cbusuttil

Active Member
Hello all! recently acquired this marvelous SX-1280 - been looking for one for a while. I also have an SX-980 which i recapped myself back in 2011. Here are some pictures that I managed to take while restoring this big old dog. re soldered all of the solder joints, full recap and replacement of all semiconductors.

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Dismanted the facade - all knobs to be polished. Facade seems to have some sticky substance round the knobs.... i don't think who owned it ever cleaned it!

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I had a 980. Fantastic receiver. It did most everything well.

Subscribed with interest. Good luck.
 
Tuner board renewed with Nichicon audio grade caps. Notice the IC bases. I 've installed IC holders so that should any IC blow up, i would not need to remove the bottom cover. Unfortunately, I blew up the 5 chips when i was testing the power supply; +13V the regulating transistor heatsink touched one of the 5151 rectifier diodes and ... well..... there's no need to get into further detail. Managed to source up the 4 chips except that pesky PA1002A, for which I am currently designing a small PCB containing a buffer to amplify the sound. Temporarily, PA1002A has pins 3 and 5, 12 and 14 shorted so that I can at least use the FM radio too. Better than hearing nothing. You hear the radio, clearly, but signal is not amplified since PA1002A has also an integrated AF amp in it. really pissed by my mistake.

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Speaker protection relay dismantled, and polished using Autosol Metal polish. I did not change this relay because it's contacts were in exceptional condition. However, i did change the soft starter relay because it had dodgy contacts and I was risking to blow up the thermal fuse, which is a pain to get to. Notice the new high power green resistors. Those are new, 5W ones..... unlike Pioneer's 2W ones.

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Tone control board, flat amp, filter board, etc all renewed with new transistors ( including the 2sk117 FET on the flat amp) and all signal diodes replaced with comparable 1N4148 diodes. 2SA726 (known to be the source of many problems!) found on the flat amp was replaced by 2SA970 - you can use 2SA992 too. higher gain than original transistors can be used and are not an issue here because the transistor has an emitter degeneration resistor to stabilize the signal (provides some sort of negative feedback so to say) - (since HFE is dependent on both bias current and temperature) - neat little circuit. All other transistors like 2SC1919, 2SC1312 etc were replaced with 2SC1845. One thing i must say, it's a hell of a nightmare to get to these boards; so once they are out, might as well replace everything. I don't want to take everything out again. no way.

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Oh well.....ohh my...... it seems someone's been here before me.... and he seems to have installed these 2STC5949 and their complementary power transistors. It is evident that in the past, this receiver was abused with very low loads.. Nevermind, i have some MJL4281s and their complementary that i will install. Some 10 years ago, I designed a high power amplifier with these - very powerful transistors, IIRC one of the most powerful with a wide SOA that come in TO264 package.


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New MJL4281As with fresh thermal compound. Changing the output transistors on a 1280 with TO264 requires some electronics expertise.THIS IS NOT A BEGINNER's modification because, unlike the 980 (and IIRC 1980 too) the 1280 has the output transistors mounted on a PCB, rather than soldered wires directly on the transistor leg (like the 980). Pinout configuration is not the standard BCE like most of todays power transistors

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The 4X S5151 dual diodes were replaced with higher current, much faster, more efficient STTH30L06-Y schottky diodes.These barely get warm at full power (because of the larger current handling die inside them) so they won't be receiving a heatsink. I also renewed their spike filtering ceramic caps. Note that I also added a small computer fan; this power supply runs extremely hot, and it would be beneficial to provide some sort of air circulation. Some of it's components run nearly up to their limits so these were changed. NPNs and PNPs used are TIP 41C and it's complementary. voltage regulation is achieved with good results. Unfortunately, while changing those transistors most of the foil on the pcb fell off! you wouldn't expect any better after running that hot for years. I had to fix the legs with some superglue. I hate it, but i had no other choice. :(


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The newly rebuilt power supply showing the small additional computer CPU cooling fan and schottky rectifiers. The protection relay transistor was upgraded to a beefier, TO126 since it is continually providing supply to the relay.


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The big dog received more punch - new, 22000uF main caps. These help aid a little bit more dynamic headroom too hence and better punch.

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Dial filament bulbs renewed with warm white LEDs. shows a warm whitish (and slightly greenish) colour. still looks vintage. I wanted to remove those old bulbs because they were cooking the clear perspex. One thing i noted on BOTH the 980 and this 1280, is that the power meters don't track that well together. One will, for example display 10W and the other might be in the 25W range. I minimised the problem by matching all of the diodes and the 2 transistors found in the left and right meter amplifiers. It helped a little, but still not dead spot on; i suspect the meter's mechanical spring has lots of tolerance. What I did to compromise this, was, I've fed the 1280 a signal so that the meters are at 180W (output has to be 37.95V, according to the service manual). Tuned them at 180W. Then attenuated the input signal (1KHz) so that the meters are at12 oclock (10W) and re set them again. This is to compromise the non linearity of both the left and right part of the physical meter's scale (spring tension non linearity). Still not spot on, but it is better than it was before. You can still see them not tracking evenly in the photo below. I would have been very, very pissed at pioneer, if i spent some 950$ in 1979 for a high end receiver like the 1280 and have the meters not track well together!!


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Note that i covered the clear perspex with shiny foil to increase the luminous transfer efficiency of the perspex light deflector. There are no more bulbs that produce huge amounts of heat. It made a slight improvement, i must say.

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I think for now we're done. Whenever i have some time, i will order all of the transistors found in the power amps. I would want to change as much as i can. The only thing that there's no need to replace for sure are the resistors.

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onwardjames - the 980 is a fantastic receiver and can really put out some decent sound with some efficient speakers. I run this 1280 with a set of 1977 Yamaha NS690II - crisp detail, and 90dB efficiency. not too bad. Thanks mate! I appreciate!
 
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In order to keep track of the amount of hours the device worked after this restoration, i installed a small hour meter. We all know capacitors have a number of hours at their rated spect; the nichicon I used have 1000 hours @ rated voltage and temperature. Since I am not running them to their maximum rating, the 1000 hours might double to 2000 hourse or even more. But it's good to have an hour meter which keeps track of how many hours the receiver worked. It cost me some 7$ from ebay and it works as it should (i checked it out with another hour meter)

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If anyone of you have some surplus PA1002A chips and is willing to sell me one or two, kindly let me know. Thank you!
 
every single solder joint in this receiver had it's solder removed, and new solder was applied.

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Phono amplifier - The main outputs have been replaced with the TO126 2SC2682 and their complementary. Honestly, I renewed the phono board so that i can say "i have a fully restored receiver" .... because I don't use Phonos.

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