Pioneer SX-434 - fuse keeps blowing - #8

[j berry]

Pioneer SX-434


fuse keeps blowing - #8 on board

Transformer issue?


Thanks for any input...
J

 

[petehall347]

short in whatever b1+ supplies or in the supply itself .
welcome to AK

 

[j berry]

Thanks!!

 

[petehall347]

it supplies AF board 25.5v . to pins 21 and 26 .

 

[cademan]

Can't remember off the top of my head but does the 424 use an amplifier block or discreet transistors?

But yeah, something is drawing too much current. In my experience it is usually a rectifier bridge, filter capacitor, or output transistor or amplifier block. The power transformer is only suspect upon the previous mentioned things being good. Power transformer failure is rare so that is why I suspect it (if at all) last.

 

[petehall347]

Can't remember off the top of my head but does the 424 use an amplifier block or discreet transistors?

But yeah, something is drawing too much current. In my experience it is usually a rectifier bridge, filter capacitor, or output transistor or amplifier block. The power transformer is only suspect upon the previous mentioned things being good. Power transformer failure is rare so that is why I suspect it (if at all) last.

discrete transistors . b1+ is derived from a single diode and a couple of transistors etc for regulation . there are many things that might be shorted . i would start at supply looking for shorted caps etc . i might disconnect the wire going to amp board first . then see if current draw drops .

 

[cademan]

Yeah, I think simply testing the transistors with a meter would be a lot less complicated. 9 times out of 10 at least in my experience, it has been an output transistor. I might even go as far as saying 99 times out of 100.

I have had shorted diodes, rectifiers, filter capacitors, and power transformers but I automatically go for the output transistors first when I see blown or blackened fuses.

 

[petehall347]

i would expect f5 f6 to be blowing if problem with outputs .

 

[cademan]

I don't have a schematic so I am assuming that this is/was the main fuse.

 

[beeluv]

Could you have the circuit board wiring post number confused with the fuse number?
I am a newbee myself, so perhaps one of the more experienced members could help with this.
I think you might have reported fuse 8 blowing, when it is really fuse 6, because you were looking at the post number, not the fuse number.
I have an SX-434 that was given to me, and it blows fused 5 and 6. So far I have been too chicken to dive into it, but I have done some research.
I have noticed that the fuses on the power supply board are not labelled, but the wiring posts that are right next to the fuses are labelled very clearly.
I believe that post 7 is connected to fuse 5, post 8 to fuse 6, post 9 to fuse 8, and post 10 to fuse 7.
I will try to upload a pic of the power supply board (from the web) and one of a portion of the schematic to show what I mean.
If I have this all wrong, will somebody please set me straight. This is the kind of thing that can really throw us newbees.

 

[Standard-SX7]

I have a similar issue. Board is the AWR-059
I recapped my power supply and when I went to check the voltages I accidentally shorted pin 4 and 5 I believe.
After replacing the fuses no voltage was right, I got 3v at the speaker terminals, so I pulled the board, tested every part, no issues, put it back together and connected only the transformer side.

everything was good.

18V AC on pin 7 + 8
27V AC on pin 9
14V AC on pin 10

DC SIDE

14,4 V on pin 2
25,5 V on pin 3

-25,5v (service manual shows 24)on pin 4
+25,5v on pin 5

but base collector and emitter voltages check out as well.

when I went on to connect the wires of the filter caps to pin 4 and 5 both 3.15A fuses blew again. main filter caps are replaced and test OK.

what did I mess up shorting pin 4 and 5 in the first place?

 

[petehall347]

I have a similar issue. Board is the AWR-059
I recapped my power supply and when I went to check the voltages I accidentally shorted pin 4 and 5 I believe.
After replacing the fuses no voltage was right, I got 3v at the speaker terminals, so I pulled the board, tested every part, no issues, put it back together and connected only the transformer side.

everything was good.

18V AC on pin 7 + 8
27V AC on pin 9
14V AC on pin 10

DC SIDE

14,4 V on pin 2
25,5 V on pin 3

-25,5v (service manual shows 24)on pin 4
+25,5v on pin 5

but base collector and emitter voltages check out as well.

when I went on to connect the wires of the filter caps to pin 4 and 5 both 3.15A fuses blew again. main filter caps are replaced and test OK.

what did I mess up shorting pin 4 and 5 in the first place?

check the rectifier diodes

 

[Standard-SX7]

check the rectifier diodes

I removed and checked every component on the power supply board. I get all the proper DC voltages on the board but blow fuses when I connect the filter caps or the AF board.

 

[petehall347]

outputs shorted then

 

[Standard-SX7]

outputs shorted then

output transistors?

 

[petehall347]

output transistors?

yes

 

[Standard-SX7]

yes

q21 and q23 are shorted between E and C. thanks

 

[Standard-SX7]

mouser is no option as I dont need a ton of part at the moment.
locally MJE1530/31 are the only transistors I can order. are they a suitable replacement?

 

[petehall347]

what are the original ones ?

 

[Standard-SX7]

SD313 and SB507, service manual mentions SC789 and SA489 as an option too