• 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

Does your SX-450 run This hot?

Greg64

Greg64

AK Subscriber
Subscriber
Just finishing up a recap and driver transistor refresh. I didn’t change out the output transistors (didn’t have any on hand). Also original filter caps as they seemed usable in the short term until Mouser gets ahold of my wallet again.

It sounds great but man, outputs are running 114-117F degrees after 30 minutes. I had cleaned and re-greased the main heat sink with thermal paste.

I’m thinking this is a bit warm. Thoughts and suggestions welcome.
 
Register to hide this ad
Greg64 said:
Just finishing up a recap and driver transistor refresh. I didn’t change out the output transistors (didn’t have any on hand). Also original filter caps as they seemed usable in the short term until Mouser gets ahold of my wallet again.

It sounds great but man, outputs are running 114-117F degrees after 30 minutes. I had cleaned and re-greased the main heat sink with thermal paste.

I’m thinking this is a bit warm. Thoughts and suggestions welcome.
Click to expand...
If by refresh you meant changing the driver transistors that may well have effected the bias.

Did you confirm bias was still in the correct range? (sounds like it may have gone higher than it was) The 450 has a weird spec without an actual target value and no way to adjust. You can at least go in and see what the bias is now and decide if it is too high.

Sounds like it was not that warm before you did the work? Possibly put the original drivers back and check bias and temps?

450bias.jpg
 
I’ll check it later today, but she wasn’t running when I got it. I put in a new old transformer. The fuse was open on the original transformer. It took right off after I got the new transformer swapped in.
 
Just had a chance to check voltages

Right channel = 84.5 VDC
Left channel = 80.2 VDC
 
The Pioneer service manual for this model is zero help, just looked at it, other than telling you how to check the idle current as stated in post #2.
 
I’m not sure but it seems a little high. Was hoping some folks might have ideas on swapping out some resistors to lower the idle to around 30 mV. Seems a lot of other receivers are dialed back to around that.
 
Greg64 said:
Just had a chance to check voltages

Right channel = 84.5 VDC
Left channel = 80.2 VDC
Click to expand...
Are you sure it is not mV. If you have 80 volts on those test points you have some real serious issues :D

Assuming it is mV DC. 80mV across .5 ohms is 160mA which is way too high, no wonder it is hot.

A more typical value is 15mV across one emitter resistor. Which would be 30mV across both resistors.

Are the jumpers cut or not?
 
mrk229 said:
Are you sure it is not mV. If you have 80 volts on those test points you have some real serious issues :D

Assuming it is mV DC. 80mV across .5 ohms is 160mA which is way too high, no wonder it is hot.

A more typical value is 15mV across one emitter resistor. Which would be 30mV across both resistors.

Are the jumpers cut or not?
Click to expand...
Jumpers are not cut
 
I’m going to check some power supply voltages since I did swap that out. I’ll report back.
 
Greg64 said:
I’ll check it later today, but she wasn’t running when I got it. I put in a new old transformer. The fuse was open on the original transformer. It took right off after I got the new transformer swapped in.
Click to expand...

Wait...you installed a new transformer and did not check power supply voltages?

Was the new transformer from another SX450? Same product code?
 
Last edited:
Guess that makes me more of a lunatic than you!

And I washed this thing in a mop tub and dried it in my oven. Without the transformer lol. IMG_9282.jpeg
 
I had a problem like this with an SX-434. The outputs were blown and it took the drivers with them. I changed them both and the bias was pretty high with the modern silicon.
I remember temping a variable resistor in to get the bias where it needed to be, and then measuring the resistor to get a value for the replacement fixed resistor.

You could try that on yours. Use a low value 220 or 470 or 500 ohm pot wired as a variable resistor in place of R221 (222). Set the pot at 82 ohms out of circuit and then temp wire it into the amp and gently tweak the pot to get the bias around 15 to 20mV. Pull the pot measure the resistance and put a fixed resistor in that is close to the pot value.


sxbias.jpg
 
This thread may be helpful:

Pioneer SX-450 DC Offset

I have a little SX-450 here that had 2 blown outputs. I replaced all 4 with MJE15032G,s and I also replaced Q201-Q204 with KSA992,s. No caps were replaced. I matched up Q201 and Q203 to within a few points of each other. There was a little larger spread between Q202 and Q204, but still less than...
audiokarma.org
 
mrk229 said:
I had a problem like this with an SX-434. The outputs were blown and it took the drivers with them. I changed them both and the bias was pretty high with the modern silicon.
I remember temping a variable resistor in to get the bias where it needed to be, and then measuring the resistor to get a value for the replacement fixed resistor.

You could try that on yours. Use a low value 220 or 470 or 500 ohm pot wired as a variable resistor in place of R221 (222). Set the pot at 82 ohms out of circuit and then temp wire it into the amp and gently tweak the pot to get the bias around 15 to 20mV. Pull the pot measure the resistance and put a fixed resistor in that is close to the pot value.


View attachment 3398753
Click to expand...
Interesting technique and I think I have a couple low value pots for such a thing. Here we go!
 
You must log in or register to reply here.
Back
Top Bottom