Pioneer SX-990 Repair questions (Added Adjustment procedure links)

[Nashou66]

I have this Sx-990 I'm working on for a local record shop . It has a bad channel and the shop owner told me his previous tech did some work on it and it still doesn't work. I removed the amp board and found it had a strange
transistor in it.

Here is the pic. the transistor is an SK 3027 . All transistors are not shorted but measure strange hFE's or I should say very low like 12, 33, 66 and the oddball IIRC was over 100 .

I also found two burnt resistors on that said channel R24 a 22 ohm and R30 a 150 ohm .

Do you think I should change out the outputs with new MJ21194G's ?, the smaller tranny's seem to be fine .


What is that SK 3027 and could it have caused a failure? It does test as an NPN . on my DCA pro 75.


EDIT: Adjustment procedure links .
Athanasios

 

[zebulon1]

That SK3027 is actually a 2SK3027 which is a Mosfet. Not compatible to the 2SC793 NPN BJT.
That 990 is lucky you came to the rescue.
You have some doing to undo.
That entire board should be checked over for values and serviceability. It won't take long if you are organized.
Replace the E-caps.
Use Google to look for replacement transistors, "2sa497 audiokarma" will give excellent results.

 

[EastPoint]

It’s actually an SK3027 output transistor as far as I know. Should work fine in there most likely but I would obviously replace it for comfort.

 

[markthefixer]

That SK3027 is actually a 2SK3027 which is a Mosfet. Not compatible to the 2SC793 NPN BJT.
That 990 is lucky you came to the rescue.
You have some doing to undo.
That entire board should be checked over for values and serviceability. It won't take long if you are organized.
Replace the E-caps.
Use Google to look for replacement transistors, "2sa497 audiokarma" will give excellent results.

(nope)
zeb

SK is an OLD replacement series back from when RCA was doing that kind of stuff. Talk about a blast from the past - that's 70's and 80's latest....

 

[Nashou66]

That SK3027 is actually a 2SK3027 which is a Mosfet. Not compatible to the 2SC793 NPN BJT.
That 990 is lucky you came to the rescue.
You have some doing to undo.
That entire board should be checked over for values and serviceability. It won't take long if you are organized.
Replace the E-caps.
Use Google to look for replacement transistors, "2sa497 audiokarma" will give excellent results.

Yep already replaced the burnt resistors that actually measured ok when taken out. I also replaced the same ones on the other channels for consistency . All caps went out and the half were way out of spec. I left the two large silver 4 pin e-caps as they measured fine .

Yeah and who ever did the previous repair unwound all the wire wraps . I hate that !!! Why cant people realize its easier to just heat up the pin and pull out the entire pin and wire!!! Uggg .

It’s actually an SK3027 output transistor as far as I know. Should work fine in there most likely but I would obviously replace it for comfort.

View attachment 2587143

(nope)
zeb

SK is an OLD replacement series back from when RCA was doing that kind of stuff. Talk about a blast from the past - that's 70's and 80's latest....

Yep after I posted I found that same pic and some places that actually sell the original RCA Sk3027 for 1.99 each. much cheaper than the MJ21194G I plan to use.

Question, should the case of the outputs have continuity to the heat sink? I think I might have some on a few of the transistors. I put two back in to test for that.



Anything else I should look for?

Athanasios

 

[Nashou66]

Here are some large pics of the amp before I began the repair work .

extra large for your viewing pleasure

I couldn't do a repair and give it back with out cleaning the unit!! This previous guy was a hack !!!

Tuning capacitors will need a good cleaning of the copper shaft contacts.

nasty solder work and dirty board . So embarrassed I would be to give this back to a customer.

Athanasios

 

[zebulon1]

I have some spare 2SC793’s if you want one.
The paste looked old to me but it kept coming up MOSFET. Arff

 

[Nashou66]

I have some spare 2SC793’s if you want one.
The paste looked old to me but it kept coming up MOSFET. Arff

Thanks for the offer but I just put an order in at digikey for the MJ21194G‎ and some caps I needed to stock up on.

Athanasios

 

[Nashou66]

Question, I cant find any side mount or long lead 30k multi turn trimpots ( I prefer them to single turn) , So I ordered 25k and 50k . Which would you all use? I assume that Id set them to 15k( mid point of the original 30k ) All 4 trimpots were bad ( 50 ohm and 30k) and what I measured on the old ones may not have been correct.

