Pioneer sa-9500 circuit diagram problems

[oz-2]

Hi all

I'm a new user from the UK and I wondered whether I could draw on your collective skills to help solve a problem with my old SA-9500.

I am trying to recap it and re-transistor it in the power supply, the protection circuit and the power amp boards.

Looking at the layout diagram given in the HIFIEngine service book download pdf and the same thing but clearer in another place I find that my power amp board looks nothing like the one in the schematics.

the schematics show the power amp transistors wired to the binding posts in groups of three - collector, emitter, base - 31, 32,33 - 34,35,36 - and so on.

My board has completely different pin groupings - it seems in groups of five and some parts of the wiring to the transistors is commoned on the transistor fitting plates.

Has anyone come across this before? Is it common?

I definitely have an sa-9500 mk1 not a mk2 It is the three mains voltage model. If I knew how to upload pictures I would. but can't at the moment.

Any advice from those in the know appreciated.

btw - measured the dc offset voltage at the speaker terminals - 260mV left and right which is what I think is causing the protection to cut in at low volumes. Am I right?

with all good wishes and thanks

David

 

[petehall347]

collectors are common for the + and - supply voltages to them .
high offset will be the protection problem
have you tried adjusting the offset down ? vr1 left and vr2 right

 

[oz-2]

collectors are common for the + and - supply voltages to them .
high offset will be the protection problem
have you tried adjusting the offset down ? vr1 left and vr2 right

Hi Pete

thanks for taking the interest in this one.
Yes, I followed the instructions in the service manual to the letter but cannot adjust down the offset below 250mv for both l and r channels.

I can adjust the bias voltage to 20mV fine.

I have taken a look at the pwr supply board and note that one of the large 470uF caps is a bit bulgy on the top - it doesn't appear to be leaking yet. So I took the decision to rebuild the pwr supply board caps and maybe also, whilst the iron is hot the transistors too. I just ordered them from Little diode and 24 hrs later here they all are. My only anxiety is that every one of the replacement caps is physically smaller then original even though, in some cases, I have gone for a higher voltage spec. I think it should be fine though - do you?
Another problem is that, although Im fine with wire wrapping the wires are very fragile once unwrapped and break off - there's no way they could be re-wrapped I dont think.

any way thanks for any advice. Much appreciated. any wisdom is always appreciated.

David

 

[petehall347]

wire wraps are normally left in place and the boards moved around with wires still attached .
its that or unsolder the pins and remove with wires still wrapped .
a wrapping tool is available but the wires will maybe be too short now .
smaller physical size caps is usual for new ones .just keep the same uf rating and same or higher voltage .
as for the offset that could be a number of things .transistor diff pair / variable resistors / fixed resistors / capacitors /diodes. power supply .
i would get the power supply rebuilt first and go from there ..things tend to work better with decent in spec power .

 

[oz-2]

yes. Thanks. Thats where we are heading. If only Farnell had sent me the complete order for the caps I could get started straight away! I will have to wait until the 10 July for the rest of the order so I can rebuild the pwr supply.

thanks for help and advice. watch this space if Wimlbledon gets too boring!

david

 

[oz-2]

OK - that's all the caps in the pwr supply replaced with new. It took me a while to work out what was going on as one of the voltages was looking pretty good but the -ve voltages were way out. I was reluctant to tweak the two pots but after watching the dvm it all came good. Output voltages are now pretty close to spec for the +/- 24v, 35v +/- 50v and the 13v. I notice that the 60v voltages coming from t he rectifier are closer to 68v. that seems very high to me. Is that a real problem?

I can hear the protection relay cutting in and out - so that will be the next step.

regards

David

 

[petehall347]

i take it the 60v is unregulated ?
don't quote me but i seem to remember 10 percent high is the max . 10 percent would be 66v
is the mains voltage selector set right ? assuming it has one .

 

[markthefixer]

Not a problem, on Raw DC, it's not really locked down to a percentage, the regulated DC output however, SHOULD be within 10 percent.

I can't hold to it, but I think that 60, is the lowest acceptable input that will still hold regulation, with a 12v input/output differential for proper operation.

Bottom line, DON'T SWEAT IT...

