Pioneer SPEC-4 Power Amplifier

Pioneer SPEC-4 Power Amplifier

Today we’re highlighting a Pioneer SPEC-4 Power Amplifier, we’ll be following up this post with the SPEC-1 Pre-Amplifier as paired. The SPEC-4 was manufactured in the late 70’s into 1980 as one of their higher end amplifiers in the DC series.

The SPEC-4 is an excellent design featuring separated dual power transformers/filter stages/ soft-start and protection relays as well as driver and input paths! just fit into one chassis. Im a big fan of the 22-stepped 0/-20dB attenuator as well.

Power Supply Stage and Rectification & Meter Assembly AWM-113

 The Spec utilizes two independent 82B+/- rail power supplies with each its own transformer.

The Driver supplies are each a bridge rectification point featuring 44,000 microfarads of filtering.

The Meter assembly is supplied by a 90V bridge rectification circuit.

Each of the 22,000/80V capacitors were replaced with high grade 22,000 mfd 100V Nippon Chem-Con capacitors. The safety capacitors were replaced with .01 X1Y2 1Kv rated and new .01 ECW PP bypass film capacitors.

The Meter assembly is best accessed by removing the front chassis plate.  The electrolytic on here were replaced with high temp (105C) long life Panasonic FR/FC capacitors and high temp (105C) low impedance Nichicon PW’s. Remaining 2473 type diodes with modern 4148 type as well.  The regulating BJT’s 2SB536//SD381 are prone to developing faults on the SPEC series, these were replaced with TO-220 MJE15028/29 devices and affixed with thermal compound. Small signal devices were updated with 2690/945 TO-92’s and TO-126 devices. 

Driver Assemblies

The driver assemblies are separated symmetric topologies. Initial gain is applied at the dual PNP deferential pair transistors with a constant current mirror network providing sufficient low distortion gain to the class-A pre-driver stage.  a 3 stage darlington array to a parallel SEPP output stage.  The protection circuit of the SPEC-4 consist of a soft-start type On-Off muting, overload and dc deviation. The PCB relays were replaced and soft start burnished with die tools.  

The differential pair package was replaced with a separated pair of 992’ type TO-92’ devices matched to 1% tolerance to ensure stabile dc offset.   2SC1775’s were replaced in pairs with 1845 TO-92’s and the pre-drivers to 3503/1381 TO-126 devices along with potentiometers and cleaned. All of the electrolytic were replaced with high temp (105C) long life Nichicon PW’s and the .22 electrolytic with a ECW polypropylene .22MFD film capacitor. All remaining .033/.01/.1 films were replaced with high grade ECW Panasonic capacitors, the input .1 film was replaced with a high grade MKP polypropylene film capacitor.

New Meter LED’s

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Marantz 2285B Restoration

Marantz 2285B Receiver 

Today were showcasing a Marantz 2285B restoration that came in recently.  The 2285B is an exceptional unit, built and sold from 1977 to 1980 its still today a highly sought after 22XX series.  The 2285B used a unique to its standard (22XX series) dual section 6800MFD filtering capacitors and a single symmetric driver PCB design with a fixed dual output power supply. The B version with integrated Dolby Noise is also rated at 94W versus 85W.

Power Supply P800

As mentioned the 2285B uses a pair of multi-section 6800MFD filter capacitors which were restuffed with 10KMFD/100V Panasonics. The power supply itself contains the dc protect and deviation circuit as well. The original Omron MY4 was replaced with a new low noise MY4-24V Omron relay. The common cathode/anode  wave rectifiers were replaced with MUR1620 TO-220 modern devices.  Remaining electrolytics were replaced with a high temp (105C) FR Panasonic and PW’ Nichicons. The 555 type diodes were replaced with 4148 type as well.  The regulator 2SD330 is a fairly problem oriented component which was replaced with a Onsemi MJE15XX30 type TO-220 with new thermal compound. Associated 1384/1318/733/945 and the 2a684 were all replaced on this assembly with 1220/2690/1485 type BJT’s. I like to over do this assembly as its sandwiched near the radiant heat of the driver assembly. 

