Pioneer SM-Q300B AM/FM Receiver Restoration

Pioneer Model SM-Q300B FM/AM valve (tube) receiver

A very good friend where I work, Robert Molnar, gave me this unit. Robert is the archetypal hoarder of the consumer electronics world. Salted away in his garage collection of audio bits and pieces is anything that once was shiny and a bit different. Perhaps the penchant for shiny objects marks him instead as a barracuda rather than simply a bowerbird! I assume Robert took pity on my burgeoning interest in valve audio, in donating this unit to "the cause".

Robert had had it boxed up and in his garage in storage for some sixteen years at least. It had been a trade-in at Natsound HiFi, a noted Melbourne audio dealer through the '70's and '80's, and where Robert worked at the time. He'd picked it up at a very reasonable price at Natsound's annual clearance sale of trade-ins and seconds and so on. Robert recalls the magic-eye tuning indicator as being what particularly grabbed his attention. [Ever the collector, he acquired another one exactly the same way, some six months after the first. I've recently offered to "do a number" on this later one too, for him, by way of thanks for the one he's given me. He's yet to take me up on the offer - I suspect because he can't find the other one in the garage, but of course Robert says otherwise!]

There appear to have been a range of dual AM receivers of this ilk, available in the mid 1960's. In addition to my unit, SM-Q300B, with dual AM receivers and an FM receiver, there's an SM-B200A, using 6BM8's in the output. There's also in existence an SM-B200B, presumably very much the same as the "A", and an SM-B161, which is AM only, not FM too, and also uses a 6BM8 based audio output stage, producing 8~10W. Restoration of this later unit was featured in a Silicon Chip, Sept. 1999 article by regular Silicon Chip Vintage Radio column contributor Rodney Champness.

Early AM stereo in Australia, using two separate radio transmissions and a receiver with two completely discrete AM receivers is discussed in the article. Apparently it was available this way from the late 1950's to the mid 1960's. At least Pioneer and Kenwood produced suitable dual AM receivers accordingly, and one can reasonably guess that there would have been other brands available too. It seems that technically the idea worked well, but nevertheless did not become popular.

It was in complete and pristine condition when I got it, and looked like having seen some service but not an awful lot. Externally, nothing was missing or even moderately marked. Inside, it needed considerable work however, and I must now have two or three dozen hours of work clocked up on it. Even when this thing was made, the Japanese manufacturing drive for ever more compact products must've been well and truly in force - the components are like sardines inside a very close fitting chassis. Consequently, although ventilation slots are provided, it runs "hot".

Even after just initial inspection of the internals, I determined not to even bother switching it on even for the hell-of-it, until I'd had a real good go at at least the electro's. They did not look good at all. Clearly too, a repair had previously been conducted or attempted on one output stage channel; quite a number of changed components could clearly be seen. The repair appeared to have been proficiently and knowledgeably conducted. At least nothing was currently blackened, burnt and catastrophically broken.

Before that though, my eyes immediately lit up at the butchered mains cord termination within the unit. It was clearly correctly set-up for 240V operation well enough, and the mains cord was a typical and correct three-wire type as is used in Australia. But the mains cord was an "add-on", and clearly not factory-original; the earth wire was simply snipped and hadn't been connected at all, soldering connection of both phase and neutral wires was rude and crude, and the single pole on-off switch switched the neutral not the phase line! Damn dangerous, but relatively straightforward to put right. Alarmingly, wiring of the mains plug such that the power switch was in the neutral rather than the phase lead was also noted in Rodney Champness' SM-B161 article referred to earlier. One has to wonder if something systematic was happening at the time these models were being imported, 35 years ago.

I found replacing all the electro's really hard and stressful. There were several of them and most were as multiple-unit cans. While I had a good collection of typically 350V rated electro's salvaged from modern equipment, they were single value cans, and in a couple of cases just a shade low in voltage rating. They got crammed in everywhere, albeit as neatly, tidily and reliably as I could. I took a lot of time with this, because while I accepted that I was taking the unit away from original condition, I earnestly wanted to maintain the integrity of the unit. At least some of the large electro's I fitted could be made to occupy the holes and clamps for the original multiple-cans. So these ended up making the chassis look quite O.K. in this regard, at least when viewed from above!

Slowly and rather laboriously I then went through the whole thing, working over the chassis, crammed with components, quadrant by quadrant. I lifted, checked and replaced any number of resistors and capacitors that in any way caught my eye; heat-marked, discoloured or otherwise likely to have been stressed during operation. I had quite a bucket of excised bits at the end. As I had no schematic, no idea where I might find one, and dreaded the thought of circuit mapping such an extensive and compacted point-to-point wiring "collection", the work proceeded very slowly and carefully indeed. If I dropped concentration and induced a fault, between the myriad changes I was making, and the total absence of any apparent symmetry in the (albeit stereo) circuit, I knew I'd have a really difficult repair challenge ahead.

Pretty soon I'd ended up mapping so many little bits of circuitry that I was able to join some of the bits up and make quite a reasonable basic schematic of the audio path. The output stage is a conventional Class AB1 push-pull circuit, using a pair of EL84's (6BQ5) driven by a 6AN8 triode-pentode. The pentode portion of the 6AN8 is used as the voltage amplifier, followed by the triode portion as concertina phase splitter. I thanked my lucky stars that there wasn't later to be any kind of problem with any of the RF sections; mapping them would have been much more difficult!

