Pioneer A400 Silent!!

[mhk1058]

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 (after swapping out the capacitor of course) 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 to look for to fix it? This amp has served me well for 25 years and I really don't want to write it off if possible.

 

[drummerman]

Reads like the relay has expired on shut.

 

[Vladimir]

The relay wont switch since it is in protection mode.

Do you have a multimeter that measures capacitance? Doesn't have to be precise, just to check if the cap you installed wrong is dead. You should also check the transistors before and after the cap. Those small black ones. They are cheap to replace and need no thermal grease.

 

[mhk1058]

The relay wont switch since it is in protection mode.

Do you have a multimeter that measures capacitance? Doesn't have to be precise, just to check if the cap you installed wrong is dead. You should also check the transistors before and after the cap. Those small black ones. They are cheap to replace and need no thermal grease.

Thanks, I'll give it a go, not sure if my multimeter measures capacitance but cheap enough to try a new one.

Will the relay come out of protection mode on it's own? I also noted there are two relays, are they both part of the protection circuit, are they channel specific or do the do completey different jobs?

Thanks again

 

[TrevC]

You need to look at the circuit diagram and find the cap you put in the wrong way to see what might happen, then it's normal fault finding. Presumably you've discarded the cap you put in incorrectly.

 

[Vladimir]

The amp will stay in protection mode untill the deffect is sorted. If the cap is faulty and you replace it with a good one, the A400 will come out of protection. If not, then there is some other dead cap/transistor/resistor/diode that needs replacing.

 

[Vladimir]

And double-check if you haven't placed other cap incorrectly. Tripple-check if needed.

 

[mhk1058]

yeh, confident all other caps are OK, checked everything I've replaced.

I've ordered new cap C237, transistor Q237 and the 100ohm resistor R289 next to the cap. It measures OK but definitely some discolouration.

Thanks for your help, I'll let you know how it goes

 

[mhk1058]

R289 is listed as a fusible resitor, although it looks like any other. Is a fusible resitor (as opposed to a fuse resistor) just a regular resistor resistor but with critical wattage parameters so it blows when required.

If my interpretation of the part no is correct it's 1/4w

 

[Vladimir]

R289 is listed as a fusible resitor, although it looks like any other. Is a fusible resitor (as opposed to a fuse resistor) just a regular resistor resistor but with critical wattage parameters so it blows when required.

If my interpretation of the part no is correct it's 1/4w

They are low current fuses for protecting circuits. They are small and look like typicall resistors. These are not used as mains fuses, those need to be easily replacable with fuse holders and more robust.

Replace it with another fusible resistor at 0.25W and same resistance.

Short and interesting read.

 

[mhk1058]

R289 is listed as a fusible resitor, although it looks like any other. Is a fusible resitor (as opposed to a fuse resistor) just a regular resistor resistor but with critical wattage parameters so it blows when required.

If my interpretation of the part no is correct it's 1/4w

They are low current fuses for protecting circuits. They are small and look like typicall resistors. These are not used as mains fuses, those need to be easily replacable with fuse holders and more robust.

Replace it with another fusible resistor at 0.25W and same resistance.

Short and interesting read.

Yes, I think I got that but really the question was is it just a good quality, accurately calibrated resistor that acts like a fuse when required? Can't find anything listed at the required ratings as 'fusible' so plan on using a good quality precision resistor.

 

[mhk1058]

Thanks for your help Vladimir, replaced the cap and 100ohm resistor, transistor and 4.7ohm resistor were OK, fired her up and heard the reassuring click of the relay and alls well... maybe for another 25 years!

Thanks again, much appreciated.

Now if anyone can tell me where to find a blue LED that will work to finish the job.. Forward voltage on blue is obviously higher than red and I can't get a blue LED to light up. Not worth monkeying around with resistors but I'm sure I've read on very old forums that this was a standard thing done on the A400 upgrade (originally by Tom Evans)

 

[Vladimir]

Glad to hear it. The giant killer lives on! *drinks*

P.S. Look for low voltage blue LEDs (Osram 2.87V). They are not terribly bright but will do the job. A blue LED is the A400 GTE cherry on top.