Pioneer SA-9500 ii rebuild protect thump fix!

[sean sweeney]

I've been dealing with this 9500ii thumping ever since I rebuilt the amp and recapped, from the protect circuit (also rebuilt with upgraded caps and transistors)! It seems the more efficient newer transistors in the protect circuit were staying on too long at shut down and coming on instantly when they were supposed to take some time! There were multiple issues but here's what worked for me. The 100uf 16v capacitor in the protect circuit should be found at that EXACT rating, because bigger voltages can stay charged longer, so some newer low esr version of that will work pretty well. The resistor used to charge that circuit can be changed, to keep it from coming on too fast, namely the 120k resistor that charges the 100uf cap can be changed to 200k or whatever works for you, if this is an issue. Going higher seems to buy you more lag time at start up and I actually got it to wait a minute and still work.

Another thing that worked and the ultimate solution for me was to run an extra circuit off of both sides of the diode that powers the relay... use another diode of the same rating, and run that through a 160ohm resistor, and then through an 8v light bulb. The bulb will slow the charging down at the start and also suck up all the residual power at the shut down. I've been trying to get this one solved for a while... and i'm sure there are muting circuits that would achieve this just as well... in fact, if you could use another switch for the power on this unit, you could simply wire the same bulb to the diode that powers the relay so that is becomes activated when you shut the power off... the light bulb will pull the power away from the relay fast and prevent the thump at shut down, and you can ad resistance to the 120k resistor to lag the start up as well. I tried playing with various values for the capacitor, but the 100uf 16v seems to be the best one for the job. I hope this helps someone else with this trouble. It took me a good while to think this up... and if someone has a better method, go head and put it on this link. Good luck!

 

[markthefixer]

please check this:

Switch Assembly Board AWS-109 has both the power supply regulators as well as the protect circuit on it. The protect circuit is a pretty standard implementation, with one change - the relay driver transistor is a dual transistor Darlington. That'll shoot some usually expected voltages in the keyster...

I looked for a transistor list to try to understand what's up, and there isn't one i can find, so... WHAT did you swap out the protect transistors with?

Q14 of the power supply makes +36v (from +48v) and the only circuit using that 36v is the protect circuit. Please check that +36v .

All your complaints about the protect circuit are familiar, and all have been associated in the past with excessive supply voltage, screwing up many elements and thresholds in the near-standard protect circuit. That -5.5v on C14 is mostly responsible for the thresholds inflexibility.

What voltage is that supposed +36v really at?

This is not a be - all and end - all post, but rather an explorative start (continuation actually) of the (my) reverse engineering of the (Darlington driven) protect circuit.

 

[sean sweeney]

Ah! Ok... I actually tried quite a few initially, but wound up going with a c1815 and an A992... because they worked! haha... and I think you meant C13 right? That's the other thing that messed with me on this, is that the theory of operation on the protect circuit lists a bunch of caps and resistors by number, and the numbers do NOT correspond to the numbers on the full schematic of AWS-109... I have no idea what numbers they DO correspond to... but that was just a little annoying... but with the little trick I figured out it seemed to have done the trick. I had fixed this thing once for the guy a while back, and he brought it back to get the pop fixed, since I didn't know it was doing that... it's back home with the guy now, so I can't say or remember too much of the original repair or restoration...

 

[markthefixer]

the numbers correspond to the example schematic right there on the same page.

also, you have to distinguish between sa-9500 and sa-9500ii or "mark 2"...
The changes in the mark 2 "ii" , ESPECIALLY the protect circuit, deserve special attention - that darlington in the relay drive circuit for example...

hearing pops on power down is an indicator of the total failure of the protect discharge circuit.
it expects a fast collapse of the negative bias when the AC is turned off.
it usually uses the lighting circuit to get this negative bias, which is why if the lights blow their fuse,
protect disconnects the speakers, it thinks the unit has been turned off,
there is a half wave rectifier and a few uf of capacitance and a significant bleeder resistor.
this circuit is supposed to decay first, before any other supply starts to sag.

The sa-9500 runs that circuit from 7.5 vac lighting circuit

the sa-9500 ii runs that circuit from FIFTY volts AC!!! Through a 13k / 3.3k voltage divider ( to 10 volts), so it's getting a lot of zap...
(7.5vac) the mk 0 has a 4.7uf cap bled by a 15k resistor, feeding a 12k bias resistor for the stage
(10vac) the mk 2 has a 2.2uf cap bled by a 5.1k resistor, feeding a 5.1k bias resistor for the stage.
thus less capacity, much more heavily bled, and heavily biased.

i'm trying to wrap my thoughts around your changes... but with no circuit to measure nothing can be proved.

MY first experimental step would have been to drop the R46 in the ac voltage divider from 3.3k to 2.2k. changing it's output from 10v ac to 7.2v ac.

I have worked on sa-8500's,(mk 0 & mk 2) as well as SA-9500's and have never run into the faults you described with those units, they are not widespread..

The sa-9500 ii is the only unit that used a Darlington pair for the relay driver.

 

[sean sweeney]

Cool. Thanks for the info... I occasionally get stuck on some of these old pieces... but typically get through about 8 or so a week... and also attend to my other stuff... but I was just trying to figure out a way to make that thing work for the guy... and i'm sure there's a better way... in fact I had a thump fix kit for one of my Carvin amps that already had the kit installed... and was trying to wrap my head around if that could work in my given situation... lol... but couldn't find the paperwork on it and just had the blank PCBs... ok... heading to bed... I've been toiling with a magnatone Custom 440 guitar amp all night... i've had that thing for years and never got the vibrato circuit to work right... it's actual pitch changing vibrato, which is awesome, like a natural tube driven phase, and the amp itself is a true gain monster for an early 60s all tube 6v6 combo... and this one uses the vibrato that employs the multiple varistors, and oscillator tube, and is just a monster of archaic principles that i'm having a hard time absorbing! lol...