Category Archives: Pioneer QX-747

More VU meters

I decided to work on the everyday QX-747 with the intention of sorting out the VU meters. The things hadn’t ever sat on zero since I got it, and that irked me.

not zero

So, off came the front.

in bits

Was also able to clean up the insides of the front panel, there was a lot of white rubber dust from when I did the lights. With the VU meter assembly in bits, I took my soldering iron and heated the glue up. The adjustment on the hair springs identical to that on a wristwatch. Moved the adjustment levers and tried it out.. got them either on or very near to zero.

quiet

loud

Bottom left bulb is very black inside, and quite dim as a result. Might get LED’s for it once they all burn out.

Phase shift

I’d been having a bit of trouble with the matrix quad audio positions on the QX-747. As in, the quad wasn’t working.

I sat down, thought critically about the operational description, looked at the block diagrams logically and studied the circuit diagrams.

2-CH worked properly, as did 4CH CD-4 but RM and SQ were marginal at best and RM favored the left channel, and both were a bit muffled sounding.

Determined that the cause must be early on in the decoder board, most likely the left channel. Popped the lid off, removed the CD-4 matrix board (sits on a bracket above) and inspected the decoder board.

pnp

Aha yes. Spot the culprit. There’s a PNP transistor early on in the left channel circuitry with the B and C legs transposed. It’ll pass current but not how it’s meant to!

board up

Popped the board off the plastic lugs, did a bit of surgery and put a brand new KSA992 in.

Result? Now it works properly, sounds clean and albums like Dark Side of the Moon play in another added dimension. Currently listening to Billy Joel. Impressive to listen to now. This is recorded (mostly) as if you’d be sat in the middle of the recording studio, listening to the band play. I like that. Worth the $0.12 transistor!

Stereo stuff and lights

Back to the Pioneer QX-747. I’d always been disappointed with the reception on the set. I had managed to tweak the AM reception, which is greatly improved now; the FM requires a plastic hexagonal tool to adjust the coil slugs.

Ordered a set of plastic tools, got them in. There’s a number of strong and some marginal stations locally, and one or to that have really picky stereo subcarrier signals, so a tweak here and there and I’m pulling in probably close to 20 stations. Happy with it now, adjusting by ear. Need to get a distortion meter ideally to get it set up fully and peaked, but right now it doesn’t pop, crackle or overdrive so it’ll be alright for the time being.

Trimming RF coil 2

Decided to set about redoing the indicator lights with LED’s, as the colors are still rich on the front gel. The holders are this funny rubber stuff which is going gooey due to the heat mostly. Pulled the lights out as best as I could and took apart the holders to rebuild them for LED’s.

twist

Decided to twist the wires on rather than solder so as not to damage the insulation.

trimmed and tucked

Trimmed the legs and tucked the wires in.

rubber bungy

Added the rubber holster

metal can

Finally slotted it into the metal can.

resistance

Added appropriate resistance. You can see the IR coming up through the board from the scale lights- the camera picking it up as purple.

Finally, together. the speaker option lights are now the correct light green, the mode lights a nice deep green.

lit up

Camera eats the colors unfortunately but it does look nice.

All pretty good now

So, the final shipment of bits arrived. Unhooked the control amp boards to recap them.

control amp board

Tada! All redone. Times two. Both the front and rear boards are identical.

recapped tone board

DeOxit’d and FaderLubed the potentiometers, then turned my attention to the pointer, which had been bugging me since I had learned that it should glow.

dial pointer in bits

Carefully bent the metal tabs up and liberated it from the scale movement. Inspected the bulb and sure enough, the filament was rolling around inside the glass, burned.

wires

Stripped the wire back carefully as it is about as thin as a hair, and tinned the conductors.

bulb

Carefully soldered on a replacement (12v 60mA one from Radio Shack, correct one should be 8v 60mA) to see if it worked.

led

I had considered a red LED, bought this side-shining one but unfortunately it was a little big and I didn’t fancy modifying it.

light

Yes! Okay, good deal. Not as bright as the original, but running at a lower voltage it should last for ages.

all glowing

Polished the plastic pointer so it wouldn’t show the light up anywhere other than the end. Looks good.

refitted

In a darkened room the effect is good, a little floating red dot on the dial scale.

nice

Enjoyable to listen to. Adjusted the AM alignment a bit and got that much better, though it needs further adjustment because it’s a bit weak on the low end of the scale. FM is better but still pretty horrendous. Thinking about sending it off to be aligned professionally. For now, it works.

