Pioneer A-X5 overheating after replacing transistor

[Gerge_Sletar]

Hello

I’m in need of some advice, I've been working on a pioneer A-X5 that is overheating after replacing a transistor in the left channel.

After much probing with a multimeter, the right channel output transistors and other components showed the correct voltage. Moving over to the left channel, I found 12v on one of the 2SA798 duo transistors, where there should be 0v. I followed the line back and tested transistors as I went. I got to the output transistors, and the base voltage should have been 0.6 / -0.6 but was showing much higher (I can't remember off the top of my head). Following down more, I came to transistor Q6 2SC2705/A/ took it out for testing, and it was completely dead, with no signs of life. I ordered a replacement from eBay.

2SC2705 Transistor Silicon NPN - CASE: TO92L MAKE: Toshiba | eBay

Find many great new & used options and get the best deals for 2SC2705 Transistor Silicon NPN - CASE: TO92L MAKE: Toshiba at the best online prices at eBay! Free delivery for many products.

www.ebay.co.uk

Which arrived last week, soldered it in and turned everything on. The relay clicked, and I had sound.

A new problem has emerged: the output transistors on the left channel got so hot in the minute, or so I had the amp on that the heat sink was hard to hold on to. Turning everything off very quickly to cool down, I inspected the new transistor (photo attached) markings where 2SC2705 Y markings. After looking on alltransistors.com, the only difference between the Y and the standard is the HFE, which is 120HFE on the Y and 80HFE on the minimum. Could this be the problem? I did some quick voltage testing with the new transistor while the amp was powered up, and all the voltage looked correct.

I was going to try to order a 2SC2705-O, which has identical specs on eBay, but the only UK supplier has told me they've run out.

If this is the case, does anyone have any suggestions on sourcing a replacement transistor?

 

[rcs16]

After changing parts did you turn output stage bias to minimum before turning it on?
I doubt the hfe grading makes a difference.
I’d have to look at the schematic to see what function the part does.
I wish folks would post pioneer related questions in the pioneer forum, you might get more answers

 

[ivandezande]

You got a transistor from eBay, it shouldn’t be a surprise…..

Get a KSC2383 from Mouser, Digi-Key or Newark.
Make sure it’s not the only bad component.

 

[rcs16]

Do you have a service manual/schematic for that design?

Pioneer A-X5​

 

[Gerge_Sletar]

After changing parts did you turn output stage bias to minimum before turning it on?
I doubt the hfe grading makes a difference.
I’d have to look at the schematic to see what function the part does.
I wish folks would post pioneer related questions in the pioneer forum, you might get more answers

I don't think you can adjust the bias on this model, no variable adjustments

 

[Gerge_Sletar]

Do you have a service manual/schematic for that design?

Pioneer A-X5​

Yep, here you go

Pioneer A-X5-min.pdf

Shared with Dropbox

www.dropbox.com

 

[rcs16]

Okay thx for the info. They have a balancing act going on there.
yeah no output stage bias current adjustment but you could add one by replacing R41, 42 with a 500 ohm trim pot.
Connect Wiper to one end so that when set to CCW position, it has the minimum terminal resistance. A short in the CCW position.

As a test you could short out R41, it will allow more current to flow through R43 increasing bias to Q7 reducing the bias V spread across Q9,11 bases.
measure DCmV bias across the emitters R55

You could also tack another resistor across R41, a few options.