Cherished 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 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.

 

[tunedwolf]

The damage could be pretty much anything. Most of the older Pioneers I have worked on were all dc coupled, so damage to one stage rippled through the others. However, that being said, from your description, it sounds like the cap may have been in the dc speaker protection circuitry, If that's the case, then the speaker protection relay will not pull in. I think I would begin by finding the output of the amp at the speaker protection relay contacts and confirm good audio, if that's all ok, locate the dc protection circuitry and begin fault finding. Pioneers were great when everything circuit wise was within spec, but at the mere sniff of any problem they had a habit of popping fuseable resistors left and right, so check em all carefully

 

[mhk1058]

Thanks very much for the insight... I do think it is going to be a bit of a chase.

 

[picbits]

Service manual here :

https://www.eserviceinfo.com/downloadsm/50032/Pioneer_A400.html

Which capacitor was it ? Can you get the number from the board ?

 

[mhk1058]

Service manual here :

https://www.eserviceinfo.com/downloadsm/50032/Pioneer_A400.html

Which capacitor was it ? Can you get the number from the board ?

Yes, I should have mentioned in my op, it's C237 (100uf 50v)

 

[picbits]

At a guess, check the resistance on R289 (100R) R287 (4R7) and Q237 (2SC1845)

 

[mhk1058]

At a guess, check the resistance on R289 (100R) R287 (4R7) and Q237 (2SC1845)

Thanks, yes, resistance at R289 and R287 are good although R289 looks discoloured. Not sure what to expect at Q237.
Will replace R289 and Q237 and see what happens.

Thanks for taking the time to check it out and reply, much appreciated.

 

[mhk1058]

It lives. Changed the capacitor and the 100ohm fusible resistor next to it and all's well.

Thanks for your help

 

[picbits]

Excellent news - hopefully you'll get another 25 years out of it now

 

[mhk1058]

Excellent news - hopefully you'll get another 25 years out of it now

Now that would be something!