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Old 21st March 2015, 06:51 PM   #1
mhk1058 is offline mhk1058  Denmark
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Default Pioneer A400 No Sound

Been doing a few stage by stage capacitor replacements and all was well up to today.

Schoolboy error, put one cap in the wrong way round. In my defence all the other channel specific capacitors are mirrored - except these two! Nevertheless, should have checked the PCB rather than using the other channel capacitor for reference.

Anyway, the result was at power on I got the usual red light, delay, then hear the relay switch as usual. Only this time it's closely followed by another relay like click... and then nothing.

Now when I switch it on I get the red light but no relay switching.... and no sound! Power is getting to the board and fuse is OK.

Anyone have any ideas what I may have done and what can be donre to fix it? This amp has served me well for 25 years and I really don't want to write it off if possible.

Last edited by mhk1058; 21st March 2015 at 11:05 PM.
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Old 21st March 2015, 07:44 PM   #2
agdr is offline agdr  United States
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Originally Posted by mhk1058 View Post
Anyone have any ideas what I may have done and what can be donre to fix it? This amp has served me well for 25 years and I really don't want to write it off if possible.
I'm not the guy that can help solve your problem, but is it a Pioneer A400 rather than Panasonic? If so here is the schematic which would help someone help you troubleshoot.

Pioneer A400 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics (their free download link is all the way at the bottom)

Also please post the capacitor number on the board, or value and where it was located, that you put in backwards.

Last edited by agdr; 21st March 2015 at 07:49 PM.
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Old 21st March 2015, 09:49 PM   #3
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There is also a Yamaha A-400 model: Yamaha A-400 Manual - Stereo Integrated Amplifier - HiFi Engine
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Old 21st March 2015, 10:57 PM   #4
mhk1058 is offline mhk1058  Denmark
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Damn, yes, Pioneer A400, spent too many hours on it today, you'd think after 25 years of staring at it I'd know who made it.

The capacitor was the 100uf 50v cap at C238
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Old 22nd March 2015, 04:25 AM   #5
agdr is offline agdr  United States
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Originally Posted by mhk1058 View Post
The capacitor was the 100uf 50v cap at C238
Found it, on page 7 of the schematic, right in the middle of the page on the base of Q238 on a power supply line that goes across the page and feed multiple transistors. Also shown on page 8.

That particular circuit is a capacitance multiplier. Q238 acts to multiply the effective capacitance - its filtering ability - of C238 by the current gain of the transistor, probably in the 30-100x range.

When electrolytic capacitors are reversed they act a lot like diodes. So when you put C238 in backwards it would have shorted the base of Q238 to point B, which is likely some power supply line I have yet to be able to trace.

But the important thing is when that happened, you also shorted that 100 ohm R290 and 4.7 ohm R288 on the collector of Q238 to point B, which probably fried them, or at least fried R290. R290 would not normally have any significant amount of current going through it, just feeding the base of Q238, so it is likely a small (1/8W, etc) reisistor. But with that C238 conducting it probably went up like a fuse. So you should find those two resistors - they should be in the neighborhood of your C238 - and *with the power off for at least 5 minutes to discharge caps* do a resistance reading across those two resistors and see if it shows they are approximately right. At the least the resistance should not read higher than each resistor's value. Also with a magnifying glass and take a look at the two resistors. That line apparently has somewhere around 42 volts on it (the Q238 collector). One or both of those resistors may be visibly burnt.

Q238 probably survived though since the conducting reversed-biased electrolytic on the base wouldn't have left much voltage to go out the emitter. Also it goes without saying that C238 is bad now and would need to be replaced again.

The Q238 capacitance multiplier appears to supply power to a bunch of parts downstream. If R290 and/or R288 did open up that would shut off the cap multiplier and effectively quit supplying power to many of the amp's parts, hence the dead amp.

Good luck!

