Pioneer SX-1250 recap and restoration thread!
- Thread starter mattsd
- Start date
Hi All,
Quick intro....
I am finally getting around to a rebuild of an SX-1250 that I got in August. Most but not all is original. Interestingly there is documentation of the prior repairs. The protection relay has been changed out twice in the last 20 years and, once again, was misbehaving and so that protection board was the first one to be rebuilt. In about 2014 a technician rebuilt both of the power amp boards and switched out most of the transistors and electrolytics (but not all). He left the blue drop EC behind and used lower gain badly unmatched NTE 234 pairs for Q1 and Q2. So it goes. The real oversight was that the stabilizer board was left untouched and that probably needed a rebuild even more. C1 and C2 (both 220uf, now 100V) were completely shot with one measured to be essentially open with zero capacitance and the second having about 50uf of capacitance. There were other issues as well but now that board specs out.
This post actually pertains to a problem I am having with the bias voltage on one of the two power amp boards. Originally it was zero and unresponsive to setting VR2. I then went ahead and did a second rebuild (with real brand name Nichicon electrolytics and various metalized polypropylene caps) . I only swapped out Q1, Q2 and Q11 (with the "standard" KSA992 and KSC1845). Now there is bias voltage....sort of. After power on the bias voltage starts at zero for a small time and then creeps up but never stabilizes . It often drops back to zero but, if I wiggle or touch the pre-out to amp-in coupler (so that my finger touches the signal ground portion of the RCA) the bias voltage pop back up, is stable for a little bit and then slowly drops out again. The only thing I can notice that may have a bearing on this are the base DC voltages at Q7 and Q8 which are supposed to be at 1.8V and -1.8V respectively so that the two series varistors (plus VR2) operate to drop the bias voltage if they get hot. [This is not appears to be a safety interlock but looks to my mind as a way of reducing the bias when not really needed (i.e., high amp power). This is only a best guess.] The issue is that these two transitor base voltages drift considerable and so that will necessary impact the operation of the bias voltage control. Short of simply replacing everything nearby in a shotgun fashion I don't really know what else to do.
Suggestion as what to try first would be welcome.
Thanks,
Michael
Quick intro....
I am finally getting around to a rebuild of an SX-1250 that I got in August. Most but not all is original. Interestingly there is documentation of the prior repairs. The protection relay has been changed out twice in the last 20 years and, once again, was misbehaving and so that protection board was the first one to be rebuilt. In about 2014 a technician rebuilt both of the power amp boards and switched out most of the transistors and electrolytics (but not all). He left the blue drop EC behind and used lower gain badly unmatched NTE 234 pairs for Q1 and Q2. So it goes. The real oversight was that the stabilizer board was left untouched and that probably needed a rebuild even more. C1 and C2 (both 220uf, now 100V) were completely shot with one measured to be essentially open with zero capacitance and the second having about 50uf of capacitance. There were other issues as well but now that board specs out.
This post actually pertains to a problem I am having with the bias voltage on one of the two power amp boards. Originally it was zero and unresponsive to setting VR2. I then went ahead and did a second rebuild (with real brand name Nichicon electrolytics and various metalized polypropylene caps) . I only swapped out Q1, Q2 and Q11 (with the "standard" KSA992 and KSC1845). Now there is bias voltage....sort of. After power on the bias voltage starts at zero for a small time and then creeps up but never stabilizes . It often drops back to zero but, if I wiggle or touch the pre-out to amp-in coupler (so that my finger touches the signal ground portion of the RCA) the bias voltage pop back up, is stable for a little bit and then slowly drops out again. The only thing I can notice that may have a bearing on this are the base DC voltages at Q7 and Q8 which are supposed to be at 1.8V and -1.8V respectively so that the two series varistors (plus VR2) operate to drop the bias voltage if they get hot. [This is not appears to be a safety interlock but looks to my mind as a way of reducing the bias when not really needed (i.e., high amp power). This is only a best guess.] The issue is that these two transitor base voltages drift considerable and so that will necessary impact the operation of the bias voltage control. Short of simply replacing everything nearby in a shotgun fashion I don't really know what else to do.
Suggestion as what to try first would be welcome.
Thanks,
Michael
This would be better as it's own troubleshooting thread.
