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

Started by checking voltage at the P700 pins - kinda close to spec (within ~10% mostly). (Ignore that picture above the chart ... that's for the Yamaha YP-D6 also on my bench that's also refusing to work properly :confused:)

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Will go upstream and check H709-H712 next. While the unit is powered on, I've observed that the DBT is lit at a dim+ level, and also that DC offset at the speaker terminals (surprisingly) is pretty low (~5mV). The resistors aren't smoking any more, but there's still a slight burning plastic smell when it's on.

I bought an infrared thermometer off Amazon that'll get here Tues, and hopefully it'll work to measure each resistor's temperature and save me from sticking my fat fingers in there only get shocked and burned.

Also confirmed that the main caps are all wired correctly. There's been mention of a brown wire that passes through the chassis from P700 to the negative terminal of the main cap C006, but mine is black, and intact.
 
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Ok ... I'm not sure what to think now.

When it's on, every single pin on P700 shows as shorted to ground.

Yeah, with my multimeter, using the beeping continuity check, every. single. one. beeps to ground.

Uhh. That can't be right. o_O

When off, I get an initial beep followed by increasing resistance and no beep on 709, 711, and 715 (and counterparts on the left channel).

I have NO idea how every pin could be shorting to ground. :oops:
 
You may be chasing your own tail.You can't use a continuity checker with the power on. There comes a time when you have to take it to a shop and get someone with more experience to help you out. It happens to all of us.Like Clint Eastwood said (a mans got to know his limits)
 
Hmm, fair point LOL! This is how I learn. :rflmao:

And that's exactly what I'm doing here ... hoping those with more experience will give a thought here and there.

Like Tim Allen said - Never give up, never surrender.
 
Today's discoveries:

(1) disconnecting C004 and C005 (the coupling caps) causes the short to disappear completely

(2) R748 and R747 get the hottest (almost up to 200 degrees before I pulled the plug)

What does this mean? I might guess that it means I'm somehow shorting through the coupling caps.

So I may try to lift something else in this path and see if the short is eliminated...

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Well, so far so good. Lifted E of H001 and R747 is much cooler. So it seems I've confirmed that's the correct short path thus far. Now I need to lift C725 and see what happens.
 
Found a cold solder joint at J726, but it didn't solve anything.

Testing continuity from J726 to chassis ground was fine ... but it also tests continuous to the + side of C006. Then I tested directly and confirmed C006 tests continuous across + to -. That seems like a short to me.
 
C006 when removed from the board is shorted out ? It's good to find something "wrong" after so much poking around . keep it up . We are all interested.
 
Nope, it wasn't shorted after all - I must not have waited for the capacitor to fully discharge before checking continuity. Another lesson learned.

Finally, back around to lifting C725 which I'll try tonight.
 
Well finally what might be a solid lead. Instead of continuing to lift leads, I first wanted to verify that all of the grounds that are supposed to be grounds (and nothing that wasn't supposed to be grounded) was correct. All was good EXCEPT for - guess what - R753 and R754! Those are the resistors that were smoking on day 1.

What I found was that neither side of R753 nor R754 tests continuous to ground. Instead, on both sides of both, it's about 150 ohms to ground.

(Edit: actually, on the capacitor side it's 158 ohms, on the other side it's 148 ohms, which makes sense since insofar as they're 10 ohm resistors - note that the capacitor goes to ground, NOT the resistors, as documented below... but I think it should be completely open to ground?? As I mention below)

One side of each is supposed to be grounded, the other side going to the 0.1uF ceramic caps that go to C004 and C005 (and are supposed to prevent oscillation?).

Next up is to figure out what the heck is causing that to happen.

Oh, and the rest of the grounds are indeed intact and correct.
 
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OOOOooookay. Duped by the factory P700 circuit diagram. Should have been looking at the PCB layout. The circuit diagram has swapped R753 with C725, and R754 with C726! If you look at the PCB layout, you'll see that C725/C726 are grounded, not the resistors.

