QRX-7500 Transistor Noise?

[W.T. Holt]

This monster quad receiver has been used pretty heavily by me for the last year or so and recently I started hearing a fluttering noise coming through the right speaker. It sounds like a fast paced ticking or popping continuously happening at a consistent rate. It is a faint noise but it seems to change its sound characteristics when I fiddle with the volume and EQ controls. I can hear it in 2 channel and 4 channel mode and it comes through on both the front and rear speaker channels.

All of this preliminary inspection has me thinking there is a bad transistor on the preamp somewhere. I don't have an oscilloscope though so I'm wondering what the next step might be toward finding and fixing the problem.

My main questions right now are:
1) Am I right in thinking that this type of noise is usually caused by faulty transistors?
2) If so what board should I be focusing my search on to find the bad one? Looking at the service manual I'm thinking either the volume circuit (F-1487) of the tone control circuit (F-1484a).

Looking forward to hearing what you all have to say, any and all help is appreciated.

Thanks!

 

[Hyperion]

You are right to think that popping (and static) type noises are often caused by aged transistors. However your description would seem to point to a PSU issue as it affects all channels, however it could still be one or more transistors requiring replacement. Also you mention 'regular' ticking and popping, this sounds like instability somewhere - PSU ? - I don't know, having never worked on a Quad receiver, I'll let others with model specific knowledge guide you further on this - good luck with it.

You might want to describe any reconditioning or recapping work performed on the QRX ?

 

[W.T. Holt]

Oh yes the power supply, I hadn't though of that, thanks!

Just to clarify though, the noise is only on the right side. It does come through on both the front and rear channels though. I'm pretty sure the front and rear channels have separate power amps which is what led me to believe the noise was in the preamp. If it were coming from the power supply I would think the sound would be heard on both the right and left side, no?

 

[Hyperion]

If it were coming from the power supply I would think the sound would be heard on both the right and left side, no?

Well exactly, yes I was basing my response on your comment...

I can hear it in 2 channel and 4 channel mode and it comes through on both the front and rear speaker channels.

I had assumed this meant all channels, but now see you had previously described it being limited to the right channel, then you are correct about the issue most likely being in the pre-amp stages, especially as you say you can alter the sound by moving the volume and EQ controls.

What news of 'work' done on the QRX? - anything?

 

[W.T. Holt]

I have done literally nothing to this receiver since I bought it at a yard sale a while back. It didn't even need to have the controls cleaned until now

 

[Hyperion]

OK, that sets the stage for someone more knowledgeable to chime in.

 

[slimecity]

It sounds like a fast paced ticking or popping continuously happening at a consistent rate

First check for something else on the same circuit that could be polluting your power - ie: have you just gotten a fridge or something? That could be sending very regular noises thru your house circuit. Ie: especially if it is a very regular ticking sound or similar.

If its less regular, then "fluttering" does sound like an unhappy trannie in your powersupply possibly.

 

[kevzep]

Shoot a video of the noise and show us how the noise changes etc etc....

Gotta be a semiconductor.....would be easy with a signal tracer/scope...

There are several dodgy transistors in these QRX's, I have come across them often, in the 9001 there are a boatload of 2SC1313 for example....the 7500 will no doubt have something similar...

 

[LBPete]

Years back I was chasing an irregular chime type sound in a receiver. It turned out it was picking up a signal from my cell phone. Turned the cell phone off, the sound disappeared. Turned it back on, noise returned.

Another simple trick is freeze spray. Chill the various semiconductors and see if the noise changes. Please post the serial number for the database. Click on the link in my signature block.

- Pete

 

[W.T. Holt]

SN is now in the database, as requested.

The sound I'm hearing is definitely a fluttering and I've plugged the amp into a bunch of different circuits at this point so I think that eliminates it being a noise from an appliance. Here's a link to a video so you can hear the noise for yourselves:

Once again, its only on the Right Channel. Does this rule out the power supply? Looks like the PS only has 3 transistors so I might just swap them out anyway...

 

[slimecity]

That still sounds like some kind of interference on the power to me. Its very regular - like you say.

 

[kevzep]

Ah, right, what you have there is a Low frequency oscillation.
Does it matter what input you have it on? Does it go away when you select the tuner or other inputs?
Basically you need to isolate it to a particular area and work from there.
It can be a power supply related issue...

It is not interference, its called "motor boating".

 

[Hyperion]

its called "motor boating"

I wonder who thought of that name for it...

 

[slimecity]

OK, so if you could see the speaker cone, you would see it cycling in and out.....I got it now.

 

[ManyMoonsAudio]

Sounds like it is before the volume control. That said, I believe it is time to recap that ol sucker.

 

[kevzep]

Sounds like it is before the volume control. That said, I believe it is time to recap that ol sucker.

Yeah I agree, failure point for this sort of symptom is usually a cap gone bad....

I wonder who thought of that name for it...

Yeah Put, put, put, put.....doesn't sound Iike any boat I know!!

OK, so if you could see the speaker cone, you would see it cycling in and out.....I got it now.

Yep, that is correct......

 

[W.T. Holt]

Ok folks, thanks for the input. I haven't been back into the Sansui recently but from the info I'm reading here I gather I should shift my focus from transistors to caps, possibly on the power supply or on circuit before the volume control.

Identifying the power supply is no problem, but the rest of the guts are packed so tightly inside the chassis its very tough to tell whats what. Any advice on which circuit boards to take a closer look at?