Q5 & Q6. Q5 is diode connected and only used as a diode!! Q6 is a current mirror load for q3/q4 differential pair (mirroring the q5 side). R16 is 220 ohms, and has 0.4v (65-64.6) across it.
Thus 1.8 milliamps. Through d1, vr2 and d2 to get the 3.6 volts (1.8 + 1.8) and vr2 makes it a bit adjustable.
HOWEVER if that 1.8 milliamps is not stable, neither will be the 3.6v and the idle current. The 1.8mA current through the diodes determines the amount of voltage reference across the diodes.
The 100 ohm resistors are not part of that. Forget them for now. Wild tangent.
These are DC characteristics - NO SIGNAL.
The wiggling/touching injects hum/ac which pushes through the bad DC conditions.
I'm sure the numbers below can be found in this thread.
q1,q2 ksa992
q3,q4 ksc3503
q5,q6 ksa1381
q7 ksc3503
q8 ksa1381
q9 ksc2073
q10 ksa940
q11 ksc1845
Q5 & Q6. Q5 is diode connected and only used as a diode!! Q6 is a current mirror load for q3/q4 differential pair (mirroring the q5 side). R16 is 220 ohms, and has 0.4v (65-64.6) across it.
Thus 1.8 milliamps. Through d1, vr2 and d2 to get the 3.6 volts (1.8 + 1.8) and vr2 makes it a bit adjustable.
HOWEVER if that 1.8 milliamps is not stable, neither will be the 3.6v and the idle current. The 1.8mA current through the diodes determines the amount of voltage reference across the diodes.
The 100 ohm resistors are not part of that. Forget them for now. Wild tangent.
These are DC characteristics - NO SIGNAL.
The wiggling/touching injects hum/ac which pushes through the bad DC conditions.
I'm sure the numbers below can be found in this thread.
q1,q2 ksa992
q3,q4 ksc3503
q5,q6 ksa1381
q7 ksc3503
q8 ksa1381
q9 ksc2073
q10 ksa940
q11 ksc1845
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Hi, thanks for the operational notes and suggestions (and quick response!). Going forward I will push this off to its own thread as suggested with my follow up notes once I make the changes. It will be a while as I don't have the requisite transistors in my stock. Shipping is also a little slow this time of year as well. Since I was at impasse with respect to the power amp board I moved on to the flat amp and front switches (with nothing of note).
M.
M.
Richard1079
Active Member
Has parts list been updated since 2012?
BobbyN
Well-Known Member
Matt, or anyone else, How about this number for C1 on the 1250 power amp board, AWH-048A... ECW-F2105JA 1uF, 250 volt.
Or... ECW-F2W105JA 1uF, 450 volt.
They're polypropylene.
Also, how critical is C7, 10pF to audio quality?
I'm getting things together here.
Thank you.
Bob
Or... ECW-F2W105JA 1uF, 450 volt.
They're polypropylene.
Also, how critical is C7, 10pF to audio quality?
I'm getting things together here.
Thank you.
Bob
Has anyone encountered any other part number changes since the 10/3/17 list update? Is there a newer list? I know it is probably cheaper to buy these parts straight from mouser, but out of ease and possibly not making mistakes on my end, what about the kits available on eBay? Peace ,love & music's kit https://www.ebay.com/itm/1256660711...uid=_pp414CQRuO&widget_ver=artemis&media=COPY shows to have 245 name brand parts in their kit & lionrr12 https://www.ebay.com/itm/3628734966...uid=_pp414CQRuO&widget_ver=artemis&media=COPY shows to have a lot of parts in their kit. Neither totally break it all down. How does the 10/3/17 list compare to these packaged kits, is the list more extensive or the same, but more cost effective. There are a lot of parts involved in a 1250 rebuild, I just don't want to miss something or get wrong or bad components. "If attaching those links is not please delete them."
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Just finished a restoration of my sx1250.
Bought the $360 kit from peace/love etc.
Excellent service, all parts there, accurate and saved tons of time looking up stuff on mouser etc.
Well worth it in my case.
took a long week, long hours....all worked out well here.
Ben (peace/love) answered all my questions asap and was nice to deal with.
Alex
Richard1079
Active Member
Ben is great and his parts and instructions are good.
Stay away from that other dude on eBay.
Stay away from that other dude on eBay.
Steve Powers
New Member
Thats a great parts list you have there! Thank you!