I recently blew the right channel of my Yamaha M-45 amplifiers.
(I have a second. . .so I can compare before powering up).
I was adjusting the bias as I do every year; but this time I dropped
the clip onto the +V rail and shorted it to the output. This instantly
sent a beautiful blue spark into the room and subsequently shut
the amplifier down (protection circuit activated).
Now. . .
The two SA1265n (PNP) output transistors are close-circuit from all
base, collector, and emitter connections. The 2SC3182n outputs seem to be fine but will be replaced so they are matched pairs.
THE QUESTIONS OF THE DAY:
(1) What else should I look for that might be fried or out of spec.
(i.e. the driver transistors, output resistors. . .etc.).
(2) When turning back on, what should I look for besides sparks,
smoke, or magic genies.
Thank-you DIYers.
(I have a second. . .so I can compare before powering up).
I was adjusting the bias as I do every year; but this time I dropped
the clip onto the +V rail and shorted it to the output. This instantly
sent a beautiful blue spark into the room and subsequently shut
the amplifier down (protection circuit activated).
Now. . .
The two SA1265n (PNP) output transistors are close-circuit from all
base, collector, and emitter connections. The 2SC3182n outputs seem to be fine but will be replaced so they are matched pairs.
THE QUESTIONS OF THE DAY:
(1) What else should I look for that might be fried or out of spec.
(i.e. the driver transistors, output resistors. . .etc.).
(2) When turning back on, what should I look for besides sparks,
smoke, or magic genies.
Thank-you DIYers.
Hi Chris,gni said:
Seems like you have a good grasp and heading in the right direction. Unless I misunderstood what you ment by matched pairs, the outputs won't be matched between PNP and NPN but yes replace all and not just shorted (might go without saying but be sure to clean old grease and use new compound with new insulators). I thinks its good practise to do the drivers as well even if they check out ok. Yes, check drivers, feed and emitter resistors. You could always power up without the outputs and see if you come back out of protection. When your ready to power up use a variac if possible or use the good old light bulb in series with mains.
Steps to replacing blow output transistors:
(1) Replace blown output transistors [2SA1265n]
(2) Replace their complementary pair [2SC3182n]
(3) Replace the emitter resistors [look like 0.22 ohm at 1W]
(4) Replace Feed Resistor [not sure where, will trace it down]
(5) Replace the Drivers [need to pull them to identify them]
(6) Replace insulators and heat sink grease [the output drivers don't
seem to have heat sink grease, just the insulators]
(7) Use a light bulb in series with mains when powering up the first
time.
That does that seem like reasonable steps. I have one advantage:
I have a second M-45 that works. . . once the repairs are made and
before I powerup, I can check the values with the meter to see if
they are within similar values.
What size light bulb do you suggest to put in series with the mains.
Here in the USA we use 120VAC. . . it also follows that different
wattage bulbs will drop the voltage to different values. . .as would
the amplifier. . . .simple series circuit with amplifier inrush. . .
Amplifier states 500 watts [ I would assume at full power. . .and
something less at idle or low power. . .maybe higher at initial
inrush].
[4.167 Amps at 120VAC for max 500 Watts, 28.8 ohms]
So, the resistance of the amplifier should go up after inrush to some
idle value. . .I could test with other amplifer, but adding up idle
current of transistors plus 20% would give me 1.75A on the secondaries. . .so something less on the primary (equal wattage).
I would go with just over an amp.. . 1.2 A at 120VAC would be
144 watts idle or 100 ohms seen by the 120VAC mains.
Light bulbs calculate out at follow [low ohms cold]:
20W [167mA at 120VAC, 720 ohms hot]
40W [333mA at 120VAC, 360 ohms hot]
60W [500mA at 120VAC, 240 ohms hot]
90W [750mA at 120VAC, 160 ohms hot]
100W [833mA at 120VAC, 144 ohms hot]
150W [1250mA at 120VAC, 96 ohms hot]
Since the light bulb is in series. . .each will only get a percentage
of the total 120VAC mains. . .at turnon the inrush resistance will
still be lower than any of the lightbulbs. The lightbulbs will never
really reach 120VAC across them.
(1) Replace blown output transistors [2SA1265n]
(2) Replace their complementary pair [2SC3182n]
(3) Replace the emitter resistors [look like 0.22 ohm at 1W]
(4) Replace Feed Resistor [not sure where, will trace it down]
(5) Replace the Drivers [need to pull them to identify them]
(6) Replace insulators and heat sink grease [the output drivers don't
seem to have heat sink grease, just the insulators]
(7) Use a light bulb in series with mains when powering up the first
time.
