Pioneer's SA500 Blowing protection fuse

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colin.hepburn
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#1 Pioneer's SA500 Blowing protection fuse

Post by colin.hepburn »

Well l I acquired a little 12+12 Watt Pioneer SA 500 today it is good cosmetically but it's not without a problem in that the left channel blows the protection fuse upon switch on I was assuming it to be the output Q9/11 gone short circuit has anyone got a clue on this and how to check for the problem here's the left channel schematic there are no visual problems that I can see no bugling caps or burnt resistors on the board
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pioneer SA500 left channal.jpg
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#2

Post by ed »

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#3

Post by colin.hepburn »

Yep seen that but it don't help much though :)
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#4

Post by Mike H »

My first suspect is VR3 bad / dirty / open-circuit :?:


 
 
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#5

Post by colin.hepburn »

Mike H wrote:My first suspect is VR3 bad / dirty / open-circuit :?:


Yep that's a possibility the pre sets at the very old open type like you got in the old black and white TVs hand adjustably but are still sealed with some blue stuff thou that don't mean much I know
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#6

Post by Mike H »

So first thing I'd do is replace with nice new ones, else it's just going to happen again ~ don't think the originals are worth keeping TBH, IMO....

PS sealed multi-turn type may be a good idea


 
 
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#7

Post by colin.hepburn »

Yep I agree new multi-turn type are the way to go if they will fit thou I would replace them if I thought that was all that is wrong is there a way to check the transistors in circuit just to be sure
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#8

Post by colin.hepburn »

I have just put the meter across the pre sets in circuit and good channel reading of 70.3 and bad channel nothing with or without the fuse in
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#9

Post by colin.hepburn »

OK VR3 100ohm seems to be a old two legged type no centre leg VR 1 30K old type ceramic with centre leg internal connected to right leg plus it's a 30k looking a rapid not available in that value ??
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#10

Post by Mike H »

30k ? :!: Image

Would have thought the "100R" written on the schematic is THE maximum value.

You could try shorting it out with a bit of wire soldered across it then see if it blows fuses then. If not, then perhaps transistors are alright after all :?:

Getting infinite resistance on the duff side makes sense given that there's 2 diodes in series, either they are reverse blocking the meter current, or the meter test Voltage is lower than a diode forward drop, which DMM's typically are. On purpose :D


 
 
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#11

Post by Mike H »

Beg your pardon just saw where VR1 is, that only sets the o/p Voltage DC doesn't it? So shouldn't make fuses blow by itself


 
 
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#12

Post by colin.hepburn »

OK just rechecked
Bad Channel
VR1 the 30k preset reads 002 ohms VR3 100ohm preset reads 001ohms
Good Channel
VR4 the 30k preset reads 005 ohms VR4 100 ohm preset reads 071ohms
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#13

Post by Mike H »

Oh I see when you said VR3 "was nothing" I thought you meant infinity.

Well that could just mean someone has reduced VR3 to zero, after damage has been done.

I presume it still blows fuses?

Any way you can test the output transistor junctions? You could have blown transistors after all


 
 
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#14

Post by colin.hepburn »

I don't think VR3 has been touched as it matches the preset on the good channel and both have a blob of blue sealing stuff on them when you said short out the preset I take it you meant V3 and not the 30k VR1 I need to get some more fuses to try this first
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#15

Post by Mike H »

Yes I did mean VR3. OK forgot about the blue sealing stuff :oops:

"it matches the preset on the good channel" you mean the position of the wiper then? But what does it measure as? Ohms

Presume you got a DMM with diode test, can you test the o/p transistor junctions and compare left and right channels


 
 
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