Recapping, Rebuilding, Restoring, Rest-o-modding... a Pioneer SX-450 receiver

dlucy

dlucy67 (Doug)
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Well, AK'ers, I owe many of you a big round of thanks. I've finished my first complete restoration, it was a SX-450, and most of what I did was a result of you all asking each other questions, answering them, and leaving pointers to more info. You guys rock! Thank you! It sounds terrific (with Polk 5's, Infinity RS-3000's, RS-10's, and JBL L46's) and is plenty loud for a bedroom or a smallish room. BOTL? Not any more, not with quieter semiconductors and resistors, bigger power supply caps, less incandescent lamp drain, a tightened-up phono stage and trimmer pots for adjusting idle. TOTL? No. But MOTL, maybe....

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The more-detailed story in pictures is here https://www.facebook.com/media/set/...535.1073741872.1208612974&type=1&l=bf3cb01537

What's next? The SX-580 the old married couple gave me for $25 because it finally stopped playing FM after 30 years. They were sad because nothing new they bought sounded anywhere NEAR as good as the Pioneer they both had when they started dating. So, I'll need help again, I'm sure, getting this one restored and returned as a surprise to the unsuspecting couple. It'll kick the jacks off the Bose Wave thing they finally settled on....
 
Nice save.

What value trimmer was used in place of R222? If you do one of these again it may be worthwhile to study the factory method. Most often the circuit is designed with a fixed resistor and a trimpot in parallel. This is done so that if the trimpot goes open the fixed resistor will still be in place to keep the bias from running away due to an open circuit. A 220Ω resistor paralleled to a 250Ω trimmer would probably get that done in the 450.

When you dive into the SX-580 rebuild I think you will find it is very familiar territory. If the STK modules are intact you will be at a much better starting point.
 
That's a beautiful 450! That model was my first receiver ever, bought with money from my job at the grocery store when I was in high school. I had a pair of those old KOSS liquid filled earpad headphones, and I remember The White Album sounding incredible...
 
What value trimmer was used in place of R222? If you do one of these again it may be worthwhile to study the factory method. Most often the circuit is designed with a fixed resistor and a trimpot in parallel. This is done so that if the trimpot goes open the fixed resistor will still be in place to keep the bias from running away due to an open circuit. A 220Ω resistor paralleled to a 250Ω trimmer would probably get that done in the 450.

I used a 100 ohm Bournes trimmer part number 3386H-1-101LF to replace the 62 ohm fixed resistor at R221 and R222. That allowed me to set the DC idle.

Now (two years later) that I better understand what's going on, I think your idea makes much more sense. A fixed resistor of 220 ohms in parallel to a 250 ohm trimpot would yield an adjustable range of 0 to 117 ohms.

Adding trimpots to SX-450.png


So, going back to the original BOM I'd

  • Change 100 ohm trimpot 3386H-1-101LF to either a 220 or 250 ohm trimpot. Mouser doesn't have the H pin arrangement in stock at the moment, but they do have the P, F, and G pinouts in 220 or 250. So, any of these would work: 3386P-1-251LF, 3386P-1-221LF, 3386F-1-251LF, or 3386G-1-221LF

  • Add a low noise Vishay CCF series 2% 220, 240 or 270 ohm metal film resistor CCF07270RGKE36, CCF07240RGKE36, or CCF07220RGKE36 or a very low noise Vishay CMF series 1% 221 ohm CMF50221R00FHEB or Vishay RN series 1% 221 ohm RN60D2210FB14

  • Bend the legs of the trimpot as I did before (middle wiper leg to one of the two sides)

  • Wrap and solder each leg of the fixed resistor to a corresponding leg of the new two lead trimpot

  • Insert the new trimpot + fixed resistor assembly in place of R221 and R222

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Realizing I never wrote up the restoration and mods, here's a shortened version for those who are working on Pioneer SX-450 stereo receivers. They are easy-to-work on, low power but fine sound, and the parts plentiful and inexpensive.

