The Pioneer SX-626 was a mid range HiFi receiver from the early to mid seventies which I actually had as a kid. It was handed down to me as the old receiver which otherwise would have been discarded. It was paired with two medium sized Criterion 100B floor speakers.

The SX-626 output 20 watts of power into 8 ohms per channel. This by no means is a “powerful” receiver, but I had assumed it was more than 20 watts up until recently when I actually looked at the owners manual. I had thought all these years that it was at least 50 watts per channel. I never did have the original owners manual back then.

The sound of the SX-626 paired with these speakers was my first real experience with “HiFi”. It was the basis for which I would compare anything else to as time went on.

I had this receiver for many years, and it worked well, other than a minor issue of a low level “scratching” or crackle sound which was noticeable only when no music was playing and it was quiet in the room.

As time went on, the receiver eventually suffered from a few blown fuses and more scratchy sounding trouble from the amplifier.

I remember at the time, knowing nothing about electronics, thinking that the wire wrap between each board was likely the culprit and fixing that would be a headache.

I was also curious about electronics and when something was broken or not working correctly, I would take it completely apart before discarding it. Unfortunately, my original Pioneer SX-626 was eventually taken apart and discarded.

Pioneer SX-626 Design

The pioneer SX-626 is a vintage HiFi receiver which is easily serviceable today. The reason for serviceability is similar to other receivers of the time – very few parts were custom manufactured for the model or brand. Parts which might normally fail over time with normal use are easily replaceable today. Everything from the capacitors and bulbs to the output transistors in the audio power amplifier are still available today.

Receivers made after the late seventies started to use custom made components, especially in the power amplifier section such as the amplifier ICs. When these fail, the only option is to pull them from a donor unit. There are exceptions however, but there certainly were a fair amount of modernized receivers that have blown final output section(s) that cannot be replaced today.

The SX-626 (as with many other Pioneer models of the time) had a separate PCB for each functional part of the receiver. This makes for diagnosis and repair quite simple. Each PCB has posts for sampling voltage, signal output or other testing methods. Each PCB is connected with wire wrap, which if done properly is a better bond than a soldered connection.

Connections

The SX-626 can support up to three sets of speakers, and two pairs can be set to output at once. Note when enabling two sets of speakers at the same time, they are actually connected in parallel and the combined impedance must be at least 4 ohms.

Two tape decks can be attached, which back then was useful for dubbing. It has two phono inputs as well. There is also an auxiliary input (which I connect my streamer to). Oddly there is a microphone input on the front.

As with many of the receivers of the time, this one has a pre-amp out and main in, allowing for connection to an external amplifier, equalizer, or other accessories.

The SX-626 is one of several models which had the infamous proprietary speaker connectors:

I have one pair of the original connectors, and usually these are not included with the purchase of one of these today. Many were lost or left attached to the speaker wires. There are some who sell replacements on eBay. These are nothing more than a lamp cord plug with one leg twisted by 90 degrees. They do work and I would rather use those than hack up the back plate to add a modern speaker terminal strip.

Replacing an Inexpensive Amazon Amplifier

Shortly after moving to where I am now, I wanted to get a small amplifier for two bookshelf speakers I had for the small room I use for my office. I ended up buying an inexpensive power amplifier from Amazon, which was not all that bad. I connected an AirPort Express (2nd Gen) to it and would use it for streaming.

While this audio amplifier worked well and sounded okay, it did not have a tuner and had only one source (input). I had been thinking about getting a Pioneer SX-626.

eBay Purchase

There are many sellers on eBay that “restore” or “service” vintage HiFi receivers. After a bad experience with one, which had not even cleaned the unit, I decided I certainly could not do a worse job of it myself and I could save a ton of money.

I added a saved search on eBay, and after a few months I saw a listing from a Goodwill location on the west coast for just over $125. Shipping was another $60 or so, and I messaged the seller to inquire about protecting the face, knobs, and double boxing. I offered to pay extra for shipping if necessary. The seller mentioned they ship these all the time and will take the proper precautions.

When the unit arrived, it was double boxed and packed well. There was no damage from shipping and it was in good condition.