That does that seem like reasonable steps. I have one advantage:
I have a second M-45 that works. . . once the repairs are made and
before I powerup, I can check the values with the meter to see if
they are within similar values.
What size light bulb do you suggest to put in series with the mains.
Here in the USA we use 120VAC. . . it also follows that different
wattage bulbs will drop the voltage to different values. . .as would
the amplifier. . . .simple series circuit with amplifier inrush. . .
Amplifier states 500 watts [ I would assume at full power. . .and
something less at idle or low power. . .maybe higher at initial
inrush].
[4.167 Amps at 120VAC for max 500 Watts, 28.8 ohms]
So, the resistance of the amplifier should go up after inrush to some
idle value. . .I could test with other amplifer, but adding up idle
current of transistors plus 20% would give me 1.75A on the secondaries. . .so something less on the primary (equal wattage).
I would go with just over an amp.. . 1.2 A at 120VAC would be
144 watts idle or 100 ohms seen by the 120VAC mains.
Light bulbs calculate out at follow [low ohms cold]:
20W [167mA at 120VAC, 720 ohms hot]
40W [333mA at 120VAC, 360 ohms hot]
60W [500mA at 120VAC, 240 ohms hot]
90W [750mA at 120VAC, 160 ohms hot]
100W [833mA at 120VAC, 144 ohms hot]
150W [1250mA at 120VAC, 96 ohms hot]
Since the light bulb is in series. . .each will only get a percentage
of the total 120VAC mains. . .at turnon the inrush resistance will
still be lower than any of the lightbulbs. The lightbulbs will never
really reach 120VAC across them.
gni said:
They are probably the rubber type insulators that serve both purposes then. If you reuse them just make sure they aren't damaged. Personally I'd lose them and use heatsink compound and mica insulators. Also, just make sure you use that other M-45 to compare with and not as a parts swapper
Good luck and keep us posted.
"Seems like a smaller bulb will result in a smaller voltage drop
across the amplifier; a larger wattage bulb will allow a larger
voltage drop across the amplifier."
True, It's a matter of how much fault current you want to let through. I use the smallest lightbulb the amp will power up with. I can run at a low listening level on most amps with the relay pulling in with only a 75W bulb.
"What would be the advantage of using heatsink compound and mica insulators versus the other type of insulator?"
There are very high performance rubber insulators available, the manufacturer of your amp didn't use them. In such a case a mica and grease is a better insulator. The rubber ones that outperfom mica and grease usually cost more than the outputs do. The best rubber insulators should not be reused either, a further expense.
across the amplifier; a larger wattage bulb will allow a larger
voltage drop across the amplifier."
True, It's a matter of how much fault current you want to let through. I use the smallest lightbulb the amp will power up with. I can run at a low listening level on most amps with the relay pulling in with only a 75W bulb.
"What would be the advantage of using heatsink compound and mica insulators versus the other type of insulator?"
There are very high performance rubber insulators available, the manufacturer of your amp didn't use them. In such a case a mica and grease is a better insulator. The rubber ones that outperfom mica and grease usually cost more than the outputs do. The best rubber insulators should not be reused either, a further expense.
The mica offers a lower thermal resistance over average rubber pads (Sil-pads). I'd like to correct my earlier statement about reusing the insualtors also. Mica can be reused (clean it) if its not damaged(mica is brittle though). DO NOT REUSE rubber types.gni said:
However its just good practise to use new insulators regardless of type you choose.
Thank you all for the great advice.
I will make sure I order new insulators and get new heat sink
compound (old tube is about 12 years old. . . ).
With Mica insulators is there more caution with how tight the
mounting screws(since they tend to be brittle)?
One last question about the light bulb: the amp will not fire up
if the light bulb is to low of wattage. . . so it is safe to start with
a low wattage bulb (40 watts) and then increase the bulb wattage
until the amp fires up?
I made a cable assembly that has a plug at one end and two
sockets at the other end: the two sockets are in series so one
can accept a light bulb fixture, while the other will accept the
amplifier plug. Took me 30 minutes and with parts in the parts
box!
I will make sure I order new insulators and get new heat sink
compound (old tube is about 12 years old. . . ).
With Mica insulators is there more caution with how tight the
mounting screws(since they tend to be brittle)?
One last question about the light bulb: the amp will not fire up
if the light bulb is to low of wattage. . . so it is safe to start with
a low wattage bulb (40 watts) and then increase the bulb wattage
until the amp fires up?