Last edited by agdr; 22nd March 2015 at 04:49 AM.
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Old 22nd March 2015, 10:37 AM   #6
mhk1058 is offline mhk1058  Denmark
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Quote:
Originally Posted by agdr View Post
Found it, on page 7 of the schematic, right in the middle of the page on the base of Q238 on a power supply line that goes across the page and feed multiple transistors. Also shown on page 8.

That particular circuit is a capacitance multiplier. Q238 acts to multiply the effective capacitance - its filtering ability - of C238 by the current gain of the transistor, probably in the 30-100x range.

When electrolytic capacitors are reversed they act a lot like diodes. So when you put C238 in backwards it would have shorted the base of Q238 to point B, which is likely some power supply line I have yet to be able to trace.

But the important thing is when that happened, you also shorted that 100 ohm R290 and 4.7 ohm R288 on the collector of Q238 to point B, which probably fried them, or at least fried R290. R290 would not normally have any significant amount of current going through it, just feeding the base of Q238, so it is likely a small (1/8W, etc) reisistor. But with that C238 conducting it probably went up like a fuse. So you should find those two resistors - they should be in the neighborhood of your C238 - and *with the power off for at least 5 minutes to discharge caps* do a resistance reading across those two resistors and see if it shows they are approximately right. At the least the resistance should not read higher than each resistor's value. Also with a magnifying glass and take a look at the two resistors. That line apparently has somewhere around 42 volts on it (the Q238 collector). One or both of those resistors may be visibly burnt.

Q238 probably survived though since the conducting reversed-biased electrolytic on the base wouldn't have left much voltage to go out the emitter. Also it goes without saying that C238 is bad now and would need to be replaced again.

The Q238 capacitance multiplier appears to supply power to a bunch of parts downstream. If R290 and/or R288 did open up that would shut off the cap multiplier and effectively quit supplying power to many of the amp's parts, hence the dead amp.

Good luck!
Hey, thanks for taking the time to look into this, very much appreciated. I'll have a look and let you know how it goes.

Thanks again
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Old 22nd March 2015, 11:14 AM   #7
mhk1058 is offline mhk1058  Denmark
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Well, R299 looks fine and meaures fine, 5.1 ohms but the other channel is the same so probably my low cost meter. R290 also measures fine at 100ohms but actually some discolouration which looks suspicious, it has obviously been hot.

I'll try new resistor, cap and transistor while I'm at it.
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Old 22nd March 2015, 02:28 PM   #8
agdr is offline agdr  United States
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Originally Posted by mhk1058 View Post
I'll try new resistor, cap and transistor while I'm at it.
Good plan. Then it would be worthwhile to measure some voltages.

The print shows -41.2Vdc coming out of the emitter of Q238, so the collector line should be a Vbe drop (0.7Vdc or so) above that plus the drop across R290, around -42Vdc. If that -42Vdc is missing then something has opened up in the power supply. If the -42Vdc is present, then you should get the -41.2Vdc out of Q238 with your new R290 and new C238 in place. If not, measure the voltage on that point "B", one leg of C238, and see what it says. It should be ground (connected to the chassis).

I was able to track down point "B". At the bottom of sheet 8 it shows point "B" in a bundle of lines. Trace that line over to the right edge of the sheet, then compare with the lines on the left edge of sheet 9, going to the power supply. If you count up from the bottom of sheet 8 at the right edge & left edge of 9 you have 7 lines, then 1, then 3, then 1, then 1, then 2, then the 3 lines that have line "B" in them, and those 3 tie to ground. So long story short, the negative leg of C238 should be zero volts with respect to the chasis.

Last edited by agdr; 22nd March 2015 at 02:50 PM.
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Old 22nd March 2015, 02:45 PM   #9
mhk1058 is offline mhk1058  Denmark
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OK, thanks a lot. Ordered the parts so it will be next weekend before I can fit them. I'll let you know how it goes.
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Old 22nd March 2015, 02:53 PM   #10
Mooly is offline Mooly  United Kingdom
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