I had also purchased an LED lamp kit for this model and a rebuild kit, mainly for a few components I knew I would want to replace and a set of good instructions.

Dim Bulb Test

Since I purchased this receiver from eBay and it was listed as “for parts” and not guaranteed, I put it on the dim bulb tester to smoke test it before going further.

The dim bulb tester will glow or not light at all if the device plugged into it is pulling low current. It will light brightly if the device plugged into it is pulling higher current, and in this case it will limit the current to the device, protecting it from self destructing, to some degree.

The receiver passed the test! The face lit up and everything appeared to work. Now that the unit is working, I can then look to some of the items that should be done as preventative maintenance.

Replacing the Filter Capacitor

There is a relatively large filter capacitor for the power supply right next to the power transformer. These caps are very high quality and long lasting in my experience (Elna brand), and incidentally very similar to the ones in my Kenwood hybrid HF rigs.

I replaced the capacitor even though the original tested fine.

Amplifier Board Variable Resistor Replacement

When I had my original Pioneer SX-626, it suffered from a few ailments. A friend of mine had the same model as a kid, and his would blow fuses (the amplifier protection fuses). Mine would have a low volume “scratching” in one of the channels.

One thing I heard (and experienced myself) is variable resistors over time can open, changing their resistance value dramatically. The older the resistor, and depending on the design, it is more likely to fail 40+ years later.

The SX-626 has two variable resistors per channel on the amplifier board. These are used to set the bias current and voltages for the output transistors for the amplifier.

Even though these variable resistors might be working now, they can fail at any time and are known to fail on these models, even when they were relatively new.

Setting the Bias on the Amplifier Board

I replaced all four of these variable resistors and set the bias and voltage according to the instructions that came with a rebuilt kit:

Once you’re done with these, test the receiver on the dim-bulb. If it works fine, it’s time to set the voltages. Use a multimeter with mini-grabber leads, NOT regular probe leads, set on DC volts. The receiver should be plugged directly into the wall (no dim-bulb), left powered OFF for now, and should have speakers attached, with volume at minimum and source selector on Aux. Setting the bias on this is easy. Remember those two wires that are taped to either of the 2200 uf caps on the amp board? For each one, attach your multimeter’s red probe to the wire, and your black probe to chassis ground, then adjust the trimmer nearest to it until you get 17mv on your multimeter. Then set the other side to 17 mv as well, using the trimmer closest to that one.

If for some reason you don’t have these wires or can’t use them, you can set the bias with the emitter resistors instead. Each side of the amp board has two larger resistors at the back of the board near the heat-sinks. On the left they are R31 and R33 and on the right they’re R32 and R34. These are 0.5 ohm 2W resistors. Attach your test leads across either resistor (one lead on one side of the resistor, one lead on the other, doesn’t matter which lead is on which side). Then power on the receiver and set your multimeter to DC volts. For the left side of the board you’ll use VR3 (further back, left side) and for the right side you’ll use VR4 (further back, right side). Carefully adjust the trimmer with a plastic trimmer adjustment tool, setting it for +/- 17mvDC on your multimeter (voltage will be negative or positive depending which way your test leads are. Set both sides.

Then it’s time to set the DC balance. Leave the black probe on chassis ground and touch your red probe to one of the metal output transistor cases (start with the left side). Then touch it to the other case on the left side. There are two output transistors on the left side-the metal case of one should read about half of the voltage of the other. Keep your probe on the one with the lower voltage and adjust trimmer VR1 (left center) until the lower voltage is exactly half of the higher voltage–your voltages will probably be around 56.5v DC (high) and 28.25v DC (low). Then do the same for the right side, using the right-side output transistors and adjusting VR2 (right center). Once you’ve set the DC balance on both sides, recheck your bias for both channels and reset it if necessary. Run the receiver on FM for an hour, touching the big metal heat-sink on both sides every ten minutes or so to make sure it’s not getting hot. This should run cool unless you’re really cranking up the volume–if it’s running hot, you need to check your voltages.

After an hour, switch back to Aux, volume at 0, and re-check bias and DC balance again and adjust if necessary.