I made a cable assembly that has a plug at one end and two
sockets at the other end: the two sockets are in series so one
can accept a light bulb fixture, while the other will accept the
amplifier plug. Took me 30 minutes and with parts in the parts
box!
gni said:
There are actual torque specs for transistor mounting. Caution should be used regardless of the insulating material. Generally transistors get tightened under 10 in lbs. Too loose and the thermal resistance goes up and too tight you can distort the case or damage the heatsink screw area causing an uneven surface. Just make sure your mating surfaces are flat and burr free and snug the transistor.
The bulb test... yes start off with say a 40 watt and if it doesn't lite then move up in bulb wattages. If the bulb stays lit you have a power supply short.
I guess a good question to state now is:
What are the states that the light bulb will be in when testing
an amplifier (and what do they mean?)?
Bulb doesn't light: amp isn't completing the circuit due to a blown
fuse or bad connection (make sure the bulb actually works!).
Bulb glows steady and amp works: both are getting some percentage less than 120VAC but sufficient to operate.
Bulb burns at full brightness and amp doesn't work: short in the power supply (bigred).
There must be more like:
glows bright then dims
Pulses
glows dim then brightens
????
I think a full list would be very helpful (not only to me) but
anybody that has or will run into amplifier problems and needs
to perform the light bulb test.
I will try a test on my working M-45 so I have a baseline for a
light bulb response.
What are the states that the light bulb will be in when testing
an amplifier (and what do they mean?)?
Bulb doesn't light: amp isn't completing the circuit due to a blown
fuse or bad connection (make sure the bulb actually works!).
Bulb glows steady and amp works: both are getting some percentage less than 120VAC but sufficient to operate.
Bulb burns at full brightness and amp doesn't work: short in the power supply (bigred).
There must be more like:
glows bright then dims
Pulses
glows dim then brightens
????
I think a full list would be very helpful (not only to me) but
anybody that has or will run into amplifier problems and needs
to perform the light bulb test.
I will try a test on my working M-45 so I have a baseline for a
light bulb response.
Light Bulb Test Circuit: 25wpc single chip amplifier
25W light bulb (208mA at 576ohms hot, 48ohms cold)
(1) Turn On = Very Bright then dims quickly
(2) Amp stable and output relays latch = very soft glow.
(3) Loud music = Bright and pulses (lags in brightness behind peaks).
The amp has a 4amp fast blow fuse in it (Accurian Mod found in the
DIY Audio thread).
Hint: A clear bulb is easier to see the faint glow.
70W light bulb (583mA at 260ohms hot, 14ohms cold)
(1) Turn On = Medium Glow, then no glow
(2) Amp stable and output relays latch = no glow
(3) Loud music = medium glow with slight pulsing not as strong as
the 25W bulb.
Will test with Rotel RB850 Saturday after replacing Bias circuit. . .
in another thread at DIY Audio. . .
25W light bulb (208mA at 576ohms hot, 48ohms cold)
(1) Turn On = Very Bright then dims quickly
(2) Amp stable and output relays latch = very soft glow.
(3) Loud music = Bright and pulses (lags in brightness behind peaks).
The amp has a 4amp fast blow fuse in it (Accurian Mod found in the
DIY Audio thread).
Hint: A clear bulb is easier to see the faint glow.
70W light bulb (583mA at 260ohms hot, 14ohms cold)
(1) Turn On = Medium Glow, then no glow
(2) Amp stable and output relays latch = no glow
(3) Loud music = medium glow with slight pulsing not as strong as
the 25W bulb.
Will test with Rotel RB850 Saturday after replacing Bias circuit. . .
in another thread at DIY Audio. . .
Working M-45 and bias set to about 16mV to 17mV
95.1 VAC CT
93.5 VAC CT Class A
25W light bulb. . .never turned on.
70W light bulb (583mA at 260ohms hot, 14ohms cold)
(1) Turn On = Bright glow for about 10 seconds.
Then protection circuit turned off.
(2) Steady running = Medium glow. . .filament visible through white.
(3) Bias down to 9mV
70VAC CT
Switch to Class A
(1) Bulb goes bright, but less than turn on bright for 3 seconds.
(2) bulb medium glow, but brighter than idle in AB mode.
61 VAC CT
95.1 VAC CT
93.5 VAC CT Class A
25W light bulb. . .never turned on.
70W light bulb (583mA at 260ohms hot, 14ohms cold)
(1) Turn On = Bright glow for about 10 seconds.
Then protection circuit turned off.
(2) Steady running = Medium glow. . .filament visible through white.
(3) Bias down to 9mV
70VAC CT
Switch to Class A
(1) Bulb goes bright, but less than turn on bright for 3 seconds.
(2) bulb medium glow, but brighter than idle in AB mode.
61 VAC CT
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