My beautiful SX-1980

Tuesday, October 3, 2017

Pioneer SX-1250 Gold Tier Restoration!


I'm pleased to announce I now offer 3 tiers of recap/restoration work. This is a recent development in light of several tests and experimentation with various types of capacitors and their affect on the sound. The first tier, silver, is the standard restoration and is what you have seen in previous posts. Next, the gold tier restoration, goes quite a bit further, replacing more caps and changing the cap dielectric of several caps. This offers an even better sounding amp compared to the standard recap. Several other components (rectifiers, diodes etc) are also upgraded depending on the amp or receiver in question. 

Finally there is the platinum tier restoration, which is a fully custom, all out electrical restore of an amp or receiver, all resistors, diodes, rectifiers, transistors and capacitors are replaced or upgraded where needed and the circuit may be modded for better performance or sound quality. Some components like outputs may and may not be replaced, but that will depend on the amp or receiver in question. The owner will be able to select the parts that go into the amp if they wish, such as a certain brand of capacitor (Nichicon KA, Elna Silmic etc). I will also soon be offering a complete performance evaluation of any amp or receiver that comes in for restore.

With that being said, here is the first SX-1250 gold tier restoration!

This was sent in by an Audiokarma member from Illinois, the receiver cosmetically was in very nice shape, some dust and dirt from age and weird white speckles all over the faceplate.

Let's dive in! Starting on the bottom boards, underneath are the power supply, tone amp, flat amp, filter amp and function switch boards.


First up, the flat amp board. Before...


And after! 24 caps and 6 transistors were replaced along with four 1S2473 getting changed to 1N4148 diodes. Cap replacements are Panasonic FM and FC for the electrolytic caps and Panasonic ECW polypropylene for the 1uF mylar film caps. Also used were Wima polypropylene caps. Transistor and diode replacements are by Fairchild.


Next up, the tone amp board. Before:


And after! 36 caps were replaced along with 4 transistors. Cap replacements are Panasonic FM for the normal electrolytic caps, Nichicon KL for the bright orange low leakage caps and the tantalum caps (they make a difference in the noise floor of the amp) and the mylar film caps are replaced with Wima and Vishay polypropylene caps. Transistor replacements are by Fairchild.


Up next, the filter amp. Before:


And after! 10 caps and 2 transistors were replaced. Cap replacements are Nichicon KL for the tantalum caps and the mylar films are replaced with a combination of Wima and Vishay polypropylene caps. Transistors are Fairchild as before.


While here I also took apart the filter switches and deep cleaned them, as they are notorious for causing channel dropouts and other problems. Just spraying Deoxit into the switch doesn't always help either. Below are a few photos of the process, all the contacts were cleaned with a eraser, soaked in Deoxit, then the switch put back together and worked around 50 times each to ensure they are scrubbed clean.




With those done, let's move onto the function switch board. Before:


And after! Just one little 220uF cap on here, replacement is a Panasonic FM.


Moving on, we now arrive at the power supply board. Before:


And after! 4 caps were replaced, I used Nichicon TVX and PW here. All fuses were checked for proper rating and to make sure they didn't have any corrosion.


Now onto the top! Here we find the 4 removable boards, left and right power amp boards, protection board and stabilizer (regulated power supply) board. Also up here is the phono amp board under the silver cover to the far right..


Let's start with the left power amp board. Before:


And after! 12 caps, 3 transistors and both trimpots for the bias and offset were replaced. Replacements are Nichicon PW for the big 470uF caps, Panasonic FM for the 220uF and Nichicon KL for the low leakage 10uF. For the 1uF input cap a Panasonic ECW polypropylene was used. Also used were Wima polypropylene caps. Bourns trimpots replaced the offset and bias trimpots. The input pair were replaced with 1% beta matched KSA992 transistors, also replaced was the overcurrent sensor transistor which is known to go bad and cause the protection circuit to trip on and off constantly when at low volume. 


Here are some photos showing the difference between the old and new caps. There is a real, measurable difference between old and new caps, even if they seem fine in capacitance there are other things such as ESR and dielectric loss. Even the film caps are not as good as the new poly caps. Here is an example of the ESR difference between the old mylar film vs the new poly film (lower is better):



A fairly sizable difference.

Moving on, the molex connectors pins are always dirty and this was no exception, cleaning these is an important part of any restore. Before:



And after!



Now for the right amp board, this is the same as the left.


And after! Same parts and process as the left channel were used.


Pins all clean!


All 8 outputs for both channels were removed, the heatsinks and old thermal paste cleaned off and new thermal paste was applied.

Now for the stabilizer (regulated power supply) board. This board was previously recapped by another shop, however they did not replace or upgrade any of the transistors. They used a mix of good Nichicon caps and cheap Chinese generic brand caps, some of Nichicon caps used however were not of a low impedance style.


And after! This board runs very hot so all diodes and transistors are replaced along with the caps. Several transistors get upgraded to bigger case styles to better handle and dissipate the heat. In total 16 caps, 13 transistors, 6 diodes and both trimpots for the +65V and -65V supplies are replaced. Nichicon PW was used for all the radial caps with a Nichicon TVX being used for the 1000uF axial cap. Transistors and diodes are by Fairchild and ON-Semi. Trimpots are made by Bourns.


And last but not least, the EQ amp (phono amp) board. 


And after! 18 caps, 14 transistors and 6 diodes were replaced. The differential pairs (the 4 with the orange dots) were replaced by 1% beta matched KSA992 transistors. Panasonic FC and FM were used for the electrolytic caps, Nichicon KL was used for the orange low leakage and tantalum caps and the 1uF input caps were replaced with Panasonic ECW polypropylenes. Transistors and diodes are by Fairchild.