Replacing the Tuning Needle Lamp

The next part of the restoration was to replace the lamps, as some burn out over time. The SX-626 (as many other similar models) have incandescent lamps behind the blue and black face plate for the tuner. There are also lamps to indicate the source (AM / FM / Aux / Phono) and the tuning needle lamp.

Any number of these incandescent bulbs can burn out at any time and the older and more used these are, the more likely they will burn out.

The most difficult of the lamps to change out is the tuning needle lamp. The needle is plastic and extremely fragile! If it breaks, the only way to get a new one is from a donor model. The LED replacement has a bulky wire harness but the LED itself will fit in place where the incandescent bulb was.

In order to proceed, the silver face plate needs to be removed. This is simple and can be done by carefully pulling each knob off and removing the nuts on the shaft of each potentiometer.

Take note of where and how many washers are on each potentiometer. These washers were used as a simple level or shim, so that the face plate plastic doesn’t bend or crack due to uneven lengths of each potentiometer.

Now that the face plate has been removed, time to move on to the needle light replacement.

The trick I found to be most effective is not to touch the needle mount where the tuning wire and track are. I did not disconnect this at all, as not only is it glued to the tuning wire, if not put back in the exact same position, you will need to adjust the location so that the dial indicator matches the actual tuned frequency. Rethreading the tuning wire is a project in and of itself.

Disconnecting the needle and lamp housing was a bit tricky, but easily done with a good set of magnified glasses and a small jeweler’s flat blade screwdriver.

I carefully lifted the tabs up with the jeweler’s screwdriver and used a miniature needle nose pliers to bend them completely upward.

Once the four tabs are lifted, the needle will be freed from the track mount and the bulb can be removed.

I then placed the LED bulb in place where the incandescent bulb was, and the wiring harness I oriented to the left. I then secured the needle assembly back onto the slide track, carefully bending the four tabs back over where they originally were.

I then took another shortcut and snipped the white wires close to the old incandescent bulb. I pulled them back through the fabric “tunnel” that they sneak under the tuning wire with, and ran the new LED wiring through there in place of where the original bulb wires went through.

In order to allow for the freedom of movement of the tuning dial with the LED wire harness attached, I secured the wire loop with a clip I made from a capacitor lead.

I connected the new LED wiring to the existing bulbs white wiring connection and I used some small heat shrink tubing to secure it after soldering.

Replacing the Lamps

I removed the fuse style incandescent bulbs behind the blue and black face plate and replaced them with an LED equivalent.

I then removed the incandescent bulbs for the source indicators and stereo light and replaced them with an LED equivalent. Since the LED bulbs were a bit smaller and not inside the same casing, I used a small amount of hot glue to hold them in place. The hot glue is not permanent and can be easily removed if needed.

There is also a fuse type lamp behind the tuning meter, which also was replaced with an LED equivalent.

After replacing all of the lights, I put the blue and black dial board back in and powered it up for a quick check that all of the lights worked.

I switched through all of the sources to check that each one was lighting up and the correct ones were lighting up, making sure I did not accidentally put a lamp in the wrong place.

I lubricated the various pulleys and gears for the tuning thread and track. These vintage receivers have a famously weighted tuning knob and feel that lets you swing the dial to the right and left with quite a bit of momentum behind it. I then put the face plate back on and gave it a spin around the dial.

Cleaning Potentiometers

Sometimes these vintage receivers will have a scratchy potentiometer or two, and the fix is a debated subject. On this model and many others, the volume and balance pots for example have a “weighted” or resistive feel when turning them.

I would argue not to use regular DeoxIT on these pots, as the result could damage specific types of plastic, which might be present in these pots. Instead, use DeoxIT Fader F5 for example. It will not destroy some of the plastics that might be present in the potentiometer.

I would not recommend dousing any pot with damping grease in it, as this will flush it away. Instead, I would take the pot out if needed, and find a spot where just the wiper(s) can be cleaned more directly, leaving any damping grease undisturbed.

Further, I never use any cleaner on any pot unless the scratching is severe, annoying, or affects operation. If it is not real bad, leave it. These pots are not easily replaced, other than sourcing from a donor unit.