I had some long lead 50 ohm pots with top adjustment and was able to instal them and bend the leads with heat string on the exposed legs to be able to lay it up against the board for a quasi side adjust ability. Could not find any 30k with long leads either to do the same hence the 25k and 50k ordered .


Also I asked earlier if the case of the output tranny's should have continuity to the heat sink? I would think not but want to be sure as I had on one i pre fit to test the sockets. I need to check all before I go further in the repair. I am using all new MJ21194G‎'s .

Athanasios

 

[markthefixer]

never continuity to heat sink. output transistor collectors are at wildly different voltages.

use 50k BUT set it at 15k initially.

 

[Nashou66]

never continuity to heat sink. output transistor collectors are at wildly different voltages.

use 50k BUT set it at 15k initially.

Thats what I thought , Thanks .

Athanasios

 

[Nashou66]

Ok part came in and installed . hopefully I'll have time Monday to instal the amp board and set the voltages .

Athanasios

 

[emsrph]

Looks good. Did you remove and re-grease the heat sink diodes or leave them as-is to avoid any potential lead breaking?

 

[Nashou66]

Looks good. Did you remove and re-grease the heat sink diodes or leave them as-is to avoid any potential lead breaking?

I left them as Is.

Now I have board reinstalled and I was able to adjust the Half point voltage to 31.5 volts each from 63 VDC I had at +B .

Now onto the DC offset. I have searched but there different approaches.

I downloaded the schematic awhile ago of using your ammeter through the fuse for DC offset ( VR3 and 4) but now I cant find the post that describes it.

Also with the amp board back in its harder to connect tot he test points on the emitter resistors.

Ugh

Athanasios

 

[Nashou66]

Well I hooked up the amp to speakers and all is well, sound is nice and even across the channels and very clear up to full volume.

I still have not adjusted the two trimpots VR3 or 4 yet. Still need some guidance on that , but I must be close cause it sounds great.

Athanasios

 

[Nashou66]

Found a link to that adjustment procedure

Athanasios

 

[emsrph]

Thanks for the post link.

To measure your bias voltage maybe you can attach negative to ground and positive meter lead carefully probing each of the test points on the foil side with an insulated probe. You can see here where the TP1 and TP2 locations are.

Or attach the positive lead to Q11 emitter, then Q12 emitter, which ever location is the safest to reach without shorting anything.

 

[Nashou66]

Thanks for the post link.

To measure your bias voltage maybe you can attach negative to ground and positive meter lead carefully probing each of the test points on the foil side with an insulated probe. You can see here where the TP1 and TP2 locations are.

Or attach the positive lead to Q11 emitter, then Q12 emitter, which ever location is the safest to reach without shorting anything.

View attachment 2597841

I'm going to use the procedure of removing the fuse and connecting my Fluke 87V in series and measure the current (mA's) and make the adjustment to 50 mA's .

SX-990 main amp adjustment

Here we are:

You don't need fancy equipment to adjust this amp. Measure the DC voltage at the power fuse, should be close to 56V, and adjust the trim pots VR1 and VR2 for exactly half this voltage (~ 28V) at the + side of the output cap. You have enough precision with your actual meter for this.

Now I will suggest something you will not find in the service manual, just for those like you, with a less sensitive voltmeter.

For bias measurement, you will remove the power fuses on the amplifier board (W15-060), and connect your meter in DC CURRENT mode, the + probe on the supply side of the fuse, the - probe on the amplifier side of the fuse. Adjust the trim pots VR3, and VR4 (with the meter in the corresponding fuse for the channel to adjust) for 40~50 mA reading on your meter. This corresponds to 1 mA in the input stage, + 5 mA in R13, + 4 mA in the driver stage, + 30~40mA (15~20mV / 0.5 Ohm) in the output stage. Adjust the Bias with no input signal.

Set your meter in the highest mA range when you will power ON the amp, since the initial current in the output capacitor may exceed the 250mA range. If 250 mA is your highest current range, connect your meter at the fuse but temporarily shunt your meter with a jumper, for a few seconds.

Good luck!

Athanasios

 

[emsrph]

Would be interesting to adjust by that mA current method and then get a spot reading of the emitter mV

 

[Nashou66]

Would be interesting to adjust by that mA current method and then get a spot reading of the emitter mV

Ahhh, wish i seen this . I took the amp back to the shop and collected the repair money .

But it sounded great after the adjustments .

Now they gave me a Yummy CR-620 to work on.

Athanasios