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Post subject: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 10:17 am
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i was playing it yesterday and it sounds like it took a crap on me. i was playing it rather loud, and the volume went way down when it was still cranked up. im about to take it apart and see whats wrong with it. also, what chassis do I have? is it the ab763, or the ac5 whatever it is?


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 10:23 am
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Check & test your tubes first, before yanking the chassis. Got any photos of the amp (re: wiring). It'll help determine exact circuit.


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 10:55 am
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here are the pics that I just took a couple of minutes ago:

Image

Image

i hope this helps.


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 11:21 am
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Looks like a AA270 circuit:

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 11:44 am
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More likely it's the 100-watt platform with the master-volume control......

http://web.archive.org/web/200710170523 ... _schem.pdf

HTH

Arjay

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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 12:14 pm
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Arjay, right again. Shoulda counted dials.

Thanks!


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 1:14 pm
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yes it is the 100 watt with the master volume just to clear things up. I should have said that earlier....


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 1:16 pm
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retroverbial, the webpage couldnt open for some reason?


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 1:24 pm
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Fit, not Arjay. But, maybe this will work:

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 1:42 pm
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The "Fender Heaven" schematic site has always been somewhat erratic. But Steve (above) has posted a link to the correct drawing for your amp.

Your amp has lost something critical -- I'd start by inspecting the voltages at the power supply.

HTH

Arjay

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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Mon Jul 09, 2012 2:21 pm
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Fit, a couple questions: Does the pilot light stay on? Are both channels down?( if so, I'd tend to suspect the phase inverter tube.) Art

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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Tue Jul 10, 2012 8:46 am
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the pilot light does stay on, and both channels are down.


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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Wed Jul 11, 2012 3:00 am
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Fit, can you try swapping in another inverter tube(V6)? You could pull (V1) and try that as a substitute for a test, of course the normal channel would be non operative. But your reverb(Vib) channel should be good to go. Art

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Post subject: Re: well it sound like my 1973 SFTR went out on me
Posted: Wed Jul 11, 2012 9:33 am
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Fit, If all the above advice still doesn't narrow down the problem... you need to whip out the old DVM! And take voltage measurements.

I'd concentrate of B+ voltages to the anodes of each tube. And the Bias Supply voltages. Good start. You can pull the tubes --- just remember the actual volts maybe higher than the nominal ones posted on the schematic (wtih no tube and today's outlet voltages). My bet is you have a bad filter cap (dead--- no conductive) or dropdown resistor in the PS.

http://www.webphix.com/schematic%20heav ... _schem.pdf


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