Tuner Alignment

I was curious if the FM tuner needed alignment due to a slight bit of noise when receiving a particularly strong FM stereo station. FM stations are not always crystal clear in my experience and sometimes very strong nearby stations can affect the receiver on these to a slight degree.

I was not sure if the fault was in the receiver, my ears, the station or environment. I decided to give the alignment a try and see where the receiver was in terms of condition and necessary adjustment. The service manual has a step by step alignment procedure for both the FM and AM tuner including MPX (stereo) adjustment. I will avoid rehashing the alignment process because it is clearly outlined in the service manual. However, I will share some experiences on a few of the steps.

First, there were two versions of not only the tuner PCB but the tuning capacitor in the frontend on the SX-626. Mine in particular was the 1973 version, which does not appear to have an adjustment for T8, required for the MPX adjustment.

Of particular note is the adjustment of T6:

T6 is a dual core transformer. This means it has a top core and bottom core. These days, this type of transformer is not used in receivers, and alignment tools are difficult to find. In order to turn the bottom core, you need an alignment tool that can pass through the top core, and fit into the bottom core.

A standard alignment kit will have at least one hex shaped head, with a narrower stem. These are designed for adjustment of dual core transformers.

The particular set of alignment tools I have had the right size hex head, but the hex part was slightly too long. It passed through the top core, but when it got to the bottom core, the hex head was still in contact with the top core.

I trimmed the length of the hex shaped head a bit so that it was shorter. This allowed the tool to pass through the top core and into the bottom core without still being in contact with the top core.

The most necessary piece of equipment for tuner alignments is a frequency and signal generator. There are many options on the used market at relatively low prices.

The signal generator needs to generate a basic tone, 400 Hz for example, both FM modulated and AM modulated. Further, it should also generate the MPX (stereo) signal as well.

I happened to find a heavily used Kenwood SG-5110 signal generator on eBay from a seller in China for a bargain price.

Of course I took a big chance on it possibly not working, but I tested the basic features by using an SDR.

I was lucky to have received a fully functional unit. I did have to change the line voltage setting as described in the user manual. I would recommend double checking this setting on any equipment purchased for use in multiple countries or if used outside the United States.

Lastly, a basic two channel oscilloscope is needed in order to view the signal output from the receiver.

I did not adjust anything on the frontend tuning capacitor, and followed most of the alignment instructions. I stopped on the MPX adjustment because I could not figure out how to adjust T8.

I did not get as far into the alignment as I wanted to. Upon digging a bit further, there were two service manuals for the SX-626. One from 1973 (the first service manual I found and followed the instructions for) and a previous service manual from 1971, which does not match my version of the SX-626. I am fairly certain my version is the 1973 version, as all of the transformer and variable resistor markings match. The only exception is T8, which does not appear to be adjustable.

This is the 1971 version of the service manual:

This is the 1973 version of the service manual:

Different AM/FM Tuner Boards

The most obvious difference between the 1971 and 1973 versions of the SX-626 is the tuner assembly:

To Recap or Not to Recap

I did not recap any of the PCBs nor did I replace any other components yet. I visually inspected each PCB and did not notice any obvious signs of leaking caps or bulging caps. Eventually they will need to be replaced, and that might be a future project, one board at a time.

Final Thoughts

The Pioneer SX-626 was the first HiFi receiver I had, and it was quite a while ago as a kid. I was surprised to see so many of these still around on auction sites. There are bargains to be had and restoring these vintage 1970s HiFi receivers is not all that challenging.

Modern receivers have many advantages compared to vintage HiFi, but they do not have that classic 70s feel and vibe. They also in many cases do not have the same sound and classic properties that you can no longer get today.

One example is while moving the tuning knob around the dial, you can hear the static between stations. These days you rarely hear static, due to modern receivers having digital tuning.

These 1970s receivers had the silver face and large lighted tuning needle, lighted AM/FM frequency scale for tuning, and the classic lighted blue on black face which is sought after by vintage HiFi fans and hobbyists to this day. I am not certain how much longer AM and FM broadcasts will continue or when they will be phased out much like analog TV broadcasts have been. But for the time being, I have enjoyed tuning around the AM and FM dial when not streaming digital music format via the auxiliary input.