**updated to include BA312 modification in 1701#249**
Today were showcasing two Marantz 2275 Receivers that came in from customers for restoration. The Marantz 2275 is one of my personal favorite Marantz receivers from the era. Unlike the popular 2270, the 2275 is built FAR better, with all of the driver issues address. One should compare the 2275 to the 2325 versus the 2270. The 2275 features a robust 26,000MFD filtering, good RF shielding, BA312 IC pre-amp chip AF stage like the 2332 (at/ 18,000MFD)
Power Supply / Protection P800
The 2275 features a 11A bridge rectifier output for the driver -/+ 48Vdc rail source. The main RF/AF circuit uses two full wave center tap rectifiers for the protection and 13.4V/35V regulated rail sources.
The two full wave center tap rectifiers were both replaced with state-of-the-art Ultra-Fast (UF) type MUR On-Semi TO-220 type devices, along with the 35V source the initial 220MFD smoothing capacitor was increased to 330MFD, along with all other electrolytic to high temp (105C) long life Panasonic FC and Nichicon PW capacitors with an increase on operating voltages. High grade WIMA polypropylene bypass capacitors were also installed. New precision Bourns potentiometers for the adjustment were added along with new low noise Fairchild TO-92 devices and TO-220 Regulators. Low noise, modern Omron 24V w/ indicators relays were installed.
BEFORE
AFTER
Filter Capacitors
Two new 18,000MFD computer grade Nippon filter capacitors per unit were installed to lower ripple effect and increase loading capacity during low frequency production at increased output levels.
Drivers P700
The 2275 driver section employs a dual differential pair which drivers the output via H703 and predriver/driver H711/12/13 devices per channel with a global feedback circuit and dynamic bias control via H709/10 and a complimentary output stage.
As mapped below the differential pairs were matched within 1% and thermally coupled together, new constant current/ detect, pre drivers were installed using low noise, modern Fairchild TO-92 and TO-126 devices. New general axial Vishay diodes installed as well.
Differential Mapping
Both potentiometers were removed, soaked, cleaned, lubricated and cleaned, as Ive mentioned several times I very much prefer these large enclosed plastic pots as they tend to be very precise and reliable. All of the electrolytic capacitors were replaced with audio grade Nichicon KT capacitors with an increase in operating voltages, all the original circuit films were replaced with high grade MKP, WIMA polypropylene film capacitors.
BEFORE
AFTER
Bias Confirmation
PE01 Tone Stage
The 2275 pre-amp/tone control stage uses a BA312 PRE-AMP IC instead of a differential pair and buffer circuit, the preceding is a npn/pnp compliment circuit.
All of the npn/pnp devices were replaced with modern, low noise Fairchild TO-92 devices. 1MFD capacitors were replaced with high grade WIMA polypropylene films along with the initial input films to high grade MKP polypropylene films. Remaining capacitors were replaced with audio grade KT and Fine-Gold Nichicon capacitors including VP BiPolar capacitors with an increase in operating voltages.
BEFORE
AFTER
W/OUT Modification
W/ BA312 Modification
P400 RIAA EQ
The phono stage (RIAA EQ) in the 2275 is a very popular design used in MANY of the 22XX series Marantz units, the RIAA EQ provides a nominal -40dB gain stage using a NPN compliment. One of the known issues with the P400 assemblies is the employment of the 2SC458 transistor which is known to become nosey and failure prone. As shown in the photos below several version of the film compliment were used by Marantz, assuming due to availability/costs at the time of production if no other reason.
All of the npn devices were replaced with modern, low noise Fairchild TO-92 devices including the 458 transistors. The epoxy varistor was replaced with a series of modern axial Vishay diodes. Both clipping adjustments were replaced with Bourns precision trimmers.
The initial input and output coupling films were replaced with high grade polypropylene film capacitors, electrolytic were replaced with a audio grade Nichicon KT type with an increase in operating voltages.
BEFORE
AFTER
RIAA Curve Response
RF (AM/FM+)
Good shielding encloses both Dolby/FM/AM RF PCB’s to help isolate noise along with the 5-gang tuning capacitor. Sadly one of the original HA1156 IC's due to a bad .47 coupling capacitor.
All of the bipolar/tantalum and electrolytic including a few mylar were all replaced with low impedance, long life Nichicon PW capacitors with an increase in operating voltages as well as high grade WIMA polypropylene film capacitors.
Hardware
Due to the heat of the original 8V incandescent bulb the vellum paper used to diffuse becomes baked and changes hue along with the meters, its important to replaced both meters and dial display vellum as shown below. Also, new 8V Custom SMD type and LED Bi-Pins installed :)
AUDIO NOTIZEN
No comments:
Post a Comment