Monthly Archives: June 2015

Stereo stereo

More of the same.


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


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.


Into the other end of the tool.


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.


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.


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:


Began setting up the idle and 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).


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.

Two channels..

Four channels….

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!

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.

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.

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.

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.


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.


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.


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

More QX-747, dials switches and the like.

Had a bit of time to put the QX-747 back on the table today.

Pulled the bottom off to begin with to eyeball it.

Yup, looks good. No signs of leaks, scorches, any signs of distress. Bottom’s been off before, but eh. Not surprising.

Seems though it’s been relatively unmolested, considering the screws have been out.

Next up, some Caig products 🙂

Applied Faderlube to the pots and gave them all a good wiggle back and forth.
DeOxit onto the rotary switches, accessible from underneath. Ran them through their selections a bunch of times.

Cleaned all the grime off the lower case cover.

Took the top lid off again and applied Faderlube to the pots accessible from the top.
With a flashlight, pulled my glasses off and leaned in at a funny angle to be able to get to all the pushbutton switches with the DeOxit. Luckily these ones have a hole in the top of the case so that got a light douse and a few-minute rapid succession switching session.

Put the bottom case back on and powered up with two test speakers connected up.
Everything now works quite well, as far as switches are concerned. With a six foot length of wire attached to the 75 Ohm antenna screw, I was able to pull in a number of stations. Only a few would come in as stereo though.

Speakers selected off, CD-4 selected on, all 4 channels set to the same level, the deflection of all 4 needles in the display is even. The bottom left one is more dim than the others, despite all 4 light bulbs working.

They idle at 1 on the scale. Not sure if that’s typical for this.

FM stations come in stereo with the needle pointed here. Not quite right.

Dial is also not right, 104.5 is at peak reception at 104.1 on the scale. 94.3 at 94.0 – might just have slipped as it seems to be fairly evenly incorrect. I’ll have to see where the needle comes to rest at on the scale at either end. Didn’t think to earlier before I packed it away.

Still, not too bad. Does produce a hefty POP switching from FM to Phono, and a bit of a snap switching to AUX. Function switch still not 100% as the illumination changes on the signal meter as I wiggle it.

Thinking of changing the lighting to LED to keep it cool, especially the mode lights, which are always on.


Cleaning up the stereo

First thing this afternoon on the receiver was to stick it up on the table and lift the lid off to have a look about inside.

Lid removed 1

Not too bad really. A bit grimy but otherwise fairly sound. No signs of being hacked about with too badly. Quite a neat design really.


Set about cleaning it up, the painted surfaces with a bit of wax polish. Well, why not?


Came up looking pretty smart.


more shiny

Noted that the wires are very colorfully coded. I guess it needs to be, there are so many. There’s the CD-4/SQ/RM decoder board harness:


I’m not looking forward to taking these switches apart to clean the contacts up though. Yes, there is a 5 gang switch hidden in there.

function wafers

The main volume dial is broken. I’m thinking about gluing half a dowel to it to seat the knob properly. Right now it falls off and scratches the faceplate.
It was also rather grimy at this point.

broken volume pot

This little metal box contains a light bulb. That has burned out. I’m not looking forward to taking that to bits to fix it. The dial pointer should glow red.

dial pointer

Pulled the faceplate off and cleaned up the push-buttons and the inside of the dial scale where it was very dusty.

faceplate gone

Removed the wooden ends and set about scrubbing the faceplate with gentle soap and a soft bristled brush. That got it mostly clean, then went about gently with WadPol in the grain and around the dial bezel to bring the clean shine back.


Reassembled it all and put the power on, eyeballing and smelling for problems. So far so good.

powered up again

All buttoned back up. Stage one complete. Next up will be a recap, and redoing the wood veneer of the case.


Looks smart 🙂

New audio. From all around you.

It came up at a good price (ever so slightly dog-eared but nothing that a bit of TLC won’t fix) locally so I scraped together the pennies and bought me a new* quadraphonic amplifier.

Pioneer QX-747

A Pioneer QX-747 Quadraphonic receiver. From 1974. It’s seen a lot of use, has a few problems, needs a recap and all the switches redoing, along with the pots.

Thankfully this one has a service manual available for it online (Yay Elektrotanya).

Shall be opening it up probably tonight, but it did pass the initial turn-on test. Wiggling the buttons and switches got all four channels operational (to a degree), and the main volume pot is broken, and a couple light bulbs are burned out.

Beyond that, it’s a beast. Not as big as some amps, but it’s not small, by any shade of the imagination. I shall need to learn wood restoration also, and it needs some new paint on the grille at the back.

This weekend, initial clean-up and inspection.