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2220 Power Amp Burnt Resistor (R753) - Cause? SOLVED

@Steven Tate Yes exactly - but I am getting conduction from -B to both +C and +E! For each of the 4 of them. No, I did not test out of circuit but I suppose I can ... that said of all the other transistors I've tested in circuit, the test has operated correctly in circuit. Although on these it may be as simple as unscrewing the nut on the C terminal.
 
I’m hoping someone more in the know will chime in on the in-circuit, out-of-circuit question. In the mean time, if it isn’t a giant job, you might pull one completely out and see if you get the same results. Even if all four outputs are toasted, it’s going to raise the question “Why?”
 
You were right, the output transistors once out of circuit tested fine. I tested all the board transistors too and they're all good.

So, ruling out any transistor failure ... back to square 1.
 
Going back to square one...are you using a dim bulb tester? First piece of knowledge you need is whether or not there is a short. A DBT will tell you that. What I’m not sure of is if the DBT would stay bright during oscillation. If not, that might be a way to separate a short from oscillation as a source.
 
A DBT is a simple device you can build from a few Home Depot parts. It’s purpose is to put a 60-100W incandescent bulb in series with the power cord. If there is no short (high current draw) in the unit, the bulb will go bright for 2-3 seconds (as the caps load and current is high), and then go dim, showing low current draw. If the bulb goes bright and stays bright (only give it a few more seconds), there is a short somewhere, and the bulb will be dissipating much of the energy, possibly saving your unit’s components. When making changes inside a vintage unit, you should always have the power coming through a DBT when you first power up to protect the unit in case you have introduced a short. When first turning on a unit new to you, DBT is needed to indicate if anything is already shorted. Once the bulb goes dim, you can disconnect it and go with straight power for your settings. A lower rated bulb, like 60W is used on lower power units, and a 100W may be needed on larger units. Why? The larger units draw more current at idle, and the lower powered bulb may not dim even though there is no short. And, again, only incandescent bulbs work for this. Here is one article on how to build a DBT. Good luck!

http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/
 
I would check to make sure all solder connections on capacitors are good and no open grounds specially in the power supply , which could cause oscillations.
 
Thank you I will do so. Appreciate it.

I forgot one thing - maybe important - that I remembered today. When I first powered it on, I heard a small "zap" and then the resistor started heating up. I wonder what that "zap" could have been.
 
Alright, dim bulb tester built and in service! I am using a standard 100W bulb. With the 2220 it is a little brighter than it is with my Onkyo garage receiver.

So I suppose this must mean I have a short somewhere - right? Since it's drawing more current at idle than my Onkyo is making music?


 
Yeah ,that looks too bright for a 2220 at idle with a 100watt bulb.
When I use a 100w DBT on say a 2270 or 2285b it goes dim (practically out) after initial cap charge.
So I'd say you have something wrong still.

Bob
 
You have a short . You might start by pulling the output transistors out . I know you said you tested them but still pull them out of the unit . Then fire up the unit with the Dim Bulb Tester . See if you get a different result .Hopefully the bulb will dim. You might want to Pull just the left channel ones first and try the "test" . Then the right if your bulb still is bright . That way you can find out if it's a "left channel" issue or a "right channel" issue . Hopefully you will be able to narrow the location of the short. Nice looking DBT .
 
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Disconnected C & B of H001 / H002 and the light stayed dim! On but very dim.

Soo. Must be a shorted output transistor after all.
 
You want to be sure that transistors H710 and H709 are in fine shape before you put in some new output transistors . Outputs can or will take out other components when they short out . The fact that the DBT showed a "dim" glow after turning on shows that you might be OK and you just need to replace the output transistors . Are both channels having the same issue ?
 
Thanks, I will check. I just now checked the other channel's resistor (R754), and it's still getting hot even though the bulb is dim now. I'm going to pull H003 & H004 too and see if the resistor cools off.
 
Took out H003 and H004, now the bulb doesn't even look lit at all and neither resistor warms up whatsoever. Also checked H709-H712 (which are labeled incorrectly in the tech manual by the way), each tests fine except each shows a C-E short in circuit, but they're coupled with a cap in circuit so seems fine. I'll order replacements anyway, just to hedge.
 
All 4 outputs are NPN (H001 - H004), they are 2SC1030. You mean those? It's starting to get weird again - all 4 measure fine out of circuit. Totally fine.

I need to pull H709-H712 and test them.
 
Johnny_Law said:
All 4 outputs are NPN (H001 - H004), they are 2SC1030. You mean those? It's starting to get weird again - all 4 measure fine out of circuit. Totally fine.

I need to pull H709-H712 and test them.
Click to expand...
my bad i am more tired than i thought .
if they are both shorted start wondering why that is so .
 
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