x 1N4148 in series.

Pretty safe to say D01 and D02 can be replaced with 1N4148 too, but just check the schem, I don't have it handy right now..

I have also been replacing Zeners have found a few bad ones lately....

 

[lbcgav]

Yes MV12 is 2 x 1N4148 in series.

Pretty safe to say D01 and D02 can be replaced with 1N4148 too, but just check the schem, I don't have it handy right now..

I have also been replacing Zeners have found a few bad ones lately....

Thanks Kev. All zeners have been replaced per Leestereo's instructions.

 

[Vintagear]

I'm getting some static sound from my speakers that does not vary with the volume control. This happens intermittently and only occurs when the flat amp is in use (goes away with the jump switch on). For now, I've been bypassing the flat amp, but I'd like to use the tone controls, so I was hoping someone had a suggestion concerning the origin of the problem.

This amp has been redone according to Leestereo's fantastic thread. None of the transistors or the dual FETs have been replaced on the flat amp. The sound is pretty loud and is a combination of static and crackling along with some intermittent pops. As stated above, it does not vary with volume and goes away entirely when the jump switch is engage.

Here is the flat amp after restoration:
View attachment 1314292

If anyone has any ideas on what could be causing the static sound, I'd be very grateful.

I hope you get this sorted out soon.

Good luck.

Interestingly and topically I have just had a wrestle with an AU-919 Flat Amp, doing exactly the same thing (left channel only), and this one had also also been reconditioned. I changed the 4 x 2SC1313's and the dual diodes - and still something wasn't right. So I changed all the remaining T092 transistors - making 14 +2 dual diodes changed. Now the Flat Amp seems quiet, you could be pretty sure it would make the noise for a short period after power up - and intermittently at other times - a sound that I would liken to 'rustling papers'.

Time will tell whether I have solved the issue, the Dual FET's and TR05 & 06 are the only semiconductors not yet changed.

These sorts of scenarios are becoming more common as these units age more. I am finding more and more components which we would never have suspected are now becoming suspect. I have had some good ones lately, involving ceramic and poly capacitors......
So we need to broaden our fault finding more now.

When my tech restored my X1 3 years ago he replaced many of the ordinary transistors that are usually not replaced in restorations. His reasoning was that the equivalent parts were inexpensive and he was already in there replacing components and reflowing the entire board anyway. Sooner or later one may act up due to age. If the unit comes home to me and one of those transistors starts to act up and do the "rustling papers" thing, it will cost me more to send it back to have him hunt down, identify and replace the culprit.

Some call this somewhat of a "shotgun" approach but I think for equivalent run of the mill transistors it makes a lot sense. Especially if you are paying a qualified tech out of state to do the work. If you're skilled at this kind of stuff and can do the repairs yourself or your tech lives nearby, that's a different story. Of course, special/rare hard to find equivalent parts like FETs and such should be left alone if they are not acting up at the time of restoration. You take your chances on those.

 

[kevzep]

Thanks Kev. All zeners have been replaced per Leestereo's instructions.

Good call on that....
I am going more thoroughly through units now with my restos, these ageing old beasts, they need a little more attention now than they did even 10 years ago....Its more than just caps these days.

 

[ManyMoonsAudio]

I am seeing more transistor problems in the last 5 years or so. The Marantz receivers are real bad about this.

 

[stereofun]

Shotgunning used to be a swear word around here, but as alluded to, with aging units it just might make sense, as you spend less time replacing a group of suspects, rather than individually desolder, test, resolder until culprit is found - only to realize that the culprit's brethren are the same age and make and really should be retired as well. Ofcourse when a burnt resistor is found, there is often a secondary trigger downstream - but maybe transistors just naturally degrade over time, and finally their 1970' ties birth certificate is expiring.

 

[Hyperion]

I was pretty well convinced it was down to one device causing the issue, but I couldn't find out which one - so, I changed them all. And although this was technically shotgunning - which is not technical I feel I exhausted all practical avenues to get this resolved in a reasonable time, I also had to pay due consideration to the amplifier's owner too.

 

[kevzep]

I was pretty well convinced it was down to one device causing the issue, but I couldn't find out which one - so, I changed them all. And although this was technically shotgunning - which is not technical I feel I exhausted all practical avenues to get this resolved in a reasonable time, I also had to pay due consideration to the amplifier's owner too.

Exactly and this is pretty much how it rolls for me too, I feel its a complete waste of time and money to try and isolate these type of issues when you'd be replacing all the silicon anyway.
Sometimes you might replace the transistors and whatnot only to find the issue is still there, but then you have eliminated a whole lot of possibilities and you can then hone in on the issue a lot quicker.

 

[slimecity]

Shotgunning used to be a swear word around here, but as alluded to, with aging units it just might make sense, as you spend less time replacing a group of suspects, rather than individually desolder, test, resolder until culprit is found - only to realize that the culprit's brethren are the same age and make and really should be retired as well. Ofcourse when a burnt resistor is found, there is often a secondary trigger downstream - but maybe transistors just naturally degrade over time, and finally their 1970' ties birth certificate is expiring.

I just had an (almost) argument with someone in another section at AK when a user complained of obvious small transistor noise - and he had a whole heap of 725' and 1313's in there. To me, shotgunning seems like a logical approach - and double-especially with transistors which are known troublemakers.