Mine was working fine and was a spare unit, so a perfect place for me to learn how to read a schematic, build a parts list, desolder parts, figure out how to correctly install new parts, deal with unexpected issues, post questions on AK, and test.

In general, I wanted to:

  • Replace all the electrolytic caps with modern equivalents, or better if available and cheap
  • Increase the power reservoir cap capacity (just because it sounded like an upgrade and would be easy to reverse if I ran into trouble)
  • Add snubber caps across the power reservoir caps (just because I was in there and others had done the same to their nicer units)
  • Replace the rectifier diodes with fancier diodes (again just because it was cheap and fun)

Along the way:

  • I noticed the tuning dial lamps would dim a bit when the amp was under heavy load
  • Discovered I could "add" a DC idle adjustment by swapping a fixed resistor with a trimpot and "upgrade" this design to a slightly "better" model
  • Found the phono stage was obvious, so I upgraded all its components to higher precision, lower noise ones (since I'd be playing records with this unit)
  • Took the opportunity to clean the power switch
  • Found I'd need to unstring the tuning string and needed to carefully photograph the string and its path through the pulleys so I could restring it
  • Replaced all the components in the power supply
  • Discovered there were some undesirable transistors in the main amp, so I replaced them
  • Was shown there is a big error in the schematic, showing a transistor the wrong way around, and a CORRECTED schematic available
  • Found a few tantalum capacitors that could be replaced with film or electrolytics
  • Considered replacing the varistors, built replacements, realized they'd be hard to mount, and left the original varistors in place
  • Got everything working fine, sounding great
  • Scraped all the woodgrain vinyl off the case, painted over it, and replaced the woodgrain vinyl particleboard sides with solid hardwood
The result sounds great, looks great, and is a solid performer. I was happy with it, took my new skills and learning to bigger, more complex units and passed the SX-450 to a friend.

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The SX-450's insides are all on two boards. Very simple layout: two similar-sized boards, side by side, one plane, no hidden boards, no daughter board, very few "hanging off something else" components.

01 IMG_2915.JPG

The first board has the power switch, the power supply, the tone controls, the preamp and the power amp (in the below photo these are roughly right-to-left):

02 IMG_3001.JPG


The second board has the tuners, the MPX stage, and the phono stage (in the below photo the phono stage is lower right and everything else is left side):

03 IMG_3408.JPG

The two boards are connected via a small number of wire-wrap posts:

04 IMG_2999.JPG 05 IMG_2998.JPG

and keep in mind there are some components hidden underneath covers along the backside of each board:

06 IMG_3409.JPG
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Opened up and de-sided, the unit looks a bit awkward:

07 IMG_2928.JPG
and the front of the two boards with their separate sets of controls are more obvious:

08 IMG_2929.JPG
 
I was thinking about upgrading the power transformer and this was my first restoration, so I photographed the wires coming in from the AC inlet, to the transformer, then off to their spots on the fuses, power switch, and board. There was nothing wrong with the power transformer and the larger power reservoir caps solved the dimming problem, so the pictures just ended up being reference material.

Note the safety cap (X2 style, I believe) in the upper left of the first photo:

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And note the resistor from the AC inlet to chassis ground:

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The power switch is worthy of note here. The whole SX-x5x series (and more, I imagine) have the problem of the power / speaker selector switch having an internal set of contacts that connect when "on" and which arc (due to nice ol' current wanting to due its job and get from the AC outlet to your waiting power supply) many thousands of times over the unit's life. These arc cause carbonization and eventually fail.

I carefully opened mine up, cleaned it up, and put it all back together. And gave it a new cap on top.

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What I would have done, if I'd known about it and this was a keeper, would have been to bypass the big-current-through-the-physical-contacts and add the triac power switch mod as here: http://www.audiokarma.org/forums/in...orth-5-and-an-hour-or-so-of-your-time.504673/
 
With a plan to replace everything in the power supply circuit, I photo'ed everything in that portion of the board:

01 IMG_3046.JPG


02 IMG_3013.JPG

The big diodes that form the rectifier bridge coming in from the power transformer's secondary:

03 IMG_3019.JPG

The voltage regulator transistor and diode:

04 IMG_3021.JPG

Removing the big power reservoir caps gave me much easier access to everything. They were the three-post type. The board was silkscreened and I verified the polarity of the caps coming out matched the silkscreen, so I didn't need to correct it. But I did use a Sharpie to start noting component numbers since the PCB did not have that:

05 IMG_3055.JPG

During this component-removing and component-marking process I found my first differs-from-the-schematic issue (covered later). A jumper in place of a resistor... hmmm.

