| gni |
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. |
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| Bigred |
| quote: | Originally posted by gni
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.
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Hi Chris,
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. |
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| gni |
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. |
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| ACD |
Use a 60 to 100W light bulp. I normally always use a 100W. Works fine for testing
;) |
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| djk |
| A 75W 120V lightbulb will start any hi-fi solid state amp less than 800W that is not class A. I've only had to use a bigger lightbulb once in the last 20years, and I can't even remember what amp it was now (must have had a lot of bias). |
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| gni |
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. |
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| Bigred |
| quote: | Originally posted by gni
the output drivers don't seem to have heat sink grease, just the insulators. |
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. |
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| gni |
I will keep the 2nd M-45 as is and use it to compare against only. . .
What would be the advantage of using heatsink compound and mica insulators versus the other type of insulator? |
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| djk |
"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. |
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| Bigred |
| quote: | Originally posted by gni
What would be the advantage of using heatsink compound and mica insulators versus the other type of insulator? |
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.
However its just good practise to use new insulators regardless of type you choose. |
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| gni |
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! |
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| djk |
Buy a $0.99 two-wire extension cord.
Buy a $0.99 lampholder.
Cut one wire on the cord. Attach those two ends to the lampholder. Screw in a 75W bulb.
Anything plugged into the extension cord will now have the lightbulb in series with it. |
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| gni |
I'll upload a picture of what I made. . . I wanted something that
was simple, but very specific for the task. . . |
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| gni |
| 120 VAC Series Receptacle |
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| Bigred |
| quote: | Originally posted by gni
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? |
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. |
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| gni |
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. |
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| gni |
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. . . |
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| gni |
Bright bulb = normal current for bulb = current in circuit
but voltage less than 120VAC at amplifier. |
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| gni |
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 |
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| gni |
Rotel RB-850
25W light bulb
(1) Turn on = very bright
(2) Steady on = above glow. . .verge of being bright
DC offset 2x to 3x in the 60mV range
70W light bulb (583mA at 260ohms hot, 14ohms cold)
(1) Turn On = very bright for 2 seconds
(2) Steady Running = low glow (less than M-45)
DC offset elevated about 1.8x
Amplifier on and running at
6.6V/8ohms = 5.4W x 2 channels
Light bulb bright but not fully bright. . . |
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| gni |
Replacement Power Transistors for Yamaha M-45
Original:
A1265n [Toshiba 2SA1265n] pnp
C3182n [Toshiba 2SC3182n] npn
Replacements:
2SA1941 pnp
or nte37(mouser) cross ref to Sanyo 2SB817 [used in RB850]
2SC5198 npn
or nte36(mouser) cross ref to Sanyo 2SD1047 [used in RB850]
Mouser has the pair 526-NTE37-MCP for $10.13! |
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| gni |
NTE Electronics. . .is that the death symbol. . .are NTE transistors bad
compaired to Sanyo or Toshiba? |
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| Bigred |
| Personally I don't think NTE and audio mix (and as you can see more expensive). In a jam they are good for replacing the horizontal output in a TV though ;) |
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| gni |
Here is a web site with a good selection of replacement transistors.
http://audiolabga.com/
might have to buy a bunch to get them to be matched.
That is good advice. NTE bad for audio (mostly at mouser). |
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| gni |
Continuing work on blow output transistors:
Traced more problems back to the driver board.
5 total transistors
2SA1306 (clamped to heatsink)
2SC3298 (clamped to heatsink)
A970
C2240 (back blown off)
'unknown' (clamped to heatsink)
3 resistors
(1) 100ohm (fried chicken) on emitter of 2SC3298
(2) 1.1kohm
2 caps
A220J Soshin
I'm wondering out loud if I should just replace all parts?
I'm seriously thinking of pulling the same channel from the
other M-45 and dropping it in. . .
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| djk |
| I would use NTE before buying anything from AudioLab in Ga. |
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| gni |
| quote: | | I would use NTE before buying anything from AudioLab in Ga. |
Is there something I should know about AudioLabs in Ga?
Is there a list at DIY audio of reputable parts suppliers? |
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| Bigred |
| I suspect its a problem with fakes issue because NTE would be an absolute last resort for me otherwise. |
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| djk |
| Bingo, got it in one. |
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| gni |
Maybe I'm slow. . . .but is the consensus that AudioLabs in GA is
supplying knockoff transistors? NTE seems to have some bad rap.
I started a thread in parts to compile a list of parts suppliers.
I want to keep it simple. . .
Example:
Company A Name: good caps, expensive shipping
Company B Name: quality transistors, limited stock
Company C Name: knowledgeable sales staff, good return policy
etc. |
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| gni |
After much time researching the replacement
transistors I've come to a cross-roads.
Option A:
Continue to hunt for quality transistors for +20 year old amplfier.
Plus: Restore original amplifer sound at reasonable cost (<$50)
Minus: Might not work, more time hunting down other problems,
replacement transistors might not be matched and/or fakes.
Option B:
Swap out amplifier section and replace with new amp modules.
Plus: Quality amplifier modules, new technology, more support.
Minus: Mod power supply to work with module, different sound quality.
In the end. . .I like having three quality working amplfiers. I would also
like to keep the power meters functioning. I will be doing some
non-distructive testing of the meters to see if it will work.
Why I'm leaning towards the ColdAmp modules.
They accept the +/- 63VDC power rails of the M-45.
They run cooler than most Class AB amps and would mount
easily on the existing heatsinks.
They are relatively cheap at $300 US dollar for two modules plus
shipping. I originally paid about $410 for the M-45.
Good opportunity to reuse existing parts (instead of landfill). |
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| gni |
I have completed the install of the Radio Shack Accurian Receiver
inside the shell of the Yamaha M-45. Bench testing now. . .listen
test this afternoon. . .I expect I will need to rotate the transformer
to reduce humm. |
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| gni |
The Radio Shack Accurian amplifier mounted in the shell of the
Yamaha M-45; space is left for the addition of two Aussie Amps later.
Cost: Accurian amp ($16.00) + solder since all parts were reused. |
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| gni |
Time to take out the original M-45 transformer and replace it
with a 12.6VAC 3A transformer for the meter circuit. I will also
pull the power supply caps. . . might as well make it lighter. . . |
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