Thursday, June 15, 2017

Steve's Marantz 2252 Receiver

Marantz 2252 Receiver

Today Im showcasing the restoration of a customers Marantz 2252 Receiver. Two version of this series exists, the 2252 and 2252B, released in later years apart the 2252 was rated at .1%THD where as the 2252B was rated at .05%THD due to some circuit advances. There are also sticking aesthetic differences and build quality between the two units. I tend to prefer the early 2252 due to build quality, sound and aesthetics personally. 

P800 Power Supply
The P800 regulated power supply feeds the +35/14Vdc supplies tied to a rectified -/+40V filtered rail supply for the drivers.

One of the major benefits of a proper restoration is the integration of todays modern components where we can achieve far better results such as with Schottky diodes. In this rebuild the original rectifier axial diode was replaced with a Zero Recovery silicon carbide Schottky diode, the Schottky is an extremely nice rectifier due to its extremely fast switching, low transient/noise and higher efficiency (better dissipation qualities.)  In addition as with other restorations the common cathode, common anode rectifier packages are replaced with a “state-of-art” On-Semi dual rectifier TO-220 devices which exhibit extremely low leakage, ultra-fast (UF) qualities. 

Along with the diodes, the regulating TO-220 was replaced with a modern robust On-Semi mJE series TO-220 device along with a new low noise Fairchild TO-92 NPN device. The adjustment point was replaced with a precision grade Bourns trimmer.



The electrolytic capacitors were replaced with a high temp (105C) low impedance Nichicon PW capacitors with an increase in operating voltage along with a high grade polypropylene ECW film capacitor at 1kV.

Confirmation of the regulated 35V and 40V rails

Filter Capacitors
The original 10,000MFD 35mm filter capacitors were replaced with a standard 40mm 12,000MFD Panasonic capacitors for a better low frequency response at higher output levels. 

P700 Driver (+Protection)
This is where the 2252 FAR EXCEEDS the 2252B (disregard performance,) the 2252B uses a single PCB for the power supply/symmetric driver and protection which is entirely susceptible to intrinsic potentials and heat issues. The 2252 NON-B as in this writeup uses a single symmetric separate assembly for the driver section with the protection positioned in-circuit.

The original differential were not thermally tied together which always improves performance, the new matched Beta (see data below), low noise TO-92 Fairchild devices were thermally coupled with compound applied. New current source and all driver/pre drivers were replaced with modern low noise Fairchild TO-92L and TO-126 devices with better dissipation. The drivers had new compound applied and remounted where the bias varistors are located.  Both offset and idle current adjustments were replaced with precision grade Bourns trimmers.  Several of the 1/2W resistors exhibited some premature heat issues and were replaced with 1/2W carbon film KOA resistors. 2473 type diodes were replaced with modern 4148 axial diodes as well as the MV-13 varistors to modern 4148 custom compliments.

All of the electrolytic in the driver AF were replaced with a audio grade Nichicon KT capacitor with an increase in operating voltages. High grade WIMA polypropylene film capacitors were installed along with a high grade MKP polypropylene 20)V film capacitor. 


Differential Pair Mapping and Matching

Protection (P700 Assembly)
The protection stage relay was replaced with a new, modern MY24V Omron Relay. The associated transistors were replaced with new low noise Fairchild TO-92L device for better dissipation. The network capacitors were replaced with a low impedance Nichicon PW capacitors with an increases in operating voltages.

Like many of the 22XX series RIAA versus the 2230,45,70 model its features the integrated Selector switch into the same assembly as the RIAA EQ which uses a simple NPN gain stage.

All of the small signal NPN devices were replaced with a modern low noise, low distortion Fairchild TO-92 transistor which significantly drops the RIAA noise floor. Electrolytic’s were replaced with a audio grade Fine-Gold and KT Nichicon capacitors with an increase in operating voltages. As noted with the 1/2W resistors in the power supply the substrate had bubbled on the large 33OHM 1/2W resistor, it was replaced with a 30OHM higher wattage resistor.



