SX990 amp on board output caps

I'm doing a recap and output transistor replacement on a SX990.
I have a thread going in another subforum, but I thought I would post a question here in the Pioneer section.

There are (2) 1000uF 35V caps that are on each end of the amp board (4 pin can style) and then there are (2) 1000uF 35V caps mounted to the chassis near the amp board in clips that are wired parallel to the ones on the board. The schematic only shows (2) 1000uF 35V caps!?!....so I don't know if this was a factory thing or something else?!

I had read on here that the thing to do is to increase the value to 3300uF 63V along with a smaller film cap.
So my question is would this be a good replacement?

Nichicon Audio Grade Electrolytic
3300uF 63V - 647-UFW1J332MRD

I could mount those in the chassis clip locations and remove the can types on the circuit board entirely.
The chassis clip cap is 22mm X 47mm and the Nichicon's are 20mm X 40mm....so they would almost fit perfect!

My other question is does anyone have a good way to remove the 4 pin tab style caps on the board....they don't seem to budge...they're really in there!

Sounds like someone already did some "hotrodding".

The original caps are glued down. After de-soldering and straightening the pins (if needed) you will have to "get physical" to get them off.

Make sure you get all the re-connections right. The originals are 3 "working" posts and a mounting post. You may need to "jump" two of the connections on the amp board.

blhagstrom said:

Sounds like someone already did some "hotrodding".

The original caps are glued down. After de-soldering and straightening the pins (if needed) you will have to "get physical" to get them off.

Make sure you get all the re-connections right. The originals are 3 "working" posts and a mounting post. You may need to "jump" two of the connections on the amp board.

Click to expand...

Thanks blhagstrom
Yeah...I finally got them out...whew!...I saw there were two negative locations to jump. My plan is to hard wire the new caps directly to the board so I'll just add a jumper there.
The weird thing is these "external caps in the metal clips have the same color metal as the chassis and are screwed into the chassis with same type of sheet metal screws? So that's why I wondered if this was a factory thing and if anyone else had seen this on one of these?

Any thoughts on the 3300uF "Audio Grade" UFW Nichicons are those OK for this application?
I can't seem to wrap my head around the "Audio Grade" series....as in... they can be used anywhere in an audio product including power supplies or are they just for actual audio path circuit applications?
Maybe someone has an opinion?

My take is that "audio grade" is a fancy word for caps that have better specs.

I try to use them for audio signal, as in this case. Non-audio application seems to be just getting "fancy".

As far as increasing the uf of those caps. I'd feel comfortable up a little.
1000 to 3300 seems like quite a jump. The small coupling cap addition may help with surge. I just don't know enough about all that.

FWIW, just straight up replacement caps worked geat on the SX-990 I re-worked. Adding capacity, I don't know.

My take on caps at this point is that anything made now is SO much better than anything as old as what they are replacing. In manufacturing process (there were no computer aided anything back then), materials and just the age of the old caps.

I read in an antique radio page that there is no use using fancy computer grade caps because the rest of the system can't really benifit from the super tight tolerances.

you must replace the trim pots. Vr3 and Vr4 are a growing point of failure, and POSSIBLY the reason the 990 popped it's output and emitter (0.5 ohm 2 w ) resistor in the first place. Replacing Vr1 and Vr2 is a very good idea as well. when the pot opens up (when the commutator hits an oxidized point in the rotation) the circuit goes open and turns the output transistors and their drivers full on - resulting in a smoking ruin....

when you read "audio capacitor" think 90% snake oil..... especially when they won't come clean as to WHY it's a better audio capacitor - the ONLY exception I have seen is the silmic caps sil = silk in the separator mic = suppressing microphonics...

audio grade can be used as (expensive) power supply caps ....

when unable to match voltages for a capacitance value, go UP in voltage.

now, 3300uf for the output capacitor... not a good idea.

LOOK at the schematic - there is feedback tapped from BOTH sides of the capacitor, 15k on the amp side and 8.2k on the speaker side. Changing the capacitance will affect the filtering equations for the entire amp. You will be putting the gift into an unknown and unexpected operating region. I would even be hesitant in an amp this old about putting stacked film caps across that output capacitor - it could be promoting high frequency instability.

The biggest question is the clipped on additional capacitors - I feel THAT is not factory, the added cost of a second cap, the clips and the labor versus a larger capacitor.

You could try a 2200uf cap on the board, but I would stick with 1000uf stock, ON the board. Caps nowadays are much smaller than they were 40 years ago. A modern caps improved characteristics is enough of a challenge for an old design, without starting to magnify the differences. In my opinion, doubly so when I won't be around to notice and correct signs of trouble.

