Pioneer QX-747 Intermittent Phono Input "Thumps"

riverty

New Member
I have a Pioneer QX-747 that works good and sounds great except for the phono section. Aux, Tape, FM, AM all sounds great and clean. Set the unit to phono and I experience intermittent and variable, what I can only describe as "thumps" in the audio. These quite noisy episodes also show on the quad display in any or all channels and at various times. The frequency and duration of these noise episodes do seem to get worse in time.

So, with this, I assume this is in the phono preamp section however, it is not clear where the phono preamp is in this unit. It seems to be incorporated into the "EQ Amp" section but I'm not sure. Has anyone else had this problem? Got any suggestions in troubleshooting this?

Regards,
 
Eq amp is the phono amp. Probably your problem is a bjt(transistor) on its way out. It could be a power supply issue as well.
Is the problem on one or both channels? Something of this age has ecaps that are also on their way out.
rebuild the phone eq amp?
 
rcs16 - Thank you for the return.

I have already replaced the main power supply filter caps (2 x 15000 uF and 2 x 1000uF) and that cleaned the audio up overall. Channel 3 was dead so I rebuilt the channel 3 final preamp (new caps and transistors) and that got channel 3 working. I went ahead and replaced all caps in the other 3 final preamp sections to be sure they were OK.

I did some more long-time testing today. I found that the amp displays these noise episodes in AM, FM, and Phono only. AUX, 2-Channel Tape and 4-Channel Tape are all quiet. So, what is engaged in AM, FM, and Phono that is not engaged in AUX, 2 and 4 Channel Tape?

I did notice that there is a slight "clicking" going on inside the amp when in phono mode and these noise episodes happen. I'm guessing the extra gain added while in phono mode simply pulls more current in the final amp and is "ticking" the power supply into protection mode.
 
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