Technical question - servicing my SX-850

RastaFish

RastaFish

Where's My Mule?!
Here's a question for the technical-minded: I want to service the amp on my sx850, nothing too involved, just adjust the idle bias and the offset. No problem, except for this - the service manual says to first pull the pre-out/main-in jumpers and put in a 5.1k ohm resistor. I can easily do this, except I have resistors that are 4.7k and 5.6k, but no 5.1k. So the question is this: Can I use either of the resistors that I have on hand, and if so, does it matter which one (is it better to be too high or too low)?

Thanks in advance for any input, I am hoping to do this tonight but if I really need the exact resistor I will wait until I have it.
 
Put two 4.7K in series then put them in parallel with the other two 5.6K which also in series.
 
Want the truth? It won't make a damn bit of difference.

The idea is to make sure that any DC or interference from the preamp doesn't affect the offset while you are trying to adjust. Pulling the jumpers and putting the resistors in is totally optional.

Put the selector switch on Aux with the volume turned all the way down, and then do your adjustments. If you have a problem setting the offset (floats more than about 5mV either way) then you might try the resistor thing, otherwise it's a time-waster.

And you can set the bias with or without the resistors...won't make diddley-squat of difference either way.
 
Thanks EchoWars, I *kind of* suspected that might be the case, but my knowledge is rather suspect so I figured when in doubt, follow the service manual to the letter. The positive out of all of this has been an increased understanding of resistors. I will probably get into my 850 tonight!
 
The puropse, as I understand things, of the resistors on the pwramp inputs is to insure those inputs are firmly at as close to zero as possible so they won't 'float' during adjustment. Remember, these are factory type adjustments and are a standard way of doing such adjustments. Yes, there are other ways of doing it which will do fine, as mentioned above. If you're curious, do the adj. without the res. 1st and note the results, then do it again with the res. and see if it's diff.
PS: I just did these adj. on my 850, as stated in the mnl. and never considered doing it any other way. I had on hand some 5.09 1% Rs so it was a no brainer for me.
And may I suggest letting it warm up 20 min before doing the adjustments cause they're gonna change during warm up anyway
 
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I was able to scrounge up the appropriate resistors from my grandfather (the Jedi Master of Electronics) so I will be into this little project very soon. I appreciate all the input and will post some feedback once the task is completed!
 
RastaFish said:
I was able to scrounge up the appropriate resistors from my grandfather (the Jedi Master of Electronics) so I will be into this little project very soon. I appreciate all the input and will post some feedback once the task is completed!
Click to expand...
I have a SX-950 and need to do the same adjust. Can't wait to read
your post!
 
Well when I did mine I could get right on the money with no probs. How far was it out of adj.?, couldn't say as I did the presetting of the pots like they said (DUMB). Doing it was easy. Shudda let it warm, then read where it was at, then preset, then adj.
You live and learn.
Don't know if it changed the sound as I did it before hooking it up.
 
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I adjusted my 850 last night, and I did the same thing as mentioned above so I don't know how far off it might have been to begin with. Anyways, it is exact now.

I found the process on the 850 to be a little more work than when I did my 880. The 880 I could simply remove the top and access everything I needed to get the job done, with the 850 I had to also remove the bottom cover and stand the unit on its side to do the adjustments. Hardly a difficult task, but still a little more work than the 880 was (I know, I am very lazy).

Best purchase under $2.00 I have ever made has to be the little clip-things that go onto the ends of the multimeter test probes. I doubt that I would have been able to do the job without them at all on the 850, I was able to do the 880 without them but it was much more of a pain in the ass than it should have been. They clipped right on to the posts and allowed me to easily make my adjustments. It was a fairly simple matter, except for those ultra-sensitive little bitch-ass pots for making the actual adjustments. The bias ones weren't as bad as the offset pots, damn those things are aggravating. I would get it so close to zero and then just nudge it a little too much and holy crap it would fly up to like 20 and I would cuss and fume and try to carefully get it back to zero and then the same damn thing all over again... :gigglemad ... But in the end I got everything right up to spec :banana: .

