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three possibly unrelated issues / 2325

F

Fisten

New Member
Hey guys.

I have a 2325 that I re-capped a few years ago. Sounds fantastic hooked up to my AR-11's.............

I have three issues, some of which I hope you lot might be able to comment on:

1) It's recently starting to hum across both channels. I pulled the lid and checked the PS: spot on 35v DC. It actually sounds like a short. When I touch (ground) the chassis, the hum diminishes. If I attach a grounding wire, it diminishes (but doesn't go away)

2) While trying to figure out the hum issue, I tested and found the amp biases to be way off, and I can't get either channel to zero out via R713. The trim just wont go that low for me. I was wondering: If trim R740 is WAY out, would that make it difficult/impossible to trim R713 to zero? I know there is some interaction between the two resistors.

3) Lastly, when I power on, one channel emits loud static for about 20 seconds, then completely goes away. It's actually been doing this for a while, but Ive ignored it. Happens every time I power on (except if I switch off, then back on almost immediately).

Any insights into any of the above gratefully welcomed!
 
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On item #3 I had a 2325 have the static issue on power-up and the issue turned out to be the wire-wound Bias/offset pots being faulty. I replaced them and the problem was solved.
 
On #2, bias is not suppose to be zero, it is like setting the idle on a car engine. Maybe you meant DC offset which is measured at the speaker connections. Doubt it though since you are measuring at a resistor on the board.
 
BmWr75 said:
On #2, bias is not suppose to be zero, it is like setting the idle on a car engine. Maybe you meant DC offset which is measured at the speaker connections. Doubt it though since you are measuring at a resistor on the board.
Click to expand...
I do mean DC off-set, yes.....which should be, but isnt zero

And I was wondering if the idle current was way off the prescribed 15mV DC, would that make trimming the speaker connections to zero DC difficult/impossible?
 

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richiem46 said:
On item #3 I had a 2325 have the static issue on power-up and the issue turned out to be the wire-wound Bias/offset pots being faulty. I replaced them and the problem was solved.
Click to expand...
That might be my issue also, as the offset pots do not modify current at all. (and possibly also relates to static on power-up...
 
Fisten said:
I do mean DC off-set, yes.....which should be, but isnt zero

And I was wondering if the idle current was way off the prescribed 15mV DC, would that make trimming the speaker connections to zero DC difficult/impossible?
Click to expand...
It can be difficult to get DC offset exactly at zero. Post 1 in this thread gives some good guidance on DC offset. Yes, Bias adjustments can affect DC offset as indicated in the screen shot you posted above.

Amplifier Distortion, DC-Offset, and You!

Time for one of those long, boring semi-technical posts that no one here reads... As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire...
audiokarma.org
 
BmWr75 said:
It can be difficult to get DC offset exactly at zero. Post 1 in this thread gives some good guidance on DC offset. Yes, Bias adjustments can affect DC offset as indicated in the screen shot you posted above.

Amplifier Distortion, DC-Offset, and You!

Time for one of those long, boring semi-technical posts that no one here reads... As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire...
audiokarma.org
Click to expand...
Thanks for the reply.

I can't get anywhere near zero. Turning the pot all the way still leaves me (edit 0.05 V) out on both channels, which might account for the humming, as there is a constant current put out.

You can see from the screenshot, the channel adjustments are a two-stage process; hence, I was wondering if a very unbalanced idle-current, which will not adjust at all, might be stopping me from being able to trim the main amp DC offset to zero.
 
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Fisten said:
Thanks for the reply.

I can't get anywhere near zero. Turning the pot all the way still leaves me 500 mV out on both channels, which might account for the humming, as there is a constant current put out.

You can see from the screenshot, the channel adjustments are a two-stage process; hence, I was wondering if a very unbalanced idle-current, which will not adjust at all, might be stopping me from being able to trim the main amp DC offset to zero.
Click to expand...
500 mV DC offset is way high. Sounds like the adjustment pots are bad as a previous poster suggested.
 
BmWr75 said:
500 mV DC offset is way high. Sounds like the adjustment pots are bad as a previous poster suggested.
Click to expand...
thanks, yeah, agreed.
Not sure about the mechanics of how those trimming pots work. Would fader lube or deoxit help? Would it hurt giving a squirt or two into the pot mech?
 
Fisten said:
thanks, yeah, agreed.
Not sure about the mechanics of how those trimming pots work. Would fader lube or deoxit help? Would it hurt giving a squirt or two into the pot mech?
Click to expand...
Doubt trying it would hurt anything, but am going on logic instead of experience.
 
I used it on the unit I serviced and it helped for about 2 months but then the crackling at power-up returned so I replaced the pots.
 
Thanks Both. Giving it a spray with some fader lube seems like a reasonable initial step. Will see how it goes!
 
So, lubing the amp trim pots completely eliminated the 20 or so seconds of static upon power up. Still unable to zero the DC across the speaker terminals. bottoms out at about 0.05 V DC for both channels, and the idle current adjustment doesn't adjust at all.

The continuous low hum is gone (could have been that I was testing with the lids removed), but I'm now noticing a faint crackle when any significant base (or maybe its just a power requirement) kicks in. I can hear it on some drum beats.
 
Fisten said:
So, lubing the amp trim pots completely eliminated the 20 or so seconds of static upon power up. Still unable to zero the DC across the speaker terminals. bottoms out at about 0.05 V DC for both channels, and the idle current adjustment doesn't adjust at all.

The continuous low hum is gone (could have been that I was testing with the lids removed), but I'm now noticing a faint crackle when any significant base (or maybe its just a power requirement) kicks in. I can hear it on some drum beats.
Click to expand...
So it's 50mV not 500mV correct? That could be the VRs going, though both at once at the same measurement seems strange.
 
I think this is the one that you need to make sure you have the input coax cable plugged into J701. Part of the PCB gets its ground connection from the cable shield.
 
So, its been about 5 or 6 years since I maintained the front-end pots. I gave them all a squirt with either Fader Lube, or Deoxit, depending on the pot type.

As mentioned, the hum, which I thought was a short, is completely gone. I probably just freaked myself out hearing it with the lid off.

Now, the only lingering issue(s) are: I hear a very very low static, which I can only hear across both channels when no music is playing. Any sound drowns it out.

And the 50 mV coming from both speaker terminals which I cant zero......and that R740 doesn't seem to adjust anything...

JJ, VR's?

dankik............sorry, you lost me with your post there? This one being the 2325?
 
Fisten said:
So, its been about 5 or 6 years since I maintained the front-end pots. I gave them all a squirt with either Fader Lube, or Deoxit, depending on the pot type.

As mentioned, the hum, which I thought was a short, is completely gone. I probably just freaked myself out hearing it with the lid off.

Now, the only lingering issue(s) are: I hear a very very low static, which I can only hear across both channels when no music is playing. Any sound drowns it out.

And the 50 mV coming from both speaker terminals which I cant zero......and that R740 doesn't seem to adjust anything...

JJ, VR's?

dankik............sorry, you lost me with your post there? This one being the 2325?
Click to expand...
Variable Resistors you make the adjustments with.
 
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