Problems with a Pioneer SA-9800 Amp

Hi,
Just wondered if anyone could help with a baffling fault that has been causing me some sleepless nights!

When I first recieved the amp, the symptoms were no output except a mild humming on both channels, and an unlit fuorescent indicator.

Subsequent investigations revealed two faulty electrolytic smoothing caps (the big expensive ones, R1 and R3! One of them had been replaced with a capacitor TEN TIMES LOWER in value than it should have been, and the other was probably leaky). One of these had been causing low output voltage on one of the main power lines (not sure now which it was).

Having replaced those, the hum reduced considerably, and the voltage was back to 55V, but still no audio from the amplifier section.

I then found that transistor Q8 in the power supply was open circuit in one junction, so I replaced it. On re-applying power, the fluorescent indicator came on, but the replacement transistor self-destructed, along with Q4 and Q6!

So, that is the latest state of play - I know this is a very complicated machine, so repairs will take time and patience, but I really need to know what to do next, as I do not want to cause any more damage to the power supply.

Any help would be gratefully received!

Greetings Potless, and welcome to AK and the Pioneer forum.

Doc

Hello, welcome

You don't start off easy, do you ?? jumped right into the deep end. did you !!:yes:

no matter...

Do you have the hi-res service manual downloaded, so that we are "on the same page"?

if the voltages on 3,4 and 5,6 are ok, 2/3 of the battle is won... because we know the master zener diode reference is ok, then all we have to do is fix the negative voltage regulators.

You may want to read these threads, as both involve 9800 power supplies. The second thread was done after the first (obviously) so I changed my philosophy towards finding individual bad transistors in a section, because as you have seen, piecemeal replacement can eat more parts.

You have to replace Q6, and Mouser is currently backordered on that jfet until october 1, which I found out to my dismay when I waited too long to stock up. I ordered an alternative from ampslabs, but they only sell 20 for $20 at a time, so I'll have to get some to you, WHEN I get them, because they're still in transit, they're NOT in my hands yet.

this is one thread, it is long, and fixes power supply and amplifier problems
This thread while short and incomplete has VALUABLE parts information in it



Because the second thread is way shorter, I would read that one first.

I have extracted the more relevent portions of the threads:
----------
sa-9800 power supply voltages

1 -52v regulated
2 -52v regulated
3 +52v regulated
4 +52v regulated
5 +32v regulated
6 +32v regulated
7 +18v regulated
8 0v ground
9 raw -48v UNregulated
10 -32v regulated
11 -32v regulated
12 raw +50v UNregulated
13 raw +68v UNregulated
14 0v ground
15 Raw -68v UNregulated

---------


The jfets may be the hard ones to find, as they are 50 volt parts (rare).
(Fairchild KSK30 has worked for me)

Thus you need 4 transistor types:
a 50v jfet
a higher power pass transistor
a complementary pair.(counts for two)



Q1, Q2 = 2SC1735 BCE 100V, 0.5A 0.8W 130MHZ use KSC2682 (power pass) ECB (leads are REVERSE order)
mouser.com part # 512-KSC2682YSTU @ $0.31 ea
These transistors are a different case style, a lot bigger than the little bitty Q2, but with ingenuity, it can be installed. The payoff is that while the old transistor was straining to hold the load, this one handles it a lot better.

Q3 = 2SA733 ECB 60V, 0.15A, 0.25W 190MHZ use KSA910 (npn of pair) ECB (leads are SAME order)
mouser.com part # 512-KSA910YBU @ $0.12 ea

Q4, Q8 = 2SA904 BCE 120V 0.05A 0.2W 150MHZ use KSA910 (another npn of pair) ECB (leads are REVERSE order)
mouser.com part # 512-KSA910YBU @ $0.12 ea

Q5, Q6 = 2SK34 SGD use KSK30 (jfet) SGD (leads are SAME order) // out of stock.......
-----
Q5, Q6 = 2SK34 SGD use NTE458 (jfet) SGD (leads are SAME order)
as much as I hate to recommend NTE parts (the absolute LAST ditch chance)
mouser.com part # 526-nte458 @ $2.00 each

Q7 = 2SC1914 BCE 90V 0.05A 0.2W 150MHZ use KSC2310 (pnp of pair) ECB (leads are REVERSE order)
mouser.com part # 512-KSC2310YBU @ $0.11 ea

Mouser has NO minimum order, or fee for a small order. and towards the end of the checkout procedure you will be able to specify first class mail shipping, which will keep costs low. Get a few extra transistors... the shipping of a second order will cost you more than a replacement set of parts.


These KSA, KSC & KSK numbers can be looked up on the web at:
http://www.fairchildsemi.com/crossref/search.jsp

BEFORE YOU INSTALL THE TRANSISTORS, CHECK THE LEAD ARRANGEMENTS OF THE NEW AGAINST THE ORIGINALS!!!

The ONLY reason I have recommended replacing all 4 transistors in each regulator is a balance between
1) the possibility of damaging (and having to get) good parts by using a piecemeal approach with MANY voltage readings and out of circuit tests that will take a while to talk you through
and
2) the probable low cost of the replacement transistors.

Plus the recommended replacements are MUCH more robust.

Once we get the power supplies behaving, then we can assess the rest of the amplifier's health. Something else could have been wrong, taking out the power supply in the process. Then we have the situation of some individual's ill-advised tampering with it's innards.

The LONG thread boils down to a double fault, a very leaky capacitor loading down the repaired power supply as well as a failed driver transistor in one of the amplifier channels also loading down the repaired power supply. Whether the bad amplifier driver transistor shorted the power supply first, applying overvoltage to the cap which then shorted out the power supply completely or vice versa will never be known, the end result was that once the power supply was repaired, we weren't done...

Thanks a lot for all the information - I have the hi-res manual downloaded, and will take on board all that's been suggested.

Sounds like I could have my work cut out on this one!