tracking down scratchiness - not pots

[rmunson]

Hello Pio friends -
Curious for some thoughts/opinions on a mysterious scratchiness/static sound I'm getting on one channel in my sx-727.

I do not believe the noise is coming from pots or controls or switches -

I hear the static-like crackle/scratchiness sound intermittently - I believe it is limited to one channel --

I have sent signal out the Pre-Outs on the back of my Pioneer into another amp - I did not notice the scratchiness from the sound coming from the 2nd external amp --

could this scratchiness be related to the protection relay being dirty?

or does this sound more like a transistor?

I"m not getting a hum just an infrequent crackliness --

The symptom is evident in aux and FM - I believe all the inputs --

Thanks for any insight into this puzzler....
Reed

 

[rmunson]

Jerry gave me a good recommendation on testing the power amp side of the receiver by sending audio from my phone or laptop into the power side -
I'll try this and report what I find -- hopefully the issue will reproduce and we will have narrowed the source of the issue to the power amp section......

Thanks guys!
R

 

[rmunson]

ok - here's what I found:

Test 1: using Pre OUT from Pioneer to another amp and speakers -

audio coming from laptop into Pioneer aux
audio goes from Pioneer Pre OUT to external amp
sound is fine from both channels -


Test 2: audio going direct from laptop into Pioneer's Main IN (both channels) using Y splitter

crackle is coming from right speaker

tested with A B and C speaker outs left is fine, right crackles on all outs

Issue appears to be with the right channel out to speaker

any ideas?
Reed

 

[quaddriver]

its possible one of the transistors - or more - got noisy and they are easily replaceable, BUT, clean the switches and pots ANYWAYS, FIRST

 

[wd409]

sounds to me like relay or transistor problem, or a bad solder joint somewhere in the amp section.

 

[rmunson]

yep - I've deoxit'ed the unit -
so the suspect transistors would most likely be power amp or protection unit? Down stream from Main IN?
R

 

[rmunson]

sounds to me like relay or transistor problem, or a bad solder joint somewhere in the amp section.

Thanks wd409- so for the relay - is that a matter of cleaning the contacts in the relay, or are we talking about replacing it? For transistors, is the approach to just replace them all on the suspect board?

I'm looking at the service manual and schematic - could it possibly be connections to the big transistors on the heat sinks?

 

[Oldsansui441]

Hi, l would start with the main amplifier input differential pair, l haven't had a chance to look at the service manual, do you know if the SX-727 uses the 2SA726's in this position? If so these are known trouble makers and can be replaced with KSA992's.

 

[Oldsansui441]

Actually the input pair will either be 2SA572's or 2SA763's, l would still be inclined to start with these though. They can be replaced with the KSA992's also.

 

[kjello]

Relays first. My 2 cents|

 

[rmunson]

OK - sorry guys - when we say Relays what are we talking about?
When I hear relay, I think protection circuit relay - the little box that goes ping when turning on/off the system --

are relays transistors that forward signal through the amp and protection unit?

Thanks!
Reed

 

[quaddriver]

tap the relay. if the scritching alters in any way - bad relay contacts. otherwise any differential input pairs and current mirrors (not having the SCM in front of me or knowing the 727 inside) - replace with each matched up and mitzvah if the inputs and mirrors match each other as well.

If the inputs are replaceable by KSA992s (easy to get) then the mirrors are most likely replaced by KSC1845s (harder to get now) or perhaps KSC1815s (ask MTF for guidance on that sub)

 

[rmunson]

ok.... will be sitting down and watching these first:

http://bfy.tw/FhlT

 

[wd409]

Thanks wd409- so for the relay - is that a matter of cleaning the contacts in the relay, or are we talking about replacing it? For transistors, is the approach to just replace them all on the suspect board?

I'm looking at the service manual and schematic - could it possibly be connections to the big transistors on the heat sinks?

regarding the relay, it would depend on if and how bad it has started pitting, if its still relatively smooth you could just spray some deoxit on a small piece of paper (not sand paper), and gently do a back and forth motion between each set of contacts.

regarding, what im going to assume are the outputs you're referencing, hell its possible, i had a kenwood once that would distort in one channel once the outputs warmed up. you could spray it with canned air and it'd start playing normally again. I would tend to think its the smaller transistors tho if thats whats causing it. keep in mind tho im no expert. I just do it as a hobby. (aka i have no idea what im talking about sometmes.)

 

[slimecity]

I second doing the input pair on both channels of the power amp. Had a few Pioneer amps where it was essential to do this.

You could confirm this by measuring the DC offset on the speaker terminals of the unit once the noise kicks in - if its jumping up and down with the noise then thats another indicator that these are the issue.

 

[rmunson]

ok - I'm going to start with cleaning the relay -
any hints on how to remove the clear plastic cover?
R

 

[petehall347]

recording of the noise might help here . i do believe faulty transistors sound different to faulty relays ..
tap the relay first though . easy things first . or crank up the volume suddenly and see what happens .

 

[rmunson]

well that was fun....

 

[petehall347]

well that was fun....

what happened ?

 

[rmunson]

just a bugger getting that plastic housing off....

I've been running strips of paper through the contacts alternating with and without a drop of deoxit on the paper.....

just gave the contacts a direct shot of deoxit....