Here are some photos showing a cap on this board and how much is has deteriorated. There were many caps I tested across all the boards with similar levels of dielectric degradation and loss.

Old:



New:



Almost done! I replaced the soft start relay as well, the old relay contacts tend to build up carbon and other impurities and or become pitted, causing the resistance between the contacts to rise and forcing power draw through the soft start resistor when the relay contacts can't keep enough current flowing. In severe cases it can blow out the soft start resistor or cause the thermal fuse to open because the receiver cannot draw enough current through the relay, overheating the resistor.

The soft start relay in the 1250 has a flanged case, meaning the new relay case will not work. Thankfully, the new Omron relay is a direct fit into the old case. All I have to do is pop the new case off and fit the old case on.

New relay on the left, old on the right


And voila! Old case on new relay!


 Mounted and wired in!


All four filter caps were also replaced with UCC 22000uF 100v replacements, they are a perfect drop in replacement for the old caps. All that has to be done is enlarge the ring terminal and ground bar holes slightly.



And all done! Fired her up on a DBT (Dim Bulb Tester) to make sure there were no problems, check the bias and made sure it set correctly, then turned it back to zero, took it off the DBT and fired it up line power. Set the offset to 0.5mV each side. The bias was set to 75mV each side, lower than the 100mV the manual calls for, but it runs cooler this way and I recommend doing it for this reason. There are no determinants to the sound or operation in doing this


All the lamps, both dial and indicator were changed to LED lamps, I kept the color as close to the originals as possible. The dial lamps were slightly different color and brighter, but the indicators are almost identical.



And we're all done! Hook up my Polk SDA-1C speakers, hook up my source and hit play. HOLY. CRAP. I know there's always an improvement in sound when I recap a receiver, but this difference is FAR beyond just a standard recap. The highs and mids are out of this world. I compared my own 1250 which just has a standard recap at the moment and yep, I am hearing the difference. I am now a firm believer in replacing mylar films with polypropylene films, the difference has to be heard to be believed. And if you're going from an un-restored, no caps replaced receiver to this, the difference will make your jaw drop.

The final thing to do was to clean the faceplate and knobs, they had some weird speckles of something all over it. I also oiled and polished the wood case and wooden blocks on the sides of the faceplate. The chassis and transformer was also cleaned. Came out looking like this. 

(Drool alert ahead, protect your keyboard. You have been warned!)










Ran and burned it in for almost 2 weeks, flawless performance. Readjusted bias and offset at the end of the burn in period, gave her the final check over and this one is on its way back!

If you like what you see, be sure to hit that +1 button. Please feel free to leave comments or questions!

Tuesday, September 19, 2017

Pioneer SA-9500 power amp board repair with a twist!


The SX-1250 gold tier restoration is going to be posted here very soon, I have to finish processing the photos. In the meantime, I thought I would showcase this "straight forward" repair that turned out to have a few surprises. 

This SA-9500 came in to my shop needed the right channel rebuilt, it suffered a nasty failure that left 3 of the 4 outputs and both driver transistors shorted out as well as a few burnt resistors and a bad emitter resistor. 


Board before, note the burnt covers on the 150 ohm resistors.


New parts! I managed to find NOS drivers and outputs, for the resistors I used Vishay/Dale resistors and Bourns trim pots, 25 turn for the offset for easier offset adjustment and single turn for the bias.


All done! Replaced all the emitter resistors as well as the base resistors. I also replaced the 2SA726 differential input transistors with a 1% beta matched set of Fairchild KSA992 transistors.


All the old parts. The 150 ohm resistors got toasty!


The bias trimpot actually melted a little from the current overload and is burnt internally.

Got everything replaced, I also replaced the trimpots and the 2SA726 differential input pair in the left channel as they were flaky. Hooked it up to the DBT and fired her up, success!.....orrrr so I thought.

A loud hum greeted me through both channels when I hooked up speakers. Drat. A hum through both channels generally indicates a ground problem or a bad power supply, so I first looked at the input/phono amp board and checked the ground traces. Aha! A ground trace had completely lifted and appears to have had a current overload, as it was falling apart and appeared slightly burnt. I scrape away the old trace so it doesn't cause problems and solder a wire in place of it. Check the rest of the ground traces and ground points, they all look good. Fire it back up, hum is lower in volume but still there. Okay, that means it must be the power supply then. I first check the 470uF smoothing caps for the +60V and -60V regulated supplies. First one I check:


Yeahhhhhh, I think we may have found the problem. That's measuring 8.39nF, it should measure around 470uF, and the ESR is off the charts.

I replace both 470uF caps and just to make sure, I check the other caps.


Supposed to be a 47uF cap, it measures 966pF. 😲


Supposed to be a 100uF cap, measures 18.579uF. ESR for both is off the charts as well.

That is just a sample of what I found, in all only 2 caps still measured even remotely close to their original specs. Time for a power supply rebuild!


Power supply before, note I have replaced the 470uF caps already.


And after! I went ahead and replaced all the diodes and transistors as well, upgrading three of them to TO-126 case transistors and two (the positive and negative feedback transistors) to a TO-92L case transistors for better heat dissipation and longer life. The two current source diodes were replaced along with the bridge rectifier diodes.


All the old parts from the power supply.

Fired it up again and success! Dead quiet in both channels. Adjusted the +48V and -48V rails to exactly 48V via the PS trimpots and reset the bias and offset again to spec and we have a fixed SA-9500 and a very happy owner!

If you liked what you see, be sure to hit that +1 button down below. Comments and critiques are always welcome!