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Rebuilding Pioneer SX-1980 power supply board

Fred Longworth

Fred Longworth

audio fanatic
JP Dylon and I are restoring a mint-looking SX-1980 one of our best customers.

The power supply board was previously "operated on" by the usual amateur brain surgeon. I have so far spent three hours testing and replacing components (the previous person had replaced about twenty things with inappropriate subs) -- and also cleaning up the ravages of his horrible soldering.

I have upgraded the main series-pass devices to TIP41CG (NPN) and TIP42CG (PNP). These are 65 watt devices in a TO-220 package. Other than that, the board is pretty much back to "stock."

At this time, the only parts remaining to replace are 2SA720 and two pieces of 2SK34.

Anybody have any suggestions on this board. Special things to watch out for. Weak points. Etc.

Thanks in advance for your gray matter.

Fred Longworth
 
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Echowars is the man to talk to. I have yet to finish my sx-1980, for our baby is taking away most of my free time. I hope to get it together in the next coming weeks. The 2sk34 fets are the weak link. Echowars fabricated two mini pc boards that function as power sources for the regulator circuit.
He was kind enough to send me two. Do a search on AK for "2sk34". There is a whole thread dedicated to the power regulator mod.

Derek
 
I have (somewhere) two different data sheets for the 2SK34...one says a maximim drain-source voltage of 30V, the other says 50V. Regardless, the JFET is the weak point in this design, and the last three 1980's I've rebuilt left here with a current source of my 'design' (the basic circuit has been around for years...I just made a minor change and had some tiny boards built).

The 2SA720 is the error amp for the negative 80V supply. I use the Zetex ZTX795A, 'cause it works well and I've always got a lot of them around. Its high-gain will assist tracking.

If you haven't (or a previous tech hasn't) already replaced the failure-prone original D212 and D213 35V zener diodes with a series pair of 17V and 18V 500mW diodes, now is the time to do so.

88gibson said:
Echowars fabricated two mini pc boards that function as power sources for the regulator circuit.
Click to expand...
They are properly called a 'current source'.

Derek..here's that thread...not terribly informative, but the attached Word doc here will help.
 

Attachments

Yup, EchoWars, I've replaced the zeners to yield 35 volts. I've PM you about those little boards that sub for the infamous 2SK34.

Fred Longworth
 
SX-1980 Power Supply...

Really tired of Pioneer's crummy upside down backward parts layout in their service manual, so I made a picture with the majority of common parts labeled from the bottom side of the board. It is in Microsoft Publisher and can email if interested.

Mike W
Vintage Audio
Ft Worth, TX
 
The two 5.6 ohm 1/2 watt series resistors replaced.

five 0.047 ceramic disk changed to Panasonic ecq film caps.

The transistor subs weren't Fairchilds, were they?
 
EchoWars; If you haven't (or a previous tech hasn't) already replaced the failure-prone original D212 and D213 35V zener diodes with a series pair of 17V and 18V 500mW diodes said:
i am working now from few days (mostly nights) on sx1980. it is in very good overall condition. it was dirty inside with full of dust and after cleaning i did some woodwork and painting. then i checked internet about sx1980 and mostly common problem; power board. somebody already did some nice work and even used your idea two zeners in series but just for one "+" voltage supplies. maybe it was because of 35V zener shortage. some of zeners on the board looks like they have more then 500mW and two zeners you recommend to replace have capability of 1W (2x500mW). what about other zeners on the power board, should they have 1W each or 500mW is enough?
Click to expand...
 
Pioneer released an engineering change which called for the replacement of the original 35V zener with two series'd 17.5V 1/2W zeners. Since those are NLA, a 17V and an 18V do the job well.

High value zeners hit their thermal limits much quicker than low-value zeners (P=IxV). The four 24V zeners (D208 through D211) are simply protection for the stock FET's. The only other two zeners are the 13V for the protection chip, and the 6.1V for the 13.5V, 8V, and 5V supply. Half-watt zeners are preferred here.
 
