schematic for Pioneer SA-510

[Nigel Goodwin]

Hi Nigel, Thank you for the advice. Excuse my ignorance, my text books referred as Vbe being "Voltage between base and emitter" will you please draw your suggestion on this? View attachment 63313

That's not the same circuit as in post #17, which I was referring to. The #17 circuit uses a Vbe multiplier, the circuit you posted uses a crappy special diode (D3)
, and is EXTREMELY dependent on the exact components being fitted supplied from Pioneer.

I would suggest making sure R59 is shorted out, and also try shorting out R61 - this will reduce the bias to the minimum possible with D3 fitted.

Personally I'd throw out D3 and fit a Vbe multiplier (with a preset resistor for setting it), then you can set it for whatever transistors you fit.

I would then like to add, when I had the non-original transistors installed, it worked fine for about five minutes. Could the fact that I did not put in new thermal grease be the cause it went into thermal runaway so quickly at low output? I just bought a new tube, since my daughter thought mine would look great in the carpets...

No, it failed because the bias was too high - thermal runaway occurs VERY quickly.

 

[unclejed613]

always clean off the old thermal grease (alcohol is a good solvent for it) and put fresh grease on, and replace the mica insulators if the originals are damaged in any way,
those STV diode stacks were very unreliable, and were prone to failing open circuit,
more modern amps do away with the STV-3 diode stack and the other diode and resistor, and replace it with an adjustable bias circuit like the one shown below. the transistor should be mounted on the heat sink between the output transistors, and also coupled to the heatsink with thermal grease.

 

[Manuel Moreno]

I need the SA-508 service manual. And need to buy the same transistor: B773A and D883A. Can you tell me where ?
email mmorenoc@hotmail.com

 

[boinaverde]

you find this schematic in hi-fi engine.and transistors is very true 2SD883A IS A NPN and 2SA773A IS A PNP.

 

[unclejed613]

i don't think you'll find the transistors. if you do find some, it's likely they're some other device that's been relabeled. they were discontinued many years ago, but there may be a legitimately licensed manufacturer making replacements.

the japanese transistor cross reference lists 2SA1061 and 2SC2485 as replacements. since they are TO-3P packages instead of TO-3 metal packages, you have to bend the B and E leads to go into the socket, and wire a ring lug to the collector lead and put the ring lead on the mounting screw. otherwise, the TO-3P package should fit where the original metal package went. you may need a longer collector screw because the plastic package is a bit thicker than the original metal tab.. you ill also need a rectangular heat sink insulator instead of the oval one.

 

[Rotarymaker]

Hi,

I have the SA-520 which had only the B773A on one channel faulty. I got the 2SA1941 pair and replaced all ouput transistors, only to blow them all, burn one of the bias resistors and blow a speaker, due to the DC passing through the shorted tranny...

I noticed, the website I used to identify the equivalent, did not take into account the Bace-Emitter voltage. Otherwise, the transistor was a good match. The voltage difference is only 1 volt less. My question, will this small difference be the reason why the transistor fries itself? The Veb of the B773A is 6v, I am considdering a transistor with a rating of 7v. This should be ok as long as the Hfe is in the same region right?

I have seen suggestions of the 2SA1104, but they are pricey.

I am considdering:

MJL3281AG with MJL1302AG or 2SA1492 and 2SA3856. If these fry themselves...

BTW, no there aint anything else wrong with the amp. I actually had no fault on the left channel prior to installing the 2SA1941.

Anyone have the schematic for the SA-520?

Thanks.

An old "trick of the electronics trade" was to place a 2200uF to 4700uF capacitor in series with each speaker when repairing dc coupled audio amplifiers ..... this acted as a dc blocker and saved your speaker coils from burning up under amplifier output fault conditions.

hope this helps,
Rotarymaker