Pioneer SA 7300 power dropping out

[bakatono]

hi guys

long time reader, first post

i have my dad's pioneer amp and it has started to drop out when using it at any volume level.

it has been matched to the same pair of speakers for 20+ years

you can hear the power relay click on and off when it does this. jiggling the power switch does not influence the problem in any way.

im wondering if anyone has come across a similar problem and can point me in the right direction to start looking at fixing this?

thanks in advance!

[jaycee]

Assuming it isnt a real fault with the amp circuit, it is possibly bad capacitors in the protection circuit making it trip. It looks like quite an old amp from the 70's and it'll be long overdue for a complete overhaul and capacitor replacement anyway.

[bakatono]

thanks jaycee, i was under the impression that it might be the amp's power supply circuit, as the supply solenoid is tripping

could it be some poor earthing?

[sakis]

jaycee is right ... all small electrolytics need to be replaced and probably big ones too

secondary power supplies and electrolytics around protection circuits are the first thigs to look at

[amptech]

These old amps need a service from time to time,bet there plenty of dust in there....

you can remove the old large filer caps say(100000uf @85c and look for any leakages all re-flow the dry pcb..

[sakis]

amptech .... in a way this the number one wrong approach ( refering to the big caps )

if for example one amplifier has leacky or dry filter capacitors will work perfectly allright until you ask ammounts of power from it .If 5% of the out is only needed will work like charm even with dry capacitors....

on the other side one amplifier with a dry capacitor in the signal/feedback chain will not play at all or like the OP above will have problems starting up if aux power supply that feeds protection and preamp circuits is under some kind of fault

protection circuits feature also caps in the signal path that also might be a reason fro not starting up properly

main filter caps might be a problem after all these years but will not effect the operation of the amp in low or very low power ...that puts the filter caps by far lower to the fault trace chain ...

kind regards sakis

[Mooly]

It may be nothing more than dry joints around the relay drive transistors or any discrete regulator series pass transistors... any that run hot, check the soldering... it's a common cause of problems.

[amptech]

1 Question.. Has the amplifier been used to driver higher wattage speaker's in it's long life...

try the re flow solder then retry with out any speaker's connected....

[bakatono]

hi everybody, thanks for the informative replies.

it sounds like a good idea for me to check the soldering along the power supply and protection circuits.

something worth noting - once a week when you power the amp up it will make a bzzzzzz sound for 1 sec then power up. it sounds similar to when the amp drops out for a second...

[mrfeedback]

Forget about capacitors for now.
Inspect throughout and resolder as required.
Look for small temperature monitor transistors in thermal contact with the main heatsinks and be sure to resolder these (if present).
If this does not cure the fault small electros in the protection stage may be the culprit.

Eric.