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Marantz 2015 Receiver Distortion

WHONOES

May 20, 2017
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Ah ha! The Marantz 2250 has the same SV-3A's
OK but, don't go ripping stuff out willy-nilly. Lets see if we can get a handle on what's going on first.
It's now early evening here so I am off out. Back on it tomorrow.
 

Ylli

Jun 19, 2018
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Don't mean to disrupt Whonoes ideas, keep following them.

I would next measure the voltages at the base of H615 and H617.

With power off, measure the combined in-circuit resistance of R627||H605. Try it with the meter leads one way, then switch them and measure again. Do this with R627 at both extremes of rotation.

With power back on, measure the voltage at H603 emitter.
 

KilgoreCemetery

Apr 12, 2017
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OK but, don't go ripping stuff out willy-nilly. Lets see if we can get a handle on what's going on first.
It's now early evening here so I am off out. Back on it tomorrow.

Woops! I put my on willy-nilly pants before I found your message. Good news is, I figured out where the constant buzzing was coming from on the Marantz 2250 receiver. That's why it was a parts unit in the first place. Bad news is, I don't have a spare set of good SV-3A's after all.

I'll put the original ones back and get you those voltage readings for tomorrow.
 

KilgoreCemetery

Apr 12, 2017
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If you can get to the bases of H611 and H613, put you dmm on those points and measure the voltage. It should be close to four Vbe's or 2.4V. Maybe a little more or a little less but not substantially so.

Base to base:
H611 to H613 = 400mV
H612 to H614 = 388mV

I would next measure the voltages at the base of H615 and H617.

Base to ground:
H615 = 25.78V
H617 = 23.89V
H616 = 27.65V
H618 = 25.52

Base to base:
H615 to H617 = 1.88V
H616 to H618 = 2.02V

With power back on, measure the voltage at H603 emitter.

600mV


With power off, measure the combined in-circuit resistance of R627||H605. Try it with the meter leads one way, then switch them and measure again. Do this with R627 at both extremes of rotation.

Zero ohms at one extreme and 156.8 at the other. Switching the leads around gives pretty much the same reading.
R628 is very similar as well. Zero to 159.9 ohms. I also sprayed these trimmers with deoxit and worked them back and forth before I posted on here, so it does seem like the resistance builds and declines gradually like it's supposed to
 

Ylli

Jun 19, 2018
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Base to base:
H615 to H617 = 1.88V
H616 to H618 = 2.02V

I should have specified where to set the pot for this measurement. Please try it with the pot set to maximum resistance.

We are looking for about 2.5 volts (three B-E drops) between the bases of H615/H617. You measured 1.4 volts across H701 (which is good). H603 appears to be drawing 6 mA through the bias network which should drop up to 1 volt (6 mA * 160 ohms) across R627/H605. That should give 2.4 volts base to base.

Anything getting suspiciously hot? (Careful, 50 volts present).
 

KilgoreCemetery

Apr 12, 2017
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Sorry for the delayed response. New school year and all that.

I should have specified where to set the pot for this measurement. Please try it with the pot set to maximum resistance.

Base to base:
H615 to H617 = 2.19V
H616 to H618 = 2.14V
 

Ylli

Jun 19, 2018
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I'm scratching my head.....

Does R621 *look* OK? The voltage drop across R623/R625 is about a volt less than it should be. The voltage drop across H701/H605/R627 is lower than it should be. That all points to the current through this series section is too low.

Yet the 0.6 volts at the emitter of H603 would indicate there is 6 mA - as long as the emitter resistor is indeed 100 ohms.

And really confusing is that this is happening on both channels which would indicate a global rather than local issue.

Have we measured the 50 volt buss yet? Have you tried adjusting R615 toward a lower resistance, raising the base voltage (and emitter voltage hence current) of H603?
 

