Crate GFX 212 very low distorted output

[doctorfate]

I have a Crate GFX 212 I'm working on for someone with a just barely audible distorted output , the type of amp which I often just suggest someone ditch and pick up a new one as the repair may cost more than the value of the amp. but as we know at times people still want it repaired.
So for anyone having the schematic handy, I've replaced R51, 55 and 63 which were all burnt ( though tested good) along with Q11 that was blown off the board and D21 shorted. Since the amp wasn't blowing fuses I hoped the Output trannies still good.
Unfortunately the problem is still there and some major heat coming off the replaced parts. So I pulled the TIP 147 pair of outputs to check them, although not completely shorted B-E , there is a very low resistance less than 1ohm and they conduct both ways with diode tester.
Would it be likely to assume these two power transx'r are the problem? I don't have any in my parts at this second so an opinion is appreciated.
Thanks

[52 Bill]

although not completely shorted B-E , there is a very low resistance less than 1ohm and they conduct both ways with diode tester.

These Darlingtons will test just like any other transistor, but you will also find a diode junction type reading from C to E.

If they read low in both directions between any two terminals they are shot.

[doctorfate]

Yeah thanks Bill, I had pretty much concluded that pair was shot with an ohms reading that low and diode tester conducting both directions on that junction, just figured it'd be blowing a fuse if they were that shorted. I guess I won't know for sure if that'll be all the issues with the amp until I grab two of them to pop in there. 3

[Enzo]

Look up a data sheet on the TIP142/147. It will have the internal schematic. The part is not just two transistors wired together, there are also resistors inside it. You can read them with an ohm meter. But they won;t remotely be as low as 1 ohm.

[doctorfate]

Ok I finally got some T147's in although they are a much smaller package than the existing ones assume that they're spec'd the same as I can find no others. anyway upon installing them my dumb ass thought I could just use grease on the heatsink as I had no silipads. OOPS , the current limiter went bright and when I pulled the speaker load still(as I didn't mean to have it connected) on the limiter something sizzled.

SO immediately realizing my mistake I undid the 147's from the sink and fired up again, This time the limiter was fine... No dc on the output to speak of. Surprisingly the replacement tranny's seem fine in there voltages are C -41.5v B 1.3v E .9v

HOWEVER the T142 seem to be the problem now! C 41.5v B .9v E 75mV
so I don't know yet exactly what happened when it sizzled but seems to me it's between the B/E of the T142's. I tested them for shorts and they seem ok. I'm gonna go pull the MPSA06 driver now and test it but ideas would be great.
I'm currently kicking myself.

[Enzo]

OH. TIP142/147 come in several packages. The small ones are TO220. The body about a half inch square plus tab. There are similar looking but larger ones in a TO218 package. Those are the most common in amps. Apparently some makers call the TO218 an SOT-93-3. Note on those the metal tab is exposed completely. There is a very similar to TO218 in size type called TO3P. Looks like those are not around any longer, but would work fine. Then there are also the larger TO247. They would work too as long as the larger size fits on your heat sink.

MOST transistors come in one size, if they want to make a larger or smaller one, they usually give it a different number. But obviously not always.


The tab on the transistor is connected in the circuit directly to the power supply. It needs therefore to be isolated. However, if you screw it to the grounded heatsink, all it generally does is short the power supply to ground through the screw. It really shouldn;t hurt the transistor.

[doctorfate]

Yeah the replacements are definitely TO220 package , the existing ones were just a larger version , the T0218 apparently.
As far as mounting them uninsulated, yes the new trxrs survived that. I am just confused on what is going on with the voltages on the T142 emmitters now. I find no burnt traces or bad components around there yet. I definitely heard something sizzle. There is no output from the speaker at all now.
thanks

[doctorfate]

No ideas on what could be going on here looking at these voltages? I'm gonna get around to checking them again hopefully tonight. thanks

[doctorfate]

I pulled this back out after putting it to the side and couldn't believe I missed that there's no negative rail vltg present across R90 (see schem) The resistor checks good in crkt and I resoldered all the components involved here but still no voltage produced across it and the resistor does get red hot compared to the dropper on the pos (+) side.
The 40v is present both positive and negative at the rectifier btw.
I guess I could cut the epoxy away and replace that 270ohm resistor anyway but I am thinking it's not the resistor. ideas

[52 Bill]

If there's no voltage on the output side of that resistor, something is shorted and is causing the voltage to drop. This is why it is heating up, all -40 volts are being dropped to zero across it.

Check D26 and C44 for shorts. If they are ok, then follow the rail through the amp until you find the shorted component. Are any of the chips getting hot?

[Enzo]

Um, if it is getting hot, then there most be voltage across it. If there is 40v at one end and zero volts at the other, there is 40v across the resistor. And about 6 watts dissipating. Yeow.

If there is no -15v at the end, then there is no voltage across D26 and C44. And then what Bill also said about checking them. More likely a shorted zener than anything else, but you never know.

That's a 5 watt zener.

[doctorfate]

sorry guys , yeah I'm still messing with this one. I initially tried replacing the resistor , even though it tested to be fine, as well as the 16v zener to no avail. I lifted the dropping end of the 270ohm resistor to make sure voltage is indeed getting across it and it does read -40v when freed up. This made me think that something is shorting the -15v to ground on down the line, I am expecting that to be a blast to try and find. One last component check I'd neglected before was C44 which showed a dead short across it! I hoped it was a shorted cap but I still measure a dead short across c44's pads with it removed from crkt.

so I think I'm close but haven't been able to track down this apparent short. ideas?

[52 Bill]

Like I said before, any of the chips getting hot?

That -15 volt rail will probably feed every chip on the board. One of them may be shorted. I find that when they are shorted, they will get hot to the touch.

It's not always a chip, but that's the first thing that I usually check. I've also found shorted bypass caps to cause this.

Start tracing the -15 rail starting at C44. When you come to a part that is connected to the rail, find a way to check it. If it's a cap remove it or unsolder one leg. Is the rail still shorted? If it is, then resolder the part and move to the next component on the rail.

Sometimes there are jumpers that connect the rail to isolated sections of the board. If you find one of these unsolder one end and see if the short is cleared. If it is then you know that the short is in that isolated section. If not, them move on to the next section.

Worst case, find the approximate center of the amp circuit and cut the pc trace carrying the -15 volt rail. If the short clears, then you only have to check half of the circuitboard.

[doctorfate]

Thanks Bill.
IC's were the first things I checked on this one cause I knew something wasn't right with the preamp, none were heating up but I replaced a couple from suspicion. Good tips for tracking it down. It's one of those real aggravating layouts , to me at least.