Pioneer SA-708 sound on left channel - UK Vintage Radio Repair and Restoration Discussion Forum

jim221 · 6th Nov 2018, 3:52 pm

My son asked me to look at his amplifier. He complained it was down on one Channel. I took the cover offhand peered inside. No smoke on turning it on and also no sound out of left channel. When moving some wires the sound magically came back. Doddle thinks me, gently moving each one I found the offending one. It was wire wrapped to a terminal and I found that the terminal was loose. Resoldered the terminal and Voila fault fixed.It has a switch which allows you to turn off the bass and treble controls and in the position where it turns off the controls the amp was very noisy. So thinks dirty switch contact so I removed the control board it was on (to be able to squirt stuff on switch) which was not easy and then refitted the control board. That was a good month ago. now I have got almost no sound out of the left channel again.I have obviously caused the problem in moving the board which had stiff connecting wires but how do I find the fault. I have a signal genie but no scopehttps://www.vintage-radio.net/forum/images/editor/color.gif. I have checked all the voltages on the transistors which seem ok. If I put in a signal at the volume control the output signal is definitely louder on the r channel although the l channel is fairly loud. I cannot fathom where the fault is,I got fid of all my test equipment some years ago as I had decided to not spend so much time in my shed. Help please there is a good service manual on hi fi engine.

vidjoman · 6th Nov 2018, 3:56 pm

probably dry joints - like the first one you found. Wire wrapped pins are often a cause of bad joints to the PCB.

Diabolical Artificer · 7th Nov 2018, 7:01 am

You should be able to check you have a signal at each part of the circuit with a DMM, it's not the same as a scope though, as you can't see it; use a PC as sig source. Failing that if you fancy a day out you can borrow my bench (sig gen, scope, dummy load, etc).

Andy.

FIXITNOW · 7th Nov 2018, 8:30 am

is this one of those with 2SA798 dual PNP transistors that play up

Mooly · 7th Nov 2018, 9:08 am

Does this amp use speaker relays ? If so then they can be a source of trouble, typically low distorted sound that 'clears' as you turn the volume up... until next time.

6th Nov 2018, 3:52 pm	#1
jim221 Pentode Join Date: Dec 2005 Location: Dereham, Norfolk, UK. Posts: 140	Pioneer SA-708 sound on left channel My son asked me to look at his amplifier. He complained it was down on one Channel. I took the cover offhand peered inside. No smoke on turning it on and also no sound out of left channel. When moving some wires the sound magically came back. Doddle thinks me, gently moving each one I found the offending one. It was wire wrapped to a terminal and I found that the terminal was loose. Resoldered the terminal and Voila fault fixed.It has a switch which allows you to turn off the bass and treble controls and in the position where it turns off the controls the amp was very noisy. So thinks dirty switch contact so I removed the control board it was on (to be able to squirt stuff on switch) which was not easy and then refitted the control board. That was a good month ago. now I have got almost no sound out of the left channel again.I have obviously caused the problem in moving the board which had stiff connecting wires but how do I find the fault. I have a signal genie but no scopehttps://www.vintage-radio.net/forum/images/editor/color.gif. I have checked all the voltages on the transistors which seem ok. If I put in a signal at the volume control the output signal is definitely louder on the r channel although the l channel is fairly loud. I cannot fathom where the fault is,I got fid of all my test equipment some years ago as I had decided to not spend so much time in my shed. Help please there is a good service manual on hi fi engine.

6th Nov 2018, 3:56 pm	#2
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,327	Re: Pioneer SA-708 sound on left channel probably dry joints - like the first one you found. Wire wrapped pins are often a cause of bad joints to the PCB.

7th Nov 2018, 7:01 am	#3
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,674	Re: Pioneer SA-708 sound on left channel You should be able to check you have a signal at each part of the circuit with a DMM, it's not the same as a scope though, as you can't see it; use a PC as sig source. Failing that if you fancy a day out you can borrow my bench (sig gen, scope, dummy load, etc). Andy. __________________ Curiosity hasn't killed this cat...so far.

7th Nov 2018, 8:30 am	#4
FIXITNOW Octode Join Date: Feb 2007 Location: Ely, Cambridgeshire, UK. Posts: 1,976	Re: Pioneer SA-708 sound on left channel is this one of those with 2SA798 dual PNP transistors that play up

7th Nov 2018, 9:08 am	#5
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Pioneer SA-708 sound on left channel Does this amp use speaker relays ? If so then they can be a source of trouble, typically low distorted sound that 'clears' as you turn the volume up... until next time.