Pioneer PA3004 Chip PCB Substitution

design by merlynski and Ashtrinidad
RIP merlynski, and thank you for everything!

The PA3004 IC chip is a speaker/output stage protection circuit which includes; DC deviation, B-/+ detection, AC detect, discharge/switching and power muting. It controls the operation of the protection relay in the Pioneer SX-780, SX-980, SA-7700 and several other common Pioneer silver-face amplifiers!

New "old stock" of the PA3004 chip is getting scarce, and prices are rising. We decided to attempt to replicate the chip functions on an easily makeable board to allow Pioneer Amplifier units using this chip to continue to be serviceable once the original chip completely disappears.

Design Goal: To replicate all functions of the original chip, allowing the board to be used as a drop-in replacement in all use cases, in as small a package as possible.

To start, we characterized an original PA3004 chip to determine the exact voltage characteristics and fault conditions of each pin. The results are here.

We designed a circuit to simulate the functions of the original chip. The schematic is shown here.

We redesigned the input detect sections for the DC offset detect (Pin 4), the negative supply detect (Pin 6) and the AC Detect (Pin 7). We were able to get similar specs to each of the original chip specs. We were able to design the timing circuit to continue to use the 33-47uF external cap on the original power supply boards, and design a relay NPN driver with enough current to easily pull in the relay.

When a fault condition is detected, Q8c goes to ground, allowing the timing cap to instantly discharge through R8 and Q8. Because of the high instantaneous current, we have specified a high current NPN for Q8 to increase reliability. If the fault condition persists, Q8c remains at ground, and the timing cap does not charge, forcing the relay driver to stay off.

If no fault is detected, Pin 8 and Q8c rises in voltage until it hits the trip point set by the zener diode. (it's Vzd + 1-2V) Once the trip point is hit, current flows through the zener diode and into the base of Q9, pulling the Q9e high, which allows Pin 3 to turn on the relay driver BJT.

All nominal voltages are similar to the orginal chip. Pin 8 may run a volt or two higher once it settles - around 10.0V. The chip is designed to work with all Pioneer SX and SA line amplifiers and receivers that use the original chip.

The builds

Two builds were created from this schematic - a surface mount (SMD) version and a thru hole component version. The SMD board is physically smaller, and may fit more easily in a tight space, but SMD reflow soldering capabilities are required to put it together. The thru hole version is slightly bigger, but can be hand soldered easily. Both versions can be fully assembled for about $7 a piece, if 10 at a time are made.

When building, we recommend that you cut the lead for pin 2, since the power supply board won't have a hole for it. It will keep you from installing the board backwards as well! We also recommend that you replace the relay driver BJT at the same time, since the BJT gain tends to degrade over time. A KSC2690AYSTU NPN is usually appropriate, but please confirm that it's the correct sub for your particular unit!

If you need assistance in ordering your own set of boards from JLCPCB.com, please contact me on audiokarma!

PA3004 sub v3.1s (SMD version)

PA3004 sub v3.1t (thru hole version)

Both downloads include the following:

Full sets of gerber files, which can be submitted to any pcb manufacturer for fabrication.
A full BOM & parts list
Graphics of the PCB layout
Eagle 7.7 schematic and layout files
The LTSpice file used to simulate this circuit

More details can be found an the audiokarma forums thread regarding this project, here:

https://audiokarma.org/forums/index.php?threads/pa3004-chip-pcb-substitute.950028/

Verified to work on the following units

Pioneer SX-1280 (verified by johnhoef on June 6, 2021)

Plenty of space - either the thru of SMD board should fit without issue.

Pioneer SX-980 (verified by Ray66 on May 3, 2022)

Enough room to install either variant!

, but best to install the 90deg header with the plastic guard removed so that the board sits close to directly over the thru holes. Otherwise caps C2 and C4 will have to be moved around to make everything fit.

Pioneer SX-880 (verified by Dustmaker on Nov 4, 2021)

Pioneer SX-780 (verified by Strgazr1967 on Mar 6, 2021)

PA3004 chip is located right against the relay enclosure - I would highly recommend the SMD variant for use in this receiver. If you use the thru hole variant, I reccommend the mod detailed below.
johnhoef: I got your clone installed, and the 780 sprang back to life! I used some solid wires that were a little smaller gauge that I had in mind, but this was all I had. There are seven of them, and that combination is quite stiff enough.

Pioneer SA-7700 (verified by ashtrinidad on Feb 10, 2021)

Plenty of room to install either variant. I installed a 8-pin socket for easier testing as well.