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Yet Another SX-1050 Restoration

Will_Emanuel

Will_Emanuel

AK Subscriber
Subscriber
Hello all,
I have just wrapped up a major restoration on a newly acquired SX-1050 that had a very rough past to me and had arrived in very rough shape. The unit had been sitting in the attic of a small stereo shop near my university for nearly 27 years, according to the repair tag's date of 1994. Despite the poor conditions in the attic and the fact that the unit had been sitting in a pile of gear when found, it was still in decent condition cosmetically with no visible rust anywhere on the chassis. After bringing it home and taking off the cabinet, it was quickly obvious that this unit was in dire need of a serious restoration. the main filter capacitors were bulging heavily on the top and it was obvious on the underside of the caps that they had started to leak electrolytes. along with this, many burnt resistors were found on the amplifier driver board as well as the protection board. A quick continuity check confirmed that all 8 of the amplifiers output transistors were dead shorted. Given the little information I had, I planned for a major restoration of the entire unit. Using BOMs provided by Stevescivic and a few others, I compiled a comprehensive BOM of all the components I used in my unit.
 

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Here are a few photos of the unit when I received it.
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and after come cleaning on the bench.
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and a photo of the leakage on the main filters.
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First order of business for me was to carefully note the orientation of the original filters and pull them out of the unit to free up some space. From here I went on to start on the AWM-090-A protection board.
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As seen, four 1.3k Ohm resistors were quite toasted and needed to be replaced. Following the BOM, these resistors along with the other transistors and capacitors on the board, as well as the protection relay were replaced.

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The rather large Dale resistors that took the place of the burnt ones were not a great replacement for the originals due to their size and cost. I have made a better suggestion for some basic 1% tolerance metal film replacements that will fit much better in the BOM.

Power Supply board B AWR-104-A, and the surge suppressor circuit AWX-098 were then addressed, just a few electrolytic caps here that needed to go.
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Next on the list was the main Power Supply Board AWR-103-A, a tightly packed board full of electrolytic caps and transistors that needed to go.
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Here, all transistors, zener diodes, a few resistors, and all capacitors were replaced.
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From here, it was time to tackle the amp driver board AWH-047. As a result of shorted output transistors, many of the emitter resistors on the driver board were open and were visibly burned, as well as a few bias resistors. I also observed that the two bias potentiometers had melted, something I haven't seen before.
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Here, all transistors were replaced, as well as the emitter resistors and bias resistors that were toasted. Electrolytic capacitors were replaced and tantalum capacitors were replaced with polypro film caps.
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The dirty heatsink was also cleaned and the original shorted output transistors and micas were removed and the heatsink was brought to the sink for a thorough cleaning. Each of the mounts for the output transistors were also cleaned and the old glue was removed.
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New transistors were then mounted with fresh micas and thermal compound.
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From here, the new filter capacitors were added and the new chassis mount soft start relay was installed.
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I was then able to fire up the receiver for the first time without the output transistors connected to confirm operation of the rebuilt power supply, protection circuit, and amplifier driver board. Both relays engaged, and I measured accurate supply voltages on the driver board as well as base and collector voltages for the output transistors. Once confirmed, The output transistors were then connected and the bias current and DC offset of the amplifier was set. A quick test with the bench speakers confirmed that the amplifier portion of the receiver was functioning as expected.
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I then moved my focus onto the equalizer amplifier assembly AWF-020-0 to start tackling the preamp section of the receiver.
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All transistors were replaced here, making sure to match transistors 1-4. All electrolytic capacitors were also replaced as well as polypro films taking the place of the original tantalums.
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The Flat amp assembly AFG-040-0 was next, starting with cleaning all the pots and switches before addressing the other components on the board.
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Here, all transistors were replaced, all electrolytics were replaced, and all tantalums were replaced with polypro films.
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The Function switch assembly AWS-094 was also pulled to replace the single electrolytic cap and clean each of the input switches. The Control Amp assembly AWG-041-A was then addressed.
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Here, all transistors were replaced, all electrolytics were replaced, and all 2.2uF electrolytics were replaced with polypro film capacitors.
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The Filter Amp assembly AWM-089 was also pulled and switches were cleaned as well as the speaker switch selector assembly. From here, it was time for reassembly.
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The unit was tested again to confirm functionality of the preamplifier and tone circuit, as well as the filter circuit and to make sure all switches were clean. The original incandescent lamps were burnt out, new lamps were installed in their place. And here we are, restoration complete. Turned out quite decent, I am pretty happy with the result.
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Great documentation of your project. The systematic approach, discipline, and attention to detail you’re currently demonstrating as an Electrical Engineering student is precisely what’s necessary for a successful career in that field.
 
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