• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Reseating power transistors on SX-950

R

rkgren1

Super Member
I will soon be recapping(+transistors) the power amp on my SX-950. Since it looks like dismounting the heat sink will be required to access the back of this board, per advice found on this forum, I figured that this is an opportune time to remount the power transistors to the heat sink.

I am looking at the heat transfer compounds available on Mouser.

Is Wakefield-Vette 120-SA appropriate for this application?

What type of insulators do I need to order to separate the transistor bodies from the heat sink?
 
Register to hide this ad
mica insulators for thermal grease
mouser 534-4662

yes, 120-SA is the heat transfer compound, but that stock number at 4 grams might be a bit small, and might not be enough for all 8 to-3's, no less any power supply work.

get the 567-120-2 2 oz jar.
 
Be very careful when resetting them. They can be placed slightly out of position enough that the leads can end up touching the heat sink. This is how I killed my first SX-1250. It blew up with a sound as loud as a shotgun. I experienced fear and sadness in the same breath.
 
What part exploded ?

QUOTE="KingBubba, post: 9855394, member: 13645"]Be very careful when resetting them. They can be placed slightly out of position enough that the leads can end up touching the heat sink. This is how I killed my first SX-1250. It blew up with a sound as loud as a shotgun. I experienced fear and sadness in the same breath.[/QUOTE]
 
One leg of the transistor was touching the chassis ground via the heatsink. The current passing that path created an electrical explosion as the leg of the transistor and about a dime sized piece of the heatsink vaporized. Scary stuff, believe me.
 
ALWAYS ohm TO-3 transistor (ANY transistor) leads to HS before applying power. And START by ohming HS to HS to establish good reference connection, checking FREQUENTLY back to the reference point. There is a LOT of energy potential in those circuits.
 
Bubba,
Thanks for the tip. That sounds like just the sort of trap I would fall into.
Mark's suggestion seems to be a very good one.
 
That suggestion is an accurate depiction of something I unfailingly do EVERY time I replace a mounted transistor.
It is "bad" until PROVEN good - that is my mantra. It accounts for my power up successes and minuscule rate of "callbacks" (just ONE - after I got out of the hospital in 2012 - a tuner wouldn't hit the bottom of the band : one tooth off when tuning cap's gears were remeshed).
 
I planning my first recap to.it's a 950 also. My game plan is to do one board at a time and power up with dbt. And also if I messed up something Ill know it's in that board. Is that a good or bad idea
 
Hi Caught,
I worked my way through one board at a time as well.

I tested the unit after finishing each board.

Based on what I have read here, I reworked the power supply board first.

I followed up with the protection board, flat amp, tone amp, and equalizer boards, and lastly the power amp.

I found this receiver to be easy to work on.

Only the tuner requires removal or wires to access the back.
 
Sounds like my plan. I have bad transistors in the protection board.so it won't power up the relay. Ill do as you. power supply,protection and so on.I ran across 3 950's with in a month. each one was a little nicer.the other two works fine. but this one is extremely clean.And the one I plan on keeping.
 
Caught,
I may be worth your while to tabulate power supply voltages before and after refurbishing the power supply board.
It's a chance to document an early success from your labor, as well as assure that all supply voltages are correct prior to moving forward.
It's worth looking hard at all of the solder joints on the power supply board. Many of mine were really crusty, so I reflowed them.
 
Jkgren1
I have. I went though the voltage. And solder joints. I really wanted it working befor I started. The protection board is were it gets funny. I can't remember witch one but the one resistor that is to go to reverse to ground isn't. it's showing positive.all the voltage going to it from the power supply are dead on spec. But it makes sense to check.Thanks for the advice.
 
You must log in or register to reply here.
Back
Top Bottom