pioneer sx-20a - UK Vintage Radio Repair and Restoration Discussion Forum

g0hze snowy · 14th Nov 2025, 7:06 pm

Hi Peeps, once again i am in need of your advice.
Whilst i can replace visually faulty components and am learning, albeit slowly, to test components that look ok but are in fact faulty.
What i am finding difficult is understanding what each part of a circuit does, at the moment i have 2 x pioneer sx-20a both exhibiting the same fault, stuck in protection mode, 1 of them i have had advice from this site re 0.27ohm resistor replacement, also the power amp transistor and its driver were faulty so they have been changed, i have tried the factory reset procedure but no joy.
So onto the question, at last you say ha ha, how can i temp bypass the protection circuit to see if the items are worth spending more time and money on them, they do represent a medium amount of loot in the purchase and the cost of spares already fitted.
so once again your advice and guidance is sought.
TIA
Snowy

g0hze snowy · 14th Nov 2025, 7:11 pm

Mediators, please move to correct post block, sorry

Diabolical Artificer · 15th Nov 2025, 8:51 am

There's a bloke called John audio tech on Youtube who did a series of video's a while back on transistor audio amplifiers EG the differential pair, the VBE multiplier etc, that might help you understand the building blocks of an amp more.

Re protection circuits I have a Pioneer amp on the bench at the moment in protection. The protection circuit is pretty simple as are most protection circuits in amps. They do 3 things 1) delay the connection of the power amp to the speakers via a relay at switch on 2) If DC is present at the OP of the power amp, the relay opens & 3) disconnect the power amp from the speakers at sw off.

The protection circuit does 1 & 3 with a simple RC time constant circuit which then turns on or off a transistor & thus the relay. Therefore check to see if that part is ok & check for DC on the OP of the power amp. That leaves the relay itself which can be checked with a meter & bench power supply.

Andy.

g0hze snowy · 15th Nov 2025, 12:59 pm

Thanks Andy, will check those suggestions tomorrow when i can get to my workshop.

14th Nov 2025, 7:06 pm	#1
g0hze snowy Hexode Join Date: Oct 2013 Location: Peterborough, Cambridgeshire, UK. Posts: 396	pioneer sx-20a Hi Peeps, once again i am in need of your advice. Whilst i can replace visually faulty components and am learning, albeit slowly, to test components that look ok but are in fact faulty. What i am finding difficult is understanding what each part of a circuit does, at the moment i have 2 x pioneer sx-20a both exhibiting the same fault, stuck in protection mode, 1 of them i have had advice from this site re 0.27ohm resistor replacement, also the power amp transistor and its driver were faulty so they have been changed, i have tried the factory reset procedure but no joy. So onto the question, at last you say ha ha, how can i temp bypass the protection circuit to see if the items are worth spending more time and money on them, they do represent a medium amount of loot in the purchase and the cost of spares already fitted. so once again your advice and guidance is sought. TIA Snowy

14th Nov 2025, 7:11 pm	#2
g0hze snowy Hexode Join Date: Oct 2013 Location: Peterborough, Cambridgeshire, UK. Posts: 396	Re: pioneer sx-20a Mediators, please move to correct post block, sorry

15th Nov 2025, 8:51 am	#3
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 8,409	Re: pioneer sx-20a There's a bloke called John audio tech on Youtube who did a series of video's a while back on transistor audio amplifiers EG the differential pair, the VBE multiplier etc, that might help you understand the building blocks of an amp more. Re protection circuits I have a Pioneer amp on the bench at the moment in protection. The protection circuit is pretty simple as are most protection circuits in amps. They do 3 things 1) delay the connection of the power amp to the speakers via a relay at switch on 2) If DC is present at the OP of the power amp, the relay opens & 3) disconnect the power amp from the speakers at sw off. The protection circuit does 1 & 3 with a simple RC time constant circuit which then turns on or off a transistor & thus the relay. Therefore check to see if that part is ok & check for DC on the OP of the power amp. That leaves the relay itself which can be checked with a meter & bench power supply. Andy. __________________ Curiosity hasn't killed this cat...so far.

15th Nov 2025, 12:59 pm	#4
g0hze snowy Hexode Join Date: Oct 2013 Location: Peterborough, Cambridgeshire, UK. Posts: 396	Re: pioneer sx-20a Thanks Andy, will check those suggestions tomorrow when i can get to my workshop.