SX-450 static in one channel

[eedork]

Hey guys -

I recently picked up a nice SX-450 at the Nearfest in Deerfield. It cleaned up nicely and now looks excellent. It does have one problem though .. when I power it up there is severe static through the left channel. The right channel works fine. After searching here I figured it might be the famous power switch issue that occurs in many of the Pioneer *50 series receivers. I followed an excellent tutorial here and was able to remove the switch and clean the contacts with 600 grit sandpaper. The contacts looked new when I reassembled everything, but unfortunately that did not solve the problem. I've it all of the controls with D5/F5 once already and confirmed that the static is present regardless of which input is selected.

Any ideas guys?

-Matt

 

[Watthour]

Does the static increase and decrease with the volume control position, or is it present even at minimum volume?

Does the loudness switch have any effect on the static?

Do the tone controls have any effect on the static?

Answers to these questions might help isolate the source of the noise to a particular area or circuit(s).

 

[eedork]

Hi Watthour,

The static is extremely loud (it sounds like it may damage the speakers, so I only leave it on for a few seconds each time). The static is present through headphones as well, and is not impacted by the volume control, loudness switch, or tone controls. Most of the testing I've done has been with a disposable pair of headphones.

I thought for sure it was dirty controls and would go away with a little D5, but that was not the case. I'll dig up a schematic at some point and start tracing it out. Since it occurs only in one channel I suppose it could be a cold solder joint. When I had the unit apart to clean the power switch I reflowed a few suspect looking joints but I may have missed a couple.

I also noticed that there are wire wrapped connections on one of the controls (balance pot maybe) .. should I be concerned about those?

Thanks for the help!

-Matt

 

[markthefixer]

Ah HAH, there are 2sa725's in the power amp. Four of them, two each channels.

Plus two in the phono amp and two in the tuner.

Replace them with mouser.com's 512-KSA992FBU

 

[eedork]

Thanks Mark! How confident are you that the 2sa725s are the culprit?

-Matt

EDIT: I have a pile of these already from the 2220B I restored last fall. Excellent! Now I just need to find them .... do I need to match them? I assume the ones in the power amp should be.

 

[Watthour]

Hi Watthour,
The static is present through headphones as well, and is not impacted by the volume control, loudness switch, or tone controls.

That's a good set of clues, and now Dr. Mark is all over it. If you're going in with a hot iron and have a "pile of these already" you might as well replace all you find until you run out of 992s.

 

[eedork]

Thank you guys. I won't get to this tonight, but I will try to swap out all of these transistors tomorrow.

-Matt

 

[qguy]

Markthefixer aka MTF is THE Pioneer guro, he knows these amp better than the persons who designed them

! How confident are you that the 2sa725s are the culprit?

-Matt

 

[markthefixer]

matching won't hurt, the difference between matched and unmatched will be a small difference in DC offset at the outputs.

 

[eedork]

I swapped out all of the 2sa725s and it works perfectly now with no static. You guys kick ass - thank you!

I haven't measured the DC offset yet, so I'll do that and button it up later today. I may recap this little guy - it would be quick and cheap since it looks like there's only 25 or so electrolytics.

-Matt

 

[Watthour]

I swapped out all of the 2sa725s and it works perfectly now with no static. You guys kick ass.

Matt - The more grammatically correct phrase would be "We kick." After all, you did all the work and are also part of this collective. As a group, it is probably safe to say that with all the resources, experience, and expertise available here we certainly DO "kick."


[EDITORIAL]

I haven't been here nearly as long as a lot of other members, but in the time since I have joined it is apparent that there is almost NO insurmountable problem with the devices we discuss here. Even the huge obstacle posed by unavailability of obsolete components can often be overcome.

The biggest single threat to achieving a successful outcome stems from some posters' inability to read and understand requests for information, to carry out simple instructions, then respond in a coherent and concise manner. Obviously, those are not impediments to you. I would expect nothing less from someone trained as an EE. It seems that's just the way we think and conduct our activities. I moderate a couple of automotive enthusiast boards and an engineering board, and the factor of effectiveness in communication seems to be a universal constant. Almost any complex or "impossible" problem which can be addressed logically can reach a successful conclusion, yet even the simplest of problems can be frustrating and protracted if communication and cooperation are ineffective.

I've often wondered just what the success percentage would be here at AK if statistics were tallied. I suspect it is into the extreme upper 90th percentile, and in those rare instances where the outcome is not favorable I would guess it would be easy to determine the reason(s).

[/EDITORIAL]

As for the SX-450, in its role as the "bottom feeder" of the Pioneer line, it still demonstrates the build quality which makes these units desirable. It may not be the most coveted of the lineup, but it's certainly worth the effort to keep running. That's why we hide our "15 kW, 18.5.1-channel, multi-modal, synthesized channel-processing, menu-driven, make-your-coffee-for-you-in-the-morning Yama-Mitsu-Denon AV Uber-Amps in a box under the flat screen and place the silver and wood vintage pieces out in the open like a fine piece of furniture.

 

[eedork]

Well said Watthour!

-Matt