SX-3900 Preventive Maintenance

[Mr_Mustrd]

Good Morning!

I’ve scored my first receiver that is over 100wpc! Pretty sure this receiver was sitting in storage for some time considering the thick layer of dust that covered it, seller stated that it did work, and he was the original owner he also had the manual! After blowing it out with the air compressor the dust was thick but fluffy and not sticky or oily.

I haven’t fired it up yet, and when I do plan on firing it up, will be with the output transistors disconnected no load and in aux to check the dc offset, if things check out fine I would like to do some preventative maintenance, someday would like to do a full recap but want to enjoy it as oem as possible for now.

From what I’ve read there are several things that I should do, and I’ve listed them below, can my research be double checked? I’ve also included some nudes, shes dirty but I’m excited to have her.

Upgrade the output transistors sockets to Phoenix Contacts: https://www.mouser.com/ProductDetail/651-1757022

Reflow all solder on the power amp board AWH-097 and replace the following transistors:

Symbol Org Replacement
Q111 2sa750 KSA992
Q112 2sa750 KSA992
Q107 2sa750 KSA992
Q108 2sa750 KSA992
Q115 2sa905 KSA1220AYS heatsink: 532-577500B00
Q116 2sa905 KSA1220AYS heatsink: 532-577500B00
Q109 2sc1915 512-KSC3503dstu heatsink: 532-577500B00
Q110 2sc1915 512-KSC3503dstu heatsink: 532-577500B00

Rebuild power supply board AWR-210 with new caps rated at 105c, new transistors, and reflow all solder on power supply board. Transistor list below:

Symbol Org Replacement

Q1 2SK34 954-E-202
Q2 2SK34 954-E-202
Q3 2SD313P MJE15032G
Q9 S2D313 MJE15032G
Q11 S2D313 MJE15032G
Q4 2SC1885 512-KSC2383
Q10 2sc2575 512-KSC2383
Q5 2SA912 512-KSA1013
Q6 2SB682 863-MJE15033G
Q7 2sc1915 KSC2690AYS
Q8 2sa905 512-KSA1220

https://audiokarma.org/forums/index...tle-background-and-a-couple-questions.759402/

https://www.audiokarma.org/forums/i...ut-no-audio-output.190127/page-2#post-2255149

 

[spark1]

Build a DBT. Use it when first turning up the unit, and after making changes.

 

[Mr_Mustrd]

Build a DBT. Use it when first turning up the unit, and after making changes.

Absolutely! I have a DBT being fed by a variac that has a meter on it. No way im connecting directly into mains the first time. It'll be brought up slowly.

 

[markthefixer]

lots to do before applying ANY power.

and it would take a LOT to persuade me to try to operate sans output transistors. NOT recommended. no load is almost as good.

Check for any burned or overheated resistors - that's a clear warning flag.

the
Q115 2sa905 KSA1220AYS heatsink: 532-577500B00
Q116 2sa905 KSA1220AYS heatsink: 532-577500B00

ksa1220 is going end of life, so use the 512- KSA1381estu, it IS the complement of the ksc3503.

When the power supply is done, look carefully at zip tying together bundles of cables running over it, to allow free-er airflow and cooling.
It made a significant improvement on the ones I have done.

why are you:
Q111 2sa750 KSA992
Q112 2sa750 KSA992
Q107 2sa750 KSA992
Q108 2sa750 KSA992

Q107 & Q108 are emitter followers and fairly innocuous
while
Q111 & Q112 are the start of the NSA circuits that we wish to keep parameters close together on.

It will take quite a bit of persuading to get me on board with just those 4 substitutions.
Quite frankly, to do that correctly , would require more a992's and ksc1845's to populate all the NSA positions.

 

[Mr_Mustrd]

lots to do before applying ANY power.

and it would take a LOT to persuade me to try to operate sans output transistors. NOT recommended. no load is almost as good.

Check for any burned or overheated resistors - that's a clear warning flag.

the
Q115 2sa905 KSA1220AYS heatsink: 532-577500B00
Q116 2sa905 KSA1220AYS heatsink: 532-577500B00

ksa1220 is going end of life, so use the 512- KSA1381estu, it IS the complement of the ksc3503.