SCHEM ERROR***

Schematic indicates that Q801 Base should be tied to ground, which it is not.

Tone Control/Pre PE01

This assembly reminds me of the 2325/2275 PE01 assembly in construction. The pre amp stage has an initial tied emitter pop gain stage and 1345, all of which on this unit were swapped with new low noise small signal 1845 transistors.  The electrolytic were replaced with a audio grade Nichicon KA and low impedance PW’s as well as 1MFD electrolytics to high grade WIMA polypropylene for the input/output couplings. 

SCHEM ERROR***

Schematic indicates CE40 wrong polarity

RIAA Phono

The phono stage here again reminds me of the array like the 2250B, it shares the common point to the selector as well, located under the chassis for space and noise reduction.  comprised of a simple pnp gain stage, all of the electrolytic were replaced with audio grade Nichicon KA capacitors. Along with the af path ceramics to a high grade coated 100Pf capacitors which helps reduce any redundant noise issues. I always reflow the selector pads on this pcbs as they can become loosened or poor with action of the hard-switching selector.

SCHEM ERROR***

Schematic indicates 330PF vs 100PF ceramics

Driver stage P700

The driver stage on the 2285B is on a single PCB featuring a complete symmetric topology. Featuring a full compliment coupled output design. The 1st order differential pair transistors were replaced with 1% matched 1845 TO-92 devices, thermally coupled together with wrap and thermal compound. Along with the pre-drivers q707/706/37/36 TO-220’s. I prefer these large stock heavy plastic potentiometers, I recommend keeping them when found, we did remove and submerge then lubricate it potentiometer after confirm tracking was consistent.  All of the electrolytic were replaced with a high temp (105C) low impedance Nichicon PW capacitor and all of the .1/.01 mylar and PP’s were replaced with high grade MKP.1 and Panasonic ECW .01 polypropylene film capacitors. TO-3’s were removed and new compound applied. 

RF

RF, unlike the top tier 23XX series the RF FM is open chassis. The multi-gang tuning capacitor was flushed and shaft lubricated/cleaned. associated .22<MFD electrolytic were replaced with stacked film ecq panasonic and remaining electrolytic with low impedance PW capacitors. 

New LED’s , switch and safety capacitors installed.

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Pioneer SX-1250 Another Restoration!

Pioneer SX-1250 Receiver. 

We’ve recently done a slew of these popular Pioneer SX-1250’s so this write up will be a little more brief.  To recap the SX-1250  was one of the foremost and sought after SX-series designs fetching still a more than attractive price for collectors. Built from 1977-1979 the 1250 features a massive toroidal transformer and a total of more than 88K microfarads filtering capacity.

New Custom built LED's

Power Supply AWR-107/AWR-106 PS and Stabilizer & AWM-091 Protection Assemblies

The large axial 470MFD capacitors were updated to a Nichicon TVX along with associated 470/35V to high temp (105C) low impedance/long life PW/FR capacitors. On this model the flyback diode was also updated to a ultra-fast #400 type diode. 

The AWR106 stabilizer  had the large axial electrolytic 1000MFD again updated to a Nichicon TVX and associated electolytics to a Panasonic FR/Nichicon PW type electrolytic. This particular stabilizer board had issues regulating the 65V B+/B- rails due to a leaking 2SB507 TO-220 driver device.  All of the small signal TO-92 and TO-220 devices were upgraded to 992/1220/1845/MJE on Semi devices and all WZ-140 with new new diodes and the 2473 to 4148 diodes. 

The AWM-091 assembly consists of the dc protection circuit.  The protection network detects dc deviations/overload parameters as well as a 48V Soft Start Relay.  The original MY4 relay was updated to a new low noise Omron MY-4 relay and associated electrolytic capacitors to low impedance, high temp (105C) Nichicon PW/FR Panansonics.  The drive transistors were updated to TO-220 2690 BJT’s and the 945 to 2383 BJT’s, remaning 2473 diodes replaced to 4148 type. 

