Hi there
Small problem with Pioneer A400x - amp works fine but the protection circuit activates when volume is turned up or down - then the relay clicks and music starts again. It appears that DC is getting to the outputs but only when volume is changed. I can measure it and also see the speaker cone move abruptly when doing so. All voltages measure OK as far as I can tell and the amp sounds good if untouched. Any guidance much appreciated!
Small problem with Pioneer A400x - amp works fine but the protection circuit activates when volume is turned up or down - then the relay clicks and music starts again. It appears that DC is getting to the outputs but only when volume is changed. I can measure it and also see the speaker cone move abruptly when doing so. All voltages measure OK as far as I can tell and the amp sounds good if untouched. Any guidance much appreciated!
Maybe so, but I've seen some pretty bad pots.
DC on the control would usually make loud noises in the speakers,
and he didn't mention that.
thanks - the pot seems clean and there is no scratchy sound when adjusting volume. There seems to be some DC at the pot. There is no DC at speaker outputs when untouched, but quickly rises on adjusting volume kicking in the relay.
There are 2 small electro caps and 4 ceramics at the pot - would they be likely suspects?
There are 2 small electro caps and 4 ceramics at the pot - would they be likely suspects?
Then next replace the electrolytic capacitors directly connected to the pot,
both input and output, observing polarity. Don't replace the ceramics.
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Have taken out the 2 2.2uF - they measure fine and look fine.
Have rechecked voltages and there is one that may be unusual. The schematic reads -11.3V at the emitter resistor and I'm getting -1.3 V - all other voltages good. The same reading both channels by the way.
Have rechecked voltages and there is one that may be unusual. The schematic reads -11.3V at the emitter resistor and I'm getting -1.3 V - all other voltages good. The same reading both channels by the way.
Attachments
Those number on the schematic are in mV. The pos side transistor emitter should have a voltage of 9.17 mV, and the negative side should be -11.3 mV. If you have -1.3 volts, that amplifier has a problem.
Ahh - well spotted thanks. Looks like the 2sc3281 2sa130 transistors then? I'll take out and test.....
No, must be an issue on the power amp input side. (It's a DC-coupled feedback circuit.) Measure DC levels there. Gain at DC should be unity, so I'd imagine your DC offset would appear there as well. Definitely something weird going on. Check input-side ground potential, too - if that is floating, strange things could happen. If it comes in via a plug-in ribbon cable, that may need cleaning up...
Power does come in on ribbon cable - checked and cleaned as well as i could. I seem to be getting around 2.5V DC on inputs - but only when I have something connected. Emitter resistors good.
What DC voltage do you see on the speaker terminals? When you say you have 'around 2.5V DC on inputs', exactly what point are you referring to?
There is no DC on speaker terminals at idle - it only appears when I turn the volume up or down. I can see the speaker move in when I turn up and push out when I turn down. I have to do this slowly or it trips the protection.
I'm measuring 2.5V DC at the + of RCA inputs when connected to an iPod.
I'm measuring 2.5V DC at the + of RCA inputs when connected to an iPod.
That shouldn't be happening. Can you verify with another source?
2.5 V sounds like an older model iPod with an AC-coupled output, +5V audio supply and output coupling caps that have seen better days. That said, the volume pot would attenuate this quite severely.
If this happens with any source and the volume pot does not reduce these 2.5 V, verify low-impedance connection between all of:
* RCA input ground
* power amp input ground (C203, R203, C211 etc.)
* speaker (-)
I wouldn't be surprised to see an open somewhere.
Incidentally, why C201/202 are oriented with (-) facing the volume pot is anyone's guess - I guess there's a slightly under 50% chance of them not working as intended if faced with severe DC either way. A pair of 4.7µs connected back-to-back ("poor man's bipolar") would be a good upgrade option if you happen to have any at hand.
Thanks you so much for your help. While messing around at the input area the problem suddenly disappeared so i assume there is some sort of short happening in that area. It seems to be working fine now - no more surging speakers or relays clicking off etc. I will try to recreate the fault otherwise it is bound to return!
Once again - thanks for your advice!
Once again - thanks for your advice!
Hi guys
i'm a newbie, obviously, but have recently had a similar experience with my A400x (clipping). Turns out I had not connected the output HF and LF biwired cables to the corresponding location on the speaker boxes, after having to disconnect them. Thought I post, just incase there are like minded people looking for answers!
mike
i'm a newbie, obviously, but have recently had a similar experience with my A400x (clipping). Turns out I had not connected the output HF and LF biwired cables to the corresponding location on the speaker boxes, after having to disconnect them. Thought I post, just incase there are like minded people looking for answers!
mike