I also would like to say at this point that the guy who was posting that thread earlier about was Pioneer ever any good clearly never saw the inside of an sx-850! Especially for a mid-line receiver the thing is absolutely built like a battleship, I mean to the point of being ridiculous. I was also realizing how lucky I was to get this particular 850, I got it from my local Goodwill for $40 a while back and have been using it ever since. When I had it all open last night I realized that it is still looking like the day it came out of the factory inside, barely any dust, all boards are clean, just beautiful! I spent a good half-hour when I was finished just checking it all out and admiring it, the sx850 is truly a beautiful piece of audio gear inside and out. :smoke:

Thanks again to all for the input and advice, I did end up soldering up a pair of plugs with the appropriate resistors for the job. As far as I was concerned it just made the job more fun! I wish I had a digital camera so I could have taken a few pics along the way, but next time...
 
SX-850 vs. SX-880

Just wondering if Rastafish or anyone else that has these two receivers can say if there is a big sound difference between the two? I'm pretty sure the 850's gonna be the better of the two just based on the weight difference, although there's not much difference as far as the watts per channel.

I would really like to adjust the idle bias and offset and whatever other adjustments would be necessary on my SX-880 (I got it from a guy who had it in his basement for 10 or 15 years and never used it), but I wouldn't have a clue where to start and what tools to use, etc. Is there a book that covers these things for an amateur like myself? I would much rather do it myself than have to pay someone else, not to mention I have many other receivers that probably need minor service too and I would really like to learn about how the internal parts work and what they do. Thanks for any suggestions!
 
In the thread (with the sticky) in the Vintage forum about DC Offset there is a scan somebody posted of the page from the SX-880 service manual that deals with adjusting bias and offset. That's what I used to do mine!

As far as sound difference between the two, I actually think I prefer the sound of my 880. I will have to see if I think the 850 sounds better or different after adjusting it last night, it was finished after the kids and wife were in bed so I could not check the fruits of my labors! But to my ears my 880 sounds a little warmer, fatter in the low end without losing any definition, if the 850 wasn't so damn purty I would probably be using the 880 much more than I do. The 850 seems to be much better built, with real wood cabinet and heavier-duty internal components, but to this point I have preferred the sound of the 880. Also I like the power meters, I wish the 850 had them too.

But here is my dark secret, since acquiring a NAD 7130 a couple of weeks ago I have been using it almost exclusively. It is by far the most pleasing and cleanest sounding receiver I have ever had. Plus the tone controls are just spot-on to my ears, and the headphone amp is absolutely dynamite! Nowhere near as beautiful as the Pioneers but oh, the wonderful sound that thing makes...
 
SX-850 vs. SX-880

Thanks for the information Rastafish! I will be searching for that scan. I know what you mean about the meters on the 880. They are definitely very nice to have and look great too. I've always wanted to check out an NAD unit but I'm way too much into the Pioneer stuff. In fact I'm enjoying my SX-838 so much I haven't used my 880 very much. Although, even when it's not in use it's nice to just see that silver face gleaming!
 
RastaFish said:
It was a fairly simple matter, except for those ultra-sensitive little bitch-ass pots for making the actual adjustments. The bias ones weren't as bad as the offset pots, damn those things are aggravating. I would get it so close to zero and then just nudge it a little too much and holy crap it would fly up to like 20 and I would cuss and fume and try to carefully get it back to zero and then the same damn thing all over again... :gigglemad ... But in the end I got everything right up to spec :banana: .
Click to expand...


I had the exact same experience with my SX-1980. :lmao: Considering how sensitive those offset pots are, it seems like they would be very vulnerable to any kind of vibration, and unlikely to stay set. This knowledge has me thinking pretty seriously about replacing them with some multi-turn pots in the future. :scratch2:
 
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