Today were featuring a very prized Marantz 2270 receiver. The 2270 is certainly one of the more popular Marantz receivers available out there today. The 2270 was produced from 1972 to 1975 with the optional wood case. This particular unit came in with massive damage to the front end chassis. A new front chassis sub-frame, new Black faceplate (on the way), new tuning hardware components and re-stringing were all required adding an exuberant amount of time to the restoration process. The final product turned out phenomenal! One of the issues I cover in this restoration is the expansion on the Bias stabilizer modifications.
Power Supply P800 & Filters
The power supply on the 2270 requires a large amount of caustic glue to be removed used for mounting the large vertical capacitors. All of the capacitors were replaced with high temp (105C) long life Panasonic FC and Nichicon PW capacitors with an increase in operating voltages. New UF (ultra-fast) diodes were installed and general 4148 type diodes. A new low noise Omron MY2 relay was installed and re-wired. Part of the 35V regulator circuit was replaced with TO-92L Fairchild low noise BJT devices.
The original 9000MFD capacitors were replaced with 12,000MFD 80V high grade/reliability Panasonic THA capacitors.
Like many of the upper end 22XX series Marantz, the 2270 utlizes two separated driver assemblies which is always ideal.
Sadly, in its original form the 2270 is incredibly unstable in its bias circuit and prone to failures. This is due simply to a piss poor design by Marantz. These issues were later acknowledged in the 2275 design where we can see them address this issue in hard form. The issue is the way the thermal tracking bias transistor and diode is mounted to the output stage heatsink, where as it should be located at the driver stage of the design to better compensate for changes in output demands.
All of the electrolytics and tantalums were replaced with audio grade Nichicon KT capacitors with an increase in operating voltages. All the original films were replaced with high grade polypropylene MKP film capacitors as well. As I mention in other Marantz writeup if these large heavy frame pots clean and track well keep them! They are far better than even new Bourns. In this case I was happy with at least two of the 2.2K pots and new precision Bourns pots were added for the Dc deviation adjustment. New 4148 general diodes and flyback diodes at the output installed.
Each of the thermally tracking H761/760 diode and BJT were replaced (MJE OnSemi TO-126 devices) and heatshrinked with leads added. Each device was then relocated under the driver stage devices and the Vbias multiplier changed to compensate for a change in potentiometer values. The final results is a rock solid 10mV bias at idle with an appropriate slope to peak to peak output and down, along with the bias modification new differential pair TO-92 low noise Fairchild devices were installed, having been matched with a 1% BETA and thermally coupled together.
In the 2270 AF Stages we start to see why the 2270 sounds so damn great….its a 2230 and 2245. The AF stage is essentially the same in each of these series, Marantz pretty much used the same damn design for each one with differences occurring the FM/AM functions, driver output and Power Supply topology.
The P700 has some inherent issues though, such as the use of 2SC458 transistors and VD1212 epoxy diodes. The 2SC458 devices were replaced with matched 1% KSC TO-92 low noise Fairchild devices. The original VD/IS diodes were all replaced with 4148 general type (series.) The tantalum and electrolytic capacitors were replaced with audio grade KT Nichicon capacitors with an increase in operating voltages along with high grade MKP polypropylenes at both input and output stages.
As above this is the same topology and board used in the afore mentioned models. A very simply gain stage and driver. The front end NPN/PNP devices along with the remaining devices were all replaced here with low noise TO-92 KSA/KSC Fairchild devices. The original input films were replaced with high grade MKP polypropylene films as well. All 1MFD electrolytic were replaced with high grade WIMA polypropylene film capacitors and remaining with audio grade KT Nichicon capacitors with an increase in operating voltages.
New Vellum and LED’s were installed on both Dial Display and Meter Assemblies. During the break down the tuning section we re-strung as well and front end AM/FM alignment completed.
Due to damage when the owner bought the unit, the Bass Potentiometer had to be replaced and cleaned and reinstalled. Along with a New EG1018 Power Switch
As noted earlier the sub-chassis had received some damage when the customer first bought the unit from jack ass off ebay to lazy to properly package the unit. Below is the new front sub-chassis being installed