What exactly is this part?

BradtheRad said:

As a few have already said, you'll probably find a shorted output transistor, whichever one is closest to your .22 resistors.

Blown output transistors are typical with an audio amp that was played too loud too long. Particularly if it was playing through low ohm speakers. Can you confirm whether the speakers were 16 or 8 or 4 ohm? Did either L or R channel have an additional speaker connected besides the main speaker? This should be avoided after you fix the unit.

You can do preliminary testing of the transistors without unsoldering them. Simple ohm readings will tell you a lot.

Warning: DO NOT use the lowest ohm setting. This could send overmuch current into a base terminal or other parts of your circuitry. (My analog meter has an internal 1.5V battery providing 60mA at the lowest ohm setting. My digital meter is different.) I don't know about your meter. Just stating this as a precaution.

Pull out the power plug. Turn on the amplifier. This should drain all capacitors. Wait 30 seconds. Check for voltage at the power supply capacitors. The idea is to avoid getting voltage at your meter.

Compare all transistor resistances across their three terminals. Put your leads across them using all six permutations. You're looking for the oddball. Probably a shorted one.

A normal transistor shows low ohms in 2 instances when it's off. Both instances are through the bias terminal. One way for NPN, the other way for PNP.

Click to expand...

Got my multimeter!

Following these steps, I found that every transistor has 2 instances of low ohms except for two in one of the amp circuits. It's in a circuit in question (happens to be the one I took the resistor out of) and whenever I try to find the Ohms, it shows zeroes across all six combinations.
Both of the transistors in this circuit do the same thing, while the rest of the transistors in the other amp circuits do as they're supposed to. (The rest of them show, as mentioned, 2 instances of ohms. Which I'm assuming is supposed to be that way.)

Is this a good troubleshoot? Should I see into ordering some new transistors? Is there anything else I can do?

I suppose I should also add: Despite staying on the same multimeter setting, I couldn't get the same resistance on a given pair of identical transistors. Does this mean, potentially, that another set of transistors is needing to be replaced?
Is there a resistance I should be looking for to make sure they're properly working?

Thanks for any responses!

Your results indicate you've tracked down one problem. Two shorted power transistors.

That explains blown fuses. By the way did you make sure the speakers aren't blown?

At this point you can find out a lot if you unsolder (or clip away) the faulty transistors. You may get sound back in the other channel(s). Assuming you have spare fuses.

It's to be expected that you won't get the same ohm readings on a pair of good transistors. An NPN type won't necessarily read the same as a PNP. Moreover since you're testing in circuit there are unknown nearby resistances in the mix.

It's up to you whether to get exact replacements. Of course that would be ideal because you need the same size, same terminal-to-pin designations.

You need the same or higher V and A and W ratings.

And preferably identical gain as you might get if you order a pair of matched transistors. If they can read 55P hfe (per post #17) so much the better.

As you remove each faulty transistor, you must look for a pad of clear mica insulator between it and the heatsink. It could be almost invisible. Avoid breaking the insulator since you can reuse it. Assuming it's there to begin with, the insulator has to be there when you install the new transistor. If it's absent then the new components will blow as soon as you turn on the amp (unless the fuse is quick to act first).

For maximum cooling, thermal heatsink grease should go between all surfaces. If you can scrape up enough grease to reuse then you might not need to buy a tube.

Guess you know that even with good transistors there's a chance something else will need fixing.

Your testing methods with the ohmeter are not clear to me. Does your meter have a diode test feature? A bad transistor will usually fail short meaning you will get a short circuit reading in both directions between at least two of the terminals.

It's extremely unlikely that you'll find the original Sanken components much less the same lot number. The NTE parts are a good match. They are available at Mouser Electronics for about $9.00 each. It's likely that other suppliers will also have them. The original Sanken datasheet specifies mica and the white silicone thermal grease is evident in photos of the unit. Buy new grease. The old stuff is dried out so it can't spread and conform correctly to the surfaces.

I figured I'd have to buy new grease anyways. It's not terribly dry as you can still spread it, but I know I shouldn't cheap out.

I also figured I wouldn't get new original transistors, either. I figured something else would work.

In terms of the speakers: pretty sure they didn't blow out, as he's using the same speakers with a new receiver just fine.
Also: Turns out my brother did end up testing this receiver before I got it, and the all of the front channels are the ones that didn't work after this problem arose, and I'm assuming this is the circuit that controlled those front channels. (That means that the rear and MAYBE the subwoofer were fine. I say maybe the sub because I don't know if that worked after or not.)

My multimeter does have a diode test feature. I'm going to test them now and see what happens. Will edit soon after.

EDIT: Just tested with the diode tester. Both of the questionable transistors gave a short when going between different terminals on two separate occasions. Does this mean it's bad? I'm confused about the test.

Mr. Fax Sender said:

EDIT: Just tested with the diode tester. Both of the questionable transistors gave a short when going between different terminals on two separate occasions. Does this mean it's bad? I'm confused about the test.

Click to expand...

The transistors sound like they're shorted.

The diode test should read 600 mV (give or take) through the bias terminal of a good transistor. All other readings should be high ohms.

That's the short and simple test. It's helpful to look at diagrams of the hookup (talkingelectronics.com website):

PNP or NPN?

Testing good transistors will let you know what to look for. However when you do it in circuit your readings may be thrown off by nearby components known and unknown.

If I am using the diode tester on a bad transistor (shorted or open), I lose track as to whether the reading changed to ohms or is still in volts. So I seldom use the diode tester.

Mr. Fax Sender said:

Just tested with the diode tester. Both of the questionable transistors gave a short when going between different terminals on two separate occasions. Does this mean it's bad? I'm confused about the test.

Click to expand...

Be sure to read the instructions in your meter manual on the use of the diode test feature. Darlingtons are a little harder to test because they often have internal resistors and diodes. You have the data sheet for these devices and known good devices in a parallel channel for reference. If testing in circuit, be sure you've replaced the resistor that was previously removed.