Pioneer SX-636.....rushing sound on left channel

[larryderouin]

Do a search of the value and voltage in mousers search box. Basically you can use any film cap you want. The stacked films at the time this list was made were being used to good effect. However the manufacturers are stopping production of them. Cornell Dublier DME's is a good choice for both. Try that.

 

[c.coyle]

Going through the BOM list, I've found that:
Mouser #: 667-ECQ-E1474KF
Mfr. #: ECQ-E1474KF
Mfr.: Panasonic
Desc.: Film Capacitors 100DC 0.47uF 10% L/S =7.5mm
(x4)
....and
Mouser #: 598-DSF050J334
Mfr. #: DSF050J334
Mfr.: Cornell Dubilier
Desc.: Film Capacitors .33UF 50V 5%
(x2)

......are no longer available.
Any suggestions for Mouser part number equivalents?
Thanks in advanced!

Just do a search on Mouser and I think you'll find suitable equivalents by WIMA, KEMET, and probably a few other brands. Same for Digikey, except they don't carry WIMA.

 

[dukeofurl]

Thanks for that, I'll keep looking. Alot of the ones with 7.5mm spacing seem to be out of stock at Mouser.

 

[larryderouin]

You know you are allowed to bend the leads slightly to engage the holes. Actually this helps with airflow around the caps and helps keep any heat from the capacitor case itself. Within a couple mm is fine. But 5mm or more doesn't look good.

 

[dukeofurl]

Noted, thanks.

 

[pdxtim]

I had 6 bad caps in my Pioneer SX 636 replaced, and it sounds MUCH better. The repairman told me to plug the unit into a surge protector type strip with an on/off switch, leave the 636 turned on, and use the switch on the surge protector to power up/power down the Pioneer. He said the power switches on the 636s often go bad and they're a mother to fix. What do y'all think of that? Do the power switches often die on the 636s? Is it OK to turn the unit on and off through the surge protector? Thanks in advance for any advice.

 

[dlucy]

My hobbyist, non-EE understanding is as follows:

In many units when they are first turned on using the power switch, there is a spark that crosses the gap between the two contacts inside the switch. After the contacts are touching, there is much less chance of a spark. The spark can causes oxidization, a kind of burnt residue (which DeOxit is designed to help dislodge), and it can cause severe pitting of the contacts. The same is true for speaker protection relays.

The difference between relay contacts and power switch contacts is the amount of voltage and amount of current going between them and something called inrush.

Inrush current is a (typical) huge spike of current, at 120 or 240 V depending on where you live, that only happens at power on. The power reservoir capacitors "need" to be filled, they are commonly pretty big, and the power switch has just given them access to the big fire hose of voltage and current that is your house AC. These capacitors want to fill quickly, they "ask" for lots of power, and the still-closing power switch accommodates with as much as the wall socket AC and your power fuses will allow. That inrush is fine, is normal, but it makes that initial contact-gap-closing power switch get a bigger, stronger spark... causing more oxidization and pitting.

There are some good articles here on AK, one in particular, on avoiding this problem completely by adding a simple TRIAC circuit to your unit. The power switch gets reassigned to switching on and off a much lower voltage and negligible current which then controls a TRIAC where the main power flows. The TRIAC is solid state, so no sparking and the original power switch never gets asked to pass much and therefore little or no spark.

http://www.audiokarma.org/forums/in...or-so-of-your-time.504673/page-2#post-6897970

 

[dlucy]

In many units when they are first turned on using the power switch, there is a spark that crosses the gap between the two contacts inside the switch. After the contacts are touching, there is much less chance of a spark. The spark can causes oxidization, a kind of burnt residue (which DeOxit is designed to help dislodge), and it can cause severe pitting of the contacts. The same is true for speaker protection relays.

A couple of examples from a hk870 power amp and a SX-450 receiver:

 

[pdxtim]

Got it....that is what I ordered to replace the A725s.

Power amp adjustment question. Page 18 of the service manual talks about measuring the DC voltage across the power transistor pairs. It specifies to use an 8 ohm dummy load across the speaker terminals while doing this measurement/adjustment. I don't have an 8 ohm dummy load. The speakers on my bench measure 3.5 ohms.

I did the measurement and read 28.5 mV left and 32.4 mV right. The manual says it should be 20.0 mV. Is using my speakers as the dummy load affecting these measurements?

Also measured DC offset now at the speaker terminals. Don't think this is adjustable on the SX-636, it is not mentioned in the manual anyway. DC offset is 17.3 mV left and 22.3 mV right.

Hi BmW - Sorry, really old post here, but I'm gonna be replacing some sa725 transistors on an SX-636. Is it necessary to do the power amp adjustment as mentioned above when new transistors are installed? Thx.