Filtering

The X4 22,000 microfarad filtering capacitors were replaced with high grade Nippon Chem-Con 22,000/100V filter capacitors and the associated .01 film upgraded to a Panasonic ECW type.

AF Stages RIAA AWF-021 Flat Amp/Control AWG-041/AWG-042/094

The SX1250 RIAA design was well done and extremely quite. using a 1st order tied emitter/SEPP gain topology and 1% resistors. The initial PNP pairs were updated to matched ksa992 groups. The large 470/220MFD) input filtering capacitors were updated with FR Panasonics and remaining AF stage with low impedance PW Nichicons. Input capacitors were replaced with high grade Panasonic ECW polypropylene film capacitors all remaining 2473 diodes replaced to 4148 type.

Remaining tantalum and electrolytic on the AM AWG-041/042/94 PCB’s were updated to low impedance Nichicon PW type and Nichicon KA audio grade electrolytic. Mylar input/out films were updated to high grade WIMA and Panasonic ECW polypropylene film capacitors. Small signal transistors within the AF front ends were replaced with matched 992/1845 type TO-92 devices.  All the functions and front end switching were removed as a pcbs and cleaned and lubricated using Caig Labs products. 

Drivers AWH-048

In earlier post I break down the drive topology as;

“ Designed around a classic a/b configuration the x1250 encompasses a 1st order differential pair gain stage tied to a constant current mirror design which mutually phases signal for a more stabile drive via Q3-4 und Q5/6 through a/b biasing resulting in a PP bias scheme.  3rd order drive is comprised of a tiered complimentary  darlington arrangement via BJT’s through to outputs.”

The differential pairs BJT’s were replaced with 1% matched ksa992 devices and Q11 to a 1845.  Pre drivers were updated to 2690/1220AYS groups. All 2473 diodes to 4148 type and ultra-fast 400#. Bias potentiometer was replaced with a precisions Bourns trimmer, all electrolytic and large 470/100V were replaced with low impedance PW Nichicons and Panasonic ECW polypropylenes replaced the original and tantalums.

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*notate the separation in comparison, we’ve perfected this protocol.

McIntosh C26 Pre-Amplifier

McIntosh C26 Pre-Amplifier 

Here we have a McIntosh C26 Pre-Amplifier that can in for a rebuild from a customer.  The C26 was sold from 1968 to 1978. The first preamplifier to feature a glass plate to match the MC2502 amplifier. The C26 featured two unique controls at the time for speaker phasing, helping to correct placement and center channel marked at -6dB/VS LR, the C26 also featured speaker outputs and a combined volume/switch potentiometer with integrated AC shielding at the contacts, a fairly revolutionary design in attenuators. To finalize a quality potted transformer was mounted to a chromed out chassis.

Power Supply - Centre CH. Assembly

The simplistic power supply is regulated by the 75V zener D303 to the NPN Q302 2N3738 transistor and the 75V filtering stage. The diode smoothing capacitors were replaced with Vishay PP type .01 and C304/305 multipack restored @ 220/220MFD 200V ratings. On the PCB mount the 100MFd electrolytic was replaced with a low impedance high temp (105C) PW Nichicon and the smoothing films to quality MKP and WIMA high grade film capacitors.

The centre channel assembly for specially recorded albums and additional output, which was set a -6db less than l/R output had the .47/.22 coupling films upgraded to a high grade MKP and WIMA film capacitors.  The Brass attenuator and phase controls were cleaned and lubricated to protect the phenolic.

AF Stage Assemblies

The C26 af stages were populated on two symmetric PCB’s on top of the chassis. All of the films were removed and upgraded to a high grade MKP and WIMA polypropylene film capacitors along with the tantalum and electrolytics to a audio grade Elna Silmic II capacitors.  Initially small signal driver pairs were updated with NOS BJT’s I located, with the batch I was able to match within about 5-7% at best.

Before

After

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