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THAT BEING THE CASE. Not sure if ~150ohms to ground on the C004 / C005 side of those resistors is OK. Those go to the + side of speakers ultimately ... THIS SEEMS LIKE A SHORT!!!!!!!!!!!!!!
 
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Maybe not. :( Looking at the diagram, I think I figured out what's going on. It's grounding through the headphone jack through 100ohm + 47 ohm resistors (if I'm reading that correctly) which ... is exactly what I'm seeing on the multimeter.

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OK ... this is my #1 priority on the workbench. It's been stumping me for too long!

HOWEVER. I just found 7.5AC volts from both of the filter cap positive terminals to ground, and 1.5VAC from the negative terminals to ground.

From the little I know about this, this is not good.

But what does it point to? I am not sure. Input appreciated!

Edit: I found a thread by @Steven Tate describing a similar issue in his 2245 here, and he was able to fix it with new filter caps (as far as I can tell).
 
This was a trip down memory lane where I learned a lot. I think you test AC across the terminals of each individual cap. But, as Tom said, some DVM’s may not block DC, so your reading might not be just the AC. AC should only be about 100 mV across the terminals. Another poster said to measure DC from terminals to ground and they should be the same. I didn’t try that since I was learning to use my scope and found the problem right away with that instrument.
 
Thanks - I couldn't get an A/C reading across the terminals, the value just bounced around. I did get a DC reading, at 18-19VDC across the terminals of the smaller (filter?) caps, and 36VDC across the terminals of the main cap.

Edit: I measured A/C at the speaker terminals, and yep, 1.3VAC across the speaker terminals.
 
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Something strange that I noticed, this is a 2220 and appears to have the dubbing inputs like the 2230 and up got, but I've never seen any other 2220s with them. "Phones" is screened on the wrong spot, the headphone jack is under the power button. Weird.

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This one is definitely not like the 2220 I rebuilt. In fact, somehow I didn’t realize we were talking 2220. I thought it was a 2245. The 2220 is a cap coupled design, like the 2230. The two smaller of the big caps are coupling caps - not filter caps. The larger cap is the filter cap - there is only one in this design. The biggest difference in this design is that there is no DC offset to test and set at the speaker terminals. If you read DC at the speaker terminals with no speakers attached, you will probably get something north of 20 volts. In direct coupled amps (most Marantz models above the 2230), you will get something near zero mV DC at the speaker terminals if things are set properly. AC at the speaker terminals is normal, as it is AC that drives the speakers, and it will vary with volume.
 
This one is definitely not like the 2220 I rebuilt. In fact, somehow I didn’t realize we were talking 2220. I thought it was a 2245. The 2220 is a cap coupled design, like the 2230. The two smaller of the big caps are coupling caps - not filter caps. The larger cap is the filter cap - there is only one in this design. The biggest difference in this design is that there is no DC offset to test and set at the speaker terminals. If you read DC at the speaker terminals with no speakers attached, you will probably get something north of 20 volts. In direct coupled amps (most Marantz models above the 2230), you will get something near zero mV DC at the speaker terminals if things are set properly. AC at the speaker terminals is normal, as it is AC that drives the speakers, and it will vary with volume.

Oh I see thanks - two coupling caps, one filter cap. But ... there's a loud hum from the speakers when connected. When I measure DC offset with no speakers connected, I get next to nothing (something like 18mV), which I thought was normal. So I guess that isn't normal! Huh.
 
I’m not an expert on the theory behind these things, but I think I read somewhere that DC measurements at the speakers - no speaker connected - would result in roughly one half the filter cap voltage. On a 2230, I have usually seen readings in the 25V DC range. The 2220 would be somewhat less, I would think. If you notice, there is no DC Offset adjustment procedure in the 2220 and 2230 Service manuals. The 1030 and 1060 integrated amps are the same design. Hopefully someone who actually understands this stuff will chime in. :biggrin:I
 
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