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PIONEER SX1500-TD Dead Channel still blowing after output replace

automaat

automaat

New Member
Hi everyone, I am a tech working in Canada. My focus is organs, synths, tape gear, etc...

My weakness is Power Amps. I took this one on thinking it had a good output section and trouble in the front end. Turns out the left channel was blown. Fuse open. One output device shorted.

I changed the output transistors as well as all the other transistors in the channel. It still acts like a short when I power up through a bulb-limiter.

What might be the next thing to check? The good channel measures the same resistance to chassis from fuse holder as the bad. The bad channel previously had a low impedance path to ground.

thanks for any help,
first post, long time lurker,

Carl

p.s.
I will soon be buying D. Self's power amp book (only own the small signal one at the moment).
 
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here is the schematic for one chan
 
Does your bulb-limiter light up when you turn the stereo on when the Pre/Main switch on the back is set to Separated?
 
The front end seems fine. Nothing unusual appearing at the input to the power amp (DC offset etc). I can jump the left side pre-amp out and send it to the working right side and it is fine.
 
Is this connected to a load or no? If it is, make sure the output coupling cap isn't shorted. Even if its bad though it should be OK into no load, just expect a bunch of DC at the speaker jacks. If it behaves with no load, replace the output caps.
 
Loaded (8 ohm speaker) or not, the bulb lights bright when the bad chan fuse is in. No DC on the output!

Crackling and fussing and flickering now with the fuse in...
1) bad soldering or stupid error? (i have checked my transistor pinouts. I use a PEAK and datasheets but I am VERY human)
2) bad biasing resistors somewhere? I didn't replace any resistors as I am not sure which would be likely culprits.
 
Total beginner here.... you changed the Output transistors?.

check components downstream of the outputs transistor ?.
 
I replaced all the transistors on the Amp Board for the affected channel. They are correctly oriented. I just pulled the whole board, reflowed all points, checked my work, no difference. The channel passes audio, but crackles and pulls a lot of current. B+ sags down from 70v to 20v. Would def be blowing the fuse without the limiter, I think.

This is what it was doing before total failure, according to the customer.

Resistors in the VAS or Dif stages?

Thanks for any help!!
 
Reliving. said:
Do you have to place the new replacements the other way around.?
Click to expand...

I used equivalent rated transistors with a different pinout. The board is clearly labeled and it is easy to be sure in this case. I checked all my replacements with a PEAK transistor analyzer to be sure of the pinout. I am a Certified Technician, I just hate power amps and have never built up a body of experience.

I don't think the channel would pass any audio if I had any transistor in wrong...
 
Crackling sounds is a symptom of bad transistors. Can you pull, test and/or replace all of the transistors on the left side of the amp board? Swapping the outputs from side to side, or just pulling the left side out might help narrow it down.
 
Hi! I did in fact replace all the transistors in that side already... It went from dead to working but crackling and sucking current. Replaced with new transistors that should be fine. The crackle has the vibe of a bad transistor though for sure. Outputs are new.

I also just replaced the output coupling capacitor and no difference...

This is a repair following an entirely blown channel w shorted output transistor and blown fuse.
 
A summary:
Pioneer amp had a blown channel (prior symptoms unknown to me, maybe crackled for a while)
Replaced ALL transistors (carefully)
Replaced biasing Diode string
Replaced all electrolytic caps (including output coupling cap)
No DC on outputs
Preamp etc working as expected

Channel now draws excessive current while passing audio and crackling badly. The problem is in the power amp, after the input trans. Most likely culprit?

i made an online order for the important resistors in the power amp. I will continue this in a few days unless someone has a "smoking gun" I haven't checked.

Thanks for any thoughts,
C
 
Reliving. said:
beginner again...... open resister???.
Click to expand...

I will see in a few days. I am just going to replace the bulk of the resistors, starting with emitter resistors. I will follow up.
Starting this thread has at least made me commit to finishing this. Don't look at the clock!
 
Check the SV03 diode (varistor), to see if it is open, or shorted. It should read as 3 simple diodes in series.
Check the bias pot to see if it is open, which will cause over current condition.
 
I was also going to suggest the bias pot, I don't remember it being particularly good quality and if the outputs shorted it very well may have roasted. At a minimum the bias does need to be checked. I recall a wire sticking down out of the amp board, and something tells me thats what you connect to in order to set the bias but I may be remembering that wrong. Possible thats for the centering control.
 
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