Replacement power transistors for Yamaha G100-212

[Jimmy74]

can I do the tests without having it connected to the speaker or does this amp need a load on the ouput?

[Jazz P Bass]

Step #1: connect an 8 ohm dummy load.

[Jimmy74]

on the service manual I have it says set VR-2 at 31v and VR1 at 5v...the one you attached says VR2 at 45 and VR1 at 10...which one is correct?

[Jimmy74]

I powered it up using the speaker as load, but it wasn't making any noises, I turned it off and it made a really loud thump. I got it ready for testing and as soon as I turned it back on again...that noise was back and smoke pouring out from the power transistor board. Sure enough the other 150ohm resistor was burnt to the bone, luckily I had taken some photos of the board a while back and was able to spot the resistor right away. I suspect crappy thermal paste may have shorted the power transistors... I was wondering why it was looking more liquid than paste!! Back to square one?

[Jimmy74]

I've taken apart everything again and sure enough the "paste" had liquified and dripped into the sockets. I tested both transistors out of the circuit and comparing them to the old MJ15015 here are the readings I got:

OLD MJ15015
between E & B = 1.164K
between B & C = 1.165K
between E & C = 0.1ohms

NEW MJ15015 1
between E & B = the meter doesn't give me a reading
between B & C = 0.3ohms
between E & C = no reading

NEW MJ15015 2
between E & B = 250ohms
between B & C = 250ohms
between E & C = 0.2ohms

[The Dude]

FYI: The meter always gives a reading. Saying no reading is confusing. It can mean no ohms or infinite ohms. It's best to specify rather than say "the meter doesn't give me a reading". That said, the transistors are shorted and will need replacing.
I'm not familiar with this unit, but there isn't possibly a mica insulator(s) that got missed when the transistors were replaced?

[Jazz P Bass]

on the service manual I have it says set VR-2 at 31v and VR1 at 5v...the one you attached says VR2 at 45 and VR1 at 10...which one is correct?

VR-2 sets the output at 1/2 of the supply voltage.
So 1/2 of whatever yours reads.

VR-1 is the 'on' bias of the output transistors.
5 or 10MA (milliamps) is fine.

From your post, I hope you didn't miss the ma.
If you set the bias to 5 Volts, well.......

[Jimmy74]

No I didn't get to do any testing whatsoever, the short happened as soon as I turned it on. The service manual states idle voltage 5 +\- 1mV

[J M Fahey]

Use a lamp bulb limiter, no load, until voltages look normal.

[R.G.]

Let me re-state some obvious things and toss in some alternate thoughts. Please bear with me on the "well, duuuh" ones.
- There is a problem with the biasing on the output stage. That's what's killing output transistors.
- Given the readings, one of the new MJ15015s is now dead, if you're measuring out of circuit.
- You can't get an accurate resistance reading of things in-circuit, because of the parallel paths into the rest of the circuit. You can get an indication of the status, whether shorted or open, or near-value-but-a little-low, but not accurate readings.
- Yes, you really, really need to be using a light bulb limiter to avoid killing a new output device every time you turn it on. You're using MJ15015s as fuses. Well, smoke generators, I guess.

Looking at the G-100 schematic you provided, this power amplifier is a bit of a relic, in that it has no current limiting at all, so there is no internal way to prevent it from smoking an output on any load that's too low. Given that, and given that you don't have much in the way of equipment, it would be a good idea to take out TR19, 20, and 21 and test them out of circuit. If you have to, you can replace the TO66 package directly with a TO220 plastic part by bending the emitter and base leads 90 degrees to go into the PCB, and clipping off the collector lead. This uses the heat sink tab for the collector connection, in the same place the single bolt holds the TO66 on your boards.

It is likely that one of the TO-66 parts is dead, since there is no limiting circuitry in the power amp.

I typed a bunch more, but upon some thought, it's better to test the TO-66s out of circuit first, given your lack of instruments.

[Jazz P Bass]

No I didn't get to do any testing whatsoever, the short happened as soon as I turned it on. The service manual states idle voltage 5 +\- 1mV

Again, that 5 is in mv's.

[Jimmy74]

I'm having problems reading your last posts...can't seem to get to page 2 on this thread...if I click on the page number 2 it just takes me to page 1 again

[The Dude]

Not sure if you'll be able to read this since you can't get to page 2, but try clearing your browser cache.

[Jimmy74]

I'm using a different browser now (chrome as opposed to friefox) it's a bit better but I'm still missing some posts. Anyways the readings I was getting for Transistor #1 were more likely infinite ohms because the tester didn't change at all when leads were applied to the transistor pins. I still think it was the crappy thermal paste, which looked more like white water, that shorted the transistors because the first time I turned it on there was no noise or smoke, just a loud thump when I turned it off...then when I turned it on again that's when I heard the noise and saw the smoke. Do you think that all the transistors are shorted or just one of the new ones? I did test them out of the circuit. And which one of the TO66's would more likely to have burnt up?

thanks

[g1]

I'm having problems reading your last posts...can't seem to get to page 2 on this thread...if I click on the page number 2 it just takes me to page 1 again

In the upper right area of this page, there is a button called 'display'. Make sure it is set to 'linear'.

Aside from the thermal paste, there should be mica (or similar) insulating spacers. They may have got lost, or have cracks/holes. If they are not correct, the transistors will short to the heatsink.