Pioneer A400 - A familiar Problem?

[Ronj]

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

[Mooly]

Are both channels affected or just one ?

[Mooly]

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 ?

[Ronj]

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...

[Mooly]

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.

[Ronj]

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....

[Mooly]

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.

[Ronj]

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...