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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Aug 14, 2019 8:55 am
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Quote:
to which caps are you referring?
I'm talking about the 5 or 6 small blue electrolytic bypass capacitors in your pictures currently rated at 22uf x 16vdc. I believe yours are all the same date group from the 24th week of 2011. Looking at the picture in my previous post from the front of the amp, it is to the right of the opto-coupler(roach).

As far as testing resistors in circuit w/ power applied for correct Ohm's value...Not a good way to test-IMO. However, testing live VDC voltage values @ resistor points is a good test to determine leakage and stray voltages.

Sometimes when replacing top of the board loaded items those that are connected below the board are displaced. They are out of sight other than the stray wire poking through. I believe if the ground isn't good the tremolo may not function correctly. An easier check(other than installing a jump wire to ground) if you have a good meter, is to just confirm that the solder point is in fact grounded.


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Fender Play November 2019
Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Aug 14, 2019 9:42 am
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sfceric64...

Ok...I just realized that when I start the amp up, come off standby...before the Tremelo kicks in, if I turn the "Speed" knob all the way to 10 I get a motor boating sound from the amp even with all other knobs (including Volume) turned all the way DOWN.
Any time the Speed knob is at 10, the sound continues...even after the Tremelo kicks in. Any time the Speed knob is at 9 or below...no motor boating.

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Aug 14, 2019 10:11 am
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I cannot read the info on your F&T caps under the doghouse from your photos but suspect a full electrolytic replacement would likely improve the overall issues you've mentioned. Especially the lower volume issue. Runaway/motor boating tremolo when on 10 isn't uncommon, fresh caps may help that as well.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 5:17 am
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royb3988 wrote:
Regarding increasing the caps from 16 to 25 or 50...to which caps are you referring?

Those little blue electrolytics... the newest-looking parts.

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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 6:57 am
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Rich,
Thanks for sorting out those pics!
Made the thread worth replying to.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 11:02 am
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The 4700 resistor in the power supply rail appears to be cracked on the end. It may be nothing, but I would replace it while replacing the caps.

A picture of the rectifier/bias power supply board would be helpful. A 50v rating for the bias supply cap is not really adequate, most people change these to 100uF 100v. Make sure the positive end of this cap goes to ground.

Looks like some of the phase inverter resistors might have been replaced, along with the 100 ohm resistor that's part of the negative feedback. ( the one with the "batwing" lead dressing) Also looks like someone maybe added 1 ohm resistors to the cathodes of the power tubes to make measuring bias easier, but I can't tell for sure. There's one visible in the lower left part of your first picture.

My 2 cents. Also curious about the motorboating, does it happen when the footswitch is unplugged, or the tremolo is switched off?


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 11:36 am
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+ 1 on the bias supply board pic
Also noticed the bright switch cap, it should be trimmed to fit better(kinda loopy); cannot tell from the pic angle if it is making contact w/ the brass grounding plate.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 5:16 pm
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Hello...

The motor boating noise ceases when the foot switch is disconnected from the amp or if the Tremolo is not engaged with the foot switch in.

Yes, someone did add 1 ohm resistors to the tube sockets...good eye.

I will get a photo of the rectifier/bias supply board up as soon as I can.

This is what I get for not finding a great tech and staying with him...most repairs seem to be necessary right before a gig and I take the amp to whichever tech is available the soonest.

Thanks,

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 5:58 pm
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Rectifier/Bias Supply Board


Image


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Aug 15, 2019 7:02 pm
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Thanks for the pics.

The more I look at it, I'm not convinced of the color of the second to last band on this resistor:
Image

It should be brown, black, brown, gold. If it's brown, black, yellow, gold then it's 100k ohms, not 100 ohms. It would be easier to tell if it wasn't partially obscured.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Aug 16, 2019 12:01 am
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Vinyl,



Image

Thanks,

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Aug 16, 2019 9:11 am
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RB3988,

Apologies for wasting your time on that 100 ohm resistor :oops:

sfceric64 wrote:
I cannot read the info on your F&T caps under the doghouse from your photos but suspect a full electrolytic replacement would likely improve the overall issues you've mentioned. Especially the lower volume issue. Runaway/motor boating tremolo when on 10 isn't uncommon, fresh caps may help that as well.

+1

F&T caps for the power supply and bias power supply, and Sprague for the cathode bias caps. I'd be careful of getting them off ebay or Amazon, they might be counterfeit Chinese junk. JMO

Regards,


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Aug 16, 2019 9:41 am
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Vinyl,

No apology necessary...time spent learning something new is never wasted.

Thanks for all the help.

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Tue Aug 27, 2019 10:50 am
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Hello again...
If it helps with a diagnosis:

I just confirmed that the Tremolo circuit is slow to come on from the time I hit the foot switch, not from the time I turn the amp on...I just played the amp for about 15 minutes and then pressed the foot switch. It took almost 30 seconds for the Tremolo effect to come on. Once it is active I can switch it off for as long as need be...it comes back immediately when I use the foot switch.

I can still use the amp, it still sounds great.

Thanks,

RoyB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Aug 28, 2019 1:04 am
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Did you even bother to check the coupling, isolation, and cathode-bypass caps in the tremolo circuit?

Arjay

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