Vintage Pioneer SX-1250 Stereo Receiver Excellent Cond. Fully Rebuilt & Tested

Description

Offered is a classic Sansui SX-1250 receiver fully refurbished and excellent in both cosmetic and working condition.  There are few of this particular model that, in my opinion, that are currently in such fine shape.
I expect this receiver to give many years of good service going forward.
I extensively document rebuilds and so many details are included fyi.   Additional notes are posted at Audiokarma.
Cosmetics:
This SX-1250 is generally in excellent cosmetic condition with no obvious defects in the front face.  Very minor blemishes are enhanced by the camera flash.  There was some minor chipping in the black paint of the heat sinks which has been touched up.  A few coats of Tung oil have been applied to the wood cover at top to freshen it up.  There are a few minor dings in the wood cover (no chipping) which can only be seen if inspected up close.  Not absolutely flawless but exceptionally nice all told.  There was no rust or corrosion anywhere inside so this receiver has been well kept over the years.  All four wedge bulbs have been replaced with white LEDs.  Note that these bulbs are AC rated (i.e., includes a SMD bridge rectifier) so that they don't flicker at 60 Hz (which some people find annoying).  I prefer to maintain as much of the original look as possible and so daylight to tungsten photographic filter strips have been added to each bulb in order to give a more classic appearance.  They can be removed if desired.  All nine of the indicator bulbs have warm white LED replacements (a DC voltage circuit so no flicker).  A larger current limiting resistor was needed for the "STEREO" LED because it outputs at a higher dc voltage than that of the other lamps.  These indicators now give a much brighter and more uniform orange glow that complements the front face lighting.
Extended rebuild notes (for those interested):
As received most everything tested appeared to be working but, as is the case in a 45+ year old receiver, there are components that should be replaced. Most but not everything appeared to be original.  Interestingly this receiver has documentation for work done in the last 20 years.  The protection relay had been changed out twice in the last ten years but, once again, was apparently misbehaving (see below for diagnosis) and so the protection board was the first one to be rebuilt. Approximately ten years ago a local service technician rebuilt both of the power amp boards and replaced many of the transistors and electrolytics (and replaced the protection relay as well).  Most were quite acceptable but certainly not all were name brand components.  There were some issues.   For example they used badly matched transistors for the Q1/Q2 pairs.  Even though these power amp boards were rebuilt there was already a problem with adjusting the bias voltage on one of the boards and, on both boards, the protection relay would engage even at relatively low amplifier power levels (ca. 10 watts).   This latter issue was traced to an improper transistor installation (i.e., the problem with the protection relay was just a symptom...it was the errant wiring of the control transistor on the two power amps boards at fault).  Hence both power amp boards have been rebuilt again (including all transistors) but now with all name brand components (Nichicon, Kemet, WIMA, Panasonic, Fairchild, etc.) and tested at high power (item 6 below).  The eight main power transistors were retained but have new mica pads and thermal grease.
A real oversight was that the stabilizer board was left untouched and this probably needed a rebuild even more as it runs very hot (a well known SX-1250 attribute).  This board's two main filter capacitors, C1 and C2 (both 220uf, now 100V) were completely shot with one measured to be essentially open with zero capacitance and the second having only 50uf of capacitance.  There were other minor issues as well.  A majority of capacitors pulled actually tested out okay but many were out of spec with unacceptably high losses and ESR (equivalent series resistance) values.   This was especially true of the smaller value caps.  All this is standard wear and tear for the SX-1250 (i.e., no burn marks on the PC boards).  This board has been fully rebuilt with all new 105C rated caps, transistors and diodes.
I generally follow the BOM (bill of materials) lists for restoration posted at Audiokarma but I have my own preferences.  All capacitors, diodes, relays & all but two transistors were sourced from Mouser or Arrow Electronics.  All electrolytic caps (on every board including the tuner board) have been replaced; often with those having higher ratings.  All electrolytic, tantulum and all but two mylar caps as recommended on the AK rebuild list (i.e. 1uf capacitance or less), have been replaced with modern Wima, Panasonic or KEMET polypropylene equivalents.  Nichicon PW 105C  caps were used in places related to the power supply.  For audio boards Nichicon FG (fine gold) were typically used (otherwise low leakage Nichicon KL, Panasonic or Elna Silmic II were substituted).  The two axial caps were replaced with Vishays.  For the four main power/filter caps I chose long-life black EPCOS (22000uF, 100V rated for a ripple current of 17 amps at 85C, bigger is better) caps as opposed to the "standard" bright blue Chemcon (14 amps at 85C).   All trim pots on the driver and stabilizer boards have been replaced with Bourn cermets.   All transistors known to age badly have been replaced as well.