When the power supply is done, look carefully at zip tying together bundles of cables running over it, to allow free-er airflow and cooling.
It made a significant improvement on the ones I have done.

Will do! I have on hand KSA1220, but will instead use the KS1381estu.

why are you:
Q111 2sa750 KSA992
Q112 2sa750 KSA992
Q107 2sa750 KSA992
Q108 2sa750 KSA992

Q107 & Q108 are emitter followers and fairly innocuous
while
Q111 & Q112 are the start of the NSA circuits that we wish to keep parameters close together on.

It will take quite a bit of persuading to get me on board with just those 4 substitutions.
Quite frankly, to do that correctly , would require more a992's and ksc1845's to populate all the NSA positions.

What to replace on the power amp is what drove me to make this post because I'm unsure, last thing I want to do is replace something that doesn't need to be replaced and cause more problems. My goal is to just replace parts that are known failure points and leave the rest. So if everything is working (and that's a big if), should the power amp be left alone? Below is the list that I've collected from other threads regarding transistor replacements for the power amp.

Symbol Org Replacement
Q139 2sa1100 KSA1013
Q111 2sa750 KSA992
Q112 2sa750 KSA992
Q107 2sa750 KSA992
Q108 2sa750 KSA992
Q121 2sa904 512-KSA1381estu
Q122 2sa904 512-KSA1381estu
Q115 2sa905 512-KSA1381estu
Q116 2sa905 512-KSA1381estu
Q140 2sc1384 KSC2073
Q119 2sc1400 KSC1845
Q120 2sc1400 KSC1845
Q113 2sc1400 KSC1845
Q114 2sc1400 KSC1845
Q101 2sc1775 KSC1845
Q102 2sc1775 KSA992
Q123 2sc1914 KSC2690AYS
Q124 2sc1914 KSC2690AYS
Q109 2sc1915 512-KSC3503dstu
Q110 2sc1915 512-KSC3503dstu
Q133 2sc2275 757-2SC5171(Q,M)
Q134 2sc2275 757-2SC5171(Q,M)
Q125 2sc2575 757-2SC5171(Q,M)
Q126 2sc2575 757-2SC5171(Q,M)
Q137 2sc2575 757-2SC5171(Q,M)
Q138 2sc2575 757-2SC5171(Q,M)

 

[Mr_Mustrd]

Found the answer! Thank you!

I hope everyone with a 3900 does these things:

check/resolder the amp board, it tends to get ring cracks in transistor joints due to insufficient wave solder and thermal cycling.
it's the usual cause of fried resistors.

check/replace the output transistor sockets, they crack and there's insufficient tension for the high current transistor lead connections.

replace 4 transistors (2sa905, 2sc1915) on the amp board with more robust transistors that are heat sinked.
The old ones had a thicker center collector lead to conduct heat out. But it had to be connected to 1 square inch of copper on the board. They didn't....

Q115 & Q116 2sa905a and
Q109, Q110 2sc1915
is better replaced by
512-KSA1381estu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.53 ea
512-KSC3503dstu to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe $0.46 ea
they can dissipate the heat better, if there is any way to use these slip on heat sinks, it would be a plus
532-577500B00 @ 0.82 ea, it is a slip on heat sink, and should keep them as cool as cucumbers IF it can fit on and in..

edit: I used to recommend using ksa1220 and ksc2690. Those were 1.2 amp collector current, while the circuits for 2sa905 & 2sc1915 in this model are 10 milliamps or less. That was grossly over specified and once the ksa1220 entered end of life status, the ksa1381 & ( ksc3503) became the best viable candidate, and with a collector current of 0.1 amps, is well capable of handling under 10 milliamps of current. So I switched the recommendation. ksa1220 will become harder and harder to find, while the ksa1381 is current production

https://audiokarma.org/forums/index.php?threads/pioneer-sx-3900-power-supply-shorted.996297/page-3