 

[pdxtim]

I've never connected dummy loads to any X3X series receiver when doing the adjustments, per past practice on various units postings per Mark. Having said that tho, I've never seen a posting on the 636. It's similar in practice to the 434 and 535, that it's probably ok to not have dummy loads connected.

No DC OFFSET adjustment. Just the BIAS. Set for AUX, Zero Volume, No input connected. Take off the speakers and leave the terminals free. Set for 20mv.

Offset is fine. Anything below 50mv is very good. It may even come down once the bias is set.

Larry

Hi Larry - Sorry, I'm resurrecting a really old post here, but I'm gonna be replacing some sa725 transistors on an SX-636 (I need to insert a nail-biting, nervous emoji here). Is it necessary to do a power amp adjustment as mentioned above when new transistors are installed? Thx.

 

[dukeofurl]

@pdxtim - Since nobody seems to have responded to your post, I'll chime in.
Yes, i would check the adjustments, especially since you are already inside!
Me; I'd do it just because!

 

[pdxtim]

@pdxtim - Since nobody seems to have responded to your post, I'll chime in.
Yes, i would check the adjustments, especially since you are already inside!
Me; I'd do it just because!

Thanks duke, I appreciate your input. I did have a tech replace some caps in the receiver, and he made some adjustments at that time. I'm going to replace the "usual suspect" transistors. I guess my question should have been, when installing new transistors, do adjustments need to be made? Or in other words, do new transistors put it "out of adjustment"? Thanks.

 

[larryderouin]

In a word NO, but there are ALWAYS EXCEPTIONS. The big exception here is the TUNER! Transistor changes can throw TUNERS out of whack as can capacitor changes, really fast unless you have a list of parts to replace from guys like MTF or Echowars, who have done the research inside and out. But since you are in there, the last thing to do is check the idle current and offset. If it's off and adjustable do so, but be very careful if you haven't changed out the pots as they are open type (not sealed ) and are subject to corrosion and such, same as switches and pots. A pair of Bournes pots cost a few bucks but will prevent a smoking pile if the originals open up while adjusting. I'd go ahead and replace them while you're in there too. But as for the PREAMP (AF ASSY) No adjustments required after cap and or Transistor changes.

 

[dlucy]

Hi Larry - Sorry, I'm resurrecting a really old post here, but I'm gonna be replacing some sa725 transistors on an SX-636 (I need to insert a nail-biting, nervous emoji here). Is it necessary to do a power amp adjustment as mentioned above when new transistors are installed? Thx.

I really like your question and I'm equally interested in the answer. Help me make sure I understand what you're asking:

You are replacing the 2SA726 PNP transistors in the preamp stage of a SX-636 (green circles in image) and you'd like to know if that necessitates adjusting the bias or idle (blue circles in image) of the power amp stage? Right?

 

[larryderouin]

No, you DO NOT NEED TO ADJUST THE IDLE CURRENT (Bias) or the OFFSET when replacing components in the AF AMP ASSY. See my previous post.

 

[pdxtim]

No, you DO NOT NEED TO ADJUST THE IDLE CURRENT (Bias) or the OFFSET when replacing components in the AF AMP ASSY. See my previous post.

Thanks Larry.

 

[Radray64]

I just picked up one of these sx-636, sounded great..some snap crackle pops..

I have read all about the notorious transistors..

I found the 2sc1451's but not any 2a725 or 2a726 yet...mine has two 2a763 and two 2a733 in the Q1-Q4 slots...
Any thoughts on the two different transistors there.?
Also read other posts on AudioKarma on the topic..
I thought since this would be my first attempt at repair.. I may just do two to four transistors first..

 

[markthefixer]

Well, it's easy enough to do on this sweetheart of a unit - do the 2sc1451's first and see if it's quiet.

The leads are larger, and care must be taken to get the proper and differently arranged leads into their correct pcb hole. The larger leads may not want to go through the hole if there is old factory wave solder flux residue in them, so a sharp wire to clean the hole OR heat will soften it.

That will give you confidence IF the snap crackle pops continue, to find the SIX 2sa725 or 2sa726 transistors in it, two in the preamp section and four in the power amp.

Remember to download the ksc3503 data sheet to get the proper NEW transistor lead arrangement (ecb) versus the OLD transistor lead arrangement of EBC IIRC.

 

[markthefixer]

I just went back and fixed the power supply C12 capacitor entry, down to 10uf 50v from 220 uf.
220uf in that position would be a problem, it is part of the feedback loop that controls the power supply regulated voltages.
I suspect maybe they wanted 22uf and had a typo, but the overwhelming evidence is that the factory wanted 10uf there.

In the 737 the corresponding cap is 22 uf, (c17 awr-058) so 220 could be a typo/error...

 

[RocknRum]

Bought one of these a few days ago and replaced the 2sc1451s with a couple of Kcs 3503 estu transistors that I had on hand. That cured the nasty static problem. This thread has been a great help.