 RIAA Curve Response

PE01 Tone Amp
The tone amp stage consists of a NPN/PNP compliment gain stage with integrated Bass/Mid/Treble tone controls all distal to the unique logarithmic balance slider and smooth gain attenuator versus the stepped 2252B model. 

All of the small signal devices were replaced with modern low noise, low distortion Fairchild TO-92 compliments, which as in the RIAA reduces the SN Ratio often significantly.   Electrolytic capacitors were replaced with Nichicon KT audio grade and low impedance PW capacitors with an increase in operating voltages. All of the 1MFD< (including BiPolar 1MFD) were replaced with a high grade WIMA polypropylene capacitors and the input .22MFD with a new high grade MKP polypropylene film capacitors. 



RF Stages
The single RF PCB is the am/fm front end and tuning capacitor. The associated tantalum and electrolytic here were all replaced with a low impedance PW and KZ Nichicon capacitors including VP BiPolar type.


The original dial did not contain vellum and the meters were quite green with a dulled stereo indicator, as with other Marantz its always important to replace the vellum behind BOTH meters and dial display! 

New custom 8V SMD LEDs were installed along with a new SMD Stereo indicator


Sunday, June 11, 2017

Norm's GAS Son of Ampzilla (SOA)

GAS Son of Ampzilla Amplifier

Today were showcasing the restoration of a customer GAS (Great American Sound) SOA (Son Of Ampzilla) amplifier! The GAS SOA is considered by many to be a top favorite amongst GAS owners. Produced from 1977 to 1982 with or with a PS the SOA is reported at 80Wrms @ .08% (8OHM)!

One of my favorite parts of the GAS SOA is the extremely simplified power supply stage. The 10,000MFD is a -/+45V rectified rail source and a two separate ac/dc bridge stages for the -/+52V driver rails.

The original 10,000MFD capacitors were replaced with a high temp computer grade Chem-Con CDE capacitor (I do NOT recommend increasing the filtering capacity here due to in-rush current limits and stressing the PS/Pformer in this particular design.)  

The original 52V AC/DC diodes were replaced with a Ultra-Fast (UF) type axial Fairchild diodes for cleaner rectification and transients. The 2200MFD capacitors were both replaced with a high grade Fine-Gold Nichicon capacitors with an increase in operating voltages. 


Their are several versions of the GAS SOA amplifier circuit out there. Many of the series feature the molex pin connectors to the output circuit PCB which makes access convenient for once.
The output stage is driven from the -/+52v driver supply and -/+45v filtered rail source via separate AC/DC source. The driver stages features a nice dual differential stage and clipping adjustments. The bias control like most GAS amplifiers are driver via the MPQ6001IC (GAS100A)

One of the major issues I have with GAS is the use of component header mounts used for the differential stages and other places in the GAS Grandson and of course the use of proprietary numbering. GAS would rename BJT devices, a late model cross reference is available but in my opinion this is a poor practice especially with an unverified cross reference which adds significant time when upgrading these and a bit of a PITA. 

Differential Matching

The differential mounts was removed and  replaced with modern low noise TO-92 Fairchild 1% matched differentials (see above) which were soldered directly to the PCB. The driver stages which are known to fail on these units were updated with modern robust On-Semi TO-220 MJE series devices also replacing the original TO-6 device along with new MICA insulators (TO-220) and thermal compound.  The bias and offset potentiometers were replaced with precisions Bourns devices.

The electrolytic were replaced with a audio grade Fine-Gold and low impedance PW Nichicons with an increase in operating voltages. The input/output and bypass films were all replaced with high grade WIMA polypropylene film capacitors.



On the output PCB is housed with GAS100A IC and replaced new Bourns potentiometer, I do like to replace the original ceramics with high grade WIMA polypropylene films as shown. 




The original input wiring was of very low quality and again a push connector, as well as loose and corroded input hardware.  New gold plated Tiffany-Style RCA inputs were added along with heavy shielded co-axial constructed input wiring installed to reduce intrinsic potentials.



(Note the impressive frequency response test out to 70Khz! extremely linear and accurate amplifier)