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Post subject: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Thu Jan 16, 2014 2:54 pm
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Anyone had any abnormal Hum & Low (distorted) output experience with Thier amp?

1) Checked All Tubes with known perfect set.

2) Checked screen grid resisters

3) Checked solder joints on tube sockets

4) I noticed when I pull the 12ax7 next to the PI the Hun
Gets louder??

5) Have not checked voltages at pins. :?:


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Thu Jan 16, 2014 3:23 pm
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Sounds like it's ready for new electrolytics.

Has the amp ever been re-capped?

Arjay

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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Thu Jan 16, 2014 3:25 pm
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Ryan41018 wrote:
Anyone had any abnormal Hum & Low (distorted) output experience with Thier amp?
No guitar pluged hum or no hum ? And what about volume ; more volume more hum ?

1) Checked All Tubes with known perfect set.
You put a complete set of 12XX7 and 6L6 tubes or only one tube at time ?

2) Checked screen grid resisters

3) Checked solder joints on tube sockets

4) I noticed when I pull the 12ax7 next to the PI the Hun
Gets louder??
Are you sure of that ? with no guitar plugged ?

5) Have not checked voltages at pins.

Check fiter caps in the " dog house"
:?:


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Thu Jan 16, 2014 10:44 pm
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HI guys - I changed tubes out one at a time.

The hum is consistent (does not increase with at higher or lower volumes).

Tried with and without guitar plugged in (hum is consistent) also tried extention cab (incase there was an issue with speakers)

Caps not changed, did readings and all are within spec.

DID NOT USE CONTACT CLEANER IN SOCKETS but didn't think arc traces would make the amp hum???


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Thu Jan 16, 2014 10:58 pm
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Contact cleaner helps insure good electrical contact between the tubes and socket. If you have carbon tracings or tracks on the sockets, you need to clean them off with rubbing alcohol.

Consistent hum, at any volume setting --- with or without a guitar plugged in --- is a sign that the main power supply caps have dried out and are now filtering properly. If they are original, mid 1990's = around 20 years. About the life of the cap types used by Fender during those years.


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sat Jan 18, 2014 11:37 am
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Used contact cleaner last night. Still Zero.

Pulled Filter caps to test and they test to spec.

Frustrated!! :oops:


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sat Jan 18, 2014 2:14 pm
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How did you test the main filter caps? They could test ok, statically, but not have the reserve needed when in the circuit. I "test" them by the weight of the caps versus new ones by same manufacturer (same mfd). Tired ones will feel lighter (less electrolyte fluid). And need replacing.

Have you measured the B+ values at the tap points off the main PSU? And the anode VDC readings at the tubes?


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sat Jan 18, 2014 4:30 pm
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Ryan41018 wrote:

Pulled Filter caps to test and they test to spec.

Frustrated!! :oops:



How do you test them ? I don't know many people with proper capacitor test equipement


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sat Jan 18, 2014 5:25 pm
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I have proper equipment / and I lift them out of the curcuit.


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sun Jan 19, 2014 12:57 am
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BMW2002Ti wrote:
Have you measured the B+ values at the tap points off the main PSU? And the anode VDC readings at the tubes?


+1

If the filter caps check as "good" then I suggest investigating the diode stack in the rectifier circuit.

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: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sun Jan 19, 2014 3:30 am
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Ryan41018 wrote:
I have proper equipment / and I lift them out of the curcuit.



I'd like to know what you name as " proper" equipment .

It is so difficult or embarrassing to mention what you use and how you test filter caps ?

There are so many people on the forum are sure they do proper test and voltage reading and many days later we read they are wrong since the beginning .

You may understand , to help you, you must answer to our question .


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Post subject: Re: Help - Mid Nineties Twin Reverb 1993 Hum Low Output
Posted: Sun Jan 19, 2014 3:40 am
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Ryan41018 wrote:
HI guys - I changed tubes out one at a time.

The hum is consistent (does not increase with at higher or lower volumes).
Filter cap defectives , heater ground defective , tubes defectives . Sorry If you say these are good ,you are wrong somewhere , It is not a hum it is another noise you can't name it . Or your tests are wrong .


Tried with and without guitar plugged in (hum is consistent) also tried extention cab (incase there was an issue with speakers)
Can't be a speaker

Caps not changed, did readings and all are within spec.
More information please

DID NOT USE CONTACT CLEANER IN SOCKETS but didn't think arc traces would make the amp hum???


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