EchoWars said:
High value zeners hit their thermal limits much quicker than low-value zeners (P=IxV). The four 24V zeners (D208 through D211) are simply protection for the stock FET's. The only other two zeners are the 13V for the protection chip, and the 6.1V for the 13.5V, 8V, and 5V supply. Half-watt zeners are preferred here.
Click to expand...

do you recommend to change all zeners, protecting and stabilizing or just stabilizing ones?
 
since the stock jfets should be replaced with the current sources, there is no need to retain

The four 24V zeners (D208 through D211) are simply protection for the stock FET's.
Click to expand...

nor r213, r215, r211, r214.

This tells me that you have NOT done enough research on the sx-1980 power supply rebuild and may end up getting some unwelcome surprises.
 
markthefixer said:
since the stock jfets should be replaced with the current sources, there is no need to retain



nor r213, r215, r211, r214.

This tells me that you have NOT done enough research on the sx-1980 power supply rebuild and may end up getting some unwelcome surprises.
Click to expand...

i knew and now everybody knows that i am a "BED GUY". unfortunately i am not perfect. but the thru is i really did not make enough research about sx-1980 yet. should i just finish myself to make world better? then who wants my junks i collected in my past? probably nobody. i have to take it with me to hell.
 
I am saying you need to do MORE research to prepare yourself and ask these and many more questions BEFORE ever heating up a soldering iron, TO PREVENT DAMAGE.

Not for ANYBODY's ego's sake, but rather to keep from damaging a fine, rare piece of equipment. The 1980 is a special case among the Pioneer line - powerful but unforgiving.

This is the equivalent of tearing into a fine racing Ferrari's engine, there are many things that can do damage accidentally.

Or jumping into deep water before you are sure you know how to swim, the consequences can hurt.

Your sarcasm carries well across the language difference, I'm sorry to say...
(I am NOT disparaging your english, I only speak ONE language - and not very well at that, so to make oneself understood in a second language is better than I can do)

I have a LOT of respect for those that restore these old units outside of the repair parts cornucopia of the continental usa .
 
markthefixer said:
I am saying you need to do MORE research to prepare yourself and ask these and many more questions BEFORE ever heating up a soldering iron, TO PREVENT DAMAGE.

Not for ANYBODY's ego's sake, but rather to keep from damaging a fine, rare piece of equipment. The 1980 is a special case among the Pioneer line - powerful but unforgiving.

This is the equivalent of tearing into a fine racing Ferrari's engine, there are many things that can do damage accidentally.

Or jumping into deep water before you are sure you know how to swim, the consequences can hurt.

Your sarcasm carries well across the language difference, I'm sorry to say...
(I am NOT disparaging your english, I only speak ONE language - and not very well at that, so to make oneself understood in a second language is better than I can do)

I have a LOT of respect for those that restore these old units outside of the repair parts cornucopia of the continental usa .
Click to expand...

thanks. you right. i do some mistakes from time to time let say mistakenly. doing my units i have to slow dawn myself all the time. it is hard to wait for final result. some pieces need just hours, others months and a line of waiting for "touching" is long and frustrating. fortunately restoring is like a dooing a mass.
 
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EchoWars said:
Pioneer released an engineering change which called for the replacement of the original 35V zener with two series'd 17.5V 1/2W zeners. Since those are NLA, a 17V and an 18V do the job well.

High value zeners hit their thermal limits much quicker than low-value zeners (P=IxV). The four 24V zeners (D208 through D211) are simply protection for the stock FET's. The only other two zeners are the 13V for the protection chip, and the 6.1V for the 13.5V, 8V, and 5V supply. Half-watt zeners are preferred here.
Click to expand...

i finally changed all electrolytics and zeners because they were all different type and of course not original. now i at least know what i have on a board. i did amplifier adjustment and set it as manual service says without a problem. "mistake transistor" was already changed. i attach a picture to show haw my power board looks in my 1980. i saw on internet a picture showing power board in 1980 without extra flat radiator. it does not look as original in mine. it is more likely added after. the big flat radiator on this board is a bit warm. is it okay? i attached it to main chases to improve cooling.
 

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