C.Dykes Marantz 3800 Pre-Amplifier Restoration

Marantz 3800 Stereo Control Console 

The Marantz 3800 pre-amplifier was released from 1973(5) to 1978 as the TOTL pre-amp to match to the popular separate power amplifier series. The 3800 is a robust pre-amplifier with a high output and low distortion. Fully integrated Dolby Noise Reduction model with unique features like multiple phono input, scope output and tape to tape continuous use for recording even different source materials!

Power Supply (MAIN PCB) + Time Delay Relay

The 3800 power plant consists of two simple AC/DC 1A Full Wave Bridge rectifiers, one for a regulated and unregulated Dual Supply and one for the Lamps. The dual supply uses a primary RCA4194 9-Pin TO-66 type voltage regulators producing a -/+ 17Vdc and unregulated -/+ 29V dual rails. The Time delay prevents loud pops upon startup with aprox 5 second delay for voltages to stabilize. 

First off both Full Wave bridges were replaced with a Ultra-Fast, Soft Recovery axial Vishay array (1A). The initial input and output capacitors were increased to 1000MFD and 220MFD for better DC filtering and power reserve with new low impedance, high temp (105C) Nichicon PW with an increase in operating voltages. As well as the C571 increased to 1000MFD for results as above. C297 of the time delay was replaced with a Nichicon PW also with an increase in operating voltage. As per almost all our rebuilds, the axials in the power supply were replaced with a better constructed radial electrolytic capacitors. 

BEFORE 

 AFTER

Regulated and Unregulated Rail Confirmation

X10 AMP —> 30Hz,9khz 5khz —> Filter Amp —> Tone Amp + OP-Amp Modification

The 3800 pre-amplifier is divided into 3 main stage; the X10 stage develops the needed amplitude for amplifier gain, the proceeding stages employ a series of R/C cut off filters 30Hz,5K and 9K to reduce origin noise composed of a non inverting, unit gain filter amp and constant current source. with a 12db octave cutoff. Signal is then fed to the Tone amplifier stage where one can attenuate with Bass/Mid/Treble controls and 250hz/4Khz turnover circuits mirror to the above similar unit gain/feedback designs. 

X10 Stage OP-Amp Modification (COURTESY NUMISTREK09)

The original design employs a 14-pin uA749 as a dual inline audio integrated circuit (IC) which at this point in its life span is prone to failures and distortion development. The non-inverting IC and a constant current source established the output with the negative feedback a gain equalizer and frequency cutoff points with a  aprox. 20dB gain.  Today’s modern OP-Amp have significantly improved specs with a decrease in distortion and noise and separation. A well documented modification involves the integration of a modern dual rail/ stereo OP-Amp, in this case we installed the high-end Burr-Brown OPA2134 device and removal of the 749 compensation circuit which is now encapsulated in modern OP-Amps. The current for this Op-Amp was equated at 8mA for 15V operating zone. 

BEFORE

AFTER

The remaining electrolytic in this stage were replaced with a audio grade KT/KA Nichicon with an increase in operating voltage along with new high-end MKP and WIMA polypropylene film capacitors. 

Filter Amp & Tone Amp

The Tone Amp stage utilizes two LM301 Op-Amps single end Op-Amps in its finalize gain stage. This stage had extensive films replaced with high-grade MKP and WIMA polypropylene films as well as associated electrolytic to a audio grade KT/KA Nichicon capacitors with an increase in operating voltages. 

BEFORE

AFTER

Upon verification reporting it was noted a eschewed dB wave upon engaging the Loudness feature. This was due to the tone amp OP-Amps, someone had replaced the original 301 type with a unknown branded/ID 8DIP device. These were both replaced with the single monolithic version of the dual 2134OPA used in the X10 stage, with 134OP devices.

RIAA/ PHONO

The phono gain stage input is carried via a differential gain and emitter tied buffer and similar output stage with a nub loop. Nominal gain rated at 40dB. 

The electrolytic capacitors were all replaced with a audio grade KT/KA Nichicon with an increase in operating voltages. High-grade WIMA polypropylene films were also installed as well as new low noise Fairchild TO92 devices. 

BEFORE 

AFTER

RIAA Curve Response

Dolby Noise Reduction & Dolby Level Meter

The 3800 features a complete Dolby Reduction design meaning Dolby Encoding and Decoding occurs simultaneously.  The Dolby processes in effect increase aptitude of low gain mid and high frequencies during recording and reduces to ideal levels during playback retaining the original recording with decreased noise potentials. 

On both assemblies all of the electrolytic capacitors were replaced with both audio grade KT/KA and low impedance PW Nichicons with an increase in operating voltages. Input 1MFD films were replaced with high grade MKP and ECW polypropylene films per application. Remaining af path films were replaced with a high grade WIMA polypropylene film capacitors. 

BEFORE 

AFTER

400hz Gen. @ (580mV) Dolby Calibration Confirmation

AUDIO NOTIZEN