An Old Friend ;)

dcibel

My father has gifted me the receiver from my childhood

It's a Realistic SA-1000, I used to listen to this with the matching Realistic tape deck or Elac turntable on a pair of Realistic Optimus-5 monkey coffins. The Optimus-5's I refurbished some time ago, re-used the 11" woofers and replaced both midrange and cone tweeter with a single Vifa TG9 and new crossover. They're still in use and sounding pretty good.

This amp isn't the most hifi thing out there, but for nostalgia sake I'm going to breath some life back into it. It's still working, but the knobs and buttons are scratchy, so I'm going to try and find replacement pots and replace a bunch of the caps while I'm at it. It's only a 20Wpc amp, with a single rail power supply so the amp output is capacitor coupled. Just replacing those two coupling caps will probably make it sound like new again I'm sure. It's easy to work in, single sided boards and before the invention of pcb connectors apparently, all the internal wiring is wire-wrapped. My dad kept the manual and even the purchase receipt, great thing is the manual has a complete schematic on the back page.

Looks pretty okay if you ask me, doesn't have VU meters unfortunately, but does have 3 band EQ, loudness button and "Quatravox"!

Tom_S

That's a gem! Looks mint too. I remember & drooled over that amp at the local Rat Shack as a teenager. I really wanted one - it would have gone perfectly with my Nova 6 speakers. But it was more than I could scrounge at the time. I settled for a little Superscope amp. They sure built gear to last back then. 

Wolf

I have one of those, but one channel is burnt out. Old Quatravox model.

Tom_S

I'm guessing the trimpots on the board  are adjust DC offset?

dcibel

It's in great shape for its age, a couple chips off the veneer on one side, and the volume knob is a bit crooked, but I'm planning on replacing that pot anyway.

I scanned the schematic if anyone wants to look:

https://drive.google.com/file/d/1ohWNA-ckqIxfGUMUuM6YNLoAJ1Pu9lZJ/view?usp=sharing

Tom_S

If you find a suitable replacement for the stacked Vol/Bal pot, let them know over on AudioKarma. I hear they have been nearly impossible to find.

dcibel

I've no problem with a little maguiver work to use a standard dual pot, but in this particular unit it'll be the thread length that makes these pots nearly irreplaceable. It'll be hard to re-attach the front plate without the long threads. The front circuit board is fixed in place by just the potentiometers, then the front plate goes over top and is fixed in place by a second set of nuts.

I haven't seen any potentiometers yet that have threads this long.

Standard replacement is a bit short. I can order them with a long shaft, but it might be hard to find the right ones with long threads.

6thplanet

I got my dad's old stuff. A Fisher receiver and Sansui speakers. I used to jam the shit outta them growing up....so did dad 😁
I feel ya, that's a cool vintage system ya got !!! 🤘🏼

dcibel

The volume pot also has a 4th terminal that is part of the loudness circuit, so even if I do find a suitable replacement, the loudness feature will be lost.

4thtry

Check coupling caps C510a and C510b connected to the volume control wiper arm (0.47uF/50V).   If they are leaky electrolytic types, this could be your noise problem (assuming, of course, by scratchy you mean a noise problem and not just scratched up physically).    

Tom_S

A pot for a Pioneer SX-990 or SX-626 look long enough, but not a PCB mount - looks like wire wrap pins or lugs. They are also larger in diameter. Might be a tight squeeze getting one to fit.

I know I've run into a few older pieces that use threaded collar nuts that screw on the pot to secure the faceplate. I don't know if those still exist, but it might be a simple project if you have a 3d printer.

dcibel

4thtry said:

Check coupling caps C510a and C510b connected to the volume control wiper arm (0.47uF/50V).   If they are leaky electrolytic types, this could be your noise problem.   

Who said anything about a noise problem? The pots are just old, worn and scratchy, and I'd rather find a suitable replacement rather than just cleaning them. Even if just the volume knob as it's the most worn. I think if I just do the volume pot, I can use a normal one, the rest of the pots can hold the faceplate on just fine without having to resort to glue or some other hacky solution.

Anyway, those small value coupling caps aren't electrolytic. Not sure what they are, they are shaped like tantalums, which aren't the best choice for an audio coupling cap if that's what they are.

dcibel

Well I've ordered a bunch of caps, about $50 to replace all the electrolytics and all coupling caps are replaced with poly type except the 2200uF output caps of course. The pots and switches I will simply clean as they're not in too bad shape apart from the volume pot.