Thanks you guys for the replies....
More to think about...
I will see if I can find some new trim pots...didn't look at those at all.
I will just go back to using (2) 1000uF 35V original spec capacitors, installed on the board and jumper that minus side and then remove the 2 external ones off the chassis.
Here's what I was going to get for those (2) caps now: 647-UKA1V102MHD1TO

On the schematic the trim pots are listed as 50ohm and 30K "semifixed".
Any recommendation on a specific style of trim pot...wattage, voltage multi-turn, single turn etc?
I've attached pictures of the existing ones plus a photo of the semi-naked amp board and the existing four caps.
Will it be possible to adjust the new trim pots to the same value as the existing or will I need to use the test points and set the bias new?
CH1 was 21ohms and 12.4K
CH2 was 33ohms and 13K
Thanks for the help...

652-3386H-1-503LF 50k ohms single turn
652-3386H-1-500LF 50 ohms single turn

markthefixer said:

652-3386H-1-503LF 50k ohms single turn
652-3386H-1-500LF 50 ohms single turn

Click to expand...

Thanks markthefixer..

Is it OK to use 50k instead of the 30k listed in the schematic or was that an oversight on your part?

Mouser is certainly the nicest way to order and find these components.

Are your pots labeled?
Mark may know for sure, but I've found mistakes in the schematic versus what is actually in the amp.
Double check the ones that are installed.

Nothing I hate more than getting parts and finding that the schematic was wrong and I didn't check the actual amp.

:gigglemad

djmtascam said:

Thanks markthefixer..

Is it OK to use 50k instead of the 30k listed in the schematic or was that an oversight on your part?

Mouser is certainly the nicest way to order and find these components.

Click to expand...

no oversight, I had a choice of 25k or 50k and while it's easy to go to a higher resistance and dial it down, it's a lot harder to dial UP past the end of the pot...

blhagstrom said:

Are your pots labeled?
Mark may know for sure, but I've found mistakes in the schematic versus what is actually in the amp.
Double check the ones that are installed.

Nothing I hate more than getting parts and finding that the schematic was wrong and I didn't check the actual amp.

:gigglemad

Click to expand...

Yes they are 30K...
the other thing is lead spacing. The existing parts have 10mm spacing between outside legs and the only ones soldered to the board.
No center leg on this 30k one. The center tab is mechanically connected to one side on the back of the pot.....both are weird designs....the 50ohm is Nichicon!
These new Bourns ones have a lead spacing of only 5mm between outside leads. I suppose if the leads are long enough I can form them to fit and then solder the center lead across one end.
Not very straightforward though....

markthefixer said:

no oversight, I had a choice of 25k or 50k and while it's easy to go to a higher resistance and dial it down, it's a lot harder to dial UP past the end of the pot...

Click to expand...

Yes...I sort of figured that was the case.
I couldn't find 30k trim pots at Mouser.....

djmtascam said:

Yes they are 30K...
the other thing is lead spacing. The existing parts have 10mm spacing between outside legs and the only ones soldered to the board.
No center leg on this 30k one. The center tab is mechanically connected to one side on the back of the pot.....both are weird designs....the 50ohm is Nichicon!
These new Bourns ones have a lead spacing of only 5mm between outside leads. I suppose if the leads are long enough I can form them to fit and then solder the center lead across one end.
Not very straightforward though....

Click to expand...

There is usually a little "modification" involved in fitting replacements. Like the caps. The trimmer leads should be long enough to bend-to-fit and you can take the center leg, bend it to the side leg and solder them together before inserting the pot. I had to do that on my SX-6000. The trick on replacement trimmers is getting them so you can access them for adjustment when the amp is all back together and installed.

blhagstrom said:

There is usually a little "modification" involved in fitting replacements. Like the caps. The trimmer leads should be long enough to bend-to-fit and you can take the center leg, bend it to the side leg and solder them together before inserting the pot. I had to do that on my SX-6000. The trick on replacement trimmers is getting them so you can access them for adjustment when the amp is all back together and installed.

Click to expand...

That's what I'll need to do then with the caps and trimmers..no 30K to be found at Mouser.

I have a question about testing the receiver with the main amp board out of the unit...
If I isolate the B+ wire (56V) coming from the main filter cap to the amp board and pull the preamp-amp RCA jumpers at the back, isolate the IN1/2 leads from the front panel switch to the amp board and any ground wires, could I power it up and not do any damage?
The reason I wanted to do this is to check the rest of the receiver out while waiting for these parts to come.
My thinking was I could put signal into the Line In with a sine wave signal generator and then put a scope on the signal path to see trace if they are OK up to the amp.
I could do a B+ voltage check (no load) check the tone circuits and make sure everything ahead of the main amp board was OK etc.....I have already found a bad mylar cap in the tone control board on one channel via resistance comparison checks so it's possible something else may be wrong besides that and was the cause of the amp stage going bad.....at least that's my logic...LOL
but I don't want to blow anything up.