KilgoreCemetery

Apr 12, 2017
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To hopefully improve clarity, this is the DC circuit we are looking at:
All the voltages listed in your drawing are within reason, if a little high. The one exception is the half a volt on H617 & H618 measures closer to 1.5V

Also, I'm not really finding anything hot. H615 - H618 are warm, but they're also larger than the other transistors. None of them are hot enough to burn a finger
 

KilgoreCemetery

Apr 12, 2017
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Does R621 *look* OK? The voltage drop across R623/R625 is about a volt less than it should be. The voltage drop across H701/H605/R627 is lower than it should be. That all points to the current through this series section is too low.

Yet the 0.6 volts at the emitter of H603 would indicate there is 6 mA - as long as the emitter resistor is indeed 100 ohms.
R621 doesn't appear damaged or anything. Brown, Black, Brown, Gold = 100 ohms. Tests at 101.5 ohms. It's counterpart at 99.5 ohms.

Have we measured the 50 volt buss yet?
The voltage at J605 is 49.6V if that's what you mean.

Have we measured the 50 volt buss yet? Have you tried adjusting R615 toward a lower resistance, raising the base voltage (and emitter voltage hence current) of H603?
As I adjust it, the waveform appears to smooth out a little, then goes back. I get the same result no matter which way I adjust it
 

Ylli

Jun 19, 2018
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the half a volt on H617 & H618 measures closer to 1.5V

1.5 volts to ground on the collectors of H617 and H618? We may have found a clue.
Do we also have 1.5 volts to ground on J612 and J614? How about on the bases of H002 and H003? Can you measure a voltage at the emitters of H002 or H003? Those *should* be at ground potential.
 

KilgoreCemetery

Apr 12, 2017
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Ok, so.. stupid question time. What is R627 supposed to actually be adjusting? The service manual says to connect to both sides of R647 and adjust until you get 7.5mV. No matter what I do, it reads zero. But, if I check the resistor itself, I get 0.5 ohms
 

Ylli

Jun 19, 2018
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R627 is adjusting quiescent current in the output pair. A class AB amplifier has each output transistor conducting a bit more than 180°, so there needs to be some current flow even with no signal present. If there is no quiescent current (which is your case) you will get crossover distortion.

When you recap'd, did you remove the amp board? Any chance you got the base and emitter connections of H002 and H003 reversed? Right now you are measuring a base-emitter voltage of 1.5 volts on the low side output transistors. That is not right for a forward bias P-N junction. So H002 and H003 are miswired or they are defective.
 

KilgoreCemetery

Apr 12, 2017
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I did have the output transistors removed. They're TO-3 ones that are mounting right to the heat sink with jumper wires going to them.

Any chance you got the base and emitter connections of H002 and H003 reversed?
At first I was like "there's no way", but the more I look at the schematic, the less I understand what pins jumper to where. For example, what pin does the collector of H001 go to?

Either way, yeah.. they appear to be reversed. The current setup matches the picture I took before disassembly, so I don't feel like a complete tool, but I'll swap them around and see if it fixes it
 

KilgoreCemetery

Apr 12, 2017
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Before I do this, though.. What happens if the schematic has the jumper numbering wrong? I do still have the current limiter, but the "before" picture that I took of the back of the heat sink appears to have factory soldering on it. I had just removed the first two nuts/bolts, pulled the collector wire out of the way and remembered to snap a picture
 

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KilgoreCemetery

Apr 12, 2017
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Ok! I did it and now adjusting the trim pots actually makes a difference. No more distortion while under load.

Thanks Ylli and WHONOES for helping me figure this out. I appreciate it
 

Ylli

Jun 19, 2018
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Congratulations! With the low side outputs effectively disabled, I'm a bit surprised you got much of an output at all.
 

KilgoreCemetery

Apr 12, 2017
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You and me both. I can't believe it didn't just shoot sparks the first time I powered it up. It wasn't even drawing much current and with just the headphones, you never would have guessed there was anything wrong.
 
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