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

So the plot thickens, out of circuit all 4 of H709, H710, H711, and H712 are fine too.

Bench testing says all 4 outputs and all 4 of these are absolutely fine.

The bulb is no longer lit when on.

Could this possibly mean that there's a short somewhere else, that somehow went through the output transistors?
 
test on the board where the junctions solder too to see if still short ..record the ohms reading .

puzzling as i dont see any way it can short apart from solder bridge .
 
What do you mean by "junctions" - sorry not familiar with that term. Thanks

Edit - Oh, you mean measure resistance to ground at each soldered junction of the output transistors. That makes sense.
 
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petehall347 said:
so you removed suspects and test ok . check where they connect to where it showed short .
Click to expand...

Hmm, the only clues as to a short was (1) the hot & smoking R753 and R754, and (2) the bright dim bulb tester. Other than that, I have no other information about the short - so it didn't really show a short anywhere?
 
petehall347 said:
if all components test good there is oscillation . for those resistors to burn .
Click to expand...

Thanks, and I admit this project is the first time I've heard of the concept. But I am not sure how this helps narrow down where to test, or where to place the short? Can you help me by telling me how oscillation would be different from a routine short, and what would I do differently to diagnose it?
 
just measure at the board where the removed transistors that test fine out of circuit that tested bad in circuit .
 
Oh I see, between C and E on H709-H712. Resistance starts at 0ohms, then gradually increases into the MOhms as I hold the leads there.
 
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Maybe the film cap in series with the burnt resistor has shorted? I've never seen one in this network do this but there's always a first time! Of course for a burnt resistor there'd have to be appreciable dc voltage there as well and in a good amp there would be very little. Hmmm.... plot thickens indeed. I'm with Pete that it seems like a solder short.
 
Thanks everyone - power was removed before measuring across C-E, output insulators are there and in good shape, and well, visually to me the solder looks a-ok, but maybe I bridged two things that aren't supposed to be connected? So maybe I should compare to the circuit diagram, step by step, tomorrow.
 
Took the day yesterday to wrench on other things ... turns out I need to order some TO-3 insulators for my NAD 3020A anyway so I'll get an extra 4 to use here.

I was thinking, I could re-install the 8 transistors that are out one at a time and test with the DBT, and see which component causes it to light up.
 
Replacements are in. H709-H712, and the output transistors. It's still broken.

Resistor isn't getting so hot so fast but ... dim bulb test is BRIGHT and connected speakers get rocked with noise so fast they'd surely have blown if I hadn't turned it off.

I tested the DBT first with H709-H712 in, and no output transistors. Practically no light at all.

Put H01 in - still no light.

Put H02 in - some light.

Put H03 & H04 in, screwed the heat sink back into the driver board, clamped the bias diodes back on the heat sink. Bright bulb.

Sigh ... what to check now.
 
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