I'm rolling the dice on what appears to be a knock off Alps pot on eBay with the loudness tap. It was only $5 and will probably take a month to get here from China, some online searches lead me to believe that it's a stepped attenuator internally but might do the trick anyway, well see. Not a big loss if it doesn't work out at only $5. Look up "RH2702" for more detail.

As an alternative, the guy at www.oldradioparts.net will build me something very close to the original pot, but the cost with shipping to Canada is a bit expensive at USD$48, doubling my restoration cost. Not too sure on that option yet.

4thtry

My mistake, when you used the word "scratchy" I misinterpreted this as a noisy pot.  Good to see that you are replacing all the tantalums.  They are notorious for going bad.  I replaced several of these types on my Dynaco ST400 and Revox A77.  

dcibel

Wow, I didn't realize it had been 2 years since I started work on this amp. I found that although it "Worked", there was something wrong with it. It clipped one side of the waveform at only about 2W of output. Internally there are several locations where should find 25-30VDC, but was only getting 10-12V. I shelved it at the time since the problem wasn't immediately obvious.

Yesterday I took another stab at it and found a bad resistor. No sign of burn or damage, but measured open circuit which was part of the voltage regulation for the smaller transistors.

Now it is all re-capped and back in service. I didn't find any replacement pots, but gave the pots and switches a good cleaning and they are working like new again. Power light needed replacing too, found a 12V automotive dash light bulb that fit perfectly as a replacement. I'll need to find some replacement for the speaker terminals at some point as the old plastic is crumbling away, otherwise this old amp should be good for another 30 years of service

Tom_S

Very cool that you got it running properly again!

I love the 60s & 70s era gear because it reminds me of a time when you could go to the record store or hifi shop and meet other music lovers who would turn you on to a new (or old) artist or band.

dcibel

This amp spent many years playing records from an ELAC turntable of equal age. If you're interested, the original receipt says this amp was purchased in 1974 for CAD$200. According to online inflation calculators, that equates to CAD$1056 today.

Billet

I like the sound of amps with cap coupled outputs.

4thtry

Good to see that you finally got this repaired and playing music again. I have an old Realistic STA-45B stereo receiver that I bought in 1970 that needs pretty much the same treatment. One of these days . . .

Tom_S

One of these days I'll put my little Pioneer SX-535 back on the bench again. Bought it cheap knowing one channel in the phono stage was a bit lower than the other. I replaced some suspect transistors, but that didn't solve the issue. Hearing about your normal looking but open resistor tells me I need to poke around again and see if I have a similar issue hiding in plain sight.

dcibel

Input selector is a physical switch on these old units, it could be as simple as just needing a spray with contact cleaner.

Most people will use that Deoxit stuff, but it's not "off the shelf" where I am, so I used some CRC QD Electronic cleaner, then added a drop of 3-in-1 oil as a dielectric protection because its what I have on hand.

dcibel

Another use for the measurement jig. 2-channel here isn't the key, but simply we are feeding the speaker level output into the audio interface input for signal analysis.

The SA-1000 includes a couple bias adjustment pots, presumably to reduce crossover distortion. The schematic simply points to a couple various DC voltages around the pots, but here's a better way. Here I simply connected the measurement jig to the amp, and used ARTA spectrum analysis. 1kHz tone generated, and exponential averaging to minimize noise. Then I simply adjusted the pot for minimum distortion products. The key here is to run a loopback test with your equipment to see where the harmonics sit before interting the amp into the equation. In my case, the 3rd harmonic here is approaching the minimum that I can reliably measure.

It would be really great if ARTA would allow to set some markers so you could monitor specific frequencies, in this case I would set a marker at each harmonic, but it doesn't appear to be that advanced of a spectrum analyzer.

If it's not evident from the graph, this old amp has a "warm" sound, it provides some of that tube-ish sound while still being a nice reliable solid state piece of equipment. Win win for that "vintage" sound

dcibel

A coat of Danish oil has brought some life back into the case.

The last thing I will do with this is try and replace the speaker terminals with something that isn’t turning to dust, and possibly replace the power cord as well, for safety.

Wolf

So did the pot w loudness tap pan out, you never said if it did.

dcibel

Oh I didn't install it. Decided to just use what was in there as-is with a good cleaning.