Finally I could see nothing more to do before switch-on. I was so proud of all the work I'd done, but knew I'd come down in a screaming heap if it blew up when I first switched on! I think I waited an entire week with it just sitting there, unpowered. You can bet I was pleased when finally I did fire it up and it all basically worked. What a rush! However, while the car might be running, the tyres were flat, the timing was way off and the gearbox didn't bear thinking about. The pot's and switches crackled outrageously, there was a monumental gain and frequency response difference between channels and after an hour or so, loud "splatts" started trying to invert any loudspeaker cone connected to the right channel.

WD40 fixed most of the pot's well enough. WD40 isn't really the thing to use for cleaning pots; it does the job satisfactorily in the short term but three or four months later you're back to crackling pot's again. I guess the oil in it actually starts collecting rather than dispelling dust, but I don't know. I assume that a proper contact cleaner is the thing to use, and when I've acquired some I'll use it!

The quad-ganged balance pot was just too bad for a quick clean and a bit of tolerance; it had to be replaced, no matter what. I even tried pulling it to pieces, thinking I could clean it more thoroughly and re-use it, but when I undid its clips it dissolved in a shower of clips, tracks, bands, shafts and so on. I wouldn't have a show of getting it back together again without a map of some kind, so I gave up.

The closest replacement I could get, from my salvage collection, with sufficient gangs, correct taper, with a still long enough shaft that would fit, was only 220kOhm, not the 500kOhm, (if I recall correctly), of the original. The lower value resulted in a substantial loss of signal level, when I fitted it and tried it out, so I padded it with 330kOhm series resistors to make up the original value. This left the actual degree of effect of the control very minimal - balance shifting only through a narrow window, when the control was rotated fully. I decreased the padding resistors to 220kOhm as a compromise, and it's like that still. I'm not at all happy with this result, but I've no hope of buying a generic pot like that.

I was able to localise the gain, frequency response and "splatt" problems to just the line level audio signal path, in just the right channel, by simply feeding a signal generator signal in via the "Aux" input terminals. Renewed component by component checking through the output stage, replacing all capacitors period, and any resistor outside a 5% of original marked value window, eventually solved all but the "splatt" problem. I actually tolerated and lived with the "splatt' problem for several months; I knew which stage it was in, but just couldn't find it. It was heat related, as it didn't appear when the unit's casing was left off. I briefly considered introducing a fan to the works, but an ugly mental picture of the result dissuaded me.

I measured the power output as about 15W~16W into 8 Ohms at clip, and frequency response measured, at about half this power level, -3dB down at something below the 20Hz limit of my signal generator, and at about 11.5kHz. I gather that this upper limit would have been consistent with the transmitted audio bandwidth of the day, itself substantially wider than the audio bandwidth of most AM receivers of the time. I certainly had nothing to grumble about there, and in fact really enjoy the sound of this unit, warts and all.

Months later I determined to mount a major final assault on the last ("splatt") problem. Must've had too little real work to do, and too much time on my hands (yeah, . . right!) . There seemed simply nothing left worth the trouble of changing. I'd long since swapped all the valves between left and right channels, to no effect at all, so I knew that they were all quite OK. The fault was in one of about fifteen remaining original passive components in the right channel output stage. I hate resorting to such methods, but I simply decided to replace five at a time, in three stages, by rote. The first five produced no fix, just as in my heart-of-hearts I expected. Took ages, but I was absolutely bloody minded by this time.

Replacing the second batch nailed it however. I was overjoyed. I was, intellectually, set to now go back and find which of the last five was the true cause, but emotionally I was sick of the whole damn think long since, and emotionalism won out. One of those five last low-power resistors in the output stage bias set-up, was in apparently good condition, measured statically within +/-5% of its nominal value, but haemoridged grossly after an hour when the unit heated up normally.

Now it worked, and now I was much happier with it. I wonder how many hours that earlier repairer expended in a futile search, for surely he must've been chasing the same ugly gremlin too.

Naturally, only now and at this point, did I stumble across Darryl Lock, a JoeNet contributor, fellow Australian, (a Queenslander, poor thing heh heh!), and talented valve audio enthusiast. Darryl maintains a well-established web site that's well worth a visit or three. Oh, marvellous timing that! Story of my life! But of course I was still very grateful for the proper and full schematic.

I exchanged a couple of emails with a very helpful Darryl. In his unit, he told me, there had been some sort of fire in the area of the output stages, and while someone had got the thing running again, they'd left all the partially charred and damaged wiring. There was a lot of work left to really clean it up.

Pioneer Model SM-Q300B FM/AM valve (tube) receiver

A great looking unit, huh?

"Mine was in fairly bad condition, but I got most of it working. It had mostly original Matsushita valves, except for the outputs; they were three Hitachi valves, and one stranger. Also, it took hours to clean the chassis. It was covered in a thick gluey sludge, like the fallout from cooking oil or smoke. I listened to it for a few months, but decided it did not sound good enough. I have built some single-ended stereo amp's, and these are much nicer. This, coupled with the burnt wiring, crowded layout etc., led me to decide not to restore it. I have reused most of the iron etc, to start construction a stereo guitar amp, (based on the VOX AC30)."

He advised that from his cannibalised SM-Q300B, (ahhh! Sacrilege!), he had some bits he could make available to me. You beauty! All except . . . you guessed it, . . . the balance control!

"Ah, such is life".That's a quote from a famous Australian, shortly before they hung him. 'Bout sum's it all up, I reckon!

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