More pics and progress

Well I stripped everything but the resistors off this amp board!
I carefully removed the SVT-3 diodes and removed the heatsink.
The heatsink was coated with some sort of glue which I have scraped off best I can.
I ordered all the parts including new grease and mica washers.
All new transistors per Markthefixer's suggestions and the Nichicon audio grade 1000uF 35V for the 2 output caps...so now just have to wait for the mail and continue my cleaning. I didn't try to fired it up yet with the amp board out.....I will wait until I get the board back in!

I measured the SVT-3 and took some closeup pictures.
They both measured on the diode setting as OL one way and 1.380V
the other.
I also measured the resistance each direction and got OL one way and 11Mohms the other. 14Mohms on the second one.
I hope these are OK as I have found out reading here they are like gold....
I need to find the posts on here about these diode/varistors and read again so I can confirm they are OK.
I think I read you need to add a 1200ohm resistor in series with it to measure them properly or maybe test them with temperature.
I carefully removed these from the heatsink as they were sort of glued in between the small fins with the same stuff I think they used on the TO-3 parts.

One of the leads goes thru a hole in the heatsink and solder to a pad under the heatsink...the other leg hangs over the edge of the heatsink and solders to the board there.

Is there something I need to use like thermal grease with epoxy on these when I re-attach them to the heatsink? Are they supposed to be glued on?
The body shape is curved one side and flat on the other (where the marking is).
My thought was to I would assume the flat surface should be up against the heatsink?
I think I would put the flat side down with grease and then press it down while soldering the leads.

This amp board certainly has some nuances to it...to get so in depth and personal with it...is really fun. I'm the elf working on this present for Santa.
I just hope it works when I get it done. Don't know yet if I'll get it up and running by Xmas....but I don't want to rush either....Ho ho ho!

http://www.audiokarma.org/forums/showthread.php?t=485379

http://www.audiokarma.org/forums/showthread.php?t=485419

both threads have measuring bias diodes in them.

basically transistors "work" at a certain voltage (0.6v or so), but as they get hotter they work at a lower voltage. If the amp continues to deliver the same voltage, the transistors will get HOTTER, the voltage will drop, in a vicious circle called thermal runaway - this is also what happens to transistors when the thermal transfer compound between the transistor and the heat sink fails (when it gets old) . The thermal bias diode attempts to REDUCE the drive voltage to the hot transistors to prevent that. The reason it won't help with bad thermal transfer compound is that the transistor gets way hotter than the heat sink and the diode.

with the way the circuits are designed (badly imho) if the diode wire breaks or the diode is put in BACKWARDS, BOTH the transistors turn on HARD, and this is suicide for them. That's why we power up amps on dim bulb testers. Look them up... a simple light bulb, but it works...

I say this because you have jumped further and faster than I expected.

also usually replacing parts is a one at a time business, then it's a lot harder to mix them up, I'm starting to get a bit worried about the chances of success on this project. I HOPE I'm wrong. I HATE it when I am correct on these issues.

markthefixer said:

http://www.audiokarma.org/forums/showthread.php?t=485379

http://www.audiokarma.org/forums/showthread.php?t=485419

both threads have measuring bias diodes in them.

basically transistors "work" at a certain voltage (0.6v or so), but as they get hotter they work at a lower voltage. If the amp continues to deliver the same voltage, the transistors will get HOTTER, the voltage will drop, in a vicious circle called thermal runaway - this is also what happens to transistors when the thermal transfer compound between the transistor and the heat sink fails (when it gets old) . The thermal bias diode attempts to REDUCE the drive voltage to the hot transistors to prevent that. The reason it won't help with bad thermal transfer compound is that the transistor gets way hotter than the heat sink and the diode.

with the way the circuits are designed (badly imho) if the diode wire breaks or the diode is put in BACKWARDS, BOTH the transistors turn on HARD, and this is suicide for them. That's why we power up amps on dim bulb testers. Look them up... a simple light bulb, but it works...

I say this because you have jumped further and faster than I expected.

also usually replacing parts is a one at a time business, then it's a lot harder to mix them up, I'm starting to get a bit worried about the chances of success on this project. I HOPE I'm wrong. I HATE it when I am correct on these issues.

Click to expand...

Thanks Mark for those links, I will test those STV-3 with the 9V and 1200R.
Those diodes are marked on the part (see close up photo) and I have the schematic and board layout, so they will go back in correctly..I promise.
These STV-3 don't look anything like the ones I've seen photos of in the other posts.
The leads of mine are completely intact and I'm fairly confident they are OK.
1.380V off my Fluke appears to be very close to the 1.477 Echowars got at 500uA in his STV test and replacement thread.
But I will still do the battery/R test just to be sure.
Crossing fingers on those as that would put a major crimp in my amp board rebuild if they are broke.

