Technics SA-5270: Need help troubleshooting

slateef

Active Member
Hello friends,

I've started to work on a Technics SA-5270. It was not powering on. The fuse inside (3A, 125 v) was burnt out. I replaced it. When I powered it on, I immediately noted smoke coming from one of the resistors, R631. After the smoke, 2-3 seconds later, the same resistor started glowing bright orange/red. I turned the power off. I have removed R631 in hopes of replacing it, but what else should I do before trying to power it on again?

And advice would be much appreciated.

ETA: Can a mod please change the thread title for me; I mistakenly typed SAS instead of SA. Thank you.
 
Also, the schmatic shows R631 to be 180ohms, 1/4w (which I'm assuming is the same thing as 250mW). Can anyone direct me to a link where I can purchase this? I looked at mouser.com, but there are too many choices!
 
R631 is associated with the left amp, on the base of TR611 which is stage before the PNP driver. You likely have some blown transistors, at least on the left channel. Suggest you do not power up the unit until the power supply and amp devices have been tested.
 
Thank you. With that in mind, I took apart the heatsink assembly and removed all 4 transistors. Both of the right sided transistors tested fine. Both transistors on the left are bad. What would be the appropriate replacements, please? The ones in the unit were NEC D586 R71 (for TR613) and NEC B616 R73 (for TR615).

Thanks for your help, gpounce.
 
Also, it seemed that someone had replaced these transistors in the past. There appeared to be excess thermal paste everywhere. It's strange, the heatsink does not have any screw holes for the bracket of the transistors to screw into to secure. Here's a picture of the heatsink:



How should I install the new transistors to the heatsink? Bracket (thermal paste) silicon pad (thermal paste) heatsink? It's strange that there is no way to secure the transistors to the heatsink; as I said the heatsink does not have any screw holes.
 
Thanks for that info. With that in mind, I removed the heatsink assembly and desoldered all 4 transistors. Both of the right sided ones tested out ok. Both of the left ones are bad. What would be appropriate replacements? The ones in the unit that need replacement are NEC D586 R71 (for TR613) and NEC B616 R73 (for TR615).

It was obvious that someone had tried to replace the transistors in the past because there was excess thermal paste everywhere! It was even covering the legs of the transistors at points. The other strange thing is that the heatsink does not have any screw holes to secure the bracket of the transistors to. How should I install? Solder the transistors in place, then apply thermal paste to the brackets, then apply a silicon pad, then more thermal paste, then heatsink? How can I be assured that the bracket won't get detached from the heatsink, since there are no screw holes? You can see a pic of the heatsink below.

Thanks, gpounce!
 

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Looks like there is something missing, there should be something holding the transistors to the heatsink, either screws or a bracket of some kind. Also don't just replace the outputs, the transistors on the heatsink, but be sure to test the driver transistors in that channel before you try to power it up again. Also check the emitter resistors on the output transistors as they are usually large wirewound types that don't show signs of being bad. And since the receiver is rather old I would take a good look at the caps in the amp section while I was there working on it, and use a dim blub tester next time you power it up.
 
What would the driver transistors look like? Or what are they labeled on the schematic, please? How about the emitter resistors?

That's what I thought about the heatsink...I've never seen one without screw holes. And there's certainly not a bracket, either. Can anyone who's worked on this model chime in?
 
Here's one thread talking about transistor subs for the drivers;

http://audiokarma.org/forums/index.php?threads/2sd586-2sb616.694577/

the annoying part of it is the bat-wing design- there are plenty of TO-220's that will work in th circuit the problemis mounting them. You could drill and tap a new hole on the heatsink to secure to-220's (thats what I would do- just did it on a Kenwood amp I'm restoring- same problem). You might be able to find actual subs for your blown drivers, though ebay etc are notorious for fakes so be sure of your supplier if you go that way.

personally I don't like the thermal compound- way too messy so I clean it all off and use Berquist silpads only.

emitter resistors are R634,R636,R638,R640 on the left amp, right amp will have similar. Measure them for resistance, sometimes they drift or fail too. Given R631 is cooked, you are probably going to need TR611 and TR609, so I'd suggest you test them as well.
 
Suggest you solder the drivers last once you have a heatsink solution figured out- the devices may end up moving around as you get it figured out; mount and secure them, then solder.
 
Thank you, guys.

What would be appropriate substitutes for NEC D586 R71 (for TR613) and NEC B616 R73 (for TR615)? Also, where did you get the Berquist silpads?
 
it looks like the clamps are missing from the outputs . i guess they used spring steel clips from the rail screwed to heat-sink to make it easier assembly . i think you could maybe add sockets to the board and install transistors to heat-sink first after drilling and tapping .
no wonder the outputs blew .
 
it looks like the clamps are missing from the outputs . i guess they used spring steel clips from the rail screwed to heat-sink to make it easier assembly . i think you could maybe add sockets to the board and install transistors to heat-sink first after drilling and tapping .
no wonder the outputs blew .

Hi Pete! Good to see you and thanks for your help.

Can you please explain what you mean adding sockets to the board and installing transistors to heatsink first after drilling and tapping?

Do you mean I should drill holes into the heatsink so that the output transistors can be screwed in?
 
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