Today were featuring a very nice and clean Marantz 2245 Receiver. Manufactured from the mid 1971 to 1976 they had a fairly long production run. The 2245 is very similar to other 22XX series Marantz. Featuring an almost identical P400/P700 RIAA EQ and Pre-Amp stage topologies and footprints. Rated at a modest 45W and no more than .3% THD+N during the restoration process we were able to significantly improve on this.
P800 Power Supply
The regulated power supply contains an integrated protection network and relay. The protection provides a soft start for the large inrush current and a dc deviation and excessive current draw from the driver output stage. The relay was replaced with a new ultra-low noise Omron MY2 type, I like these heavy lock relays, their more robust than their normal pin-type MY2 relays just cost more. The electrolytic on the P800 PCB were replaced with high temp (105C), long life PW and Panasonic FC capacitors with a increased in operating voltage to extend life. S1B1 diodes were all upgraded to ultra-fast UF400X type diodes.
The original power switches in practically all Marantz receivers have a tendency to get stuck and begin to arch and eventually fail. As standard a new power switch was installed and a X1Y2 Vishay Safety Capacitor installed.
The original 9000MFD capacitors were replaced with an increase in microfarads, this gives us a more higher current ceiling to operate with for the sub-sonic and peak to peak output as well as better filter characteristics. New Cornell computer grade filter caps were installed and increased to 12,000MFD/80V capacitors.
P400 PreAmp Stage
As mentioned the 2245 shares a similar topology to the 2230/2270 etc models with its basic NPN/PNP gain stage and coupled input/outputs. One of the biggest benefit we obtain from this stage is the upgrade of the input/out capacitors.
High grade MKP and WIMA polypropylene film capacitors were used in the .22 input and 1MFD coupled output section. The core electrolytic were replaced with audio grade Nichicon KT and Fine Gold capacitors as with each 1MFD upgraded to a high grade WIMA polypropylene radial film capacitor. All electrolytic and films were increased in voltage capacity to extend the life. All of the small-signal transistors were replaced with compliment low noise modern 992/1845 BJ-transistors. Originally I had planned to use MKP for the input and output but decided on the latter ECW combination.
The RIAA EQ stage in the Marantz 2245 as the P400 PCB is shared amongst many 22XX series Marantz with only slightly deviations developed throughout the latter models, the RIAA EQ has a nominal gain amplitude of 40dB. These are considered by many vintage fans and audiophiles are some of the best RIAA stages designed!
The original glass epoxy diodes were replaced with modern 4148 and a custom series 4148 diodes. As with many 22XX series phono stages the 2245 P400 is integrated with a 2SC458 transistors, these are well known to break down in current and cause leaking or just develop intrinsic noise, these were both replaced with modern low noise 1845 matched within 1%.
The two tantalum and electrolytic capacitors were replaced with audio grade Nichicon KT , the 100MFD filter capacitor was increased to a 220/35V to provide cleaner output. The input films were upgraded to smaller 1MFD high grade polypropylene Panasonic ECW and the output films to high grade MKP polypropylene film capacitors. Below is the only reported RIAA curve online to date we tested and complied.
RIAA Curve Response
The 2245 driver sections features some, what I call for marantz, older circuits. Relying on a thermally coupled driver and bias using the collector of the output stage. Later on advances can be seen in the 2275 design. Fly back diodes using Ultra-Fast modern diodes were added as protection at each NPN/PNP output device. The initial differential gain stage transistors were replaced with 1% matched modern low noise 1845 transistors thermally coupled. The pre and driver stage TO-18/TO-66 had new thermal compound applied to each PNP/NPN. New current limiting and Bias transitors were replaced using TO-92 low noise transistors as well.
The electrolytics and the 22MFD tantalum were replaced with audio grade Nichicon KT and Fine Gold capacitors. All the films in this stage were replaced with high grade MKP polypropylene films at .1-.22 with increased voltage rating. The input film capacitor which was the oil type was replaced again with a high grade polypropylene 1MFD MKP film capacitor.
The precision clipping pots were replaced with modern precision Bourn trimmers and the original white robust pots removed/cleaned and treat with a lipid lubricant. The original wire-wound cement cast emitter resistors (.47OHM) typically show signs of heat break down, these were replaced with new precision tolerant Dale .47 5W resistors. The TO-3’s were removed with new thermal compound applied.
The tuning capacitor was cleaned and detailed. all of the electrolytic were replaced with low impedance long life Nichicon PW capacitors.
New custom made high grade 8V LED’s were installed provided by Mr.Mirabella out of California. New Vellum paper and indicators installed as well for a clean even dispersal of the dial lamps.
At some point, which is common due to use the Main-In Pre out had been damaged. A NOS mount and RCA input was installed and re-soldered using a star ground this time around.