Yes...I'm probably an idiot for removing all of the parts and jumping ahead of myself with ordering parts. I've already realized I forgot something.
I did document all the parts of the board in detail on paper with locations and of course photos. I have the correct schematic for this FVW model as well as the SM with PL and board layouts.
I'm being very careful and always double check myself with everything before powering up.
I did find a SX990 bias adjustment post here with attached current values and procedure, so I will review that when the time comes and when the time comes I'm sure I will be asking questions here.

One thing I did find last night was this "external" 2 caps thing I had talked about earlier.
These must have been factory installed according to the schematic!
On the block diagram for the FVW model at the outputs are C15/16 1000uF/35V off the amp board. The negative side goes to OUT1/2 and the positive goes to the plus side of the board caps which is where they were soldered.
Those are not C13/14 1000uF that are mounted on the amp board!
So it looks like they did want 2000-2200uF/35V at the outputs.
In fact one of the other models I found used 2200uF caps "on the board" and the block diagram doesn't show those C15/16.
The bad thing is they do not show up on the schematic anywhere or in the PL for the main amp board. Only place they are listed and shown is the block diagram.
So now I need to either change the on board cap to 2200uF/35 (another order and shipping to Mouser) or just leave those existing one on the chassis and use the 647-UKA1V102MHD1TO I already bought on the board....any thoughts on this??...I'm sort of leaning towards 2 new 2200's on the board?
I've attached a picture of the two "off board" caps...
Yes I do have a DBT which I built myself.
I have a couple of antique tube radios that I have rebuilt as well as a couple vintage Tascam tape decks and mixer.

Just so you know...this is very much a fun and relaxing hobby for me and this particular receiver I'm restoring is a favor to my best friend who is buying all the parts as a Xmas present for his brother who is the original owner.
He wants to give it to his brother and I quote "as if it were your own most precious possession that you have had since you were 13 and the only way you could listen to the Beatles". Were all pushing 60 here and my friend's brother is 68!
So I want to take my time and be meticulous and listen to Sgt. Pepper's!...
I've already told him it might not be finished before Xmas and that's OK with him, but I am very determined to fix this like new.
It did have a blown channel fuse, output transistor and emitter resistor when I first got it from him, so this is why I'm rebuilding the amp board to "like new" condition.
I had also found a failed mylar cap in the tone control circuit board which I've already replaced....
So thank you for all the encouragement and appreciate both you and blhagstrom for all of your help and knowledge...!!:thmbsp:

Wow, a thanks to me with MarkTheFixer...I'm honored. Mark's the expert. I'm just a lucky hack with a solder gun and enough knowlege to be dangerous.

Mark's concern is that there are known mistakes in schematics and board markings. I don't know for sure if the SX-990 schematic has any errors or product changes. The extra caps in your unit are puzzling.

Since you mention that this is headed for TT use and you may need more parts, did you do anything to the phono pre-amp section? Recapping and retransistoring that may be worth while.

Well you have helped with the questions you've asked me for sure.
I'm in the same boat...I probably know way too much to be even more dangerous as I can jump to conclusions on things.
Electronics and the nuances of a particular device or design can be very unique in knowledge and understanding as I know from doing a fair bit of reverse engineering in my day and even though I have a good grasp of electronics and electrical systems in theory and practice, it's some of the little details (like the STV-3 bias) for me that is more fun with the understanding about the details and that's why Mark's Pioneer knowledge and attention to detail is so important and welcome.
Anytime you can connect with the designer or factory techs who worked on these things day in and day out on a particular device or system you always learn something new!

I have not done anything to any other module "yet" on this receiver.
There are some dial fuse lamps I'm replacing and that tone control board I fixed mentioned before..but as far as the phono preamp and tuner.....well I don't have the proper signal generators for doing the tuner alignment.
I don't have a serious TT myself. Maybe something I should ask my buddy about...what his brother has for the rest of this system!
I can certainly put a scope on the preamp once I get this amp board back in and working. I was going to power this up without the amp board in there and do some other tests like that...but I think I'll wait now until the board is back in!
I have also not looked at the power supply...
I would like to know and possibly find the proper test equipment for tuner alignment though and try that. I have done some tape deck alignments but Santa would have to bring me tuner test equipment to add to my meager lab workbench.
But then I'd have to find more tuners in the future to work on....:scratch2:!

I myself have a Pioneer Elite receiver, Pioneer RT909 tape deck, Pioneer CD and Pioneer Elite LD/CD player all working great.
I have several speaker systems to use JBL, EV, Klipsch, KRK and SpeakerLab 7's...so I'm pretty good with sound system for my use and a big Pioneer fan from way back in the day !
Oh and if you can't tell I like posting and probably have too much time on my hands and type too much information...for which I apologize!