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Thread: 1976 Phase 90 Caps

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    1976 Phase 90 Caps

    My old P-90 seems to have lost some zing..It seems to not whoosh as deep as it did 40 yrs ago Playing Pink Floyd type riffs it sounds OK,but doing Van Halen type riffs and noise it is not as strong sounding. Would replacing the electrolytic caps solve some of this.. thanks DAB

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    it only has 2 tant polarized caps 4 green.05 caps, 6 741 and a few ceramic caps,and 2 blue mylar.1 and 4 t12n5952 .this might be a script logo P-90

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    Quote Originally Posted by dumbassbob View Post
    it only has 2 tant polarized caps 4 green.05 caps, 6 741 and a few ceramic caps,and 2 blue mylar.1 and 4 t12n5952 .this might be a script logo P-90
    This is the week to work on MXR Phase 90's, apparently. Randall attached the following schematic to help troubleshoot a failing zener diode in his bench project. Yours pre-dates his, having 6 discrete IC's, but this looks like a circuit we can follow.

    Click image for larger version. 

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    Jazz P-Bass provided this version of the schematic:

    MXR_Phase90_1993.pdf

    This later version (1st schematic) replaced the slower/lower bandwidth 741 IC's with 3 Dual TL062 Bi-FET op amps (incorrectly labeled TL061's). TL-072's will also work there, but, as you indicated this Phase 90 has lost it's character over the years, I wouldn't expect the IC character to be at fault.

    C8 & C15 would be your two Dip Tantalum polarized caps. They're not in the signal path, but C8 is the one most likely to fail first. Not an issue at this time. The 4 green 0.05 caps are C1, C3, C6 & C9 in the phase shift network That leaves the few ceramic caps to be C2 (maybe) on the output of Q2, C5 (maybe) on the input, C4 across the bias trim pot and C7 feedback cap across IC1B. I would replace the input cap C5 and output cap C2. The green 0.05uF mylar caps in the phase shift network are probably ok, but, we could replace those as well with new 0.047uF film caps. I'm guessing the blue 0.1uF mylar caps you mentioned...one is the output cap, the other isn't shown on the upper schematic, but is in the lower one, in the feedback shunt network C2. If the ceramic caps are both the input and output caps, then definitely replace those with new film caps.

    Size is an issue with the replacement caps. I think the lead spacing on your board for the film and ceramic caps is 0.2" (5mm). You could try small footprint metallized polypropylene or metallized mylar...I've attached some data sheets to look at, available thru Mouser or Digi-Key.

    WIMA_MKP_5mmLS.pdf
    WIMA_MKS_2-5mm LS.pdf
    Vishay-BC mkp-5mm LS.pdf

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    Last edited by nevetslab; 11-16-2018 at 09:26 PM.
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