amr2
Well-Known Member
Repairing this model can be more complex than other amps, so here are some basic tips that may help someone. Please comment if something needs to be added or removed. (I apologize for any mistakes in my English, is not my native language)
Useful links:
General maintenance:
Beware of counterfeits when changing transistors. I think it's better to use an original equivalent than a counterfeit one. Always get them from legitimate sources.
Most common parts
Other parts (not tested)
Notes:
Useful links:
General maintenance:
- Remove all the dried glue that Yamaha used to fasten the 1000uf/100vdc caps. Usually this requires cleaning the copper bus bar because it has corrosion on it. The same happens with a couple small wire jumpers between the copper rails. There are some resistors, zeners, diodes and caps in this area that need to be checked too. This must be done even if your amp is in working condition, to prevent a future failure. See this thread
- Check all the electrolytic capacitors, they may need to be replaced due to age, stress or excess heat. Be careful with C164, the value and type on the schematic are wrong. See paragraph “Replacements”.
- Protection led ON
Could be some damage done by the dried glue. Remove it, clean the board and restore the damaged parts. The worst scenario is a blown channel. This amp has 8 “big transistors” per side, but only 4 are outputs (The upper boards are the X-power supply). Check output transistors, driver, predrivers, diodes, zeners (especially the ones in near the 1000uf caps in the center, D111 to D118) and the 0.22 ohm 5w cement emitter resistors too. Note that the drivers (TR119-125 and TR120-126) are mounted in the power supply board, not in the main board. I’ll recommend check all the Voltage Amplifier Stage (VAS) and Output Stage. In this model the damage could reach up to the input stage (!!). - Changing the output transistors
- Don't install them at first power up. Solder a 470 Ohm/1-2W resistor across the base and emitter pads of every PNP and NPN output.
- Set the bias current to cero (turn VR103-VR104 fully counterclockwise).
- When you fire it up check for sane voltages on the output transistor pads. The absolute collector voltages should be the same on the NPN and PNP sides (Beware that if the power supply hasn’t the outputs load, LP-RP could be higher than expected. You can wire a 1K/5W resistor between the collector and the emitter of the power transistor pads, to simulate the bias current).
- Base voltage should be about < 600mv.
- If everything is OK, you can install the power transistor and adjust bias.
- Adjusting bias
See adjustments chapter of the service manual. Take into account that serials 11051 and greater, voltage=15.5mV. Serials less than 11051 voltage=10.0mV. I recommend changing the variable resistors VR103-104 by new ones, the old ones may not be reliable even if you Deoxit them. If you have an oscilloscope, you can look for any symptom of oscillation at the output while biasing. - Checking the power supply
Beware the voltages of the service manual, they are not accurate.- First check voltages with no load. As guidance:
- HB = 89.7v
- LB = 39.5v
- RP-LP = 39.5v
- Take into account that may be some differences depending on the mains voltage, as the power supply is unregulated. Check also the mains voltage selector.
- Second, check the voltage switching operation. Connect an 8Ohm dummy resistor, 150W min. You can build your own dummy resistor with some cheap 100W resistors found in ebay (They need heatsink!). See adjustments chapter of the service manual, you will need an oscilloscope.
- First check voltages with no load. As guidance:
Beware of counterfeits when changing transistors. I think it's better to use an original equivalent than a counterfeit one. Always get them from legitimate sources.
Most common parts
- Transistor 2SC2773 --> 2SC3858 BDEnt,Digikey
- Transistor 2SA1169 --> 2SA1494 BDEnt,Digikey
- Transistor 2SC3298 --> 2SC4793 BDEnt ,Mouser,Digikey
- Transistor 2SA1306 --> 2SA1837 BDEnt,Mouser,Digikey
- Transistor 2SC3416 BDEnt --> KSC3503DS Digikey,Mouser
- Transistor 2SA1352 BDEnt --> KSA1381ES Digikey,Mouser
- Transistor 2SC1953 BDEnt --> KSC2690AYS Digikey,Mouser --> KSC3503DS Digikey,Mouser
- Diode 1SS82 --> BAV21 Digikey,Mouser
- Diode 1S1555 --> 1N4148 Digikey,Mouser
- Zeners: Use any 0.5W zener
- Capacitors C162-163: 1000uF/100V --> UFW2A102MHD - Nichicon (1000uF/100V FW series) Digikey,Mouser
- Capacitor C164: 47uF/25V Bipolar --> R2B-100V470MI5 Mouser. It’s a little big (100V), but fits well. R2B-25V470MI5 (25V version) was not available when I placed the order. Be careful, the service manual indicates a 100uf 25V polar capacitor, that’s wrong. There is also an error with the silkscreen on the board for this capacitor (polarity marks are reversed). So if you install a polar capacitor following the polarity marks, it will start to bulge on top…
- VR103-104 (bias adjust). You will need to bend the legs since they don't match with the holes:
- Bourns 3299Y-1-102LF (multiturn) Digikey,Mouser
- Bourns 3386G-1-102LF (one turn) Digikey,Mouser
- RY101 (Main speaker output relay - ea.JC2aD-DC24V): Omron G4W-2212P-US-TV5 24VDC Digikey, Mouser
- RY102-RY103 (MS4U): Omron MY4-02 DC24 Digikey, Mouser
Other parts (not tested)
- 2SC2240 --> KSC1845
- 2SA970 --> KSA992
- 2SA1145 --> KSA1381 (closest), KSA1013YTA
- 2SC2705 --> KSC3503 (closest), KSC2383YTA
- 2SC2603-2SC2634 --> KSC945C / KSC1815
- 2SA1115-2SA1127 --> KSA733C / KSA1015
Notes:
- D119-120 (3V zeners) measures about 2.4V
- The bias current is very sensitive to external air flow. It drops close to 0 only blowing on the pcb. I've seen people that puts a fan over the cover to cool it down, but this could be not a good idea. If the bias current drops close to 0 means large crossover distortion because the amp is working in class B.
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Can't say it is 100% correct if there are other TSB out there.
