SA9100 protection

asterkal

New Member
Hi all,
I'm new to this site where I have seen there's a lot of information about Pioneer classic SA9100 amplifier. I'm in the process of repairing one which came to me with many parts blown and burned in right power amplifier, and had been badly recapped, some electrolitic capacitors replaced in the wrong position in power supply and protection circuit.
After replacing bad parts and reversing capacitors to the right position, at power on nothing burns nor shorts, but protection lihgt stays solid red. I have tested power supply and protection circuit voltages according to the indications given in some previous threads in this forum, all voltages are good except in awr-025 (protection module) pins 1 and 3, those sould be 0 volts and are actually about 40 volts.
My question is, that means some failure in protection circuit, or means the normal condition of pins 1 and 3 when protection is activated?
 
Welcome to AK!

Protection pins 1 and 3 are connected to the amplifier L & R output and monitors them for the presence of excessive DC voltage. So, the protection circuit is doing it's job by preventing 40 VDC from reaching your speakers. At this point, I'd start by removing and checking the amplifier output transistors to see if any are blown.

Les
 
Thanks for the answer Les! I have rechecked the output power transistors (MJ21193G, substitute for 2SA679 and MJ21194G susbtitute for 2SC1079), they check good, and so the drivers and the other transistors and diodes in power amplifier module. I have been studying the schematics to understand how DC voltage can reach the output, but I don't find any short, nor any defective part, should I search in other modules?
I have used as replacement the following transistors in power amplifier module:
Q1 BC557
Q2 BC557
Q3 BF422
Q4 BF422
Q5 BC557
Q6 2SC3503
Q7 2SC3503
Q8 2SD358
Q9 2SB528
No one of them checks bad now.
 
It seems odd that you are getting +40V at both amp outputs. Did you do your initial power up on a DBT and if so, did the bulb dim? It might be interesting to see the voltages on the four output transistors (pins 15 to 20). For the purposes of testing, you probably should separate the preamp from the main amp if you haven't already done so.
 
Yes, I powered up the amplifier with a dumb light bulb for protection, and the bulb dimmed satisfactorily. I didnt mention in my post that the 40 volts DC is negative. Now I have checked the voltages on the four power output transistors, and there's the same -40 DC voltage on all of them, on base, collector and emitter of all four transistors (and none of them checks short, in circuit nor out of circuit). I haven't disconnected the preamps, should I disconnect only the power lines, or also the signal lines? Could the negative 40 volts line go across some shorted semiconductor in the preamp stages? Perhaps I should take a much closer look at the schematics, as I didn't service none of this model before.
 
First up, just because a transistor diode tests ok, does not mean the transistor is ok/fault free, some need a "few" volta/amps/whatever before they fall over.
Since you have -40V or so on the base of the outputs then I doubt that they are at fault, will download the schematic and confirm. Regarding this fault, it's
a labour intensive task to track down the problem, need to take dc voltage measurements and move in the direction of increasing voltage, I try and get
some test points to get you started.
 
Identify faulty channel
- Measure dc voltage between Pin 1 (red probe) and Pin 3 (black)
on protection circuit (AWM-025). Will need mV scale, note sign +/-.

If reading is negative, -xxxmV then problem on left power amp, positive->right.
 
Circuit uses STV-4A, suggest incircuit diode test with amp powered off/unplugged. Failed o/c? Maybe Q7?
Suggest measure dc voltages at base of Q5, 6,7,8,9. Apart from Q7b, they should be less than 1-2 volts.
 
Back
Top Bottom