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Post subject: Hum in Bassman 100 Master volume
Posted: Wed Dec 11, 2019 11:10 pm
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OK, my apprentice has a Bassman 100 that hums loudly and crackles between 1 and 2 on the master volume, then cleans up at higher volume. He replaced the volume control with no change. He replaced the .02 and .01 coupling caps on each side of the Master, lifted the capacitor connections from the eyelets, removed the preamp tubes, pulled the .02. All with no change. Pulling the .01 from the wiper of the Master and its quiet.
With both caps disconnected and the connections lifted from the board, he reports 2.6 volts DC on the Master at full volume.
Caps are original and tubes are new.
I'm guessing somehow its bad bias filter caps, but only because we exhausted other possibilities over the phone for the past hour. There's a NFB loop on the PI I haven't seen before, but that .01 was replaced too.
Any ideas??
https://www.thetubestore.com/lib/thetub ... ematic.pdf


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Wed Dec 11, 2019 11:35 pm
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Those "original" caps are at least forty years old. Likely it's time for replacement (the bias-supply electrolytic as well).

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Thu Dec 12, 2019 1:49 am
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No noise a volume higher than 2 !!

Tim, you are the king here, how can anybody help you ?


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Fri Dec 13, 2019 10:06 am
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If a cap job doesn't do it, then I would look at some weird ground loop that goes away as the resistance is increased between the wiper and ground. Are the wires going to the master control coax?


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Fri Dec 13, 2019 10:21 am
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Heater hum balance circuit issue ?


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Fri Dec 13, 2019 5:47 pm
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Captain Obvious strikes again. The two coax wires in and out of the Master are twisted together. They were hooked up with the wiper and the tap mixed up. Not sure why it would do that... might have been some of my work :oops: from back in the day....


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Fri Dec 13, 2019 6:27 pm
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Glad you found it, and thanks for coming back and letting us know what you found.


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Fri Dec 13, 2019 8:00 pm
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vinyl wrote:
Glad you found it, and thanks for coming back and letting us know what you found.


+1

So the amp is performing correctly now?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sat Dec 14, 2019 2:09 am
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I was sure you'll found without our " help" .


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sat Dec 14, 2019 11:04 am
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Well, it was such a unique symptom, I was hoping someone had run across it before. I'm sure I won't forget that one.
I was a hundred miles from the amp that night. The phone conversation was with a hot soldering iron and all the brainstorming we could pack into a single call. Yes, if I had a visual, it might have been sorted out in a few minutes.
Technician error is one of the toughest to sort out. Its just not what we're looking for or expecting to find. Props to my apprentice...


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sat Dec 14, 2019 12:07 pm
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Kudos to you as well, Tim. You're one of the most thorough techs I know.

Hand salute!

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sat Dec 14, 2019 2:12 pm
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" Yes, if I had a visual, it might have been sorted out in a few minutes. "

-Tim

Not in few minutes Tim, in few second.


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sat Dec 14, 2019 4:14 pm
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stratele52 wrote:
Not in few minutes Tim, in few second.


Probably. I'm humbled by anyone who can build an amp by starting with a bare chassis.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sun Dec 15, 2019 9:00 am
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TimsAudio wrote:
The two coax wires in and out of the Master are twisted together. They were hooked up with the wiper and the tap mixed up. Not sure why it would do that... might have been some of my work :oops: from back in the day....

I find it hard to believe that the error was yours, Tim.
Retroverbial wrote:
Kudos to you as well, Tim. You're one of the most thorough techs I know.
Hand salute! Arjay

Which is why I think the error is just as likely to have occurred before the amp left Fender.
I have read elsewhere that in the 70's, the circuit boards were populated and soldered in Ensenada.

I think Tim deserves kudos as well, because he recognizes that working musicians don't have unlimited resources, and makes a conscious effort to not do or charge for unnecessary work.

Working with a flawed schematic, with no layout, there's already 2 strikes against you.

Yes, I think there are certain things "missing" from that particular schematic.

We all learn from posts made here, and I have learned much from Tim, and others who post here.

Many thanks to you.

regards,
vinyl


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Post subject: Re: Hum in Bassman 100 Master volume
Posted: Sun Dec 15, 2019 9:37 am
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The only person who never made a mistake is the person who never did anything. My initials are on the chassis in black Sharpie with the note...Volume control.
That leaves wiggle room, but it still passed through my hands without recognizing the problem.
There is only a limited amount of time in a repair business to thoroughly vet an amp the way I would like. There is a stack of gear behind you with other musicians waiting. Doing 200 amps a year, there are things that get by you. Mistakes are made. Its part of being human. Its partly why I retired.


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