First time restorer sanity check

[asolochek]

Hi,

I recently picked up a non-functional marantz 2238b and I've gotten it mostly working, but there is significant 60Hz noise/hum. Is that kind of expected when you're operating an amp all disassembled and splayed out on a bench? Basically, I'm not sure if this is a problem that I should be concerned about at this point. Since putting this together and taking it apart again is a pain, I'm trying to avoid having to do that as a check. This is the noise:


Is this something I should be concerned about, or no?

 

[Blue Shadow]

Have you changed any components in it?

 

[DatunWalnut]

I certainly wouldn't rule it out. I've had terrible hums just through not tightening down a heatsink or a volume pot.

 

[asolochek]

Have you changed any components in it?

I replaced all the electrolytics (and some lamps and wiring). I'm going to check a few of the film caps to see if they are still within spec.

Also in the past someone has added a bunch of extra caps to the unit -- a 0.047uF between ground and -36V, a 10uF between ground and the relay coil, and 3x 0.01uF between the chassis and grounds on the phono, aux, and the insert RCA jacks. I don't know what issue they were trying to solve with those.

 

[kwayde]

Some Marantz do that with the covers removed.

Significant is subjective.

There is a forum for just Marantz you may want to have this moved to.

 

[elvi]

I replaced all the electrolytics (and some lamps and wiring). I'm going to check a few of the film caps to see if they are still within spec.

Also in the past someone has added a bunch of extra caps to the unit -- a 0.047uF between ground and -36V, a 10uF between ground and the relay coil, and 3x 0.01uF between the chassis and grounds on the phono, aux, and the insert RCA jacks. I don't know what issue they were trying to solve with those.

The extra small value caps are to improve ripple on the main filter caps, and the .01s to DC isolate inputs a touch noise.
I do these mods to my kit.

Yes, the chassis does shield induced hum. Try powering down all powered items in the area, and use headphones, no attached speaker wire.
The advice to move the thread to the dedicated forum is spot on

 

[asolochek]

Some Marantz do that with the covers removed.

Significant is subjective.

There is a forum for just Marantz you may want to have this moved to.

Yeah, I considered posting it there, but I thought this question wasn't actually brand specific and was hoping to get answers from pros who might not read all the manufacturer forums. But if that is the appropriate place I can move it (is that something I can do, or do I need to request an admin to move it?)

 

[kwayde]

Yeah, I considered posting it there, but I thought this question wasn't actually brand specific and was hoping to get answers from pros who might not read all the manufacturer forums. But if that is the appropriate place I can move it (is that something I can do, or do I need to request an admin to move it?)

At the bottom left of your posts there's a Report link. Just click that then ask the mods to move it for you. It will be done.

 

[ocdecio]

This may be a tad late to say, but I never, ever, disconnect any wires or boards unless absolutely necessary. It is very easy to introduce all kinds of noise, hum you name it by not properly replicating the factory grounding.

 

[asolochek]

Yeah, it wasn't on purpose

This may be a tad late to say, but I never, ever, disconnect any wires or boards unless absolutely necessary. It is very easy to introduce all kinds of noise, hum you name it by not properly replicating the factory grounding.

yeah, it wasn't on purpose, but one of the wires broke. It's a bummer because I don't think that wire is made anymore.

 

[Title17]

yeah how significant is significant? Like others have said when they're apart its something that can happen.

I've had my fluorescent desk light cause noticeable audio interference on these receivers while working on them.

 

[dankik]

@asolochek, where did you measure this signal, on the audio output signal? Was the volume turned all the way down?
Your noise spikes are 160usec apart, with a max of 1/2 volt.
160usec = about 6,250Hz, so these spikes are not 60Hz line voltage hum.
From your scope, it looks like an impulse noise spike (with dampened ringing), with alternating polarity. I don't think this is internally generated by the 2238B.
Do you have any LED lights, WiFi routers, cellphones, nearby that could be causing EMI interference (like @Title17 said)

 

[asolochek]

@asolochek, where did you measure this signal, on the audio output signal? Was the volume turned all the way down?
Your noise spikes are 160usec apart, with a max of 1/2 volt.
160usec = about 6,250Hz, so these spikes are not 60Hz line voltage hum.
From your scope, it looks like an impulse noise spike (with dampened ringing), with alternating polarity. I don't think this is internally generated by the 2238B.
Do you have any LED lights, WiFi routers, cellphones, nearby that could be causing EMI interference (like @Title17 said)

View attachment 3503262

The 100 us/div is the bottom zoomed in window. Those spikes are actually occurring at 120Hz (I just verified that). And that was probed at the speaker output.

 

[dankik]

Thanks for the clarification. Did you check/set the DC bias? Also, check the DC offset at the speakers.

 

[asolochek]

Thanks for the clarification. Did you check/set the DC bias? Also, check the DC offset at the speakers.

View attachment 3503674
View attachment 3503678

Yep, I have that dialed in to within tenths of a millivolt. I think the original issue was caused by how I had things arranged on the bench. Now, although I can still see the tiny spikes at 120hz on the scope, there is almost no hum on the line level inputs unless you put your ear right next to the speaker. The phono, on the other hand, does still hum, so I'm going to look into that one tomorrow.

 

[GlasgowGrip]

Now, although I can still see the tiny spikes at 120hz on the scope, there is almost no hum on the line level inputs unless you put your ear right next to the speaker. The phono, on the other hand, does still hum, so I'm going to look into that one tomorrow.

Having all the shields and covers in place can only help. So are you listening for the hum with those inputs disconnected? What happens if you short the inputs (use 600 ohms shorting plugs)? And not surprising at all that you would pick up some hum on disconnected Phono inputs given the roughly 40dB extra gain of the Phono amp.

 

[asolochek]

Having all the shields and covers in place can only help. So are you listening for the hum with those inputs disconnected? What happens if you short the inputs (use 600 ohms shorting plugs)? And not surprising at all that you would pick up some hum on disconnected Phono inputs given the roughly 40dB extra gain of the Phono amp.

I did have the phono inputs connected to a powered off exteral preamp at one point, but some of the noise was noticed while they were disconnected. But I just put the whole thing back together and now there is very very little noise on the phono, and on the other inputs you can only even hear a hiss if you put your ear right next to the speaker and crack the volume, so I'm quite happy with the state of it now!