AU-999 with new Caps still "pops" - diyAudio
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Old 3rd May 2015, 08:33 PM   #1
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Default AU-999 with new Caps still "pops"

Hello,
I am hoping someone can point me in the right direction to solve a mystery. I have a Sansui AU-999 with new caps installed. The amp "pops" during any one of the following conditions: 1. Turning the power switch "off". 2. Switching the "Balance check" to "Test". 3. Turning the "speaker Selector" between "System A" and "System A+B". 4. Turning the "Source Selector" to "MIC", "PHONO 1", and "PHONO 2" BUT NOT "AUX" Also, the "pops" occur randomly during play. I am a newbie to this and trying to gain some knowledge by reading the threads. So far I have learned the transistors could be a source of the problem but that's it. Thanks in advance for any advice.
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Old 3rd May 2015, 09:17 PM   #2
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I would check the muting circuit.
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Old 4th May 2015, 12:05 AM   #3
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The power off click is possibly the mains switch arcing.
A 100nf 250VAC capacitor across it should help.

You could try a make before break source selector.
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Old 4th May 2015, 06:17 AM   #4
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"Pops" occurring when any signal routing or diversion switch is operated, are obviously associated with the signal switching but random pops during play, also suggests a level of DC floating around the signal routes where it should not be. That's why isolating caps are used when 0V DC balance of the signal is not possible. There could be several reasons for the intermittent problem but assuming this has only emerged since reacapping, I would suspect errors were made in the process and either the caps are incorrectly fitted, unwisely omitted, inappropriate types or some component damage has occurred in the process.

If you have a proper schematic, it's not difficult to trace DC problems with a simple DVM in the low level (preamp, tone, switching etc) circuits. By disconnecting the signal from the various sections to the power amplifier, you can also isolate the possible fault locations(s). Also check the power amplifier basics like DC across the output terminals, overheating, rough sound etc. Note the DC offset pots VR801 for fine adjustment to zero, if not done already.

There is no speaker relay or muting circuit to prevent the "pops" on the AU999, as was common back in the day but the various stages are all AC coupled with electrolytic caps and these may still be a problem. Recheck your work for solder bridges, opens, sensitivity to slight pressure applied to the boards, connectors etc. You don't say anything about the condition of the amp. before the recap, but was the sound reasonable for the age, no pops etc?
Sansui AU-999 Manual - Solid State Integrated Stereo Amplifier - HiFi Engine
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Old 4th May 2015, 01:49 PM   #5
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@John Snell
I looked at the schematic and seems that i am not able to understand what are you talking about
@nigelwright
Japanese consumer audio hardly ever behaves like that if a popping sound exists and could be cured by a capacitor this is already done in the factory Its a situation that doesn't occur with time
@reality
Your sayings
Turning the power switch "off" Normally this happens when Ltp is getting out of balance during shut down . Are you sure that capacitors you changed are placed in the same value or you took the authority to increase any values ?
Turning the "speaker Selector" between "System A" and "System A+B"
That is clearly an indication of offset in the output ( small amount of DC ) indicating that something is wrong with the amp
Turning the "Source Selector" to "MIC", "PHONO 1", and "PHONO 2" BUT NOT "AUX" That makes some sense cause often in many Japanese amplifiers when NOT set at MIC or PHONO inputs then the switch rests at ground to reduce noises and often that can cause complications .

Now for some real causes
1)Your description indicates offset in the output Did you tune both offset and bias according to the service manual ?Random pops though indicate small transistor failure .
2) Most Japanese amplifiers of that era today often present small leaks in the small transistors of the preamp or the first stages of the amp I could isolate pre and main amplifier to trace the fault
3) this amplifier features crowbar protection i could also remove that to make sure that nothing is on the way at least fro the testing and repair.
4) very often a DC leak on the signal path in the area of pre may sound like a scratchy pot likewise it may present similar behavior at the changeover of switches producing thumps and pops usually this also indicates a small transistor fault
Except mistakes that you ve made the usual suspects are poor soldering and poor ground connections

Then again you haven't described if we talk about on ch or both that has a lot to do also ...

Finally expect that small things like that is harder to trace especially for a NB

Kind regards
Sakis
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Old 4th May 2015, 06:31 PM   #6
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Thank you all for your suggestions. I sent it out to be repaired and it returned with new caps and the "pops". I don't think it "popped" before then. I suppose the first thing to do is verify the caps are correct, then adjust the offset and bias if it has not already been done? It has been recommended in other blogs to remove the F1313/F1334 "protection board" as it is ineffective. Is that theory still valid?
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Old 4th May 2015, 06:59 PM   #7
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Go do some searches on 'AudioKarma.' There are several threads on restoring AU-999's . If I remember correctly they also have several 'flying saucer' transistors that need to be replaced as they go noisy and make 'pops.' Lots of info on that site.

Good luck.
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Old 4th May 2015, 07:01 PM   #8
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As you sent the amplifier "out to be repaired" we can assume it was a professional repairer who did the work. The service work and parts should be warranted and pops and other noises now occurring randomly should be covered accordingly. Give the repairer the option to fix their work first ASAP, since as Sakis suggests, tracking down the actual fault may be extremely difficult for an inexperienced DIY and some repair warranties can lapse before you get to follow up on the problems.
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Old 4th May 2015, 07:14 PM   #9
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Ian-The repair shop took 3 months to do the repairs. During this time my emails went unanswered. When I finally heard back he claimed he had some "person problems" and provided a discount for my troubles. I was happy to get the amp back-fixed or not. I will never send anything out to that shop again.
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Old 4th May 2015, 07:38 PM   #10
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most of audiokarma articles need a hell of a lot of filtering before put to practice There is a lot of people there that dont have an idea on what are they doing.

Many suggestions there are under the logic ""i listen to it and it plays fine "" or its ok ""since its playing"".

Most of the problems come in the transistor replacement suggestion

Watch out ..

Protection circuit of this style works only if the amplifier is toasted totally. It works by shorting the output to ground hopping that the fuses will blow to avoid catastrophe of your precious speakers ...

There is numerous reports that protections like that false activated resulting a totally blown amplifier ...Or a minor damage in the amplifier might trigger the protect resulting also a blown amplifier

If it was mine i could remove this style of protection totally and replace it with a normal DC protection circuit that does also a few extra things like delayed turn on , quick shut of and noiseless together with AC loss detect function all very useful to audio amplifiers ...

Kind regards
sakis
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