Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 3rd June 2011, 04:07 PM   #1
Ronj is offline Ronj  Singapore
diyAudio Member
 
Join Date: Jun 2010
Location: Singapore
Default Pioneer A400 - A familiar Problem?

I have a Pioneer A400 Amplifier which suffers from, what seems to be a typical problem in A400.

My problem is on cold start(read fully discharged) after the delayed power relay click (which comes almost 20-30secs afterwards); amp first start with a loud continuous popping/motor-boating sound(what seems to be DC) on the speakers. Amp motor-boats and it has killed one of my design acoustics tweeters once. Once switched off from this state, the very second time amps starts with a loud hum on the Right channel and normal operation on the Left channel. And usually on one more power cycling, amp starts working normally.

One way to circumvent this problem is start the amp with headphone plugged in with some decent volume and wait for few minutes and unplug the headphone. Amp works normally in that case.

After searching various forums, I got few leads, but all the solutions become futile.

Trying to rectify the problem, I have replaced the 10000MFD/50V reservoir caps (C505 and C506) with Panasonic FC caps of the same value.

I also replaced the other power supply related capacitors (esp to the OP-AMP) C501, C502 (10MFD/50V) and C503 and C504 (47MFD/25V). I used NICHICON Aluminium Electrolytic of same value for C501, C502 (10MFD/50V). While I found that on the circuit daiagram C503 is shown as 10MFD/50V while C504 shown as (47MFD/25V). Anyway I replaced both with 100MFD/35V capacitors.

I also replaced the 2.2MFD/50V Caps C201,C202 and C301,C302 with EVOX RIFA PHE series polypropylene capacitor of 2.2MFD/400V.

Anyway despite all these replacements, my problem persists… And I need to cycle power using the front panel switch for 2-3 time before the Amp starts working after the loud motor-boating. By the way I did check the rail voltages and they are normal. I really appreciate your inputs….

Circuit Diagram can be found here
Pioneer A400 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

Or
Here as JPEG

Click the image to open in full size.
  Reply With Quote
Old 3rd June 2011, 04:52 PM   #2
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Are both channels affected or just one ?
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 3rd June 2011, 05:02 PM   #3
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Hmmm... I see you mention that in second paragraph. It was hard to follow at first as the jpeg kept opening full size with massive margins.

Just to be sure can you clarify from a cold start whether both channels are affected ?

Does the volume control setting make any difference ?

When you say "start with headphones plugged in", is it OK then from cold ? Plays normally ? Thats very important ?

What is the DC offset on each channel output (before relay) from cold and with no load attached ?
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 3rd June 2011, 05:35 PM   #4
Ronj is offline Ronj  Singapore
diyAudio Member
 
Join Date: Jun 2010
Location: Singapore
Both the channels are affected initially. Second powercycling, only Right Channel is affected though... But this time no motor-boating, but just severe hum on the right channel with no audio. Left channel works normally this time. Speaker relay continuously clicks during the motor -boating.

Volume control level doesn't have any effect on Initial Motor-boating or subsequent severe hum on Right channel after second power cycle.

With headphone plugged in, the amp works well even on cold start... And if I unplug the headphone after few minutes I could hear the normal relay click and the amp functions normally with the speaker load.... Once settled subsequent power cycles don't have any effect up to couple of minutes and Amp continues to work normally after each power on... :Longer power-downs repeats the problem...
  Reply With Quote
Old 3rd June 2011, 05:54 PM   #5
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
The fact headphones work from cold could be a clue. I would definitely measure the DC offset of both channels from cold which should be essentially zero of course.

Just some thoughts... I suspect a DC offset from cold that is causing the speaker to draw a large current, that in turn pulls the rails down and would affect both channels although it sounds like just the right channel is the culprit.

Just based on experience... I would look very carefully at and resolder anyway all the power transistors on that channel. The solder should take cleanly to the leads of course... just try it... it can be a common problem.

The other thing is that (for no good reason, again just from experience) is that certain Japanese power transistors seem to have a failure mode of going intermitent Base to Emitter. Thats a real generalisation but I have seen it countless times in the past although in the TV trade rather than audio.

Check the soldering first even if it looks OK.
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 4th June 2011, 05:00 AM   #6
Ronj is offline Ronj  Singapore
diyAudio Member
 
Join Date: Jun 2010
Location: Singapore
Yeah you are right.

DC Offeset on the RIght Channel is an issue.
Righ Channel reads up to -1.5to -2V DC offset on Cold start in alternate fashion. Which is causing the loud pops along with relay cuts. subsequent power cycling it starts reducing eventually to settle around -75mV to -90mV. :Left channel stays at +20mV through out.
What could be the reason? failing transistor?

I re-soldered all power transistors, but the problem persists....
  Reply With Quote
Old 4th June 2011, 06:58 AM   #7
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
This is going to be a tough one to crack I think.

It could be anything... transistor or a cap such as the very small value ones.

Very hard to say without seeing this so I'm going to say connect up the meter again to the output so you can measure the offset and then have a good tap and poke around using a plastic handled screwdriver to see if it is at all physical in nature. Lets be absolutely certain on this. Tap around all the transistors in that channel. Push and bend the PCB too and see if the reading alters.

I know I mentioned power transitors and soldering... with a fault like this its worth now very carefully going over all joints in the suspect channel if you haven't already done so. Resolder them all and make sure the solder takes cleanly to all parts, its only a few minutes of a job. Sometimes even when looking with a magnifying and light its hard to be sure... I have been there so many times as a repair tech and the only sure way is to resolder the lot.

So I wouldn't just jump in and start changing parts yet... I certainly wouldn't bet on it not being a physical issue at this stage.

But if you are 100% sure that its not in any way physical then very careful use of freezer spray would be my next step. Don't blast any component, just drip the spray a single drop at a time on any suspected transistors first. Ones that run hot... and that means some of the small ones in the VAS stages of the power amp would be first. And that dual transistor at the input. Caps... any near heat sources would be first to try with freezer too.

The offset of 90mv sounds a bit high even when it's working... and although I wouldn't normally advise swapping parts left to right its worth jus trying with that dual transistor.
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 4th June 2011, 11:32 AM   #8
Ronj is offline Ronj  Singapore
diyAudio Member
 
Join Date: Jun 2010
Location: Singapore
Thanks for your input... I think I will put another days effort in to it... Else there is always another A400 on sale And I could use this chassis to power a LM3886 twin gain clone PCBs.... Guess I could still use the pre Amp... Power transformers, speaker terminals etc on the chassis...
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pioneer A400 power on relay problem redart64 Solid State 7 7th February 2011 11:13 PM
pioneer a400 reddish75 Solid State 24 21st September 2008 11:14 PM
pioneer a400 schematic reddish75 Solid State 2 19th September 2008 11:31 PM
bias for pioneer a400? lt cdr data Solid State 7 18th November 2006 10:02 PM
Buzz on Pioneer A400 Smiffy Solid State 4 16th January 2006 05:38 PM


New To Site? Need Help?

All times are GMT. The time now is 11:56 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2