Pioneer Hi-Fi Receiver - Pops on Switch due to DC Present. Why? - Page 1
Forum section: electronics / repair

author: garytoosweet
author_rank: Contributor
timestamp: May 30, 2024, 05:15:14 pm
content: Hello all,I am having a bit of a perplexing (to me) issue and hoping someone can clue me in as to what I am missing.I have a Pioneer SA-9100 integrated amplifier that has a pop on the "Low Filter" switch due to DC present.When powered on there is DC on the switch side of the two 'floating' capacitors (i.e. no ground connection) that are engaged when selecting the 'Subsonic' or '30Hz' .There is about -3VDC coming from the 'Filter Amp' board downstream, when the amp turns on there is the full -3V passing through to the connection point of the capacitor and switch, which settles after a few seconds to around 300mV or so. This 300mV is present until the switch is moved to one of the positions which engage one of the coupling caps, and once there is a path to ground, and after an audible pop, the voltage quickly drains. The switch can be moved around with no popping sound until it returns to the 'Off' position, then the charge slowly builds back up over a few minutes, and it will pop again.I serviced the amp, replaced all electrolytic and tantalum capacitors, new transistors and whatever else where needed. Has been fully went through, tested for output and THD and is in great shape now outside of this pop.Original capacitors were 0.47uF low leakage electrolytic, and 0.1uF mylar. I have replaced with Metallized PET Stacked capacitors, 0.47uF@63V and 0.1uF@63V.S20(a+b) in the attached block diagram/schematic. C15+C11 and C16+C12 in the Switch C schematic.Shouldn't the PET stacked caps be much lower leakage than the electrolytic replaced? Should they not block DC fully even when floating?The pop was there (and much louder) prior to replacing caps and servicing the unit. The original low leakage electrolytic was a variety known to fail and the DC present would have likely been higher.When I measure the voltage with a multimeter it bleeds off in a few seconds. Fluke 179 with >10M impedance.I could strap a 10M resistor to ground at each connection point between the switch and coupling cap which would kill the pop as long as the switch is not used until a few seconds after turn on. But I'd like to more clearly understand what is happening and see if there is a more elegant solution.Thanks in advance for any pointers.

author: garytoosweet
author_rank: Contributor
timestamp: May 30, 2024, 08:35:00 pm
content: I suppose it is as simple as a capacitor is a short until charged and a floating/open circuit capacitor is not able to fully charged.But in restoring many pieces of vacuum tube gear I have lifted one end of a coupling cap from circuit to confirm it is blocking DC. If you can read any DC on the lifted end, the cap is leaking. In most cases, the capacitor is good and will show near 0mV on the lifted end.So, what is different here?Is there a better solution that I am not thinking about other than wiring four separate pull down resistors from each junction of the capacitor and switch to ground? (~10M or so).Thanks again,

author: FIXITNOW2003
author_rank: Regular Contributor
timestamp: May 31, 2024, 06:31:29 am
content: is this the range that suffers with those noisy 2SA726 PNP transistors ?

author: garytoosweet
author_rank: Contributor
timestamp: June 01, 2024, 08:26:05 pm
content: Quote from: FIXITNOW2003 on May 31, 2024, 06:31:29 amSometimes, yes. However this particular unit did not have any.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 02, 2024, 12:17:30 am
content: REF: "SA-9100 Switch C Board.pdf"It's odd that they have resistors (R11,13,12,14) on C8,10,13,14 to fix that issue but not on C11,12,15,16.

author: mzacharias
author_rank: Frequent Contributor
timestamp: June 04, 2024, 12:45:13 am
content: The pop is due to leaky coupling capacitor(s) in the tone circuit. It is a common problem, due to the crappy Sanyo capacitors of the day.

author: garytoosweet
author_rank: Contributor
timestamp: June 09, 2024, 12:40:41 am
content: Quote from: mzacharias on June 04, 2024, 12:45:13 amIn my original post I mentioned the capacitors had already been changed. The remains pop was due to circuit design.

author: garytoosweet
author_rank: Contributor
timestamp: June 09, 2024, 12:48:12 am
content: Quote from: Kim Christensen on June 02, 2024, 12:17:30 amBeen a bit since the receiver was on my bench, but from memory, those resistors are there to form a passive high pass filter. They also prevent a pop, but that is not their primary purpose. The other positions are just coupling capacitors, no filtering, so no resistors needed.