One of the recurring issues reported for many Pioneer receivers of this vintage, including the SX-1250,  is a somewhat unpleasant background level of hiss at idle originating from the tone board.  Tracing down its origin was an interesting circuit design exercise with conclusions posted at an Audiokarma forum thread (
index.php?threads/pioneer-sx-
1250-power-amp-board-rebuild-
issue.941923/page-2#post-
14522667
, Ebay removes url links).  Spice circuit simulations suggest that it is a design issue originating from the relatively large resistances used in the signal path in combination with two sequential tone control stages.  A modest redesign of just the 1st stage resistor values yielded a moderate reduction in the audible level of this hiss with no meaningful changes to the original gain/loss frequency profile.  This redesign has been incorporated and fully tested.  The tone board rebuild also includes replacement of the original carbon resistors (in the signal path) with low noise metal film resistors and one 2nd stage ceramic cap (also in the signal path) with a polypropylene cap (overlooked in the posted BOM list) and all new transistors.  You will not find another SX-1250 with a quieter tone control board.
Standard service items:
1.  Solder joints inspected and remelted in many places
2.  AM and FM
tuner
front end and MPX stereo alignments.  Both front ends had drifted noticeably.  The FM tuner sensitivity is noteworthy (it bests my bench test unit).
3.  Light cleaning of the interior and exterior.
4.  Amplifier offset and bias adjusted.  Bias was lowered from 100mV to 85mV to reduce idle power somewhat.  With this and the LED upgrade the SX-1250 still uses about 80W when idle (still rather high for a solid-state receiver).  This bias is intended to reduced amplifier distortion at low volume levels.
5.  Pots and switches cleaned with Deoxit.  Some very light static can still be heard with the audio push button switches; especially for the two phono switches.  The left channel of the balance pot still exhibits a bit of static if turned quickly (despite multiple applications of Deoxit).
6.  Amplifier outputs were tested at 1kHz up to 150W in each channel.  They start clipping at just above 38V_rms so, in principle, they can deliver ~180W each into an 8 ohm load.  Testing or running at this power for extended periods is not recommended because of heat build up inside the receiver (which will reduce the component life).  There is limited venting in the bottom metal panel to allow for proper airflow.
Operational quirks noted:
1.  When the tuner dial is swept (mode: FM with muting ON) and muting occurs then there is a peculiar audible artifact that sounds as if the tuner string is slipping and vibrating.  It actually comes from a Reed relay module on the tuner board (common to all high-end Pioneer tuners from this vintage) which switches the FM audio outputs off when muting is turned on.  Unlike the protection and soft start relays,  Reed relays are hermetically sealed and so are not exposed to atmospheric conditions; thus they last far longer.
2A.  Crosstalk in right channel:
If using sensitive headphones or speakers there can be a very faint but just audible FM audio background (and only if no audio signal) in just the right channel.  It is independent of the volume setting.
This crosstalk originates in filter circuit board.
The first FM tuner IC (an HA1137) is always on and it provides an FM audio signal for both the stereo decoder chip and multipath monitoring. The right channel output trace is nearly adjacent to that for the FM multipath signal.  Although there is an intervening ground trace between the two for shielding, it isn't quite 100% effective.
2B. This crosstalk is only audible if both frequency filters, low and high, are disengaged.  If either of these is switched on, rerouting the signal path on the board, the crosstalk is completely absent.  Another option, only if not using the AM tuner, is to tune the dial to a frequency that has no FM station.
Available upon request:
1.  A print out from a scanned copy of the owner's manual.   The scan quality is very mediocre.
2.  All old parts except the four large filter caps.
3.  Photos for many of the rebuilt boards.
***Important:
A minimum of 30 buyer feedbacks are required (and at least five in the last year) or the sale will likely be cancelled.
This item comes with a 30 day return; buyer pays return shipping.  Of course the receiver will be double boxed, fully insured and require adult delivery signature.
***International buyers (Two important items of note):
1. No returns on international shipping (and destination shipping via Ebay international shipping agents will be very expensive).
2. The receiver was intended for just the US/Canada markets and so is only configured for 120V service.  If you reside in a country that has higher voltage AC power then you will need to use an in-line step down transformer.   I do not know if a change from 60 to 50 cycle service will matter but I suspect not.
Please ask a question if you have one and, as always, thanks for looking.