Thursday, December 29, 2016

Jim's Yamaha M-80 M-85 and C-85 Restorations

Yamaha M-80 & M-85 Power Amplifier & Yamaha C-85 Pre-Amplifier 

The Yamaha M-80 and M-85 produced during the latter/mid 1980’s (84-86) were part of the top tier M-Series Amplifiers each with a matching pre stage. The M-80 and M-85 exhibit very close characteristics/design and output with the M-85 adapting the known service bulletins applied to the M-80 versions for stability and performance.  The performance parameters of theses amplifiers continue to impress with a rated output of 250/W @ .003% distortion and capable of driving 2-ohm loads and a incredibly accurate wide-range LED peak power indicator circuit. Both units involve a Class-A operation which is highly recommended NOT TO USE due to severe overheating issues.

Yamaha M-80 & Yamaha M-85
This M-80/M-85 pair came in with some issues. The M-85 needed new binding post and the chassis straighten due to damage and the M-80 had several blown output devices and notable failure. The speaker binding posts on these amplifiers is relatively easy to upgrade and frankly a must, the original plastic mounts becomes very brittle due to the heat of the unit and will often crack and break. New Relays (MS4U/G4W 24V & JC2aD 24V) were installed and the lead wire/post for the c+ banks removed and hard-wired for safety/longevity.

On the M-80 we had extensive cascade damage noted with; X3 open emitter resistors, x2 open 2.2OHM output tied resistors and one 2.2 skewed to 10Ohms. Three of the four output devices were blown, bias serve transistors, two 7.5V diodes and a handful of resistor arrays.  The M-80 does require significant work before regular operation to install important service bulletin #665 (03/15/1986) to protect further damage in the power supply and output stage (cascade failure prevention..too late on this model :( )  In addition many of the amplifiers are probably improperly biased due to an error in the SM indicated a bias over two of the compliment device emitter resistors incorrectly. 

Another serious notation is both amplifiers used original 1000MFD/100V filter capacitor with a large amount of glue used for vertical orientation. This glue at this point HAS damaged basically any associated components…completely eating away at the metal of resistors/diodes and the copper busses. Its important not too forget the electrolytic for the front LED wide-band display as well which had new low impedance PW’s installed.

All of the electrolytic on the Main-PCB of the M-80/85 were replaced with high temp (105C) Nichicon PW capacitors with an increase in operating voltages including the large 1000MFD/100V with high grade WIMA polypropylene film capacitors (.01/001/450V). A SM/Schematic error indicators  a 100MFD polar capacitor but should be a 47MFD BP, this was replaced with a Nichicon VP 47MFD BiPolar capacitor.  Both 1K potentiometers were replaced with precision 1Kohm Bourns trimmers.

The differentials stage consists of a dual JFET 2Sk389 device and a NPN stage, the differentials were replaced with modern low noise Fairchild TO-92 devices matched within 1% including constant current, Vbias and pre-driver and driver stage transistors updated (1381/3503 devices with a 47 ohm resistor tied to each collector (TO-126 devices) plastic tabbed TO-220-3 Toshiba type 4793/1837 driver devices were upgraded/installed as well. New 1/2 W KOA metal film resistors and .22/5W wire-wound emitter resistor installed.

The additional PCB is mounted to the heatsink’s in long narrow PCB strips with the other compliment of Sanken devices, the regulator BJT’s replaced with plastic tabbed Toshiba 1837/4793 installed and new TO-92L fairchild devices for better heat dissipation, new 100MFD low impedance Nichicon PW electrolytic and WIMA polypropylene film capacitors installed as well. 




Yamaha C-85 Pre-Amplifier 

The matching pre-amplifier to the M-85 Power Amplifier, the C-85 is an incredibly well built pre-amplifier. With exception power supplies and filtering, quality original components plus a 4-gang enclosed volume potentiometers (the balance is installed very weird) and a parametric EQ control!

The C-85 uses a large amount of IC gain stages (1st order Tone Control; BA4581S) where as the second tier 6dB Flat amp uses a Dual JFET 2SK389 differential stage as the above M-80/85’s do to BA4561 IC’s.

Dual PS
All of the electrolytics within the two power supply networks were replaced with low impedance long life Nichicon PW capacitors with an increase in operating voltages. The two TO-220 heatsink mounted regulators 2SD1275/SB949 were exhibited large amounts of radiant heat damage to the PCB and surrounding area, these were upgrade to modern heavy duty MJE TO-220 devices with new thermal compound.

The AF stages had all electrolytics replaced with audio grade Nichicon KT and Fine Gold (FG) capacitors along with high grade WIMA polypropylene film capacitors and ECW film capacitors. All of the large 22MFD BP capacitors were replaced with Nichicon VP BiPolar capacitors. The remaining 2S1555 diodes upgraded to modern 4148 type and the driver stage 2SB650/438 TO-92 devices upgraded to modern  Fairchild TO-126 2690/1220 devices


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