06 IMG_3087.JPG

Those caps gone, the power supply part of the board looks pretty stark. Simple. Good.

07 IMG_3088.JPG

Since this was my first recap and I didn't have any confidence in "yes, I will always replace electrolytic caps" yet, I measured every cap coming out with one of those $20 cheap component testers from eBay http://audiokarma.org/forums/index.php?threads/cheap-component-tester.749729/

I definitely learned the two sides of the argument over recapping. Not all the caps had drifted or gone bad yet, but their ESRs were not as low as new caps and some caps had dramatically lost capacitance. At an average price of $0.20 each and a guarantee of another 10+ yers of service, it helped me reach my "always recap" mentality. This is counter to plenty of people's position and that's fine. They can do whatever they like. For me, the choice is easy.


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Your orginal write up and facebook photos were a big help to me when I redid the shop SX-450 I bought 40 years ago. I, too, redid everything on the power supply board and added the Bournes trimmers you did. I also completely bypassed that blankityblank alpin power switch, though. Put a DPDT toggle on the back rear with an X2 snubber. Front looks proper and correct, you reach over and flick the toggle and music flows, maybe another 40 years!
 
Your orginal write up and facebook photos were a big help to me when I redid the shop SX-450 I bought 40 years ago. I, too, redid everything on the power supply board and added the Bournes trimmers you did. I also completely bypassed that blankityblank alpin power switch, though. Put a DPDT toggle on the back rear with an X2 snubber. Front looks proper and correct, you reach over and flick the toggle and music flows, maybe another 40 years!

Awesome! If you have any photos of that bypass, feel free to add them!
 
On choosing a replacement electrolytic capacitor for the Nichicon FW series 6800 uF 35 VDC UFW1V682MRDtwo 4700 uF power reservoir and filtering positions C309 and C310:

The schematic calls for 4,700 uF 35V and the transformer secondaries + rectifier bridge should be delivering only 25V to them, so 35 VDC should be plenty to handle the voltage coming in and provide long life.

SX-450 power supply primary closeup.png

The two 4700 uF are not in series, but rather parallel as each supply one rail of the two rail power supply. So, you've only got 4700 uF "reserve". I don't know much about design, but that sounds like plenty of reserve for a 20W receiver. That being said, after upgrading the bridge like I did, I installed two 10,000 uF caps here and have had no failures due to too much in-rush current when first turning on the power and asking the AC wall outlet to "fill up" these two twice-as-big caps.

So, this is a power application, so we don't need fancy "audio grade" caps or actual "audio path, low leakage" caps. We want high ripple, low ESR caps. They need to be 25mm in diameter or thinner and we'd like the lead spacing to be 10mm (to fit the original PCB holes), but we could always drill new holes if we don't have any good choices. The go-to Nichicon UPW series don't go this high, so the next-best from Nichicon in regular leads are FW and KW.

As of today, April 2018, Mouser has these choices which should all work fine:

  • Nichicon FW series 6800 uF 35 VDC UFW1V682MRD
  • Nichicon KW series 6800 uF 35 VDC UKW1V682MRD
  • Nichicon KW series 4700 uF 50 VDC UKW1H472MRD
  • Nichicon KW series 10000 uF 35 VDC UFW1V103MRD
  • Nichicon KW series 6800 uF 50 VDC UKW1H682MRD

They are all between $3 and $5 each, so price isn't a big factor. You could choose the 10000 uF ones for most reserve, the 4700 uF for closest-to-original, the 6800 for a safe upgrade, and/or 50 VDC for longer life.
 
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