Yamaha M-45 Blown Output Transistors

[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.

[Bigred]

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.

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.

[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.

[Jan Dupont]

Use a 60 to 100W light bulp. I normally always use a 100W. Works fine for testing

[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).

[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.

[Bigred]

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.

[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?

[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.

[Bigred]

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.