 

[oz-2]

Thanks Pete and Mark the fixer. Much appreciate your encouragement.

I have had a good start to the day and a not so good middle!

After recapping the pwr supply and eventually working out how to adjust all those voltages I then set about recapping the pwr amp board which proved much easier than I thought. So, all new caps, apply power and adjust the Dc offset down to zero - or within a gnats nadger. Set bias to 20mv. All looking good although the protection circuit is behaving strangely - I'm sure it's permanently off. But I will come to that next.

It's was when I was putting that pwr amp board all back together that disaster struck. I accidentally touched the heatsink of Q8 against the chassis which resulted in a massive spark and crack - those 18000uf caps in the pwr supply! I really don't know what's been buggered but I suspect something has and perhaps in the protection circuit as the relay now clicks in and out. The amp will play music and sounds ok so perhaps nothing too serious. But it was a sod and I didn't expect it.

Just keeping the stream updated....

David

 

[oz-2]

Can anyone help me interpret this lot of readings, please?

:tears:Since my last update I have developed further problems with my amp power supply. I did have it working very nicely after replacing all the caps but an inadvertent short between chassis and the collector of one of the pwr amp transistors seems to have thrown everything off course.

I dont have much time to look at this problem and Im not sure i really understand what's gone wrong.

If you look at the attached pdf you can see that I have a problem with the voltage outputs of Q2 and Q4 and Q6.

I don't really know where to start with the fault finding here.

 

[markthefixer]

:sigh:

"not a lot of time" - miracles on a timetable....

q2 is POSITIVE voltages???

q1, q3, q5, q6 are working fine

assuming q2 is NEGATIVE voltages, q2 and q4 need replacing - one or both is zorched. Q4 definitely

Q6 is working. when the -48v instead of -61 is at pins 4,5,6 the voltage at pins 12,13 will be correct

 

[oz-2]

Thanks will check asap after short break away.
David

 

[oz-2]

Yes. Sometimes we have to learn the hard way...

 

[markthefixer]

I keep a couple of 10 ohm 10+W resistors, with a clip lead cut in the middle, and soldered to each end of the resistor. Clip them on when I'm working, BUT be SURE to remove them before applying power!!

 

[oz-2]

That sounds like sage advice. Now looking for 10ohm resistors... And croc clips too.
Thanks

 

[oz-2]

I was reading something else by Mark about those temperature compensation diodes on the power amp and how fragile they are and how unobtainium they are... And wondered whether there is some kind of circuit that could be built that would do the same job of adjusting the bias as temperature fluctuates. I couldn't do it but perhaps there is a small circuit using a chip that could be built and fitted that will do the same thing.

 

[oz-2]

Do all diodes change characteristics with temperature, then?

 

[markthefixer]

I was reading something else by Mark about those temperature compensation diodes on the power amp and how fragile they are and how unobtainium they are... And wondered whether there is some kind of circuit that could be built that would do the same job of adjusting the bias as temperature fluctuates. I couldn't do it but perhaps there is a small circuit using a chip that could be built and fitted that will do the same thing.

there are, but it is reengineering, touchy, but some models use those circuits, NO CHIP, transistors and resistors, including a trim pot, called a Vbe multiplier...

connecting 2-3 diodes in series will do the same thingy

that will get you by, but the temperature / voltage curves haven't even been checked by EchoWars (or me or anybody else). A VBE multiplier would be better.

Do all diodes change characteristics with temperature, then?

heck, everything affects them, don't fart nearby....
seriously the voltage changes with current AND temperature.

don't be stirring up trubble - just don't break the damn leads, I HATE fixing them, excavating down to the metal, soldering tiny wires, putting the damn thing in a mold, and forcing in epoxy, THEN peeling the gol durn thing OUT of the mold a day later. The time I spend on fixing those things ain't nearly covered by the 20 bucks.

 

[oz-2]

thanks for that info Markthefixer. yes, it sounds as though care is the best solution. If those diodes go down is it frying time at the pwr amp circuit board? or does it just feel poorly?

 

[markthefixer]

they rarely go down, just break their wires, and YES, both haves of the push pull circuit from the drivers through the output transistors turn on HARD, resulting in excessive current, smoke, sometimes flames, and multiple wrecked components.