Stereo stereo

More of the same.

tool

Unwound the wires from the CD-4 sub-channel decoder board.

unwind

That allowed the board to be removed for easy access. There were 3 pages of capacitors, and I didn’t feel like fighting with it.

cd4 board

Stripped the wires back ready to be rewound.

tool2

Into the other end of the tool.

recapped

Recapped and back in place, in record time.

eq board

EQ and preamp board popped out next. This one had wires that were long enough to flip it up for access.

redone eq

EQ board done and snapped back into place.

protection

The protection board was similarly easy to access. Pop up and redo. The 100uF 16V cap I accidentally ordered came in handy as it replaced the one I accidentally blew up on the PSU by putting it in backwards.

protect done

Pulled the power amplifier module up and out so I could pull board 2 off its’ pegs.

power amp

With burned fingers, pliers and tweezers, working in this space (looks bigger than it is, could just about get a finger in that gap) redid the 4 caps on the second board.

tight space

Clipped back in and all done.

power amp board 2 done

Have amassed quite a collection.

trash

Pulled out the service manual and started to look at the power/amp board adjustments. It stated the inputs of the amp board needed to be terminated across the pins with 5.1kOhm resistors. Wound 4 on:

terminated

Began setting up the idle and bias:

bias

These pots are real touchy, takes a very steady hand to adjust.

touchy pots

All set up (as close to 0.02000 Volt that it would go).

20mV

Buttoned it back up and set it up. Played it for a couple hours, and it sounds good.
Got some parts coming in Tuesday to redo the control amplifiers.

Tuner and power and stuff

Been busy with the stereo the past couple days, to keep my mind off everything.

Power amplifier board got a full recap. One channel.
ch1

Two channels..
ch2

Four channels….
ch4

Got 4 caps tucked away down the back that look hard to get to also. They got put on the “another day” list.
caps somewhere

Nice and even!
quad

Next up, the tuner board. Got it unscrewed and gently lifted up.
tuner board

Access wasn’t very good. Ended up having to check solder joints with a mirror.
mirror

That went back in, all very well. FM and AM reception improved significantly, but is still poor.
Tuner board done

Had an extra pair of hands for this one. Thanks, honey. More stations pull in stereo now.
fm stereo

Can get some deflection on the meter with a strong local AM station also.
am

Moved on to the power supply board. Got the thing unclipped, and again with minimal access due to wires.
power board

41 years of product development, things have become more compact.
old vs new

Some of the caps had gone bad, a couple had legs that had corroded away to nothing.
bad cap

That had made a mess on the board.
cap barf

Cleaned it up with Q-tip and isopropyl alcohol.
clean board

Refitted everything once complete. Looks good.
psu done

Powered it up, and there’s a bad smell and one of the fuses pop. Oh dear. One of the caps has gone off like a popcorn kernel.
pop cap

Pulled it off the board, and sure enough, the markings on the board are wrong for + -. Replaced it with a cap of the same type destined for the protection board.
incorrect

All done. Played ok for a while, switched off. More to follow.

Decoder board

Decided to continue on with the Pioneer and do another board tonight.

This time, one board nested under the matrix board done last night, we have the decoder board. I think the top one is the RM (QS) board, this the SQ board, because the CD-4 board is underneath.
Pulled it up off its lugs after having removed the frame and matrix board.

decoder board

Fast forward an hour and a half:

decoder board done

All redone and a fair bit tidier than the original. It’s not all haphazard. Screwed it down, I’ll come back and redo the transistors.. wanted to check this had not broken anything.

all back in

Powered up, all good on all functions. Next up, these troublemakers. They are known to fail spectacularly and bring the rest of the circuitry down with them. They came out.

2sc1451

Replaced them with some new variants that are better specced.

new transistors

After repeatedly checking the leg orientation and order, they were soldered in and the unit tested. All operational!

Enough for one night. More to come.

More in the way of vintage hi-fi.

Decided to set the QX-747 up where the SX-650 had been, to make comparison between the two. That and to make comparison between rebuilt and needing rebuild.
It sounded horrible for the first twenty minutes or so as the caps began to re-form (bad sign!) but dialed up a little… it really sounds good. It also has a better response curve than the 650 and as such the deep bass rumbles it makes, even at low volume, are impressive.

stack

So, with a cautionary warning about the transistors in one stage of the power amplifier, I set about making a list of all the capacitors I needed. Placed the order with Mouser. Came to $73 +s&h. I was expecting it to be more.
Anyway, it arrived yesterday.

box of capacitors

Big, full box of goodies!

Started to break down by circuit board the capacitors. I think I have a good scheme. Taped each one next to its description and location identifier. Stuck that in a Ziploc to make sure I don’t lose any in case they come unstuck.

system

I think that should work. Gotta do the rest now.